ContextFinder.java

/*******************************************************************************
 * Copyright (c) 2005, 2010 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<List<ClassLoader>> {
	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<Set<String>> cycleDetector = new ThreadLocal<Set<String>>();
	static ClassLoader finderClassLoader;
	static Finder contextFinder;
	static {
		AccessController.doPrivileged(new PrivilegedAction<Object>() {
			public Object run() {
				finderClassLoader = ContextFinder.class.getClassLoader();
				contextFinder = new Finder();
				return null;
			}
		});
	}

	private static Class<ContextFinder> THIS = ContextFinder.class;

	private final ClassLoader parentContextClassLoader;

	public ContextFinder(ClassLoader contextClassLoader) {
		super(contextClassLoader);
		this.parentContextClassLoader = contextClassLoader != null ? contextClassLoader : new ClassLoader(Object.class.getClassLoader()) {/*boot classloader*/};
	}

	// 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.
	List<ClassLoader> basicFindClassLoaders() {
		Class<?>[] stack = contextFinder.getClassContext();
		List<ClassLoader> result = new ArrayList<ClassLoader>(1);
		ClassLoader previousLoader = null;
		for (int i = 1; i < stack.length; i++) {
			ClassLoader tmp = stack[i].getClassLoader();
			if (stack[i] != THIS && tmp != null && tmp != this) {
				if (checkClassLoader(tmp)) {
					if (previousLoader != tmp) {
						result.add(tmp);
						previousLoader = 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 List<ClassLoader> findClassLoaders() {
		if (System.getSecurityManager() == null)
			return basicFindClassLoaders();
		return AccessController.doPrivileged(this);
	}

	public List<ClassLoader> 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<String> classesAndResources = cycleDetector.get();
		if (classesAndResources != null && classesAndResources.contains(name))
			return false;

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

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

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

		try {
			List<ClassLoader> toConsult = findClassLoaders();
			for (Iterator<ClassLoader> loaders = toConsult.iterator(); loaders.hasNext();)
				try {
					return loaders.next().loadClass(arg0);
				} catch (ClassNotFoundException e) {
					// go to the next class loader
				}
			// avoid calling super.loadClass here because it checks the local cache (bug 127963)
			return parentContextClassLoader.loadClass(arg0);
		} finally {
			stopLoading(arg0);
		}
	}

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

	protected Enumeration<URL> findResources(String arg0) throws IOException {
		//Shortcut cycle
		if (startLoading(arg0) == false) {
			@SuppressWarnings("unchecked")
			Enumeration<URL> result = Collections.enumeration(Collections.EMPTY_LIST);
			return result;
		}
		try {
			List<ClassLoader> toConsult = findClassLoaders();
			for (Iterator<ClassLoader> loaders = toConsult.iterator(); loaders.hasNext();) {
				Enumeration<URL> result = loaders.next().getResources(arg0);
				if (result != null && result.hasMoreElements())
					return result;
				// go to the next class loader
			}
			return super.findResources(arg0);
		} finally {
			stopLoading(arg0);
		}
	}
}