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package javax.tools;

import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Objects;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;

Provides methods for locating tool providers, for example, providers of compilers. This class complements the functionality of ServiceLoader. 
Author:
Peter von der Ahé
Since:
1.6/**
 * Provides methods for locating tool providers, for example,
 * providers of compilers.  This class complements the
 * functionality of {@link java.util.ServiceLoader}.
 *
 * @author Peter von der Ahé
 * @since 1.6
 */
public class ToolProvider {

    private static final String systemJavaCompilerModule = "jdk.compiler";
    private static final String systemJavaCompilerName   = "com.sun.tools.javac.api.JavacTool";

    private ToolProvider() {}

    
Returns the Java programming language compiler provided
with this platform.
The file manager returned by calling getStandardFileManager on this compiler supports paths provided by any filesystem.
Returns:
the compiler provided with this platform or null if no compiler is provided
Implementation Note:
This implementation returns the compiler provided by the jdk.compiler module if that module is available, and null otherwise./**
     * Returns the Java programming language compiler provided
     * with this platform.
     * <p>The file manager returned by calling
     * {@link JavaCompiler#getStandardFileManager getStandardFileManager}
     * on this compiler supports paths provided by any
     * {@linkplain java.nio.file.FileSystem filesystem}.</p>
     * @return the compiler provided with this platform or
     * {@code null} if no compiler is provided
     * @implNote This implementation returns the compiler provided
     * by the {@code jdk.compiler} module if that module is available,
     * and {@code null} otherwise.
     */
    public static JavaCompiler getSystemJavaCompiler() {
        return getSystemTool(JavaCompiler.class,
                systemJavaCompilerModule, systemJavaCompilerName);
    }

    private static final String systemDocumentationToolModule = "jdk.javadoc";
    private static final String systemDocumentationToolName = "jdk.javadoc.internal.api.JavadocTool";

    
Returns the Java programming language documentation tool provided
with this platform.
The file manager returned by calling getStandardFileManager on this tool supports paths provided by any filesystem.
Returns:
the documentation tool provided with this platform or null if no documentation tool is provided
Implementation Note:
This implementation returns the tool provided by the jdk.javadoc module if that module is available, and null otherwise./**
     * Returns the Java programming language documentation tool provided
     * with this platform.
     * <p>The file manager returned by calling
     * {@link DocumentationTool#getStandardFileManager getStandardFileManager}
     * on this tool supports paths provided by any
     * {@linkplain java.nio.file.FileSystem filesystem}.</p>
     * @return the documentation tool provided with this platform or
     * {@code null} if no documentation tool is provided
     * @implNote This implementation returns the tool provided
     * by the {@code jdk.javadoc} module if that module is available,
     * and {@code null} otherwise.
     */
    public static DocumentationTool getSystemDocumentationTool() {
        return getSystemTool(DocumentationTool.class,
                systemDocumentationToolModule, systemDocumentationToolName);
    }

    
Returns a class loader that may be used to load system tools, or null if no such special loader is provided. 
Implementation Requirements:
This implementation always returns null.
Deprecated:
This method is subject to removal in a future version of Java SE. Use the system tool provider or service loader mechanisms to locate system tools as well as user-installed tools.
Returns:
a class loader, or null/**
     * Returns a class loader that may be used to load system tools,
     * or {@code null} if no such special loader is provided.
     * @implSpec This implementation always returns {@code null}.
     * @deprecated This method is subject to removal in a future version of
     * Java SE.
     * Use the {@link java.util.spi.ToolProvider system tool provider} or
     * {@link java.util.ServiceLoader service loader} mechanisms to
     * locate system tools as well as user-installed tools.
     * @return a class loader, or {@code null}
     */
    @Deprecated(since="9")
    public static ClassLoader getSystemToolClassLoader() {
        return null;
    }

    
Get an instance of a system tool using the service loader.
Params:
clazz –      the interface of the tool
moduleName – the name of the module containing the desired implementation
className –  the class name of the desired implementation
Type parameters:
<T> –        the interface of the tool
Implementation Note:
        By default, this returns the implementation in the specified module.
                  For limited backward compatibility, if this code is run on an older version
                  of the Java platform that does not support modules, this method will
                  try and create an instance of the named class. Note that implies the
                  class must be available on the system class path.
Returns:
the specified implementation of the tool/**
     * Get an instance of a system tool using the service loader.
     * @implNote         By default, this returns the implementation in the specified module.
     *                   For limited backward compatibility, if this code is run on an older version
     *                   of the Java platform that does not support modules, this method will
     *                   try and create an instance of the named class. Note that implies the
     *                   class must be available on the system class path.
     * @param <T>        the interface of the tool
     * @param clazz      the interface of the tool
     * @param moduleName the name of the module containing the desired implementation
     * @param className  the class name of the desired implementation
     * @return the specified implementation of the tool
     */
    private static <T> T getSystemTool(Class<T> clazz, String moduleName, String className) {

        try {
            ServiceLoader<T> sl = ServiceLoader.load(clazz, ClassLoader.getSystemClassLoader());
            for (T tool : sl) {
                if (matches(tool, moduleName))
                    return tool;
            }
        } catch (ServiceConfigurationError e) {
            throw new Error(e);
        }
        return null;
    }

    
Determine if this is the desired tool instance.
Params:
tool –              the instance of the tool
moduleName –        the name of the module containing the desired implementation
Type parameters:
<T> –               the interface of the tool
Returns:
true if and only if the tool matches the specified criteria/**
     * Determine if this is the desired tool instance.
     * @param <T>               the interface of the tool
     * @param tool              the instance of the tool
     * @param moduleName        the name of the module containing the desired implementation
     * @return true if and only if the tool matches the specified criteria
     */
    private static <T> boolean matches(T tool, String moduleName) {
        PrivilegedAction<Boolean> pa = () -> {
            Module toolModule = tool.getClass().getModule();
            String toolModuleName = toolModule.getName();
            return Objects.equals(toolModuleName, moduleName);
        };
        return AccessController.doPrivileged(pa);
    }
}