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 * Copyright (c) 2005, 2016, Oracle and/or its affiliates. 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
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 * questions.
 */

package javax.tools;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Iterator;
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 {@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";

    /**
     * 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&trade; 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 {@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
    public static ClassLoader getSystemToolClassLoader() {
        return null;
    }

    private static final boolean useLegacy;

    static {
        Class<?> c = null;
        try {
            c = Class.forName("java.lang.reflect.Module");
        } catch (Throwable t) {
        }
        useLegacy = (c == null);
    }

    /**
     * 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) {
        if (useLegacy) {
            try {
                return Class.forName(className, true, ClassLoader.getSystemClassLoader()).
                    asSubclass(clazz).getConstructor().newInstance();
            } catch (ReflectiveOperationException e) {
                throw new Error(e);
            }
        }

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

    /**
     * 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 = () -> {
            // for now, use reflection to implement
            //      return moduleName.equals(tool.getClass().getModule().getName());
            try {
                Method getModuleMethod = Class.class.getDeclaredMethod("getModule");
                Object toolModule = getModuleMethod.invoke(tool.getClass());
                Method getNameMethod = toolModule.getClass().getDeclaredMethod("getName");
                String toolModuleName = (String) getNameMethod.invoke(toolModule);
                return moduleName.equals(toolModuleName);
            } catch (InvocationTargetException | NoSuchMethodException | IllegalAccessException e) {
                return false;
            }
        };
        return AccessController.doPrivileged(pa);
    }
}