ClassLoaderUtils.java

/*
 * $Id$
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.struts2.util;

import java.io.IOException;
import java.io.InputStream;
import java.net.URL;


/**
 * This class is extremely useful for loading resources and classes in a fault tolerant manner
 * that works across different applications servers.
 * <p/>
 * It has come out of many months of frustrating use of multiple application servers at Atlassian,
 * please don't change things unless you're sure they're not going to break in one server or another!
 *
 */
public class ClassLoaderUtils {

    /**
     * Load a given resource.
     * <p/>
     * This method will try to load the resource using the following methods (in order):
     * <ul>
     * <li>From {@link Thread#getContextClassLoader() Thread.currentThread().getContextClassLoader()}
     * <li>From {@link Class#getClassLoader() ClassLoaderUtil.class.getClassLoader()}
     * <li>From the {@link Class#getClassLoader() callingClass.getClassLoader() }
     * </ul>
     *
     * @param resourceName The name of the resource to load
     * @param callingClass The Class object of the calling object
     */
    public static URL getResource(String resourceName, Class callingClass) {
        URL url = null;

        url = Thread.currentThread().getContextClassLoader().getResource(resourceName);

        if (url == null) {
            url = ClassLoaderUtils.class.getClassLoader().getResource(resourceName);
        }

        if (url == null) {
            url = callingClass.getClassLoader().getResource(resourceName);
        }

        return url;
    }

    /**
     * This is a convenience method to load a resource as a stream.
     * <p/>
     * The algorithm used to find the resource is given in getResource()
     *
     * @param resourceName The name of the resource to load
     * @param callingClass The Class object of the calling object
     */
    public static InputStream getResourceAsStream(String resourceName, Class callingClass) {
        URL url = getResource(resourceName, callingClass);

        try {
            return (url != null) ? url.openStream() : null;
        } catch (IOException e) {
            return null;
        }
    }

    /**
     * Load a class with a given name.
     * <p/>
     * It will try to load the class in the following order:
     * <ul>
     * <li>From {@link Thread#getContextClassLoader() Thread.currentThread().getContextClassLoader()}
     * <li>Using the basic {@link Class#forName(java.lang.String) }
     * <li>From {@link Class#getClassLoader() ClassLoaderUtil.class.getClassLoader()}
     * <li>From the {@link Class#getClassLoader() callingClass.getClassLoader() }
     * </ul>
     *
     * @param className    The name of the class to load
     * @param callingClass The Class object of the calling object
     * @throws ClassNotFoundException If the class cannot be found anywhere.
     */
    public static Class loadClass(String className, Class callingClass) throws ClassNotFoundException {
        try {
            return Thread.currentThread().getContextClassLoader().loadClass(className);
        } catch (ClassNotFoundException e) {
            try {
                return Class.forName(className);
            } catch (ClassNotFoundException ex) {
                try {
                    return ClassLoaderUtils.class.getClassLoader().loadClass(className);
                } catch (ClassNotFoundException exc) {
                    return callingClass.getClassLoader().loadClass(className);
                }
            }
        }
    }

    /**
     * Prints the current classloader hierarchy - useful for debugging.
     */
    public static void printClassLoader() {
        System.out.println("ClassLoaderUtils.printClassLoader");
        printClassLoader(Thread.currentThread().getContextClassLoader());
    }

    /**
     * Prints the classloader hierarchy from a given classloader - useful for debugging.
     */
    public static void printClassLoader(ClassLoader cl) {
        System.out.println("ClassLoaderUtils.printClassLoader(cl = " + cl + ")");

        if (cl != null) {
            printClassLoader(cl.getParent());
        }
    }
}