ClassLoaderUtils.java

/*
 * Copyright 1999-2010 University of Chicago
 *
 * 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.globus.util;

import java.io.InputStream;

/**
 * Various classloader utils. Extends the standard ways of loading classes or
 * resources with a fallback mechanism to the thread context classloader.
 */
public class ClassLoaderUtils {

	private static DummySecurityManager MANAGER = new DummySecurityManager();

	private static class DummySecurityManager extends SecurityManager {
		public Class[] getClassContext() {
			return super.getClassContext();
		}
	}

	/**
	 * Returns the current execution stack as an array of classes.
	 * <p>
	 * The length of the array is the number of methods on the execution stack.
	 * The element at index <code>0</code> is the class of the currently
	 * executing method, the element at index <code>1</code> is the class of
	 * that method's caller, and so on.
	 *
	 * @return the execution stack.
	 */
	public static Class[] getClassContext() {
		return MANAGER.getClassContext();
	}

	/**
	 * Returns a class at specified depth of the current execution stack.
	 *
	 * @return the class at the specified depth of the current execution stack.
	 *         Migth return null if depth is out of range.
	 */
	public static Class getClassContextAt(int i) {
		Class[] classes = MANAGER.getClassContext();
		if (classes != null && classes.length > i) {
			return classes[i];
		}
		return null;
	}

	/**
	 * Returns a classloader at specified depth of the current execution stack.
	 *
	 * @return the classloader at the specified depth of the current execution
	 *         stack. Migth return null if depth is out of range.
	 */
	public static ClassLoader getClassLoaderContextAt(int i) {
		Class[] classes = MANAGER.getClassContext();
		if (classes != null && classes.length > i) {
			return classes[i].getClassLoader();
		}
		return null;
	}

	/**
	 * Gets an InputStream to a resource of a specified name. First, the
	 * caller's classloader is used to load the resource and if it fails the
	 * thread's context classloader is used to load the resource.
	 */
	public static InputStream getResourceAsStream(String name) {
		// try with caller classloader
		ClassLoader loader = getClassLoaderContextAt(3);
		InputStream in = (loader == null) ? null : loader.getResourceAsStream(name);
		if (in == null) {
			// try with context classloader if set & different
			ClassLoader contextLoader = Thread.currentThread().getContextClassLoader();
			if (contextLoader != null && contextLoader != loader) {
				in = contextLoader.getResourceAsStream(name);
			}
		}
		return in;
	}

	/**
	 * Loads a specified class. First, the caller's classloader is used to load
	 * the class and if it fails the thread's context classloader is used to
	 * load the specified class.
	 */
	public static Class forName(String name) throws ClassNotFoundException {
		// try with caller classloader
		ClassLoader loader = getClassLoaderContextAt(3);
		try {
			return Class.forName(name, true, loader);
		} catch (ClassNotFoundException e) {
			// try with context classloader if set & different
			ClassLoader contextLoader = Thread.currentThread().getContextClassLoader();
			if (contextLoader == null || contextLoader == loader) {
				throw e;
			} else {
				return Class.forName(name, true, contextLoader);
			}
		}
	}

	public static Class<?> loadClass(String className, Class<?> callingClass) throws ClassNotFoundException {
		try {
			ClassLoader cl = Thread.currentThread().getContextClassLoader();
			if (cl != null) {
				return cl.loadClass(className);
			}
		} catch (ClassNotFoundException e) {
			// ignore
		}
		return loadClass2(className, callingClass);
	}

	private static Class<?> loadClass2(String className, Class<?> callingClass) throws ClassNotFoundException {
		try {
			return Class.forName(className);
		} catch (ClassNotFoundException ex) {
			try {
				if (ClassLoaderUtils.class.getClassLoader() != null) {
					return ClassLoaderUtils.class.getClassLoader().loadClass(className);
				}
			} catch (ClassNotFoundException exc) {
				if (callingClass != null && callingClass.getClassLoader() != null) {
					return callingClass.getClassLoader().loadClass(className);
				}
			}
			throw ex;
		}
	}

}