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Struts example source code file (ClassLoaderUtils.java)

This example Struts source code file (ClassLoaderUtils.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Struts tags/keywords

class, class, classloaderutils, classloaderutils, classnotfoundexception, classnotfoundexception, inputstream, io, ioexception, net, network, url, url

The Struts ClassLoaderUtils.java source code

/*
 * $Id: ClassLoaderUtils.java 651946 2008-04-27 13:41:38Z apetrelli $
 *
 * 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());
        }
    }
}

Other Struts examples (source code examples)

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