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package com.sun.org.apache.bcel.internal.util;


import java.util.Hashtable;
import java.io.*;
import com.sun.org.apache.bcel.internal.*;
import com.sun.org.apache.bcel.internal.classfile.*;


Drop in replacement for the standard class loader of the JVM. You can use it
in conjunction with the JavaWrapper to dynamically modify/create classes
as they're requested.


This class loader recognizes special requests in a distinct
format, i.e., when the name of the requested class contains with
"$$BCEL$$" it calls the createClass() method with that name
(everything bevor the $$BCEL$$ is considered to be the package
name. You can subclass the class loader and override that
method. "Normal" classes class can be modified by overriding the
modifyClass() method which is called just before defineClass().


There may be a number of packages where you have to use the default
class loader (which may also be faster). You can define the set of packages
where to use the system class loader in the constructor. The default value contains
"java.", "sun.", "javax."


Author: M. Dahm
See Also:
  • JavaWrapper
  • ClassPath
/**
 * <p>Drop in replacement for the standard class loader of the JVM. You can use it
 * in conjunction with the JavaWrapper to dynamically modify/create classes
 * as they're requested.</p>
 *
 * <p>This class loader recognizes special requests in a distinct
 * format, i.e., when the name of the requested class contains with
 * "$$BCEL$$" it calls the createClass() method with that name
 * (everything bevor the $$BCEL$$ is considered to be the package
 * name. You can subclass the class loader and override that
 * method. "Normal" classes class can be modified by overriding the
 * modifyClass() method which is called just before defineClass().</p>
 *
 * <p>There may be a number of packages where you have to use the default
 * class loader (which may also be faster). You can define the set of packages
 * where to use the system class loader in the constructor. The default value contains
 * "java.", "sun.", "javax."</p>
 *
 * @author  <A HREF="mailto:markus.dahm@berlin.de">M. Dahm</A>
 * @see JavaWrapper
 * @see ClassPath
 */
public class ClassLoader extends java.lang.ClassLoader {
  private Hashtable classes = new Hashtable(); // Hashtable is synchronized thus thread-safe
  private String[] ignored_packages = {
    "java.", "javax.", "sun."
  };
  private Repository repository = SyntheticRepository.getInstance();
  private java.lang.ClassLoader deferTo = ClassLoader.getSystemClassLoader();

  public ClassLoader() {
  }

  public ClassLoader(java.lang.ClassLoader deferTo) {
    this.deferTo = deferTo;
    this.repository = new ClassLoaderRepository(deferTo);
  }

  
Params:
  • ignored_packages – classes contained in these packages will be loaded
with the system class loader
/** @param ignored_packages classes contained in these packages will be loaded
   * with the system class loader
   */
  public ClassLoader(String[] ignored_packages) {
    addIgnoredPkgs(ignored_packages);
  }

  public ClassLoader(java.lang.ClassLoader deferTo, String [] ignored_packages) {
    this.deferTo = deferTo;
    this.repository = new ClassLoaderRepository(deferTo);

    addIgnoredPkgs(ignored_packages);
  }

  private void addIgnoredPkgs(String[] ignored_packages) {
    String[] new_p = new String[ignored_packages.length + this.ignored_packages.length];

    System.arraycopy(this.ignored_packages, 0, new_p, 0, this.ignored_packages.length);
    System.arraycopy(ignored_packages, 0, new_p, this.ignored_packages.length,
                     ignored_packages.length);

    this.ignored_packages = new_p;
  }

  protected Class loadClass(String class_name, boolean resolve)
    throws ClassNotFoundException
  {
    Class cl = null;

    /* First try: lookup hash table.
     */
    if((cl=(Class)classes.get(class_name)) == null) {
      /* Second try: Load system class using system class loader. You better
       * don't mess around with them.
       */
      for(int i=0; i < ignored_packages.length; i++) {
        if(class_name.startsWith(ignored_packages[i])) {
          cl = deferTo.loadClass(class_name);
          break;
        }
      }

      if(cl == null) {
        JavaClass clazz = null;

        /* Third try: Special request?
         */
        if(class_name.indexOf("$$BCEL$$") >= 0)
          clazz = createClass(class_name);
        else { // Fourth try: Load classes via repository
          if ((clazz = repository.loadClass(class_name)) != null) {
            clazz = modifyClass(clazz);
          }
          else
            throw new ClassNotFoundException(class_name);
        }

        if(clazz != null) {
          byte[] bytes  = clazz.getBytes();
          cl = defineClass(class_name, bytes, 0, bytes.length);
        } else // Fourth try: Use default class loader
          cl = Class.forName(class_name);
      }

      if(resolve)
        resolveClass(cl);
    }

    classes.put(class_name, cl);

    return cl;
  }

  
Override this method if you want to alter a class before it gets actually
loaded. Does nothing by default.

/** Override this method if you want to alter a class before it gets actually
   * loaded. Does nothing by default.
   */
  protected JavaClass modifyClass(JavaClass clazz) {
    return clazz;
  }

  
Override this method to create you own classes on the fly. The
name contains the special token $$BCEL$$. Everything before that
token is consddered to be a package name. You can encode you own
arguments into the subsequent string. You must regard however not
to use any "illegal" characters, i.e., characters that may not
appear in a Java class name too

The default implementation interprets the string as a encoded compressed
Java class, unpacks and decodes it with the Utility.decode() method, and
parses the resulting byte array and returns the resulting JavaClass object.

Params:
  • class_name – compressed byte code with "$$BCEL$$" in it
/**
   * Override this method to create you own classes on the fly. The
   * name contains the special token $$BCEL$$. Everything before that
   * token is consddered to be a package name. You can encode you own
   * arguments into the subsequent string. You must regard however not
   * to use any "illegal" characters, i.e., characters that may not
   * appear in a Java class name too<br>
   *
   * The default implementation interprets the string as a encoded compressed
   * Java class, unpacks and decodes it with the Utility.decode() method, and
   * parses the resulting byte array and returns the resulting JavaClass object.
   *
   * @param class_name compressed byte code with "$$BCEL$$" in it
   */
  protected JavaClass createClass(String class_name) {
    int    index     = class_name.indexOf("$$BCEL$$");
    String real_name = class_name.substring(index + 8);

    JavaClass clazz = null;
    try {
      byte[]      bytes  = Utility.decode(real_name, true);
      ClassParser parser = new ClassParser(new ByteArrayInputStream(bytes), "foo");

      clazz = parser.parse();
    } catch(Throwable e) {
      e.printStackTrace();
      return null;
    }

    // Adapt the class name to the passed value
    ConstantPool cp = clazz.getConstantPool();

    ConstantClass cl = (ConstantClass)cp.getConstant(clazz.getClassNameIndex(),
                                                     Constants.CONSTANT_Class);
    ConstantUtf8 name = (ConstantUtf8)cp.getConstant(cl.getNameIndex(),
                                                     Constants.CONSTANT_Utf8);
    name.setBytes(class_name.replace('.', '/'));

    return clazz;
  }
}