/*
 * Copyright (c) 2013, 2018, Oracle and/or its affiliates. All rights reserved.
 * Copyright (c) 2019, Azul Systems, Inc. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.lang;

import java.io.InputStream;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.net.URL;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.CodeSource;
import java.security.PrivilegedAction;
import java.security.ProtectionDomain;
import java.security.cert.Certificate;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collections;
import java.util.Deque;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Hashtable;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.Vector;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Supplier;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

import jdk.internal.loader.BuiltinClassLoader;
import jdk.internal.perf.PerfCounter;
import jdk.internal.loader.BootLoader;
import jdk.internal.loader.ClassLoaders;
import jdk.internal.misc.Unsafe;
import jdk.internal.misc.VM;
import jdk.internal.ref.CleanerFactory;
import jdk.internal.reflect.CallerSensitive;
import jdk.internal.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;
import sun.security.util.SecurityConstants;

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the binary name of a class, a class loader should attempt to
locate or generate data that constitutes a definition for the class.  A
typical strategy is to transform the name into a file name and then read a
"class file" of that name from a file system.
 Every Class object contains a reference to the ClassLoader that defined it. 
 Class objects for array classes are not created by class loaders, but are created automatically as required by the Java runtime. The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type; if the element type is a primitive type, then the array class has no class loader. 
 Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes. 
 Class loaders may typically be used by security managers to indicate
security domains.
 In addition to loading classes, a class loader is also responsible for locating resources. A resource is some data (a ".class" file, configuration data, or an image for example) that is identified with an abstract '/'-separated path name. Resources are typically packaged with an application or library so that they can be located by code in the application or library. In some cases, the resources are included so that they can be located by other libraries. 
 The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will usually delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. 
 Class loaders that support concurrent loading of classes are known as
parallel capable class loaders and are required to register themselves at their class initialization time by invoking the ClassLoader.registerAsParallelCapable method. Note that the ClassLoader class is registered as parallel capable by default. However, its subclasses still need to register themselves if they are parallel capable. In environments in which the delegation model is not strictly hierarchical, class loaders need to be parallel capable, otherwise class loading can lead to deadlocks because the loader lock is held for the duration of the class loading process (see 
loadClass methods). 
 Run-time Built-in Class Loaders
The Java run-time has the following built-in class loaders:

Bootstrap class loader. It is the virtual machine's built-in class loader, typically represented as null, and does not have a parent.
Platform class loader. All platform classes are visible to the platform class loader that can be used as the parent of a ClassLoader instance. Platform classes include Java SE platform APIs, their implementation classes and JDK-specific run-time classes that are defined by the platform class loader or its ancestors. 
 To allow for upgrading/overriding of modules defined to the platform
    class loader, and where upgraded modules read modules defined to class
    loaders other than the platform class loader and its ancestors, then
    the platform class loader may have to delegate to other class loaders,
    the application class loader for example.
    In other words, classes in named modules defined to class loaders
    other than the platform class loader and its ancestors may be visible
    to the platform class loader. 
System class loader. It is also known as application class loader and is distinct
    from the platform class loader.
    The system class loader is typically used to define classes on the
    application class path, module path, and JDK-specific tools.
    The platform class loader is a parent or an ancestor of the system class
    loader that all platform classes are visible to it.

 Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. However, some classes may not originate from a file; they may originate from other sources, such as the network, or they could be constructed by an application. The method 
defineClass converts an array of bytes into an instance of class Class. Instances of this newly defined class can be created using Class.newInstance. 
 The methods and constructors of objects created by a class loader may reference other classes. To determine the class(es) referred to, the Java virtual machine invokes the loadClass method of the class loader that originally created the class. 
 For example, an application could create a network class loader to
download class files from a server.  Sample code might look like:
  ClassLoader loader = new NetworkClassLoader(host, port);
  Object main = loader.loadClass("Main", true).newInstance();
       . . .

 The network class loader subclass must define the methods findClass and loadClassData to load a class from the network. Once it has downloaded the bytes that make up the class, it should use the method defineClass to create a class instance. A sample implementation is: 
    class NetworkClassLoader extends ClassLoader {
        String host;
        int port;
        public Class findClass(String name) {
            byte[] b = loadClassData(name);
            return defineClass(name, b, 0, b.length);
        }
        private byte[] loadClassData(String name) {
            // load the class data from the connection
             . . .
        }
    }

 Binary names 
 Any class name provided as a String parameter to methods in ClassLoader must be a binary name as defined by The Java™ Language Specification.
 Examples of valid class names include:
  "java.lang.String"
  "javax.swing.JSpinner$DefaultEditor"
  "java.security.KeyStore$Builder$FileBuilder$1"
  "java.net.URLClassLoader$3$1"

 Any package name provided as a String parameter to methods in ClassLoader must be either the empty string (denoting an unnamed package) or a fully qualified name as defined by The Java™ Language Specification.
See Also:
resolveClass(Class<?>)
@jls
6.7  Fully Qualified Names
@jls
13.1 The Form of a Binary
Since:
1.0
@revised
9
@spec
JPMS/**
 * A class loader is an object that is responsible for loading classes. The
 * class {@code ClassLoader} is an abstract class.  Given the <a
 * href="#binary-name">binary name</a> of a class, a class loader should attempt to
 * locate or generate data that constitutes a definition for the class.  A
 * typical strategy is to transform the name into a file name and then read a
 * "class file" of that name from a file system.
 *
 * <p> Every {@link java.lang.Class Class} object contains a {@link
 * Class#getClassLoader() reference} to the {@code ClassLoader} that defined
 * it.
 *
 * <p> {@code Class} objects for array classes are not created by class
 * loaders, but are created automatically as required by the Java runtime.
 * The class loader for an array class, as returned by {@link
 * Class#getClassLoader()} is the same as the class loader for its element
 * type; if the element type is a primitive type, then the array class has no
 * class loader.
 *
 * <p> Applications implement subclasses of {@code ClassLoader} in order to
 * extend the manner in which the Java virtual machine dynamically loads
 * classes.
 *
 * <p> Class loaders may typically be used by security managers to indicate
 * security domains.
 *
 * <p> In addition to loading classes, a class loader is also responsible for
 * locating resources. A resource is some data (a "{@code .class}" file,
 * configuration data, or an image for example) that is identified with an
 * abstract '/'-separated path name. Resources are typically packaged with an
 * application or library so that they can be located by code in the
 * application or library. In some cases, the resources are included so that
 * they can be located by other libraries.
 *
 * <p> The {@code ClassLoader} class uses a delegation model to search for
 * classes and resources.  Each instance of {@code ClassLoader} has an
 * associated parent class loader. When requested to find a class or
 * resource, a {@code ClassLoader} instance will usually delegate the search
 * for the class or resource to its parent class loader before attempting to
 * find the class or resource itself.
 *
 * <p> Class loaders that support concurrent loading of classes are known as
 * <em>{@linkplain #isRegisteredAsParallelCapable() parallel capable}</em> class
 * loaders and are required to register themselves at their class initialization
 * time by invoking the {@link
 * #registerAsParallelCapable ClassLoader.registerAsParallelCapable}
 * method. Note that the {@code ClassLoader} class is registered as parallel
 * capable by default. However, its subclasses still need to register themselves
 * if they are parallel capable.
 * In environments in which the delegation model is not strictly
 * hierarchical, class loaders need to be parallel capable, otherwise class
 * loading can lead to deadlocks because the loader lock is held for the
 * duration of the class loading process (see {@link #loadClass
 * loadClass} methods).
 *
 * <h3> <a id="builtinLoaders">Run-time Built-in Class Loaders</a></h3>
 *
 * The Java run-time has the following built-in class loaders:
 *
 * <ul>
 * <li><p>Bootstrap class loader.
 *     It is the virtual machine's built-in class loader, typically represented
 *     as {@code null}, and does not have a parent.</li>
 * <li><p>{@linkplain #getPlatformClassLoader() Platform class loader}.
 *     All <em>platform classes</em> are visible to the platform class loader
 *     that can be used as the parent of a {@code ClassLoader} instance.
 *     Platform classes include Java SE platform APIs, their implementation
 *     classes and JDK-specific run-time classes that are defined by the
 *     platform class loader or its ancestors.
 *     <p> To allow for upgrading/overriding of modules defined to the platform
 *     class loader, and where upgraded modules read modules defined to class
 *     loaders other than the platform class loader and its ancestors, then
 *     the platform class loader may have to delegate to other class loaders,
 *     the application class loader for example.
 *     In other words, classes in named modules defined to class loaders
 *     other than the platform class loader and its ancestors may be visible
 *     to the platform class loader. </li>
 * <li><p>{@linkplain #getSystemClassLoader() System class loader}.
 *     It is also known as <em>application class loader</em> and is distinct
 *     from the platform class loader.
 *     The system class loader is typically used to define classes on the
 *     application class path, module path, and JDK-specific tools.
 *     The platform class loader is a parent or an ancestor of the system class
 *     loader that all platform classes are visible to it.</li>
 * </ul>
 *
 * <p> Normally, the Java virtual machine loads classes from the local file
 * system in a platform-dependent manner.
 * However, some classes may not originate from a file; they may originate
 * from other sources, such as the network, or they could be constructed by an
 * application.  The method {@link #defineClass(String, byte[], int, int)
 * defineClass} converts an array of bytes into an instance of class
 * {@code Class}. Instances of this newly defined class can be created using
 * {@link Class#newInstance Class.newInstance}.
 *
 * <p> The methods and constructors of objects created by a class loader may
 * reference other classes.  To determine the class(es) referred to, the Java
 * virtual machine invokes the {@link #loadClass loadClass} method of
 * the class loader that originally created the class.
 *
 * <p> For example, an application could create a network class loader to
 * download class files from a server.  Sample code might look like:
 *
 * <blockquote><pre>
 *   ClassLoader loader&nbsp;= new NetworkClassLoader(host,&nbsp;port);
 *   Object main&nbsp;= loader.loadClass("Main", true).newInstance();
 *       &nbsp;.&nbsp;.&nbsp;.
 * </pre></blockquote>
 *
 * <p> The network class loader subclass must define the methods {@link
 * #findClass findClass} and {@code loadClassData} to load a class
 * from the network.  Once it has downloaded the bytes that make up the class,
 * it should use the method {@link #defineClass defineClass} to
 * create a class instance.  A sample implementation is:
 *
 * <blockquote><pre>
 *     class NetworkClassLoader extends ClassLoader {
 *         String host;
 *         int port;
 *
 *         public Class findClass(String name) {
 *             byte[] b = loadClassData(name);
 *             return defineClass(name, b, 0, b.length);
 *         }
 *
 *         private byte[] loadClassData(String name) {
 *             // load the class data from the connection
 *             &nbsp;.&nbsp;.&nbsp;.
 *         }
 *     }
 * </pre></blockquote>
 *
 * <h3> <a id="binary-name">Binary names</a> </h3>
 *
 * <p> Any class name provided as a {@code String} parameter to methods in
 * {@code ClassLoader} must be a binary name as defined by
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * <p> Examples of valid class names include:
 * <blockquote><pre>
 *   "java.lang.String"
 *   "javax.swing.JSpinner$DefaultEditor"
 *   "java.security.KeyStore$Builder$FileBuilder$1"
 *   "java.net.URLClassLoader$3$1"
 * </pre></blockquote>
 *
 * <p> Any package name provided as a {@code String} parameter to methods in
 * {@code ClassLoader} must be either the empty string (denoting an unnamed package)
 * or a fully qualified name as defined by
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * @jls 6.7  Fully Qualified Names
 * @jls 13.1 The Form of a Binary
 * @see      #resolveClass(Class)
 * @since 1.0
 * @revised 9
 * @spec JPMS
 */
public abstract class ClassLoader {

    private static native void registerNatives();
    static {
        registerNatives();
    }

    // The parent class loader for delegation
    // Note: VM hardcoded the offset of this field, thus all new fields
    // must be added *after* it.
    private final ClassLoader parent;

    // class loader name
    private final String name;

    // the unnamed module for this ClassLoader
    private final Module unnamedModule;

    // a string for exception message printing
    private final String nameAndId;

    
Encapsulates the set of parallel capable loader types.
/**
     * Encapsulates the set of parallel capable loader types.
     */
    private static class ParallelLoaders {
        private ParallelLoaders() {}

        // the set of parallel capable loader types
        private static final Set<Class<? extends ClassLoader>> loaderTypes =
            Collections.newSetFromMap(new WeakHashMap<>());
        static {
            synchronized (loaderTypes) { loaderTypes.add(ClassLoader.class); }
        }

        
Registers the given class loader type as parallel capable. Returns true is successfully registered; false if loader's super class is not registered. /**
         * Registers the given class loader type as parallel capable.
         * Returns {@code true} is successfully registered; {@code false} if
         * loader's super class is not registered.
         */
        static boolean register(Class<? extends ClassLoader> c) {
            synchronized (loaderTypes) {
                if (loaderTypes.contains(c.getSuperclass())) {
                    // register the class loader as parallel capable
                    // if and only if all of its super classes are.
                    // Note: given current classloading sequence, if
                    // the immediate super class is parallel capable,
                    // all the super classes higher up must be too.
                    loaderTypes.add(c);
                    return true;
                } else {
                    return false;
                }
            }
        }

        
Returns true if the given class loader type is registered as parallel capable. /**
         * Returns {@code true} if the given class loader type is
         * registered as parallel capable.
         */
        static boolean isRegistered(Class<? extends ClassLoader> c) {
            synchronized (loaderTypes) {
                return loaderTypes.contains(c);
            }
        }
    }

    // Maps class name to the corresponding lock object when the current
    // class loader is parallel capable.
    // Note: VM also uses this field to decide if the current class loader
    // is parallel capable and the appropriate lock object for class loading.
    private final ConcurrentHashMap<String, Object> parallelLockMap;

    // Maps packages to certs
    private final Map <String, Certificate[]> package2certs;

    // Shared among all packages with unsigned classes
    private static final Certificate[] nocerts = new Certificate[0];

    // The classes loaded by this class loader. The only purpose of this table
    // is to keep the classes from being GC'ed until the loader is GC'ed.
    private final Vector<Class<?>> classes = new Vector<>();

    // The "default" domain. Set as the default ProtectionDomain on newly
    // created classes.
    private final ProtectionDomain defaultDomain =
        new ProtectionDomain(new CodeSource(null, (Certificate[]) null),
                             null, this, null);

    // Invoked by the VM to record every loaded class with this loader.
    void addClass(Class<?> c) {
        classes.addElement(c);
    }

    // The packages defined in this class loader.  Each package name is
    // mapped to its corresponding NamedPackage object.
    //
    // The value is a Package object if ClassLoader::definePackage,
    // Class::getPackage, ClassLoader::getDefinePackage(s) or
    // Package::getPackage(s) method is called to define it.
    // Otherwise, the value is a NamedPackage object.
    private final ConcurrentHashMap<String, NamedPackage> packages
            = new ConcurrentHashMap<>();

    /*
     * Returns a named package for the given module.
     */
    private NamedPackage getNamedPackage(String pn, Module m) {
        NamedPackage p = packages.get(pn);
        if (p == null) {
            p = new NamedPackage(pn, m);

            NamedPackage value = packages.putIfAbsent(pn, p);
            if (value != null) {
                // Package object already be defined for the named package
                p = value;
                // if definePackage is called by this class loader to define
                // a package in a named module, this will return Package
                // object of the same name.  Package object may contain
                // unexpected information but it does not impact the runtime.
                // this assertion may be helpful for troubleshooting
                assert value.module() == m;
            }
        }
        return p;
    }

    private static Void checkCreateClassLoader() {
        return checkCreateClassLoader(null);
    }

    private static Void checkCreateClassLoader(String name) {
        if (name != null && name.isEmpty()) {
            throw new IllegalArgumentException("name must be non-empty or null");
        }

        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkCreateClassLoader();
        }
        return null;
    }

    private ClassLoader(Void unused, String name, ClassLoader parent) {
        this.name = name;
        this.parent = parent;
        this.unnamedModule = new Module(this);
        if (ParallelLoaders.isRegistered(this.getClass())) {
            parallelLockMap = new ConcurrentHashMap<>();
            package2certs = new ConcurrentHashMap<>();
            assertionLock = new Object();
        } else {
            // no finer-grained lock; lock on the classloader instance
            parallelLockMap = null;
            package2certs = new Hashtable<>();
            assertionLock = this;
        }
        this.nameAndId = nameAndId(this);
    }

    
If the defining loader has a name explicitly set then
      '' @
If the defining loader has no name then
       @
If it's built-in loader then omit `@` as there is only one instance.
/**
     * If the defining loader has a name explicitly set then
     *       '<loader-name>' @<id>
     * If the defining loader has no name then
     *       <qualified-class-name> @<id>
     * If it's built-in loader then omit `@<id>` as there is only one instance.
     */
    private static String nameAndId(ClassLoader ld) {
        String nid = ld.getName() != null ? "\'" + ld.getName() + "\'"
                                          : ld.getClass().getName();
        if (!(ld instanceof BuiltinClassLoader)) {
            String id = Integer.toHexString(System.identityHashCode(ld));
            nid = nid + " @" + id;
        }
        return nid;
    }

    
Creates a new class loader of the specified name and using the
specified parent class loader for delegation.
Params:
name –  class loader name; or null if not named
parent – the parent class loader
Throws:
IllegalArgumentException – if the given name is empty.
SecurityException –  If a security manager exists and its SecurityManager.checkCreateClassLoader() method doesn't allow creation of a new class loader.
API Note:
If the parent is specified as null (for the bootstrap class loader) then there is no guarantee that all platform classes are visible.
Since:
 9
@spec
JPMS/**
     * Creates a new class loader of the specified name and using the
     * specified parent class loader for delegation.
     *
     * @apiNote If the parent is specified as {@code null} (for the
     * bootstrap class loader) then there is no guarantee that all platform
     * classes are visible.
     *
     * @param  name   class loader name; or {@code null} if not named
     * @param  parent the parent class loader
     *
     * @throws IllegalArgumentException if the given name is empty.
     *
     * @throws SecurityException
     *         If a security manager exists and its
     *         {@link SecurityManager#checkCreateClassLoader()}
     *         method doesn't allow creation of a new class loader.
     *
     * @since  9
     * @spec JPMS
     */
    protected ClassLoader(String name, ClassLoader parent) {
        this(checkCreateClassLoader(name), name, parent);
    }

    
Creates a new class loader using the specified parent class loader for
delegation.
 If there is a security manager, its checkCreateClassLoader method is invoked. This may result in a security exception. 
Params:
parent – 
        The parent class loader
Throws:
SecurityException –  If a security manager exists and its checkCreateClassLoader method doesn't allow creation of a new class loader.
API Note:
If the parent is specified as null (for the bootstrap class loader) then there is no guarantee that all platform classes are visible.
Since:
 1.2/**
     * Creates a new class loader using the specified parent class loader for
     * delegation.
     *
     * <p> If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader() checkCreateClassLoader} method
     * is invoked.  This may result in a security exception.  </p>
     *
     * @apiNote If the parent is specified as {@code null} (for the
     * bootstrap class loader) then there is no guarantee that all platform
     * classes are visible.
     *
     * @param  parent
     *         The parent class loader
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@code checkCreateClassLoader} method doesn't allow creation
     *          of a new class loader.
     *
     * @since  1.2
     */
    protected ClassLoader(ClassLoader parent) {
        this(checkCreateClassLoader(), null, parent);
    }

    
Creates a new class loader using the ClassLoader returned by the method 
getSystemClassLoader() as the parent class loader. 
 If there is a security manager, its 
checkCreateClassLoader method is invoked. This may result in a security exception. 
Throws:
SecurityException –  If a security manager exists and its checkCreateClassLoader method doesn't allow creation of a new class loader./**
     * Creates a new class loader using the {@code ClassLoader} returned by
     * the method {@link #getSystemClassLoader()
     * getSystemClassLoader()} as the parent class loader.
     *
     * <p> If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * checkCreateClassLoader} method is invoked.  This may result in
     * a security exception.  </p>
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@code checkCreateClassLoader} method doesn't allow creation
     *          of a new class loader.
     */
    protected ClassLoader() {
        this(checkCreateClassLoader(), null, getSystemClassLoader());
    }

    
Returns the name of this class loader or null if this class loader is not named. 
API Note:
This method is non-final for compatibility.  If this
method is overridden, this method must return the same name
as specified when this class loader was instantiated.
Returns:
name of this class loader; or null if this class loader is not named.
Since:
9
@spec
JPMS/**
     * Returns the name of this class loader or {@code null} if
     * this class loader is not named.
     *
     * @apiNote This method is non-final for compatibility.  If this
     * method is overridden, this method must return the same name
     * as specified when this class loader was instantiated.
     *
     * @return name of this class loader; or {@code null} if
     * this class loader is not named.
     *
     * @since 9
     * @spec JPMS
     */
    public String getName() {
        return name;
    }

    // package-private used by StackTraceElement to avoid
    // calling the overrideable getName method
    final String name() {
        return name;
    }

    // -- Class --

    
Loads the class with the specified binary name. This method searches for classes in the same manner as the loadClass(String, boolean) method. It is invoked by the Java virtual machine to resolve class references. Invoking this method is equivalent to invoking loadClass(name,
false). 
Params:
name – 
        The binary name of the class
Throws:
ClassNotFoundException – 
         If the class was not found
Returns:
 The resulting Class object/**
     * Loads the class with the specified <a href="#binary-name">binary name</a>.
     * This method searches for classes in the same manner as the {@link
     * #loadClass(String, boolean)} method.  It is invoked by the Java virtual
     * machine to resolve class references.  Invoking this method is equivalent
     * to invoking {@link #loadClass(String, boolean) loadClass(name,
     * false)}.
     *
     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @return  The resulting {@code Class} object
     *
     * @throws  ClassNotFoundException
     *          If the class was not found
     */
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false);
    }

    
Loads the class with the specified binary name.  The
default implementation of this method searches for classes in the
following order:

  
 Invoke findLoadedClass(String) to check if the class has already been loaded. 
  
 Invoke the loadClass method on the parent class loader. If the parent is null the class loader built into the virtual machine is used, instead. 
  
 Invoke the findClass(String) method to find the class. 

 If the class was found using the above steps, and the resolve flag is true, this method will then invoke the resolveClass(Class<?>) method on the resulting Class object. 
 Subclasses of ClassLoader are encouraged to override findClass(String), rather than this method. 
 Unless overridden, this method synchronizes on the result of getClassLoadingLock method during the entire class loading process. 
Params:
name – 
        The binary name of the class
resolve –  If true then resolve the class
Throws:
ClassNotFoundException – 
         If the class could not be found
Returns:
 The resulting Class object/**
     * Loads the class with the specified <a href="#binary-name">binary name</a>.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * <ol>
     *
     *   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  </p></li>
     *
     *   <li><p> Invoke the {@link #loadClass(String) loadClass} method
     *   on the parent class loader.  If the parent is {@code null} the class
     *   loader built into the virtual machine is used, instead.  </p></li>
     *
     *   <li><p> Invoke the {@link #findClass(String)} method to find the
     *   class.  </p></li>
     *
     * </ol>
     *
     * <p> If the class was found using the above steps, and the
     * {@code resolve} flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting {@code Class} object.
     *
     * <p> Subclasses of {@code ClassLoader} are encouraged to override {@link
     * #findClass(String)}, rather than this method.  </p>
     *
     * <p> Unless overridden, this method synchronizes on the result of
     * {@link #getClassLoadingLock getClassLoadingLock} method
     * during the entire class loading process.
     *
     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @param  resolve
     *         If {@code true} then resolve the class
     *
     * @return  The resulting {@code Class} object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

    
Loads the class with the specified binary name in a module defined to this class loader. This method returns null if the class could not be found. 
Params:
module – 
        The module
name – 
        The binary name of the class
API Note:
This method does not delegate to the parent class loader.
Implementation Requirements:
The default implementation of this method searches for classes
in the following order:

  
Invoke findLoadedClass(String) to check if the class has already been loaded.
  
Invoke the findClass(String, String) method to find the class in the given module.
Returns:
The resulting Class object in a module defined by this class loader, or null if the class could not be found./**
     * Loads the class with the specified <a href="#binary-name">binary name</a>
     * in a module defined to this class loader.  This method returns {@code null}
     * if the class could not be found.
     *
     * @apiNote This method does not delegate to the parent class loader.
     *
     * @implSpec The default implementation of this method searches for classes
     * in the following order:
     *
     * <ol>
     *   <li>Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.</li>
     *   <li>Invoke the {@link #findClass(String, String)} method to find the
     *   class in the given module.</li>
     * </ol>
     *
     * @param  module
     *         The module
     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @return The resulting {@code Class} object in a module defined by
     *         this class loader, or {@code null} if the class could not be found.
     */
    final Class<?> loadClass(Module module, String name) {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                c = findClass(module.getName(), name);
            }
            if (c != null && c.getModule() == module) {
                return c;
            } else {
                return null;
            }
        }
    }

    
Returns the lock object for class loading operations.
For backward compatibility, the default implementation of this method
behaves as follows. If this ClassLoader object is registered as
parallel capable, the method returns a dedicated object associated
with the specified class name. Otherwise, the method returns this
ClassLoader object.
Params:
className – 
        The name of the to-be-loaded class
Throws:
NullPointerException –  If registered as parallel capable and className is null
See Also:
loadClass(String, boolean)
Returns:
the lock for class loading operations
Since:
 1.7/**
     * Returns the lock object for class loading operations.
     * For backward compatibility, the default implementation of this method
     * behaves as follows. If this ClassLoader object is registered as
     * parallel capable, the method returns a dedicated object associated
     * with the specified class name. Otherwise, the method returns this
     * ClassLoader object.
     *
     * @param  className
     *         The name of the to-be-loaded class
     *
     * @return the lock for class loading operations
     *
     * @throws NullPointerException
     *         If registered as parallel capable and {@code className} is null
     *
     * @see #loadClass(String, boolean)
     *
     * @since  1.7
     */
    protected Object getClassLoadingLock(String className) {
        Object lock = this;
        if (parallelLockMap != null) {
            Object newLock = new Object();
            lock = parallelLockMap.putIfAbsent(className, newLock);
            if (lock == null) {
                lock = newLock;
            }
        }
        return lock;
    }

    // Invoked by the VM after loading class with this loader.
    private void checkPackageAccess(Class<?> cls, ProtectionDomain pd) {
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            if (ReflectUtil.isNonPublicProxyClass(cls)) {
                for (Class<?> intf: cls.getInterfaces()) {
                    checkPackageAccess(intf, pd);
                }
                return;
            }

            final String packageName = cls.getPackageName();
            if (!packageName.isEmpty()) {
                AccessController.doPrivileged(new PrivilegedAction<>() {
                    public Void run() {
                        sm.checkPackageAccess(packageName);
                        return null;
                    }
                }, new AccessControlContext(new ProtectionDomain[] {pd}));
            }
        }
    }

    
Finds the class with the specified binary name. This method should be overridden by class loader implementations that follow the delegation model for loading classes, and will be invoked by the loadClass method after checking the parent class loader for the requested class. 
Params:
name – 
        The binary name of the class
Throws:
ClassNotFoundException – 
         If the class could not be found
Implementation Requirements:
The default implementation throws ClassNotFoundException.
Returns:
 The resulting Class object
Since:
 1.2/**
     * Finds the class with the specified <a href="#binary-name">binary name</a>.
     * This method should be overridden by class loader implementations that
     * follow the delegation model for loading classes, and will be invoked by
     * the {@link #loadClass loadClass} method after checking the
     * parent class loader for the requested class.
     *
     * @implSpec The default implementation throws {@code ClassNotFoundException}.
     *
     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @return  The resulting {@code Class} object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @since  1.2
     */
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

    
Finds the class with the given binary name
in a module defined to this class loader.
Class loader implementations that support loading from modules
should override this method.
Params:
moduleName –  The module name; or null to find the class in the unnamed module for this class loader
name – 
        The binary name of the class
API Note:
This method returns null rather than throwing ClassNotFoundException if the class could not be found.
Implementation Requirements:
The default implementation attempts to find the class by invoking findClass(String) when the moduleName is null. It otherwise returns null.
Returns:
The resulting Class object, or null if the class could not be found.
Since:
9
@spec
JPMS/**
     * Finds the class with the given <a href="#binary-name">binary name</a>
     * in a module defined to this class loader.
     * Class loader implementations that support loading from modules
     * should override this method.
     *
     * @apiNote This method returns {@code null} rather than throwing
     *          {@code ClassNotFoundException} if the class could not be found.
     *
     * @implSpec The default implementation attempts to find the class by
     * invoking {@link #findClass(String)} when the {@code moduleName} is
     * {@code null}. It otherwise returns {@code null}.
     *
     * @param  moduleName
     *         The module name; or {@code null} to find the class in the
     *         {@linkplain #getUnnamedModule() unnamed module} for this
     *         class loader

     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @return The resulting {@code Class} object, or {@code null}
     *         if the class could not be found.
     *
     * @since 9
     * @spec JPMS
     */
    protected Class<?> findClass(String moduleName, String name) {
        if (moduleName == null) {
            try {
                return findClass(name);
            } catch (ClassNotFoundException ignore) { }
        }
        return null;
    }


    
Converts an array of bytes into an instance of class Class. Before the Class can be used it must be resolved. This method is deprecated in favor of the version that takes a binary name as its first argument, and is more secure.
Params:
b –  The bytes that make up the class data. The bytes in positions off through off+len-1 should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.
off –  The start offset in b of the class data
len – 
        The length of the class data
Throws:
ClassFormatError – 
         If the data did not contain a valid class
IndexOutOfBoundsException –  If either off or len is negative, or if off+len is greater than b.length.
SecurityException –  If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or if an attempt is made to define a class in a package with a fully-qualified name that starts with "java.".
See Also:
loadClass(String, boolean)
resolveClass(Class<?>)
Returns:
 The Class object that was created from the specified class data
Deprecated:
 Replaced by 
defineClass(String, byte[], int, int)/**
     * Converts an array of bytes into an instance of class {@code Class}.
     * Before the {@code Class} can be used it must be resolved.  This method
     * is deprecated in favor of the version that takes a <a
     * href="#binary-name">binary name</a> as its first argument, and is more secure.
     *
     * @param  b
     *         The bytes that make up the class data.  The bytes in positions
     *         {@code off} through {@code off+len-1} should have the format
     *         of a valid class file as defined by
     *         <cite>The Java&trade; Virtual Machine Specification</cite>.
     *
     * @param  off
     *         The start offset in {@code b} of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @return  The {@code Class} object that was created from the specified
     *          class data
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  IndexOutOfBoundsException
     *          If either {@code off} or {@code len} is negative, or if
     *          {@code off+len} is greater than {@code b.length}.
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if an attempt is made
     *          to define a class in a package with a fully-qualified name
     *          that starts with "{@code java.}".
     *
     * @see  #loadClass(String, boolean)
     * @see  #resolveClass(Class)
     *
     * @deprecated  Replaced by {@link #defineClass(String, byte[], int, int)
     * defineClass(String, byte[], int, int)}
     */
    @Deprecated(since="1.1")
    protected final Class<?> defineClass(byte[] b, int off, int len)
        throws ClassFormatError
    {
        return defineClass(null, b, off, len, null);
    }

    
Converts an array of bytes into an instance of class Class. Before the Class can be used it must be resolved. 
 This method assigns a default 
ProtectionDomain to the newly defined class. The ProtectionDomain is effectively granted the same set of permissions returned when 
Policy.getPolicy().getPermissions(new CodeSource(null, null)) is invoked. The default protection domain is created on the first invocation of defineClass, and re-used on subsequent invocations. 
 To assign a specific ProtectionDomain to the class, use the defineClass method that takes a ProtectionDomain as one of its arguments. 
 This method defines a package in this class loader corresponding to the package of the Class (if such a package has not already been defined in this class loader). The name of the defined package is derived from the binary name of the class specified by the byte array b. Other properties of the defined package are as specified by Package. 
Params:
name – 
        The expected binary name of the class, or null if not known
b –  The bytes that make up the class data. The bytes in positions off through off+len-1 should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.
off –  The start offset in b of the class data
len – 
        The length of the class data
Throws:
ClassFormatError – 
         If the data did not contain a valid class
IndexOutOfBoundsException –  If either off or len is negative, or if off+len is greater than b.length.
SecurityException –  If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class (which is unsigned), or if name begins with "java.".
See Also:
loadClass(String, boolean)
resolveClass(Class<?>)
CodeSource
SecureClassLoader
Returns:
 The Class object that was created from the specified class data.
Since:
 1.1
@revised
9
@spec
JPMS/**
     * Converts an array of bytes into an instance of class {@code Class}.
     * Before the {@code Class} can be used it must be resolved.
     *
     * <p> This method assigns a default {@link java.security.ProtectionDomain
     * ProtectionDomain} to the newly defined class.  The
     * {@code ProtectionDomain} is effectively granted the same set of
     * permissions returned when {@link
     * java.security.Policy#getPermissions(java.security.CodeSource)
     * Policy.getPolicy().getPermissions(new CodeSource(null, null))}
     * is invoked.  The default protection domain is created on the first invocation
     * of {@link #defineClass(String, byte[], int, int) defineClass},
     * and re-used on subsequent invocations.
     *
     * <p> To assign a specific {@code ProtectionDomain} to the class, use
     * the {@link #defineClass(String, byte[], int, int,
     * java.security.ProtectionDomain) defineClass} method that takes a
     * {@code ProtectionDomain} as one of its arguments.  </p>
     *
     * <p>
     * This method defines a package in this class loader corresponding to the
     * package of the {@code Class} (if such a package has not already been defined
     * in this class loader). The name of the defined package is derived from
     * the <a href="#binary-name">binary name</a> of the class specified by
     * the byte array {@code b}.
     * Other properties of the defined package are as specified by {@link Package}.
     *
     * @param  name
     *         The expected <a href="#binary-name">binary name</a> of the class, or
     *         {@code null} if not known
     *
     * @param  b
     *         The bytes that make up the class data.  The bytes in positions
     *         {@code off} through {@code off+len-1} should have the format
     *         of a valid class file as defined by
     *         <cite>The Java&trade; Virtual Machine Specification</cite>.
     *
     * @param  off
     *         The start offset in {@code b} of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @return  The {@code Class} object that was created from the specified
     *          class data.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  IndexOutOfBoundsException
     *          If either {@code off} or {@code len} is negative, or if
     *          {@code off+len} is greater than {@code b.length}.
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class (which is unsigned), or if
     *          {@code name} begins with "{@code java.}".
     *
     * @see  #loadClass(String, boolean)
     * @see  #resolveClass(Class)
     * @see  java.security.CodeSource
     * @see  java.security.SecureClassLoader
     *
     * @since  1.1
     * @revised 9
     * @spec JPMS
     */
    protected final Class<?> defineClass(String name, byte[] b, int off, int len)
        throws ClassFormatError
    {
        return defineClass(name, b, off, len, null);
    }

    /* Determine protection domain, and check that:
        - not define java.* class,
        - signer of this class matches signers for the rest of the classes in
          package.
    */
    private ProtectionDomain preDefineClass(String name,
                                            ProtectionDomain pd)
    {
        if (!checkName(name))
            throw new NoClassDefFoundError("IllegalName: " + name);

        // Note:  Checking logic in java.lang.invoke.MemberName.checkForTypeAlias
        // relies on the fact that spoofing is impossible if a class has a name
        // of the form "java.*"
        if ((name != null) && name.startsWith("java.")
                && this != getBuiltinPlatformClassLoader()) {
            throw new SecurityException
                ("Prohibited package name: " +
                 name.substring(0, name.lastIndexOf('.')));
        }
        if (pd == null) {
            pd = defaultDomain;
        }

        if (name != null) {
            checkCerts(name, pd.getCodeSource());
        }

        return pd;
    }

    private String defineClassSourceLocation(ProtectionDomain pd) {
        CodeSource cs = pd.getCodeSource();
        String source = null;
        if (cs != null && cs.getLocation() != null) {
            source = cs.getLocation().toString();
        }
        return source;
    }

    private void postDefineClass(Class<?> c, ProtectionDomain pd) {
        // define a named package, if not present
        getNamedPackage(c.getPackageName(), c.getModule());

        if (pd.getCodeSource() != null) {
            Certificate certs[] = pd.getCodeSource().getCertificates();
            if (certs != null)
                setSigners(c, certs);
        }
    }

    
Converts an array of bytes into an instance of class Class, with a given ProtectionDomain. 
 If the given ProtectionDomain is null, then a default protection domain will be assigned to the class as specified in the documentation for defineClass(String, byte[], int, int). Before the class can be used it must be resolved. 
 The first class defined in a package determines the exact set of certificates that all subsequent classes defined in that package must contain. The set of certificates for a class is obtained from the CodeSource within the ProtectionDomain of the class. Any classes added to that package must contain the same set of certificates or a SecurityException will be thrown. Note that if name is null, this check is not performed. You should always pass in the binary name of the
class you are defining as well as the bytes.  This ensures that the
class you are defining is indeed the class you think it is.
 If the specified name begins with "java.", it can only be defined by the 
platform class loader or its ancestors; otherwise SecurityException will be thrown. If name is not null, it must be equal to the binary name of the class specified by the byte array b, otherwise a NoClassDefFoundError will be thrown. 
 This method defines a package in this class loader corresponding to the package of the Class (if such a package has not already been defined in this class loader). The name of the defined package is derived from the binary name of the class specified by the byte array b. Other properties of the defined package are as specified by Package. 
Params:
name – 
        The expected binary name of the class, or null if not known
b –  The bytes that make up the class data. The bytes in positions off through off+len-1 should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.
off –  The start offset in b of the class data
len – 
        The length of the class data
protectionDomain –  The ProtectionDomain of the class
Throws:
ClassFormatError – 
         If the data did not contain a valid class
NoClassDefFoundError –  If name is not null and not equal to the binary name of the class specified by b
IndexOutOfBoundsException –  If either off or len is negative, or if off+len is greater than b.length.
SecurityException –  If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or if name begins with "java." and this class loader is not the platform class loader or its ancestor.
Returns:
 The Class object created from the data, and ProtectionDomain.
@revised
9
@spec
JPMS/**
     * Converts an array of bytes into an instance of class {@code Class},
     * with a given {@code ProtectionDomain}.
     *
     * <p> If the given {@code ProtectionDomain} is {@code null},
     * then a default protection domain will be assigned to the class as specified
     * in the documentation for {@link #defineClass(String, byte[], int, int)}.
     * Before the class can be used it must be resolved.
     *
     * <p> The first class defined in a package determines the exact set of
     * certificates that all subsequent classes defined in that package must
     * contain.  The set of certificates for a class is obtained from the
     * {@link java.security.CodeSource CodeSource} within the
     * {@code ProtectionDomain} of the class.  Any classes added to that
     * package must contain the same set of certificates or a
     * {@code SecurityException} will be thrown.  Note that if
     * {@code name} is {@code null}, this check is not performed.
     * You should always pass in the <a href="#binary-name">binary name</a> of the
     * class you are defining as well as the bytes.  This ensures that the
     * class you are defining is indeed the class you think it is.
     *
     * <p> If the specified {@code name} begins with "{@code java.}", it can
     * only be defined by the {@linkplain #getPlatformClassLoader()
     * platform class loader} or its ancestors; otherwise {@code SecurityException}
     * will be thrown.  If {@code name} is not {@code null}, it must be equal to
     * the <a href="#binary-name">binary name</a> of the class
     * specified by the byte array {@code b}, otherwise a {@link
     * NoClassDefFoundError NoClassDefFoundError} will be thrown.
     *
     * <p> This method defines a package in this class loader corresponding to the
     * package of the {@code Class} (if such a package has not already been defined
     * in this class loader). The name of the defined package is derived from
     * the <a href="#binary-name">binary name</a> of the class specified by
     * the byte array {@code b}.
     * Other properties of the defined package are as specified by {@link Package}.
     *
     * @param  name
     *         The expected <a href="#binary-name">binary name</a> of the class, or
     *         {@code null} if not known
     *
     * @param  b
     *         The bytes that make up the class data. The bytes in positions
     *         {@code off} through {@code off+len-1} should have the format
     *         of a valid class file as defined by
     *         <cite>The Java&trade; Virtual Machine Specification</cite>.
     *
     * @param  off
     *         The start offset in {@code b} of the class data
     *
     * @param  len
     *         The length of the class data
     *
     * @param  protectionDomain
     *         The {@code ProtectionDomain} of the class
     *
     * @return  The {@code Class} object created from the data,
     *          and {@code ProtectionDomain}.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class
     *
     * @throws  NoClassDefFoundError
     *          If {@code name} is not {@code null} and not equal to the
     *          <a href="#binary-name">binary name</a> of the class specified by {@code b}
     *
     * @throws  IndexOutOfBoundsException
     *          If either {@code off} or {@code len} is negative, or if
     *          {@code off+len} is greater than {@code b.length}.
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if {@code name} begins with
     *          "{@code java.}" and this class loader is not the platform
     *          class loader or its ancestor.
     *
     * @revised 9
     * @spec JPMS
     */
    protected final Class<?> defineClass(String name, byte[] b, int off, int len,
                                         ProtectionDomain protectionDomain)
        throws ClassFormatError
    {
        protectionDomain = preDefineClass(name, protectionDomain);
        String source = defineClassSourceLocation(protectionDomain);
        Class<?> c = defineClass1(this, name, b, off, len, protectionDomain, source);
        postDefineClass(c, protectionDomain);
        return c;
    }

    
Converts a ByteBuffer into an instance of class Class, with the given ProtectionDomain. If the given ProtectionDomain is null, then a default protection domain will be assigned to the class as specified in the documentation for defineClass(String, byte[], int, int). Before the class can be used it must be resolved. 
The rules about the first class defined in a package determining the set of certificates for the package, the restrictions on class names, and the defined package of the class are identical to those specified in the documentation for defineClass(String, byte[], int, int, ProtectionDomain). 
 An invocation of this method of the form
cl.defineClass(name, bBuffer, pd) yields exactly the same result as the statements 
 
...
 byte[] temp = new byte[bBuffer.remaining()];
 bBuffer.
get(temp);
 return 
cl.defineClass(name, temp, 0, temp.length, pd);


Params:
name – 
        The expected binary name. of the class, or null if not known
b –  The bytes that make up the class data. The bytes from positions b.position() through b.position() + b.limit() -1
         should have the format of a valid class file as defined by The Java™ Virtual Machine Specification.
protectionDomain –  The ProtectionDomain of the class, or null.
Throws:
ClassFormatError – 
         If the data did not contain a valid class.
NoClassDefFoundError –  If name is not null and not equal to the binary name of the class specified by b
SecurityException –  If an attempt is made to add this class to a package that contains classes that were signed by a different set of certificates than this class, or if name begins with "java.".
See Also:
defineClass(String, byte[], int, int, ProtectionDomain)
Returns:
 The Class object created from the data, and ProtectionDomain.
Since:
 1.5
@revised
9
@spec
JPMS/**
     * Converts a {@link java.nio.ByteBuffer ByteBuffer} into an instance
     * of class {@code Class}, with the given {@code ProtectionDomain}.
     * If the given {@code ProtectionDomain} is {@code null}, then a default
     * protection domain will be assigned to the class as
     * specified in the documentation for {@link #defineClass(String, byte[],
     * int, int)}.  Before the class can be used it must be resolved.
     *
     * <p>The rules about the first class defined in a package determining the
     * set of certificates for the package, the restrictions on class names,
     * and the defined package of the class
     * are identical to those specified in the documentation for {@link
     * #defineClass(String, byte[], int, int, ProtectionDomain)}.
     *
     * <p> An invocation of this method of the form
     * <i>cl</i>{@code .defineClass(}<i>name</i>{@code ,}
     * <i>bBuffer</i>{@code ,} <i>pd</i>{@code )} yields exactly the same
     * result as the statements
     *
     *<p> <code>
     * ...<br>
     * byte[] temp = new byte[bBuffer.{@link
     * java.nio.ByteBuffer#remaining remaining}()];<br>
     *     bBuffer.{@link java.nio.ByteBuffer#get(byte[])
     * get}(temp);<br>
     *     return {@link #defineClass(String, byte[], int, int, ProtectionDomain)
     * cl.defineClass}(name, temp, 0,
     * temp.length, pd);<br>
     * </code></p>
     *
     * @param  name
     *         The expected <a href="#binary-name">binary name</a>. of the class, or
     *         {@code null} if not known
     *
     * @param  b
     *         The bytes that make up the class data. The bytes from positions
     *         {@code b.position()} through {@code b.position() + b.limit() -1
     *         } should have the format of a valid class file as defined by
     *         <cite>The Java&trade; Virtual Machine Specification</cite>.
     *
     * @param  protectionDomain
     *         The {@code ProtectionDomain} of the class, or {@code null}.
     *
     * @return  The {@code Class} object created from the data,
     *          and {@code ProtectionDomain}.
     *
     * @throws  ClassFormatError
     *          If the data did not contain a valid class.
     *
     * @throws  NoClassDefFoundError
     *          If {@code name} is not {@code null} and not equal to the
     *          <a href="#binary-name">binary name</a> of the class specified by {@code b}
     *
     * @throws  SecurityException
     *          If an attempt is made to add this class to a package that
     *          contains classes that were signed by a different set of
     *          certificates than this class, or if {@code name} begins with
     *          "{@code java.}".
     *
     * @see      #defineClass(String, byte[], int, int, ProtectionDomain)
     *
     * @since  1.5
     * @revised 9
     * @spec JPMS
     */
    protected final Class<?> defineClass(String name, java.nio.ByteBuffer b,
                                         ProtectionDomain protectionDomain)
        throws ClassFormatError
    {
        int len = b.remaining();

        // Use byte[] if not a direct ByteBuffer:
        if (!b.isDirect()) {
            if (b.hasArray()) {
                return defineClass(name, b.array(),
                                   b.position() + b.arrayOffset(), len,
                                   protectionDomain);
            } else {
                // no array, or read-only array
                byte[] tb = new byte[len];
                b.get(tb);  // get bytes out of byte buffer.
                return defineClass(name, tb, 0, len, protectionDomain);
            }
        }

        protectionDomain = preDefineClass(name, protectionDomain);
        String source = defineClassSourceLocation(protectionDomain);
        Class<?> c = defineClass2(this, name, b, b.position(), len, protectionDomain, source);
        postDefineClass(c, protectionDomain);
        return c;
    }

    static native Class<?> defineClass1(ClassLoader loader, String name, byte[] b, int off, int len,
                                        ProtectionDomain pd, String source);

    static native Class<?> defineClass2(ClassLoader loader, String name, java.nio.ByteBuffer b,
                                        int off, int len, ProtectionDomain pd,
                                        String source);

    // true if the name is null or has the potential to be a valid binary name
    private boolean checkName(String name) {
        if ((name == null) || (name.isEmpty()))
            return true;
        if ((name.indexOf('/') != -1) || (name.charAt(0) == '['))
            return false;
        return true;
    }

    private void checkCerts(String name, CodeSource cs) {
        int i = name.lastIndexOf('.');
        String pname = (i == -1) ? "" : name.substring(0, i);

        Certificate[] certs = null;
        if (cs != null) {
            certs = cs.getCertificates();
        }
        Certificate[] pcerts = null;
        if (parallelLockMap == null) {
            synchronized (this) {
                pcerts = package2certs.get(pname);
                if (pcerts == null) {
                    package2certs.put(pname, (certs == null? nocerts:certs));
                }
            }
        } else {
            pcerts = ((ConcurrentHashMap<String, Certificate[]>)package2certs).
                putIfAbsent(pname, (certs == null? nocerts:certs));
        }
        if (pcerts != null && !compareCerts(pcerts, certs)) {
            throw new SecurityException("class \"" + name
                + "\"'s signer information does not match signer information"
                + " of other classes in the same package");
        }
    }

    
check to make sure the certs for the new class (certs) are the same as
the certs for the first class inserted in the package (pcerts)
/**
     * check to make sure the certs for the new class (certs) are the same as
     * the certs for the first class inserted in the package (pcerts)
     */
    private boolean compareCerts(Certificate[] pcerts,
                                 Certificate[] certs)
    {
        // certs can be null, indicating no certs.
        if ((certs == null) || (certs.length == 0)) {
            return pcerts.length == 0;
        }

        // the length must be the same at this point
        if (certs.length != pcerts.length)
            return false;

        // go through and make sure all the certs in one array
        // are in the other and vice-versa.
        boolean match;
        for (Certificate cert : certs) {
            match = false;
            for (Certificate pcert : pcerts) {
                if (cert.equals(pcert)) {
                    match = true;
                    break;
                }
            }
            if (!match) return false;
        }

        // now do the same for pcerts
        for (Certificate pcert : pcerts) {
            match = false;
            for (Certificate cert : certs) {
                if (pcert.equals(cert)) {
                    match = true;
                    break;
                }
            }
            if (!match) return false;
        }

        return true;
    }

    
Links the specified class. This (misleadingly named) method may be used by a class loader to link a class. If the class c has already been linked, then this method simply returns. Otherwise, the class is linked as described in the "Execution" chapter of The Java™ Language Specification.
Params:
c – 
        The class to link
Throws:
NullPointerException –  If c is null.
See Also:
defineClass(String, byte[], int, int)/**
     * Links the specified class.  This (misleadingly named) method may be
     * used by a class loader to link a class.  If the class {@code c} has
     * already been linked, then this method simply returns. Otherwise, the
     * class is linked as described in the "Execution" chapter of
     * <cite>The Java&trade; Language Specification</cite>.
     *
     * @param  c
     *         The class to link
     *
     * @throws  NullPointerException
     *          If {@code c} is {@code null}.
     *
     * @see  #defineClass(String, byte[], int, int)
     */
    protected final void resolveClass(Class<?> c) {
        if (c == null) {
            throw new NullPointerException();
        }
    }

    
Finds a class with the specified binary name,
loading it if necessary.
 This method loads the class through the system class loader (see getSystemClassLoader()). The Class object returned might have more than one ClassLoader associated with it. Subclasses of ClassLoader need not usually invoke this method, because most class loaders need to override just findClass(String). 
Params:
name – 
        The binary name of the class
Throws:
ClassNotFoundException – 
         If the class could not be found
See Also:
ClassLoader(ClassLoader)
getParent()
Returns:
 The Class object for the specified name/**
     * Finds a class with the specified <a href="#binary-name">binary name</a>,
     * loading it if necessary.
     *
     * <p> This method loads the class through the system class loader (see
     * {@link #getSystemClassLoader()}).  The {@code Class} object returned
     * might have more than one {@code ClassLoader} associated with it.
     * Subclasses of {@code ClassLoader} need not usually invoke this method,
     * because most class loaders need to override just {@link
     * #findClass(String)}.  </p>
     *
     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @return  The {@code Class} object for the specified {@code name}
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @see  #ClassLoader(ClassLoader)
     * @see  #getParent()
     */
    protected final Class<?> findSystemClass(String name)
        throws ClassNotFoundException
    {
        return getSystemClassLoader().loadClass(name);
    }

    
Returns a class loaded by the bootstrap class loader;
or return null if not found.
/**
     * Returns a class loaded by the bootstrap class loader;
     * or return null if not found.
     */
    Class<?> findBootstrapClassOrNull(String name) {
        if (!checkName(name)) return null;

        return findBootstrapClass(name);
    }

    // return null if not found
    private native Class<?> findBootstrapClass(String name);

    
Returns the class with the given binary name if this
loader has been recorded by the Java virtual machine as an initiating
loader of a class with that binary name. Otherwise null is returned. 
Params:
name – 
        The binary name of the class
Returns:
 The Class object, or null if the class has not been loaded
Since:
 1.1/**
     * Returns the class with the given <a href="#binary-name">binary name</a> if this
     * loader has been recorded by the Java virtual machine as an initiating
     * loader of a class with that <a href="#binary-name">binary name</a>.  Otherwise
     * {@code null} is returned.
     *
     * @param  name
     *         The <a href="#binary-name">binary name</a> of the class
     *
     * @return  The {@code Class} object, or {@code null} if the class has
     *          not been loaded
     *
     * @since  1.1
     */
    protected final Class<?> findLoadedClass(String name) {
        if (!checkName(name))
            return null;
        return findLoadedClass0(name);
    }

    private final native Class<?> findLoadedClass0(String name);

    
Sets the signers of a class.  This should be invoked after defining a
class.
Params:
c –  The Class object
signers – 
        The signers for the class
Since:
 1.1/**
     * Sets the signers of a class.  This should be invoked after defining a
     * class.
     *
     * @param  c
     *         The {@code Class} object
     *
     * @param  signers
     *         The signers for the class
     *
     * @since  1.1
     */
    protected final void setSigners(Class<?> c, Object[] signers) {
        c.setSigners(signers);
    }


    // -- Resources --

    
Returns a URL to a resource in a module defined to this class loader.
Class loader implementations that support loading from modules
should override this method.
Params:
moduleName –  The module name; or null to find a resource in the unnamed module for this class loader
name – 
        The resource name
Throws:
IOException – 
        If I/O errors occur
See Also:
ModuleReader.find(String)
API Note:
This method is the basis for the Class.getResource, 
Class.getResourceAsStream, and 
Module.getResourceAsStream methods. It is not subject to the rules for encapsulation specified by Module.getResourceAsStream.
Implementation Requirements:
The default implementation attempts to find the resource by invoking findResource(String) when the moduleName is null. It otherwise returns null.
Returns:
A URL to the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, access to the resource is denied by the security manager, or there isn't a module of the given name defined to the class loader.
Since:
9
@spec
JPMS/**
     * Returns a URL to a resource in a module defined to this class loader.
     * Class loader implementations that support loading from modules
     * should override this method.
     *
     * @apiNote This method is the basis for the {@link
     * Class#getResource Class.getResource}, {@link Class#getResourceAsStream
     * Class.getResourceAsStream}, and {@link Module#getResourceAsStream
     * Module.getResourceAsStream} methods. It is not subject to the rules for
     * encapsulation specified by {@code Module.getResourceAsStream}.
     *
     * @implSpec The default implementation attempts to find the resource by
     * invoking {@link #findResource(String)} when the {@code moduleName} is
     * {@code null}. It otherwise returns {@code null}.
     *
     * @param  moduleName
     *         The module name; or {@code null} to find a resource in the
     *         {@linkplain #getUnnamedModule() unnamed module} for this
     *         class loader
     * @param  name
     *         The resource name
     *
     * @return A URL to the resource; {@code null} if the resource could not be
     *         found, a URL could not be constructed to locate the resource,
     *         access to the resource is denied by the security manager, or
     *         there isn't a module of the given name defined to the class
     *         loader.
     *
     * @throws IOException
     *         If I/O errors occur
     *
     * @see java.lang.module.ModuleReader#find(String)
     * @since 9
     * @spec JPMS
     */
    protected URL findResource(String moduleName, String name) throws IOException {
        if (moduleName == null) {
            return findResource(name);
        } else {
            return null;
        }
    }

    
Finds the resource with the given name.  A resource is some data
(images, audio, text, etc) that can be accessed by class code in a way
that is independent of the location of the code.
 The name of a resource is a '/'-separated path name that identifies the resource. 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally (even if the caller of this method is in the same module as the resource). 
Params:
name – 
        The resource name
Throws:
NullPointerException – If name is null
Implementation Requirements:
The default implementation will first search the parent class loader for the resource; if the parent is null the path of the class loader built into the virtual machine is searched. If not found, this method will invoke findResource(String) to find the resource.
API Note:
Where several modules are defined to the same class loader, and where more than one module contains a resource with the given name, then the ordering that modules are searched is not specified and may be very unpredictable. When overriding this method it is recommended that an implementation ensures that any delegation is consistent with the getResources(String) method.
Returns:
 URL object for reading the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Since:
 1.1
@revised
9
@spec
JPMS/**
     * Finds the resource with the given name.  A resource is some data
     * (images, audio, text, etc) that can be accessed by class code in a way
     * that is independent of the location of the code.
     *
     * <p> The name of a resource is a '{@code /}'-separated path name that
     * identifies the resource. </p>
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally (even if the caller of this method is in the
     * same module as the resource). </p>
     *
     * @implSpec The default implementation will first search the parent class
     * loader for the resource; if the parent is {@code null} the path of the
     * class loader built into the virtual machine is searched. If not found,
     * this method will invoke {@link #findResource(String)} to find the resource.
     *
     * @apiNote Where several modules are defined to the same class loader,
     * and where more than one module contains a resource with the given name,
     * then the ordering that modules are searched is not specified and may be
     * very unpredictable.
     * When overriding this method it is recommended that an implementation
     * ensures that any delegation is consistent with the {@link
     * #getResources(java.lang.String) getResources(String)} method.
     *
     * @param  name
     *         The resource name
     *
     * @return  {@code URL} object for reading the resource; {@code null} if
     *          the resource could not be found, a {@code URL} could not be
     *          constructed to locate the resource, the resource is in a package
     *          that is not opened unconditionally, or access to the resource is
     *          denied by the security manager.
     *
     * @throws  NullPointerException If {@code name} is {@code null}
     *
     * @since  1.1
     * @revised 9
     * @spec JPMS
     */
    public URL getResource(String name) {
        Objects.requireNonNull(name);
        URL url;
        if (parent != null) {
            url = parent.getResource(name);
        } else {
            url = BootLoader.findResource(name);
        }
        if (url == null) {
            url = findResource(name);
        }
        return url;
    }

    
Finds all the resources with the given name. A resource is some data
(images, audio, text, etc) that can be accessed by class code in a way
that is independent of the location of the code.
 The name of a resource is a /-separated path name that identifies the resource. 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally (even if the caller of this method is in the same module as the resource). 
Params:
name – 
        The resource name
Throws:
IOException – 
         If I/O errors occur
NullPointerException – If name is null
Implementation Requirements:
The default implementation will first search the parent class loader for the resource; if the parent is null the path of the class loader built into the virtual machine is searched. It then invokes findResources(String) to find the resources with the name in this class loader. It returns an enumeration whose elements are the URLs found by searching the parent class loader followed by the elements found with findResources.
API Note:
Where several modules are defined to the same class loader, and where more than one module contains a resource with the given name, then the ordering is not specified and may be very unpredictable. When overriding this method it is recommended that an implementation ensures that any delegation is consistent with the getResource(String) method. This should ensure that the first element returned by the Enumeration's nextElement method is the same resource that the getResource(String) method would return.
Returns:
 An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, are not returned in the enumeration.
Since:
 1.2
@revised
9
@spec
JPMS/**
     * Finds all the resources with the given name. A resource is some data
     * (images, audio, text, etc) that can be accessed by class code in a way
     * that is independent of the location of the code.
     *
     * <p> The name of a resource is a {@code /}-separated path name that
     * identifies the resource. </p>
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally (even if the caller of this method is in the
     * same module as the resource). </p>
     *
     * @implSpec The default implementation will first search the parent class
     * loader for the resource; if the parent is {@code null} the path of the
     * class loader built into the virtual machine is searched. It then
     * invokes {@link #findResources(String)} to find the resources with the
     * name in this class loader. It returns an enumeration whose elements
     * are the URLs found by searching the parent class loader followed by
     * the elements found with {@code findResources}.
     *
     * @apiNote Where several modules are defined to the same class loader,
     * and where more than one module contains a resource with the given name,
     * then the ordering is not specified and may be very unpredictable.
     * When overriding this method it is recommended that an
     * implementation ensures that any delegation is consistent with the {@link
     * #getResource(java.lang.String) getResource(String)} method. This should
     * ensure that the first element returned by the Enumeration's
     * {@code nextElement} method is the same resource that the
     * {@code getResource(String)} method would return.
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of {@link java.net.URL URL} objects for the
     *          resource. If no resources could be found, the enumeration will
     *          be empty. Resources for which a {@code URL} cannot be
     *          constructed, are in a package that is not opened
     *          unconditionally, or access to the resource is denied by the
     *          security manager, are not returned in the enumeration.
     *
     * @throws  IOException
     *          If I/O errors occur
     * @throws  NullPointerException If {@code name} is {@code null}
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     */
    public Enumeration<URL> getResources(String name) throws IOException {
        Objects.requireNonNull(name);
        @SuppressWarnings("unchecked")
        Enumeration<URL>[] tmp = (Enumeration<URL>[]) new Enumeration<?>[2];
        if (parent != null) {
            tmp[0] = parent.getResources(name);
        } else {
            tmp[0] = BootLoader.findResources(name);
        }
        tmp[1] = findResources(name);

        return new CompoundEnumeration<>(tmp);
    }

    
Returns a stream whose elements are the URLs of all the resources with
the given name. A resource is some data (images, audio, text, etc) that
can be accessed by class code in a way that is independent of the
location of the code.
 The name of a resource is a /-separated path name that identifies the resource. 
 The resources will be located when the returned stream is evaluated. If the evaluation results in an IOException then the I/O exception is wrapped in an UncheckedIOException that is then thrown. 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally (even if the caller of this method is in the same module as the resource). 
Params:
name – 
        The resource name
Throws:
NullPointerException – If name is null
Implementation Requirements:
The default implementation invokes 
getResources to find all the resources with the given name and returns a stream with the elements in the enumeration as the source.
API Note:
When overriding this method it is recommended that an implementation ensures that any delegation is consistent with the getResource(String) method. This should ensure that the first element returned by the stream is the same resource that the getResource(String) method would return.
Returns:
 A stream of resource URL objects. If no resources could be found, the stream will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, will not be in the stream.
Since:
 9/**
     * Returns a stream whose elements are the URLs of all the resources with
     * the given name. A resource is some data (images, audio, text, etc) that
     * can be accessed by class code in a way that is independent of the
     * location of the code.
     *
     * <p> The name of a resource is a {@code /}-separated path name that
     * identifies the resource.
     *
     * <p> The resources will be located when the returned stream is evaluated.
     * If the evaluation results in an {@code IOException} then the I/O
     * exception is wrapped in an {@link UncheckedIOException} that is then
     * thrown.
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally (even if the caller of this method is in the
     * same module as the resource). </p>
     *
     * @implSpec The default implementation invokes {@link #getResources(String)
     * getResources} to find all the resources with the given name and returns
     * a stream with the elements in the enumeration as the source.
     *
     * @apiNote When overriding this method it is recommended that an
     * implementation ensures that any delegation is consistent with the {@link
     * #getResource(java.lang.String) getResource(String)} method. This should
     * ensure that the first element returned by the stream is the same
     * resource that the {@code getResource(String)} method would return.
     *
     * @param  name
     *         The resource name
     *
     * @return  A stream of resource {@link java.net.URL URL} objects. If no
     *          resources could  be found, the stream will be empty. Resources
     *          for which a {@code URL} cannot be constructed, are in a package
     *          that is not opened unconditionally, or access to the resource
     *          is denied by the security manager, will not be in the stream.
     *
     * @throws  NullPointerException If {@code name} is {@code null}
     *
     * @since  9
     */
    public Stream<URL> resources(String name) {
        Objects.requireNonNull(name);
        int characteristics = Spliterator.NONNULL | Spliterator.IMMUTABLE;
        Supplier<Spliterator<URL>> si = () -> {
            try {
                return Spliterators.spliteratorUnknownSize(
                    getResources(name).asIterator(), characteristics);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        };
        return StreamSupport.stream(si, characteristics, false);
    }

    
Finds the resource with the given name. Class loader implementations
should override this method.
 For resources in named modules then the method must implement the rules for encapsulation specified in the Module getResourceAsStream method. Additionally, it must not find non-".class" resources in packages of named modules unless the package is opened unconditionally. 
Params:
name – 
        The resource name
Implementation Requirements:
The default implementation returns null.
Returns:
 URL object for reading the resource; null if the resource could not be found, a URL could not be constructed to locate the resource, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Since:
 1.2
@revised
9
@spec
JPMS/**
     * Finds the resource with the given name. Class loader implementations
     * should override this method.
     *
     * <p> For resources in named modules then the method must implement the
     * rules for encapsulation specified in the {@code Module} {@link
     * Module#getResourceAsStream getResourceAsStream} method. Additionally,
     * it must not find non-"{@code .class}" resources in packages of named
     * modules unless the package is {@link Module#isOpen(String) opened}
     * unconditionally. </p>
     *
     * @implSpec The default implementation returns {@code null}.
     *
     * @param  name
     *         The resource name
     *
     * @return  {@code URL} object for reading the resource; {@code null} if
     *          the resource could not be found, a {@code URL} could not be
     *          constructed to locate the resource, the resource is in a package
     *          that is not opened unconditionally, or access to the resource is
     *          denied by the security manager.
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     */
    protected URL findResource(String name) {
        return null;
    }

    
Returns an enumeration of URL objects representing all the resources with the given name. Class loader implementations should override this method. 
 For resources in named modules then the method must implement the rules for encapsulation specified in the Module getResourceAsStream method. Additionally, it must not find non-".class" resources in packages of named modules unless the package is opened unconditionally. 
Params:
name – 
        The resource name
Throws:
IOException – 
         If I/O errors occur
Implementation Requirements:
The default implementation returns an enumeration that
contains no elements.
Returns:
 An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, are not returned in the enumeration.
Since:
 1.2
@revised
9
@spec
JPMS/**
     * Returns an enumeration of {@link java.net.URL URL} objects
     * representing all the resources with the given name. Class loader
     * implementations should override this method.
     *
     * <p> For resources in named modules then the method must implement the
     * rules for encapsulation specified in the {@code Module} {@link
     * Module#getResourceAsStream getResourceAsStream} method. Additionally,
     * it must not find non-"{@code .class}" resources in packages of named
     * modules unless the package is {@link Module#isOpen(String) opened}
     * unconditionally. </p>
     *
     * @implSpec The default implementation returns an enumeration that
     * contains no elements.
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of {@link java.net.URL URL} objects for
     *          the resource. If no resources could  be found, the enumeration
     *          will be empty. Resources for which a {@code URL} cannot be
     *          constructed, are in a package that is not opened unconditionally,
     *          or access to the resource is denied by the security manager,
     *          are not returned in the enumeration.
     *
     * @throws  IOException
     *          If I/O errors occur
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     */
    protected Enumeration<URL> findResources(String name) throws IOException {
        return Collections.emptyEnumeration();
    }

    
Registers the caller as parallel capable. The registration succeeds if and only if all of the following conditions are met: 

 no instance of the caller has been created
 all of the super classes (except class Object) of the caller are
registered as parallel capable

Note that once a class loader is registered as parallel capable, there
is no way to change it back.
See Also:
isRegisteredAsParallelCapable()
Returns:
 true if the caller is successfully registered as parallel capable and false if otherwise.
Since:
  1.7/**
     * Registers the caller as
     * {@linkplain #isRegisteredAsParallelCapable() parallel capable}.
     * The registration succeeds if and only if all of the following
     * conditions are met:
     * <ol>
     * <li> no instance of the caller has been created</li>
     * <li> all of the super classes (except class Object) of the caller are
     * registered as parallel capable</li>
     * </ol>
     * <p>Note that once a class loader is registered as parallel capable, there
     * is no way to change it back.</p>
     *
     * @return  {@code true} if the caller is successfully registered as
     *          parallel capable and {@code false} if otherwise.
     *
     * @see #isRegisteredAsParallelCapable()
     *
     * @since   1.7
     */
    @CallerSensitive
    protected static boolean registerAsParallelCapable() {
        Class<? extends ClassLoader> callerClass =
            Reflection.getCallerClass().asSubclass(ClassLoader.class);
        return ParallelLoaders.register(callerClass);
    }

    
Returns true if this class loader is registered as parallel capable, otherwise false. 
See Also:
registerAsParallelCapable()
Returns:
 true if this class loader is parallel capable, otherwise false.
Since:
  9/**
     * Returns {@code true} if this class loader is registered as
     * {@linkplain #registerAsParallelCapable parallel capable}, otherwise
     * {@code false}.
     *
     * @return  {@code true} if this class loader is parallel capable,
     *          otherwise {@code false}.
     *
     * @see #registerAsParallelCapable()
     *
     * @since   9
     */
    public final boolean isRegisteredAsParallelCapable() {
        return ParallelLoaders.isRegistered(this.getClass());
    }

    
Find a resource of the specified name from the search path used to load classes. This method locates the resource through the system class loader (see getSystemClassLoader()). 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally. 
Params:
name – 
        The resource name
Returns:
 A URL to the resource; 
         null if the resource could not be found, a URL could not be constructed to locate the resource, the resource is in a package that is not opened unconditionally or access to the resource is denied by the security manager.
Since:
 1.1
@revised
9
@spec
JPMS/**
     * Find a resource of the specified name from the search path used to load
     * classes.  This method locates the resource through the system class
     * loader (see {@link #getSystemClassLoader()}).
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally. </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  A {@link java.net.URL URL} to the resource; {@code
     *          null} if the resource could not be found, a URL could not be
     *          constructed to locate the resource, the resource is in a package
     *          that is not opened unconditionally or access to the resource is
     *          denied by the security manager.
     *
     * @since  1.1
     * @revised 9
     * @spec JPMS
     */
    public static URL getSystemResource(String name) {
        return getSystemClassLoader().getResource(name);
    }

    
Finds all resources of the specified name from the search path used to load classes. The resources thus found are returned as an Enumeration of URL objects. 
 The search order is described in the documentation for getSystemResource(String). 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally. 
Params:
name – 
        The resource name
Throws:
IOException – 
         If I/O errors occur
Returns:
 An enumeration of URL objects for the resource. If no resources could be found, the enumeration will be empty. Resources for which a URL cannot be constructed, are in a package that is not opened unconditionally, or access to the resource is denied by the security manager, are not returned in the enumeration.
Since:
 1.2
@revised
9
@spec
JPMS/**
     * Finds all resources of the specified name from the search path used to
     * load classes.  The resources thus found are returned as an
     * {@link java.util.Enumeration Enumeration} of {@link
     * java.net.URL URL} objects.
     *
     * <p> The search order is described in the documentation for {@link
     * #getSystemResource(String)}.  </p>
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally. </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An enumeration of {@link java.net.URL URL} objects for
     *          the resource. If no resources could  be found, the enumeration
     *          will be empty. Resources for which a {@code URL} cannot be
     *          constructed, are in a package that is not opened unconditionally,
     *          or access to the resource is denied by the security manager,
     *          are not returned in the enumeration.
     *
     * @throws  IOException
     *          If I/O errors occur
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     */
    public static Enumeration<URL> getSystemResources(String name)
        throws IOException
    {
        return getSystemClassLoader().getResources(name);
    }

    
Returns an input stream for reading the specified resource.
 The search order is described in the documentation for getResource(String). 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally. 
Params:
name – 
        The resource name
Throws:
NullPointerException – If name is null
Returns:
 An input stream for reading the resource; null if the resource could not be found, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Since:
 1.1
@revised
9
@spec
JPMS/**
     * Returns an input stream for reading the specified resource.
     *
     * <p> The search order is described in the documentation for {@link
     * #getResource(String)}.  </p>
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally. </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An input stream for reading the resource; {@code null} if the
     *          resource could not be found, the resource is in a package that
     *          is not opened unconditionally, or access to the resource is
     *          denied by the security manager.
     *
     * @throws  NullPointerException If {@code name} is {@code null}
     *
     * @since  1.1
     * @revised 9
     * @spec JPMS
     */
    public InputStream getResourceAsStream(String name) {
        Objects.requireNonNull(name);
        URL url = getResource(name);
        try {
            return url != null ? url.openStream() : null;
        } catch (IOException e) {
            return null;
        }
    }

    
Open for reading, a resource of the specified name from the search path used to load classes. This method locates the resource through the system class loader (see getSystemClassLoader()). 
 Resources in named modules are subject to the encapsulation rules specified by Module.getResourceAsStream. Additionally, and except for the special case where the resource has a name ending with ".class", this method will only find resources in packages of named modules when the package is 
opened unconditionally. 
Params:
name – 
        The resource name
Returns:
 An input stream for reading the resource; null if the resource could not be found, the resource is in a package that is not opened unconditionally, or access to the resource is denied by the security manager.
Since:
 1.1
@revised
9
@spec
JPMS/**
     * Open for reading, a resource of the specified name from the search path
     * used to load classes.  This method locates the resource through the
     * system class loader (see {@link #getSystemClassLoader()}).
     *
     * <p> Resources in named modules are subject to the encapsulation rules
     * specified by {@link Module#getResourceAsStream Module.getResourceAsStream}.
     * Additionally, and except for the special case where the resource has a
     * name ending with "{@code .class}", this method will only find resources in
     * packages of named modules when the package is {@link Module#isOpen(String)
     * opened} unconditionally. </p>
     *
     * @param  name
     *         The resource name
     *
     * @return  An input stream for reading the resource; {@code null} if the
     *          resource could not be found, the resource is in a package that
     *          is not opened unconditionally, or access to the resource is
     *          denied by the security manager.
     *
     * @since  1.1
     * @revised 9
     * @spec JPMS
     */
    public static InputStream getSystemResourceAsStream(String name) {
        URL url = getSystemResource(name);
        try {
            return url != null ? url.openStream() : null;
        } catch (IOException e) {
            return null;
        }
    }


    // -- Hierarchy --

    
Returns the parent class loader for delegation. Some implementations may use null to represent the bootstrap class loader. This method will return null in such implementations if this class loader's parent is the bootstrap class loader. 
Throws:
SecurityException –  If a security manager is present, and the caller's class loader is not null and is not an ancestor of this class loader, and the caller does not have the RuntimePermission("getClassLoader")
Returns:
 The parent ClassLoader
Since:
 1.2/**
     * Returns the parent class loader for delegation. Some implementations may
     * use {@code null} to represent the bootstrap class loader. This method
     * will return {@code null} in such implementations if this class loader's
     * parent is the bootstrap class loader.
     *
     * @return  The parent {@code ClassLoader}
     *
     * @throws  SecurityException
     *          If a security manager is present, and the caller's class loader
     *          is not {@code null} and is not an ancestor of this class loader,
     *          and the caller does not have the
     *          {@link RuntimePermission}{@code ("getClassLoader")}
     *
     * @since  1.2
     */
    @CallerSensitive
    public final ClassLoader getParent() {
        if (parent == null)
            return null;
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            // Check access to the parent class loader
            // If the caller's class loader is same as this class loader,
            // permission check is performed.
            checkClassLoaderPermission(parent, Reflection.getCallerClass());
        }
        return parent;
    }

    
Returns the unnamed Module for this class loader. 
See Also:
Module.isNamed()
Returns:
The unnamed Module for this class loader
Since:
9
@spec
JPMS/**
     * Returns the unnamed {@code Module} for this class loader.
     *
     * @return The unnamed Module for this class loader
     *
     * @see Module#isNamed()
     * @since 9
     * @spec JPMS
     */
    public final Module getUnnamedModule() {
        return unnamedModule;
    }

    
Returns the platform class loader.  All
platform classes are visible to
the platform class loader.
Throws:
SecurityException –  If a security manager is present, and the caller's class loader is not null, and the caller's class loader is not the same as or an ancestor of the platform class loader, and the caller does not have the RuntimePermission("getClassLoader")
Implementation Note:
The name of the builtin platform class loader is "platform".
Returns:
 The platform ClassLoader.
Since:
9
@spec
JPMS/**
     * Returns the platform class loader.  All
     * <a href="#builtinLoaders">platform classes</a> are visible to
     * the platform class loader.
     *
     * @implNote The name of the builtin platform class loader is
     * {@code "platform"}.
     *
     * @return  The platform {@code ClassLoader}.
     *
     * @throws  SecurityException
     *          If a security manager is present, and the caller's class loader is
     *          not {@code null}, and the caller's class loader is not the same
     *          as or an ancestor of the platform class loader,
     *          and the caller does not have the
     *          {@link RuntimePermission}{@code ("getClassLoader")}
     *
     * @since 9
     * @spec JPMS
     */
    @CallerSensitive
    public static ClassLoader getPlatformClassLoader() {
        SecurityManager sm = System.getSecurityManager();
        ClassLoader loader = getBuiltinPlatformClassLoader();
        if (sm != null) {
            checkClassLoaderPermission(loader, Reflection.getCallerClass());
        }
        return loader;
    }

    
Returns the system class loader. This is the default delegation parent for new ClassLoader instances, and is typically the class loader used to start the application. 
 This method is first invoked early in the runtime's startup sequence, at which point it creates the system class loader. This class loader will be the context class loader for the main application thread (for example, the thread that invokes the main method of the main class). 
 The default system class loader is an implementation-dependent
instance of this class.
 If the system property "java.system.class.loader" is defined when this method is first invoked then the value of that property is taken to be the name of a class that will be returned as the system class loader. The class is loaded using the default system class loader and must define a public constructor that takes a single parameter of type ClassLoader which is used as the delegation parent. An instance is then created using this constructor with the default system class loader as the parameter. The resulting class loader is defined to be the system class loader. During construction, the class loader should take great care to avoid calling getSystemClassLoader(). If circular initialization of the system class loader is detected then an IllegalStateException is thrown. 
Throws:
SecurityException –  If a security manager is present, and the caller's class loader is not null and is not the same as or an ancestor of the system class loader, and the caller does not have the RuntimePermission("getClassLoader")
IllegalStateException –  If invoked recursively during the construction of the class loader specified by the "java.system.class.loader" property.
Error –  If the system property "java.system.class.loader" is defined but the named class could not be loaded, the provider class does not define the required constructor, or an exception is thrown by that constructor when it is invoked. The underlying cause of the error can be retrieved via the Throwable.getCause() method.
Implementation Note:
The system property to override the system class loader is not
examined until the VM is almost fully initialized. Code that executes
this method during startup should take care not to cache the return
value until the system is fully initialized.
 The name of the built-in system class loader is "app". The system property "java.class.path" is read during early initialization of the VM to determine the class path. An empty value of "java.class.path" property is interpreted differently depending on whether the initial module (the module containing the main class) is named or unnamed: If named, the built-in system class loader will have no class path and will search for classes and resources using the application module path; otherwise, if unnamed, it will set the class path to the current working directory.
Returns:
 The system ClassLoader
@revised
 1.4
@revised
9
@spec
JPMS/**
     * Returns the system class loader.  This is the default
     * delegation parent for new {@code ClassLoader} instances, and is
     * typically the class loader used to start the application.
     *
     * <p> This method is first invoked early in the runtime's startup
     * sequence, at which point it creates the system class loader. This
     * class loader will be the context class loader for the main application
     * thread (for example, the thread that invokes the {@code main} method of
     * the main class).
     *
     * <p> The default system class loader is an implementation-dependent
     * instance of this class.
     *
     * <p> If the system property "{@code java.system.class.loader}" is defined
     * when this method is first invoked then the value of that property is
     * taken to be the name of a class that will be returned as the system
     * class loader.  The class is loaded using the default system class loader
     * and must define a public constructor that takes a single parameter of
     * type {@code ClassLoader} which is used as the delegation parent.  An
     * instance is then created using this constructor with the default system
     * class loader as the parameter.  The resulting class loader is defined
     * to be the system class loader. During construction, the class loader
     * should take great care to avoid calling {@code getSystemClassLoader()}.
     * If circular initialization of the system class loader is detected then
     * an {@code IllegalStateException} is thrown.
     *
     * @implNote The system property to override the system class loader is not
     * examined until the VM is almost fully initialized. Code that executes
     * this method during startup should take care not to cache the return
     * value until the system is fully initialized.
     *
     * <p> The name of the built-in system class loader is {@code "app"}.
     * The system property "{@code java.class.path}" is read during early
     * initialization of the VM to determine the class path.
     * An empty value of "{@code java.class.path}" property is interpreted
     * differently depending on whether the initial module (the module
     * containing the main class) is named or unnamed:
     * If named, the built-in system class loader will have no class path and
     * will search for classes and resources using the application module path;
     * otherwise, if unnamed, it will set the class path to the current
     * working directory.
     *
     * @return  The system {@code ClassLoader}
     *
     * @throws  SecurityException
     *          If a security manager is present, and the caller's class loader
     *          is not {@code null} and is not the same as or an ancestor of the
     *          system class loader, and the caller does not have the
     *          {@link RuntimePermission}{@code ("getClassLoader")}
     *
     * @throws  IllegalStateException
     *          If invoked recursively during the construction of the class
     *          loader specified by the "{@code java.system.class.loader}"
     *          property.
     *
     * @throws  Error
     *          If the system property "{@code java.system.class.loader}"
     *          is defined but the named class could not be loaded, the
     *          provider class does not define the required constructor, or an
     *          exception is thrown by that constructor when it is invoked. The
     *          underlying cause of the error can be retrieved via the
     *          {@link Throwable#getCause()} method.
     *
     * @revised  1.4
     * @revised 9
     * @spec JPMS
     */
    @CallerSensitive
    public static ClassLoader getSystemClassLoader() {
        switch (VM.initLevel()) {
            case 0:
            case 1:
            case 2:
                // the system class loader is the built-in app class loader during startup
                return getBuiltinAppClassLoader();
            case 3:
                String msg = "getSystemClassLoader cannot be called during the system class loader instantiation";
                throw new IllegalStateException(msg);
            default:
                // system fully initialized
                assert VM.isBooted() && scl != null;
                SecurityManager sm = System.getSecurityManager();
                if (sm != null) {
                    checkClassLoaderPermission(scl, Reflection.getCallerClass());
                }
                return scl;
        }
    }

    static ClassLoader getBuiltinPlatformClassLoader() {
        return ClassLoaders.platformClassLoader();
    }

    static ClassLoader getBuiltinAppClassLoader() {
        return ClassLoaders.appClassLoader();
    }

    /*
     * Initialize the system class loader that may be a custom class on the
     * application class path or application module path.
     *
     * @see java.lang.System#initPhase3
     */
    static synchronized ClassLoader initSystemClassLoader() {
        if (VM.initLevel() != 3) {
            throw new InternalError("system class loader cannot be set at initLevel " +
                                    VM.initLevel());
        }

        // detect recursive initialization
        if (scl != null) {
            throw new IllegalStateException("recursive invocation");
        }

        ClassLoader builtinLoader = getBuiltinAppClassLoader();

        // All are privileged frames.  No need to call doPrivileged.
        String cn = System.getProperty("java.system.class.loader");
        if (cn != null) {
            try {
                // custom class loader is only supported to be loaded from unnamed module
                Constructor<?> ctor = Class.forName(cn, false, builtinLoader)
                                           .getDeclaredConstructor(ClassLoader.class);
                scl = (ClassLoader) ctor.newInstance(builtinLoader);
            } catch (Exception e) {
                Throwable cause = e;
                if (e instanceof InvocationTargetException) {
                    cause = e.getCause();
                    if (cause instanceof Error) {
                        throw (Error) cause;
                    }
                }
                if (cause instanceof RuntimeException) {
                    throw (RuntimeException) cause;
                }
                throw new Error(cause.getMessage(), cause);
            }
        } else {
            scl = builtinLoader;
        }
        return scl;
    }

    /*
     * Initialize default paths for native libraries search.
     * Must be done early as JDK may load libraries during bootstrap.
     *
     * @see java.lang.System#initPhase1
     */
    static void initLibraryPaths() {
        usr_paths = initializePath("java.library.path");
        sys_paths = initializePath("sun.boot.library.path");
    }

    // Returns true if the specified class loader can be found in this class
    // loader's delegation chain.
    boolean isAncestor(ClassLoader cl) {
        ClassLoader acl = this;
        do {
            acl = acl.parent;
            if (cl == acl) {
                return true;
            }
        } while (acl != null);
        return false;
    }

    // Tests if class loader access requires "getClassLoader" permission
    // check.  A class loader 'from' can access class loader 'to' if
    // class loader 'from' is same as class loader 'to' or an ancestor
    // of 'to'.  The class loader in a system domain can access
    // any class loader.
    private static boolean needsClassLoaderPermissionCheck(ClassLoader from,
                                                           ClassLoader to)
    {
        if (from == to)
            return false;

        if (from == null)
            return false;

        return !to.isAncestor(from);
    }

    // Returns the class's class loader, or null if none.
    static ClassLoader getClassLoader(Class<?> caller) {
        // This can be null if the VM is requesting it
        if (caller == null) {
            return null;
        }
        // Circumvent security check since this is package-private
        return caller.getClassLoader0();
    }

    /*
     * Checks RuntimePermission("getClassLoader") permission
     * if caller's class loader is not null and caller's class loader
     * is not the same as or an ancestor of the given cl argument.
     */
    static void checkClassLoaderPermission(ClassLoader cl, Class<?> caller) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            // caller can be null if the VM is requesting it
            ClassLoader ccl = getClassLoader(caller);
            if (needsClassLoaderPermissionCheck(ccl, cl)) {
                sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
            }
        }
    }

    // The system class loader
    // @GuardedBy("ClassLoader.class")
    private static volatile ClassLoader scl;

    // -- Package --

    
Define a Package of the given Class object. If the given class represents an array type, a primitive type or void, this method returns null. This method does not throw IllegalArgumentException. /**
     * Define a Package of the given Class object.
     *
     * If the given class represents an array type, a primitive type or void,
     * this method returns {@code null}.
     *
     * This method does not throw IllegalArgumentException.
     */
    Package definePackage(Class<?> c) {
        if (c.isPrimitive() || c.isArray()) {
            return null;
        }

        return definePackage(c.getPackageName(), c.getModule());
    }

    
Defines a Package of the given name and module
This method does not throw IllegalArgumentException.
Params:
name – package name
m –    module/**
     * Defines a Package of the given name and module
     *
     * This method does not throw IllegalArgumentException.
     *
     * @param name package name
     * @param m    module
     */
    Package definePackage(String name, Module m) {
        if (name.isEmpty() && m.isNamed()) {
            throw new InternalError("unnamed package in  " + m);
        }

        // check if Package object is already defined
        NamedPackage pkg = packages.get(name);
        if (pkg instanceof Package)
            return (Package)pkg;

        return (Package)packages.compute(name, (n, p) -> toPackage(n, p, m));
    }

    /*
     * Returns a Package object for the named package
     */
    private Package toPackage(String name, NamedPackage p, Module m) {
        // define Package object if the named package is not yet defined
        if (p == null)
            return NamedPackage.toPackage(name, m);

        // otherwise, replace the NamedPackage object with Package object
        if (p instanceof Package)
            return (Package)p;

        return NamedPackage.toPackage(p.packageName(), p.module());
    }

    
Defines a package by name in this ClassLoader. 

Package names must be unique within a class loader and
cannot be redefined or changed once created.
 If a class loader wishes to define a package with specific properties, such as version information, then the class loader should call this definePackage method before calling defineClass. Otherwise, the defineClass method will define a package in this class loader corresponding to the package of the newly defined class; the properties of this defined package are specified by Package. 
Params:
name – 
        The package name
specTitle – 
        The specification title
specVersion – 
        The specification version
specVendor – 
        The specification vendor
implTitle – 
        The implementation title
implVersion – 
        The implementation version
implVendor – 
        The implementation vendor
sealBase –  If not null, then this package is sealed with respect to the given code source URL object. Otherwise, the package is not sealed.
Throws:
NullPointerException –  if name is null.
IllegalArgumentException –  if a package of the given name is already defined by this class loader
See Also:

     *      The JAR File Specification: Package Sealing
API Note:
 A class loader that wishes to define a package for classes in a JAR typically uses the specification and implementation titles, versions, and vendors from the JAR's manifest. If the package is specified as sealed in the JAR's manifest, the URL of the JAR file is typically used as the sealBase. If classes of package 'p' defined by this class loader are loaded from multiple JARs, the Package object may contain different information depending on the first class of package 'p' defined and which JAR's manifest is read first to explicitly define package 'p'. 
 It is strongly recommended that a class loader does not call this
method to explicitly define packages in named modules; instead, the package will be automatically defined when a class is being defined. If it is desirable to define Package explicitly, it should ensure that all packages in a named module are defined with the properties specified by Package. Otherwise, some Package objects in a named module may be for example sealed with different seal base.
Returns:
 The newly defined Package object
Since:
 1.2
@revised
9
@spec
JPMS
@jvms
5.3 Run-time package/**
     * Defines a package by <a href="#binary-name">name</a> in this {@code ClassLoader}.
     * <p>
     * <a href="#binary-name">Package names</a> must be unique within a class loader and
     * cannot be redefined or changed once created.
     * <p>
     * If a class loader wishes to define a package with specific properties,
     * such as version information, then the class loader should call this
     * {@code definePackage} method before calling {@code defineClass}.
     * Otherwise, the
     * {@link #defineClass(String, byte[], int, int, ProtectionDomain) defineClass}
     * method will define a package in this class loader corresponding to the package
     * of the newly defined class; the properties of this defined package are
     * specified by {@link Package}.
     *
     * @apiNote
     * A class loader that wishes to define a package for classes in a JAR
     * typically uses the specification and implementation titles, versions, and
     * vendors from the JAR's manifest. If the package is specified as
     * {@linkplain java.util.jar.Attributes.Name#SEALED sealed} in the JAR's manifest,
     * the {@code URL} of the JAR file is typically used as the {@code sealBase}.
     * If classes of package {@code 'p'} defined by this class loader
     * are loaded from multiple JARs, the {@code Package} object may contain
     * different information depending on the first class of package {@code 'p'}
     * defined and which JAR's manifest is read first to explicitly define
     * package {@code 'p'}.
     *
     * <p> It is strongly recommended that a class loader does not call this
     * method to explicitly define packages in <em>named modules</em>; instead,
     * the package will be automatically defined when a class is {@linkplain
     * #defineClass(String, byte[], int, int, ProtectionDomain) being defined}.
     * If it is desirable to define {@code Package} explicitly, it should ensure
     * that all packages in a named module are defined with the properties
     * specified by {@link Package}.  Otherwise, some {@code Package} objects
     * in a named module may be for example sealed with different seal base.
     *
     * @param  name
     *         The <a href="#binary-name">package name</a>
     *
     * @param  specTitle
     *         The specification title
     *
     * @param  specVersion
     *         The specification version
     *
     * @param  specVendor
     *         The specification vendor
     *
     * @param  implTitle
     *         The implementation title
     *
     * @param  implVersion
     *         The implementation version
     *
     * @param  implVendor
     *         The implementation vendor
     *
     * @param  sealBase
     *         If not {@code null}, then this package is sealed with
     *         respect to the given code source {@link java.net.URL URL}
     *         object.  Otherwise, the package is not sealed.
     *
     * @return  The newly defined {@code Package} object
     *
     * @throws  NullPointerException
     *          if {@code name} is {@code null}.
     *
     * @throws  IllegalArgumentException
     *          if a package of the given {@code name} is already
     *          defined by this class loader
     *
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     *
     * @jvms 5.3 Run-time package
     * @see <a href="{@docRoot}/../specs/jar/jar.html#package-sealing">
     *      The JAR File Specification: Package Sealing</a>
     */
    protected Package definePackage(String name, String specTitle,
                                    String specVersion, String specVendor,
                                    String implTitle, String implVersion,
                                    String implVendor, URL sealBase)
    {
        Objects.requireNonNull(name);

        // definePackage is not final and may be overridden by custom class loader
        Package p = new Package(name, specTitle, specVersion, specVendor,
                                implTitle, implVersion, implVendor,
                                sealBase, this);

        if (packages.putIfAbsent(name, p) != null)
            throw new IllegalArgumentException(name);

        return p;
    }

    
Returns a Package of the given name that
has been defined by this class loader.
Params:
name – The package name
Throws:
NullPointerException –  if name is null.
Returns:
The Package of the given name that has been defined by this class loader, or null if not found
@jvms
5.3 Run-time package
Since:
 9
@spec
JPMS/**
     * Returns a {@code Package} of the given <a href="#binary-name">name</a> that
     * has been defined by this class loader.
     *
     * @param  name The <a href="#binary-name">package name</a>
     *
     * @return The {@code Package} of the given name that has been defined
     *         by this class loader, or {@code null} if not found
     *
     * @throws  NullPointerException
     *          if {@code name} is {@code null}.
     *
     * @jvms 5.3 Run-time package
     *
     * @since  9
     * @spec JPMS
     */
    public final Package getDefinedPackage(String name) {
        Objects.requireNonNull(name, "name cannot be null");

        NamedPackage p = packages.get(name);
        if (p == null)
            return null;

        return definePackage(name, p.module());
    }

    
Returns all of the Packages that have been defined by this class loader. The returned array has no duplicated Packages of the same name. 
API Note:
This method returns an array rather than a Set or Stream for consistency with the existing getPackages method.
Returns:
The array of Package objects that have been defined by this class loader; or an zero length array if no package has been defined by this class loader.
@jvms
5.3 Run-time package
Since:
 9
@spec
JPMS/**
     * Returns all of the {@code Package}s that have been defined by
     * this class loader.  The returned array has no duplicated {@code Package}s
     * of the same name.
     *
     * @apiNote This method returns an array rather than a {@code Set} or {@code Stream}
     *          for consistency with the existing {@link #getPackages} method.
     *
     * @return The array of {@code Package} objects that have been defined by
     *         this class loader; or an zero length array if no package has been
     *         defined by this class loader.
     *
     * @jvms 5.3 Run-time package
     *
     * @since  9
     * @spec JPMS
     */
    public final Package[] getDefinedPackages() {
        return packages().toArray(Package[]::new);
    }

    
Finds a package by name in this class loader and its ancestors.
 If this class loader defines a Package of the given name, the Package is returned. Otherwise, the ancestors of this class loader are searched recursively (parent by parent) for a Package of the given name. 
Params:
name – 
        The package name
Throws:
NullPointerException –  if name is null.
See Also:
getDefinedPackage(String)
API Note:
The platform class loader may delegate to the application class loader but the application class loader is not its ancestor. When invoked on the platform class loader, this method will not find packages defined to the application class loader.
Returns:
The Package of the given name that has been defined by this class loader or its ancestors, or null if not found.
Deprecated:
 If multiple class loaders delegate to each other and define classes with the same package name, and one such loader relies on the lookup behavior of getPackage to return a Package from a parent loader, then the properties exposed by the Package may not be as expected in the rest of the program. For example, the Package will only expose annotations from the package-info.class file defined by the parent loader, even if annotations exist in a package-info.class file defined by a child loader. A more robust approach is to use the getDefinedPackage method which returns a Package for the specified class loader.
Since:
 1.2
@revised
9
@spec
JPMS/**
     * Finds a package by <a href="#binary-name">name</a> in this class loader and its ancestors.
     * <p>
     * If this class loader defines a {@code Package} of the given name,
     * the {@code Package} is returned. Otherwise, the ancestors of
     * this class loader are searched recursively (parent by parent)
     * for a {@code Package} of the given name.
     *
     * @apiNote The {@link #getPlatformClassLoader() platform class loader}
     * may delegate to the application class loader but the application class
     * loader is not its ancestor.  When invoked on the platform class loader,
     * this method  will not find packages defined to the application
     * class loader.
     *
     * @param  name
     *         The <a href="#binary-name">package name</a>
     *
     * @return The {@code Package} of the given name that has been defined by
     *         this class loader or its ancestors, or {@code null} if not found.
     *
     * @throws  NullPointerException
     *          if {@code name} is {@code null}.
     *
     * @deprecated
     * If multiple class loaders delegate to each other and define classes
     * with the same package name, and one such loader relies on the lookup
     * behavior of {@code getPackage} to return a {@code Package} from
     * a parent loader, then the properties exposed by the {@code Package}
     * may not be as expected in the rest of the program.
     * For example, the {@code Package} will only expose annotations from the
     * {@code package-info.class} file defined by the parent loader, even if
     * annotations exist in a {@code package-info.class} file defined by
     * a child loader.  A more robust approach is to use the
     * {@link ClassLoader#getDefinedPackage} method which returns
     * a {@code Package} for the specified class loader.
     *
     * @see ClassLoader#getDefinedPackage(String)
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     */
    @Deprecated(since="9")
    protected Package getPackage(String name) {
        Package pkg = getDefinedPackage(name);
        if (pkg == null) {
            if (parent != null) {
                pkg = parent.getPackage(name);
            } else {
                pkg = BootLoader.getDefinedPackage(name);
            }
        }
        return pkg;
    }

    
Returns all of the Packages that have been defined by this class loader and its ancestors. The returned array may contain more than one Package object of the same package name, each defined by a different class loader in the class loader hierarchy. 
See Also:
getDefinedPackages()
API Note:
The platform class loader may delegate to the application class loader. In other words, packages in modules defined to the application class loader may be visible to the platform class loader. On the other hand, the application class loader is not its ancestor and hence when invoked on the platform class loader, this method will not return any packages defined to the application class loader.
Returns:
 The array of Package objects that have been defined by this class loader and its ancestors
Since:
 1.2
@revised
9
@spec
JPMS/**
     * Returns all of the {@code Package}s that have been defined by
     * this class loader and its ancestors.  The returned array may contain
     * more than one {@code Package} object of the same package name, each
     * defined by a different class loader in the class loader hierarchy.
     *
     * @apiNote The {@link #getPlatformClassLoader() platform class loader}
     * may delegate to the application class loader. In other words,
     * packages in modules defined to the application class loader may be
     * visible to the platform class loader.  On the other hand,
     * the application class loader is not its ancestor and hence
     * when invoked on the platform class loader, this method will not
     * return any packages defined to the application class loader.
     *
     * @return  The array of {@code Package} objects that have been defined by
     *          this class loader and its ancestors
     *
     * @see ClassLoader#getDefinedPackages()
     *
     * @since  1.2
     * @revised 9
     * @spec JPMS
     */
    protected Package[] getPackages() {
        Stream<Package> pkgs = packages();
        ClassLoader ld = parent;
        while (ld != null) {
            pkgs = Stream.concat(ld.packages(), pkgs);
            ld = ld.parent;
        }
        return Stream.concat(BootLoader.packages(), pkgs)
                     .toArray(Package[]::new);
    }



    // package-private

    
Returns a stream of Packages defined in this class loader
/**
     * Returns a stream of Packages defined in this class loader
     */
    Stream<Package> packages() {
        return packages.values().stream()
                       .map(p -> definePackage(p.packageName(), p.module()));
    }

    // -- Native library access --

    
Returns the absolute path name of a native library. The VM invokes this method to locate the native libraries that belong to classes loaded with this class loader. If this method returns null, the VM searches the library along the path specified as the "java.library.path" property. 
Params:
libname – 
        The library name
See Also:
System.loadLibrary(String)
System.mapLibraryName(String)
Returns:
 The absolute path of the native library
Since:
 1.2/**
     * Returns the absolute path name of a native library.  The VM invokes this
     * method to locate the native libraries that belong to classes loaded with
     * this class loader. If this method returns {@code null}, the VM
     * searches the library along the path specified as the
     * "{@code java.library.path}" property.
     *
     * @param  libname
     *         The library name
     *
     * @return  The absolute path of the native library
     *
     * @see  System#loadLibrary(String)
     * @see  System#mapLibraryName(String)
     *
     * @since  1.2
     */
    protected String findLibrary(String libname) {
        return null;
    }

    
The inner class NativeLibrary denotes a loaded native library instance. Every classloader contains a vector of loaded native libraries in the private field nativeLibraries. The native libraries loaded into the system are entered into the systemNativeLibraries vector. 
 Every native library requires a particular version of JNI. This is denoted by the private jniVersion field. This field is set by the VM when it loads the library, and used by the VM to pass the correct version of JNI to the native methods. 
See Also:
ClassLoader
Since:
   1.2/**
     * The inner class NativeLibrary denotes a loaded native library instance.
     * Every classloader contains a vector of loaded native libraries in the
     * private field {@code nativeLibraries}.  The native libraries loaded
     * into the system are entered into the {@code systemNativeLibraries}
     * vector.
     *
     * <p> Every native library requires a particular version of JNI. This is
     * denoted by the private {@code jniVersion} field.  This field is set by
     * the VM when it loads the library, and used by the VM to pass the correct
     * version of JNI to the native methods.  </p>
     *
     * @see      ClassLoader
     * @since    1.2
     */
    static class NativeLibrary {
        // the class from which the library is loaded, also indicates
        // the loader this native library belongs.
        final Class<?> fromClass;
        // the canonicalized name of the native library.
        // or static library name
        final String name;
        // Indicates if the native library is linked into the VM
        final boolean isBuiltin;

        // opaque handle to native library, used in native code.
        long handle;
        // the version of JNI environment the native library requires.
        int jniVersion;

        native boolean load0(String name, boolean isBuiltin);

        native long findEntry(String name);

        NativeLibrary(Class<?> fromClass, String name, boolean isBuiltin) {
            this.name = name;
            this.fromClass = fromClass;
            this.isBuiltin = isBuiltin;
        }

        /*
         * Loads the native library and registers for cleanup when its
         * associated class loader is unloaded
         */
        boolean load() {
            if (handle != 0) {
                throw new InternalError("Native library " + name + " has been loaded");
            }

            if (!load0(name, isBuiltin)) return false;

            // register the class loader for cleanup when unloaded
            // built class loaders are never unloaded
            ClassLoader loader = fromClass.getClassLoader();
            if (loader != null &&
                loader != getBuiltinPlatformClassLoader() &&
                loader != getBuiltinAppClassLoader()) {
                CleanerFactory.cleaner().register(loader,
                        new Unloader(name, handle, isBuiltin));
            }
            return true;
        }

        static boolean loadLibrary(Class<?> fromClass, String name, boolean isBuiltin) {
            ClassLoader loader =
                fromClass == null ? null : fromClass.getClassLoader();

            synchronized (loadedLibraryNames) {
                Map<String, NativeLibrary> libs =
                    loader != null ? loader.nativeLibraries() : systemNativeLibraries();
                if (libs.containsKey(name)) {
                    return true;
                }

                if (loadedLibraryNames.contains(name)) {
                    throw new UnsatisfiedLinkError("Native Library " + name +
                        " already loaded in another classloader");
                }

                /*
                 * When a library is being loaded, JNI_OnLoad function can cause
                 * another loadLibrary invocation that should succeed.
                 *
                 * We use a static stack to hold the list of libraries we are
                 * loading because this can happen only when called by the
                 * same thread because this block is synchronous.
                 *
                 * If there is a pending load operation for the library, we
                 * immediately return success; otherwise, we raise
                 * UnsatisfiedLinkError.
                 */
                for (NativeLibrary lib : nativeLibraryContext) {
                    if (name.equals(lib.name)) {
                        if (loader == lib.fromClass.getClassLoader()) {
                            return true;
                        } else {
                            throw new UnsatisfiedLinkError("Native Library " +
                                name + " is being loaded in another classloader");
                        }
                    }
                }
                NativeLibrary lib = new NativeLibrary(fromClass, name, isBuiltin);
                // load the native library
                nativeLibraryContext.push(lib);
                try {
                    if (!lib.load()) return false;
                } finally {
                    nativeLibraryContext.pop();
                }
                // register the loaded native library
                loadedLibraryNames.add(name);
                libs.put(name, lib);
            }
            return true;
        }

        // Invoked in the VM to determine the context class in JNI_OnLoad
        // and JNI_OnUnload
        static Class<?> getFromClass() {
            return nativeLibraryContext.peek().fromClass;
        }

        // native libraries being loaded
        static Deque<NativeLibrary> nativeLibraryContext = new ArrayDeque<>(8);

        /*
         * The run() method will be invoked when this class loader becomes
         * phantom reachable to unload the native library.
         */
        static class Unloader implements Runnable {
            // This represents the context when a native library is unloaded
            // and getFromClass() will return null,
            static final NativeLibrary UNLOADER =
                new NativeLibrary(null, "dummy", false);
            final String name;
            final long handle;
            final boolean isBuiltin;

            Unloader(String name, long handle, boolean isBuiltin) {
                if (handle == 0) {
                    throw new IllegalArgumentException(
                        "Invalid handle for native library " + name);
                }

                this.name = name;
                this.handle = handle;
                this.isBuiltin = isBuiltin;
            }

            @Override
            public void run() {
                synchronized (loadedLibraryNames) {
                    /* remove the native library name */
                    loadedLibraryNames.remove(name);
                    nativeLibraryContext.push(UNLOADER);
                    try {
                        unload(name, isBuiltin, handle);
                    } finally {
                        nativeLibraryContext.pop();
                    }

                }
            }
        }

        // JNI FindClass expects the caller class if invoked from JNI_OnLoad
        // and JNI_OnUnload is NativeLibrary class
        static native void unload(String name, boolean isBuiltin, long handle);
    }

    // The paths searched for libraries
    private static String usr_paths[];
    private static String sys_paths[];

    private static String[] initializePath(String propName) {
        String ldPath = System.getProperty(propName, "");
        int ldLen = ldPath.length();
        char ps = File.pathSeparatorChar;
        int psCount = 0;

        if (ClassLoaderHelper.allowsQuotedPathElements &&
            ldPath.indexOf('\"') >= 0) {
            // First, remove quotes put around quoted parts of paths.
            // Second, use a quotation mark as a new path separator.
            // This will preserve any quoted old path separators.
            char[] buf = new char[ldLen];
            int bufLen = 0;
            for (int i = 0; i < ldLen; ++i) {
                char ch = ldPath.charAt(i);
                if (ch == '\"') {
                    while (++i < ldLen &&
                        (ch = ldPath.charAt(i)) != '\"') {
                        buf[bufLen++] = ch;
                    }
                } else {
                    if (ch == ps) {
                        psCount++;
                        ch = '\"';
                    }
                    buf[bufLen++] = ch;
                }
            }
            ldPath = new String(buf, 0, bufLen);
            ldLen = bufLen;
            ps = '\"';
        } else {
            for (int i = ldPath.indexOf(ps); i >= 0;
                 i = ldPath.indexOf(ps, i + 1)) {
                psCount++;
            }
        }

        String[] paths = new String[psCount + 1];
        int pathStart = 0;
        for (int j = 0; j < psCount; ++j) {
            int pathEnd = ldPath.indexOf(ps, pathStart);
            paths[j] = (pathStart < pathEnd) ?
                ldPath.substring(pathStart, pathEnd) : ".";
            pathStart = pathEnd + 1;
        }
        paths[psCount] = (pathStart < ldLen) ?
            ldPath.substring(pathStart, ldLen) : ".";
        return paths;
    }

    // Invoked in the java.lang.Runtime class to implement load and loadLibrary.
    static void loadLibrary(Class<?> fromClass, String name,
                            boolean isAbsolute) {
        ClassLoader loader =
            (fromClass == null) ? null : fromClass.getClassLoader();
        assert sys_paths != null : "should be initialized at this point";
        assert usr_paths != null : "should be initialized at this point";

        if (isAbsolute) {
            if (loadLibrary0(fromClass, new File(name))) {
                return;
            }
            throw new UnsatisfiedLinkError("Can't load library: " + name);
        }
        if (loader != null) {
            String libfilename = loader.findLibrary(name);
            if (libfilename != null) {
                File libfile = new File(libfilename);
                if (!libfile.isAbsolute()) {
                    throw new UnsatisfiedLinkError(
                        "ClassLoader.findLibrary failed to return an absolute path: " + libfilename);
                }
                if (loadLibrary0(fromClass, libfile)) {
                    return;
                }
                throw new UnsatisfiedLinkError("Can't load " + libfilename);
            }
        }
        for (String sys_path : sys_paths) {
            File libfile = new File(sys_path, System.mapLibraryName(name));
            if (loadLibrary0(fromClass, libfile)) {
                return;
            }
            libfile = ClassLoaderHelper.mapAlternativeName(libfile);
            if (libfile != null && loadLibrary0(fromClass, libfile)) {
                return;
            }
        }
        if (loader != null) {
            for (String usr_path : usr_paths) {
                File libfile = new File(usr_path, System.mapLibraryName(name));
                if (loadLibrary0(fromClass, libfile)) {
                    return;
                }
                libfile = ClassLoaderHelper.mapAlternativeName(libfile);
                if (libfile != null && loadLibrary0(fromClass, libfile)) {
                    return;
                }
            }
        }
        // Oops, it failed
        throw new UnsatisfiedLinkError("no " + name +
            " in java.library.path: " + Arrays.toString(usr_paths));
    }

    private static native String findBuiltinLib(String name);

    private static boolean loadLibrary0(Class<?> fromClass, final File file) {
        // Check to see if we're attempting to access a static library
        String name = findBuiltinLib(file.getName());
        boolean isBuiltin = (name != null);
        if (!isBuiltin) {
            name = AccessController.doPrivileged(
                new PrivilegedAction<>() {
                    public String run() {
                        try {
                            return file.exists() ? file.getCanonicalPath() : null;
                        } catch (IOException e) {
                            return null;
                        }
                    }
                });
            if (name == null) {
                return false;
            }
        }
        return NativeLibrary.loadLibrary(fromClass, name, isBuiltin);
    }

    /*
     * Invoked in the VM class linking code.
     */
    private static long findNative(ClassLoader loader, String entryName) {
        Map<String, NativeLibrary> libs =
            loader != null ? loader.nativeLibraries() : systemNativeLibraries();
        if (libs.isEmpty())
            return 0;

        // the native libraries map may be updated in another thread
        // when a native library is being loaded.  No symbol will be
        // searched from it yet.
        for (NativeLibrary lib : libs.values()) {
            long entry = lib.findEntry(entryName);
            if (entry != 0) return entry;
        }
        return 0;
    }

    // All native library names we've loaded.
    // This also serves as the lock to obtain nativeLibraries
    // and write to nativeLibraryContext.
    private static final Set<String> loadedLibraryNames = new HashSet<>();

    // Native libraries belonging to system classes.
    private static volatile Map<String, NativeLibrary> systemNativeLibraries;

    // Native libraries associated with the class loader.
    private volatile Map<String, NativeLibrary> nativeLibraries;

    /*
     * Returns the native libraries map associated with bootstrap class loader
     * This method will create the map at the first time when called.
     */
    private static Map<String, NativeLibrary> systemNativeLibraries() {
        Map<String, NativeLibrary> libs = systemNativeLibraries;
        if (libs == null) {
            synchronized (loadedLibraryNames) {
                libs = systemNativeLibraries;
                if (libs == null) {
                    libs = systemNativeLibraries = new ConcurrentHashMap<>();
                }
            }
        }
        return libs;
    }

    /*
     * Returns the native libraries map associated with this class loader
     * This method will create the map at the first time when called.
     */
    private Map<String, NativeLibrary> nativeLibraries() {
        Map<String, NativeLibrary> libs = nativeLibraries;
        if (libs == null) {
            synchronized (loadedLibraryNames) {
                libs = nativeLibraries;
                if (libs == null) {
                    libs = nativeLibraries = new ConcurrentHashMap<>();
                }
            }
        }
        return libs;
    }

    // -- Assertion management --

    final Object assertionLock;

    // The default toggle for assertion checking.
    // @GuardedBy("assertionLock")
    private boolean defaultAssertionStatus = false;

    // Maps String packageName to Boolean package default assertion status Note
    // that the default package is placed under a null map key.  If this field
    // is null then we are delegating assertion status queries to the VM, i.e.,
    // none of this ClassLoader's assertion status modification methods have
    // been invoked.
    // @GuardedBy("assertionLock")
    private Map<String, Boolean> packageAssertionStatus = null;

    // Maps String fullyQualifiedClassName to Boolean assertionStatus If this
    // field is null then we are delegating assertion status queries to the VM,
    // i.e., none of this ClassLoader's assertion status modification methods
    // have been invoked.
    // @GuardedBy("assertionLock")
    Map<String, Boolean> classAssertionStatus = null;

    
Sets the default assertion status for this class loader. This setting determines whether classes loaded by this class loader and initialized in the future will have assertions enabled or disabled by default. This setting may be overridden on a per-package or per-class basis by invoking setPackageAssertionStatus(String, boolean) or setClassAssertionStatus(String, boolean). 
Params:
enabled –  true if classes loaded by this class loader will henceforth have assertions enabled by default, false if they will have assertions disabled by default.
Since:
 1.4/**
     * Sets the default assertion status for this class loader.  This setting
     * determines whether classes loaded by this class loader and initialized
     * in the future will have assertions enabled or disabled by default.
     * This setting may be overridden on a per-package or per-class basis by
     * invoking {@link #setPackageAssertionStatus(String, boolean)} or {@link
     * #setClassAssertionStatus(String, boolean)}.
     *
     * @param  enabled
     *         {@code true} if classes loaded by this class loader will
     *         henceforth have assertions enabled by default, {@code false}
     *         if they will have assertions disabled by default.
     *
     * @since  1.4
     */
    public void setDefaultAssertionStatus(boolean enabled) {
        synchronized (assertionLock) {
            if (classAssertionStatus == null)
                initializeJavaAssertionMaps();

            defaultAssertionStatus = enabled;
        }
    }

    
Sets the package default assertion status for the named package.  The
package default assertion status determines the assertion status for
classes initialized in the future that belong to the named package or
any of its "subpackages".
 A subpackage of a package named p is any package whose name begins with "p.". For example, javax.swing.text is a subpackage of javax.swing, and both java.util and java.lang.reflect are subpackages of java. 
 In the event that multiple package defaults apply to a given class, the package default pertaining to the most specific package takes precedence over the others. For example, if javax.lang and javax.lang.reflect both have package defaults associated with them, the latter package default applies to classes in javax.lang.reflect. 
 Package defaults take precedence over the class loader's default assertion status, and may be overridden on a per-class basis by invoking setClassAssertionStatus(String, boolean). 
Params:
packageName –  The name of the package whose package default assertion status is to be set. A null value indicates the unnamed package that is "current" (see section 7.4.2 of The Java™ Language Specification.)
enabled –  true if classes loaded by this classloader and belonging to the named package or any of its subpackages will have assertions enabled by default, false if they will have assertions disabled by default.
Since:
 1.4/**
     * Sets the package default assertion status for the named package.  The
     * package default assertion status determines the assertion status for
     * classes initialized in the future that belong to the named package or
     * any of its "subpackages".
     *
     * <p> A subpackage of a package named p is any package whose name begins
     * with "{@code p.}".  For example, {@code javax.swing.text} is a
     * subpackage of {@code javax.swing}, and both {@code java.util} and
     * {@code java.lang.reflect} are subpackages of {@code java}.
     *
     * <p> In the event that multiple package defaults apply to a given class,
     * the package default pertaining to the most specific package takes
     * precedence over the others.  For example, if {@code javax.lang} and
     * {@code javax.lang.reflect} both have package defaults associated with
     * them, the latter package default applies to classes in
     * {@code javax.lang.reflect}.
     *
     * <p> Package defaults take precedence over the class loader's default
     * assertion status, and may be overridden on a per-class basis by invoking
     * {@link #setClassAssertionStatus(String, boolean)}.  </p>
     *
     * @param  packageName
     *         The name of the package whose package default assertion status
     *         is to be set. A {@code null} value indicates the unnamed
     *         package that is "current"
     *         (see section 7.4.2 of
     *         <cite>The Java&trade; Language Specification</cite>.)
     *
     * @param  enabled
     *         {@code true} if classes loaded by this classloader and
     *         belonging to the named package or any of its subpackages will
     *         have assertions enabled by default, {@code false} if they will
     *         have assertions disabled by default.
     *
     * @since  1.4
     */
    public void setPackageAssertionStatus(String packageName,
                                          boolean enabled) {
        synchronized (assertionLock) {
            if (packageAssertionStatus == null)
                initializeJavaAssertionMaps();

            packageAssertionStatus.put(packageName, enabled);
        }
    }

    
Sets the desired assertion status for the named top-level class in this
class loader and any nested classes contained therein.  This setting
takes precedence over the class loader's default assertion status, and
over any applicable per-package default.  This method has no effect if
the named class has already been initialized.  (Once a class is
initialized, its assertion status cannot change.)
 If the named class is not a top-level class, this invocation will
have no effect on the actual assertion status of any class. 
Params:
className – 
        The fully qualified class name of the top-level class whose
        assertion status is to be set.
enabled –  true if the named class is to have assertions enabled when (and if) it is initialized, false if the class is to have assertions disabled.
Since:
 1.4/**
     * Sets the desired assertion status for the named top-level class in this
     * class loader and any nested classes contained therein.  This setting
     * takes precedence over the class loader's default assertion status, and
     * over any applicable per-package default.  This method has no effect if
     * the named class has already been initialized.  (Once a class is
     * initialized, its assertion status cannot change.)
     *
     * <p> If the named class is not a top-level class, this invocation will
     * have no effect on the actual assertion status of any class. </p>
     *
     * @param  className
     *         The fully qualified class name of the top-level class whose
     *         assertion status is to be set.
     *
     * @param  enabled
     *         {@code true} if the named class is to have assertions
     *         enabled when (and if) it is initialized, {@code false} if the
     *         class is to have assertions disabled.
     *
     * @since  1.4
     */
    public void setClassAssertionStatus(String className, boolean enabled) {
        synchronized (assertionLock) {
            if (classAssertionStatus == null)
                initializeJavaAssertionMaps();

            classAssertionStatus.put(className, enabled);
        }
    }

    
Sets the default assertion status for this class loader to false and discards any package defaults or class assertion status settings associated with the class loader. This method is provided so that class loaders can be made to ignore any command line or persistent assertion status settings and "start with a clean slate." 
Since:
 1.4/**
     * Sets the default assertion status for this class loader to
     * {@code false} and discards any package defaults or class assertion
     * status settings associated with the class loader.  This method is
     * provided so that class loaders can be made to ignore any command line or
     * persistent assertion status settings and "start with a clean slate."
     *
     * @since  1.4
     */
    public void clearAssertionStatus() {
        /*
         * Whether or not "Java assertion maps" are initialized, set
         * them to empty maps, effectively ignoring any present settings.
         */
        synchronized (assertionLock) {
            classAssertionStatus = new HashMap<>();
            packageAssertionStatus = new HashMap<>();
            defaultAssertionStatus = false;
        }
    }

    
Returns the assertion status that would be assigned to the specified
class if it were to be initialized at the time this method is invoked.
If the named class has had its assertion status set, the most recent
setting will be returned; otherwise, if any package default assertion
status pertains to this class, the most recent setting for the most
specific pertinent package default assertion status is returned;
otherwise, this class loader's default assertion status is returned.

Params:
className – 
        The fully qualified class name of the class whose desired
        assertion status is being queried.
See Also:
setClassAssertionStatus(String, boolean)
setPackageAssertionStatus(String, boolean)
setDefaultAssertionStatus(boolean)
Returns:
 The desired assertion status of the specified class.
Since:
 1.4/**
     * Returns the assertion status that would be assigned to the specified
     * class if it were to be initialized at the time this method is invoked.
     * If the named class has had its assertion status set, the most recent
     * setting will be returned; otherwise, if any package default assertion
     * status pertains to this class, the most recent setting for the most
     * specific pertinent package default assertion status is returned;
     * otherwise, this class loader's default assertion status is returned.
     * </p>
     *
     * @param  className
     *         The fully qualified class name of the class whose desired
     *         assertion status is being queried.
     *
     * @return  The desired assertion status of the specified class.
     *
     * @see  #setClassAssertionStatus(String, boolean)
     * @see  #setPackageAssertionStatus(String, boolean)
     * @see  #setDefaultAssertionStatus(boolean)
     *
     * @since  1.4
     */
    boolean desiredAssertionStatus(String className) {
        synchronized (assertionLock) {
            // assert classAssertionStatus   != null;
            // assert packageAssertionStatus != null;

            // Check for a class entry
            Boolean result = classAssertionStatus.get(className);
            if (result != null)
                return result.booleanValue();

            // Check for most specific package entry
            int dotIndex = className.lastIndexOf('.');
            if (dotIndex < 0) { // default package
                result = packageAssertionStatus.get(null);
                if (result != null)
                    return result.booleanValue();
            }
            while(dotIndex > 0) {
                className = className.substring(0, dotIndex);
                result = packageAssertionStatus.get(className);
                if (result != null)
                    return result.booleanValue();
                dotIndex = className.lastIndexOf('.', dotIndex-1);
            }

            // Return the classloader default
            return defaultAssertionStatus;
        }
    }

    // Set up the assertions with information provided by the VM.
    // Note: Should only be called inside a synchronized block
    private void initializeJavaAssertionMaps() {
        // assert Thread.holdsLock(assertionLock);

        classAssertionStatus = new HashMap<>();
        packageAssertionStatus = new HashMap<>();
        AssertionStatusDirectives directives = retrieveDirectives();

        for(int i = 0; i < directives.classes.length; i++)
            classAssertionStatus.put(directives.classes[i],
                                     directives.classEnabled[i]);

        for(int i = 0; i < directives.packages.length; i++)
            packageAssertionStatus.put(directives.packages[i],
                                       directives.packageEnabled[i]);

        defaultAssertionStatus = directives.deflt;
    }

    // Retrieves the assertion directives from the VM.
    private static native AssertionStatusDirectives retrieveDirectives();


    // -- Misc --

    
Returns the ConcurrentHashMap used as a storage for ClassLoaderValue(s)
associated with this ClassLoader, creating it if it doesn't already exist.
/**
     * Returns the ConcurrentHashMap used as a storage for ClassLoaderValue(s)
     * associated with this ClassLoader, creating it if it doesn't already exist.
     */
    ConcurrentHashMap<?, ?> createOrGetClassLoaderValueMap() {
        ConcurrentHashMap<?, ?> map = classLoaderValueMap;
        if (map == null) {
            map = new ConcurrentHashMap<>();
            boolean set = trySetObjectField("classLoaderValueMap", map);
            if (!set) {
                // beaten by someone else
                map = classLoaderValueMap;
            }
        }
        return map;
    }

    // the storage for ClassLoaderValue(s) associated with this ClassLoader
    private volatile ConcurrentHashMap<?, ?> classLoaderValueMap;

    
Attempts to atomically set a volatile field in this object. Returns true if not beaten by another thread. Avoids the use of AtomicReferenceFieldUpdater in this class. /**
     * Attempts to atomically set a volatile field in this object. Returns
     * {@code true} if not beaten by another thread. Avoids the use of
     * AtomicReferenceFieldUpdater in this class.
     */
    private boolean trySetObjectField(String name, Object obj) {
        Unsafe unsafe = Unsafe.getUnsafe();
        Class<?> k = ClassLoader.class;
        long offset;
        offset = unsafe.objectFieldOffset(k, name);
        return unsafe.compareAndSetObject(this, offset, null, obj);
    }
}

/*
 * A utility class that will enumerate over an array of enumerations.
 */
final class CompoundEnumeration<E> implements Enumeration<E> {
    private final Enumeration<E>[] enums;
    private int index;

    public CompoundEnumeration(Enumeration<E>[] enums) {
        this.enums = enums;
    }

    private boolean next() {
        while (index < enums.length) {
            if (enums[index] != null && enums[index].hasMoreElements()) {
                return true;
            }
            index++;
        }
        return false;
    }

    public boolean hasMoreElements() {
        return next();
    }

    public E nextElement() {
        if (!next()) {
            throw new NoSuchElementException();
        }
        return enums[index].nextElement();
    }
}