ChildFirstClassLoader.java

/*
 * RED5 Open Source Flash Server - http://code.google.com/p/red5/
 * 
 * Copyright 2006-2012 by respective authors (see below). All rights reserved.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.red5.classloading;

import java.net.URL;
import java.net.URLClassLoader;

/**
 * An almost trivial no-fuss implementation of a class loader following the
 * child-first delegation model. 
 * <i>Based on code from Ceki Gulcu</i>
 * 
 * @author Paul Gregoire (mondain@gmail.com)
 */
public final class ChildFirstClassLoader extends URLClassLoader {

	private ClassLoader parent = null;
	private ClassLoader parentParent = null;
	private ClassLoader system = null;
	
	public ChildFirstClassLoader(URL[] urls) {
		super(urls);
		this.parent = super.getParent();
		system = getSystemClassLoader();		
		//if we have a parent of the parent and its not the system classloader
		parentParent = this.parent.getParent() != system ? this.parent.getParent() : null;
		
		dumpClassLoaderNames();
	}

	public ChildFirstClassLoader(URL[] urls, ClassLoader parent) {
		super(urls, parent);
		this.parent = parent;
		system = getSystemClassLoader();		
		if (parent != null) {
		//if we have a parent of the parent and its not the system classloader
		parentParent = this.parent.getParent() != system ? this.parent.getParent() : null;
		}
		dumpClassLoaderNames();
	}
	
	private void dumpClassLoaderNames() {
		System.out.printf("[ChildFirstClassLoader] Classloaders:\nSystem %s\nParents Parent %s\nParent %s\nThis class %s\nTCL %s\n\n", 
				system, parentParent, this.parent, ChildFirstClassLoader.class.getClassLoader(), Thread.currentThread().getContextClassLoader());
	}

	@Override
	public Class<?> loadClass(String name) throws ClassNotFoundException {
		return loadClass(name, false);
	}

	/**
	 * We override the parent-first behavior established by
	 * java.lang.Classloader.
	 * <p>
	 * The implementation is surprisingly straightforward.
	 * 
	 * @param name
	 *            the name of the class to load, should not be <code>null</code>
	 *            .
	 * 
	 * @param resolve
	 *            flag that indicates whether the class should be resolved.
	 * 
	 * @return the loaded class, never <code>null</code>.
	 * 
	 * @throws ClassNotFoundException
	 *             if the class could not be loaded.
	 */
	@Override
	protected Class<?> loadClass(String name, boolean resolve)
			throws ClassNotFoundException {

		// First, check if the class has already been loaded
		Class<?> c = findLoadedClass(name);

		// if not loaded, search the local (child) resources
		if (c == null) {
			try {
				c = findClass(name);
			} catch (ClassNotFoundException cnfe) {
				// ignore
			}
		}
		
		// If we could not find it, delegate to parent
		// Note that we do not attempt to catch any ClassNotFoundException
		if (c == null) {
			try {
				c = this.parent.loadClass(name);
			} catch (Exception e) {
				//ignore the Spring "BeanInfo" class lookup errors
				//if (e.getMessage().indexOf("BeanInfo") == -1) {
				//	log.warn("Exception {}", e);
				//}
			}
			if (c == null && parentParent != null) {
    			try {
    				c = parentParent.loadClass(name);
    			} catch (Exception e) {
    				//if (e.getMessage().indexOf("BeanInfo") == -1) {
    				//	log.warn("Exception {}", e);
    				//}
    			}
			}
			if (c == null) {
				try {
					c = system.loadClass(name);
				} catch (Exception e) {
					//if (e.getMessage().indexOf("BeanInfo") == -1) {
					//	log.warn("Exception {}", e);
					//}
				}
			}
		}
		
		// resolve if requested
		if (resolve) {
			resolveClass(c);
		}

		return c;
	}
	
	/**
	 * Override the parent-first resource loading model established by
	 * java.lang.Classloader with child-first behavior.
	 * 
	 * @param name the name of the resource to load, should not be
	 *            <code>null</code>.
	 * 
	 * @return a {@link URL} for the resource, or <code>null</code> if it could
	 *         not be found.
	 */
	@Override
	public URL getResource(String name) {
		URL url = findResource(name);
		// If local search failed, delegate to parent
		if (url == null) {
			url = this.parent.getResource(name);
		}
		return url;
	}
}