bundles/org.eclipse.osgi/eclipseAdaptor/src/org/eclipse/core/runtime/internal/adaptor/ContextFinder.java

/*******************************************************************************
 * Copyright (c) 2005, 2006 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.core.runtime.internal.adaptor;

import java.io.IOException;
import java.net.URL;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.*;
import org.eclipse.osgi.framework.adaptor.BundleClassLoader;

public class ContextFinder extends ClassLoader implements PrivilegedAction {
	static final class Finder extends SecurityManager {
		public Class[] getClassContext() {
			return super.getClassContext();
		}
	}
	//This is used to detect cycle that could be caused while delegating the loading to other classloaders
	//It keeps track on a thread basis of the set of requested classes and resources
	private static ThreadLocal cycleDetector = new ThreadLocal();
	static ClassLoader finderClassLoader;
	static Finder contextFinder;
	static {
		AccessController.doPrivileged(new PrivilegedAction() {
			public Object run() {
				finderClassLoader = ContextFinder.class.getClassLoader();
				contextFinder = new Finder();
				return null;
			}
		});
	}

	public ContextFinder(ClassLoader contextClassLoader) {
		super(contextClassLoader);
	}

	// Return a list of all classloaders on the stack that are neither the 
	// ContextFinder classloader nor the boot classloader.  The last classloader
	// in the list is either a bundle classloader or the framework's classloader
	// We assume that the bootclassloader never uses the context classloader to find classes in itself.
	ArrayList basicFindClassLoaders() {
		Class[] stack = contextFinder.getClassContext();
		ArrayList result = new ArrayList(1);
		for (int i = 1; i < stack.length; i++) {
			ClassLoader tmp = stack[i].getClassLoader();
			if (stack[i] != ContextFinder.class && tmp != null && tmp != this) {
				if (checkClassLoader(tmp))
					result.add(tmp);
				// stop at the framework classloader or the first bundle classloader
				if (tmp == finderClassLoader || tmp instanceof BundleClassLoader)
					break;
			}
		}
		return result;
	}

	// ensures that a classloader does not have the ContextFinder as part of the 
	// parent hierachy.  A classloader which has the ContextFinder as a parent must
	// not be used as a delegate, otherwise we endup in endless recursion.
	private boolean checkClassLoader(ClassLoader classloader) {
		if (classloader == null || classloader == getParent())
			return false;
		for (ClassLoader parent = classloader.getParent(); parent != null; parent = parent.getParent())
			if (parent == this)
				return false;
		return true;
	}

	private ArrayList findClassLoaders() {
		if (System.getSecurityManager() == null)
			return basicFindClassLoaders();
		return (ArrayList) AccessController.doPrivileged(this);
	}

	public Object run() {
		return basicFindClassLoaders();
	}

	//Return whether the request for loading "name" should proceed.
	//False is returned when a cycle is being detected 
	private boolean startLoading(String name) {
		Set classesAndResources = (Set) cycleDetector.get();
		if (classesAndResources != null && classesAndResources.contains(name))
			return false;

		if (classesAndResources == null) {
			classesAndResources = new HashSet(3);
			cycleDetector.set(classesAndResources);
		}
		classesAndResources.add(name);
		return true;
	}

	private void stopLoading(String name) {
		((Set) cycleDetector.get()).remove(name);
	}

	protected synchronized Class loadClass(String arg0, boolean arg1) throws ClassNotFoundException {
		//Shortcut cycle
		if (startLoading(arg0) == false)
			throw new ClassNotFoundException(arg0);

		try {
			ArrayList toConsult = findClassLoaders();
			for (Iterator loaders = toConsult.iterator(); loaders.hasNext();)
				try {
					return ((ClassLoader) loaders.next()).loadClass(arg0);
				} catch (ClassNotFoundException e) {
					// go to the next class loader
				}
			return super.loadClass(arg0, arg1);
		} finally {
			stopLoading(arg0);
		}
	}

	public URL getResource(String arg0) {
		//Shortcut cycle
		if (startLoading(arg0) == false)
			return null;
		try {
			ArrayList toConsult = findClassLoaders();
			for (Iterator loaders = toConsult.iterator(); loaders.hasNext();) {
				URL result = ((ClassLoader) loaders.next()).getResource(arg0);
				if (result != null)
					return result;
				// go to the next class loader
			}
			return super.getResource(arg0);
		} finally {
			stopLoading(arg0);
		}
	}

	protected Enumeration findResources(String arg0) throws IOException {
		//Shortcut cycle
		if (startLoading(arg0) == false)
			return null;
		try {
			ArrayList toConsult = findClassLoaders();
			for (Iterator loaders = toConsult.iterator(); loaders.hasNext();) {
				Enumeration result = ((ClassLoader) loaders.next()).getResources(arg0);
				if (result != null && result.hasMoreElements())
					return result;
				// go to the next class loader
			}
			return super.findResources(arg0);
		} finally {
			stopLoading(arg0);
		}
	}
}