Struts example source code file (ClassLoaderUtil.java)

This example Struts source code file (ClassLoaderUtil.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

aggregateiterator, aggregateiterator, class, class, classloader, classnotfoundexception, classnotfoundexception, e, e, enumeration, io, ioexception, iterator, linkedlist, net, network, url, util

The Struts ClassLoaderUtil.java source code

/*
 * Copyright 2002-2003,2009 The Apache Software Foundation.
 * 
 * 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 com.opensymphony.xwork2.util;

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


/**
 * This class is extremely useful for loading resources and classes in a fault tolerant manner
 * that works across different applications servers.
 *
 * 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!
 * 
 * It was brought in from oscore trunk revision 147.
 *
 * @author $Author: martinc $
 * @version $Revision: 894090 $
 */
public class ClassLoaderUtil {
    //~ Methods ////////////////////////////////////////////////////////////////

    /**
     * Load all resources with a given name, potentially aggregating all results 
     * from the searched classloaders.  If no results are found, the resource name
     * is prepended by '/' and tried again.
     *
     * This method will try to load the resources using the following methods (in order):
     * <ul>
     *  <li>From Thread.currentThread().getContextClassLoader()
     *  <li>From ClassLoaderUtil.class.getClassLoader()
     *  <li>callingClass.getClassLoader()
     * </ul>
     *
     * @param resourceName The name of the resources to load
     * @param callingClass The Class object of the calling object
     */
     public static Iterator<URL> getResources(String resourceName, Class callingClass, boolean aggregate) throws IOException {

         AggregateIterator<URL> iterator = new AggregateIterator();

         iterator.addEnumeration(Thread.currentThread().getContextClassLoader().getResources(resourceName));

         if (!iterator.hasNext() || aggregate) {
             iterator.addEnumeration(ClassLoaderUtil.class.getClassLoader().getResources(resourceName));
         }

         if (!iterator.hasNext() || aggregate) {
             ClassLoader cl = callingClass.getClassLoader();

             if (cl != null) {
                 iterator.addEnumeration(cl.getResources(resourceName));
             }
         }

         if (!iterator.hasNext() && (resourceName != null) && ((resourceName.length() == 0) || (resourceName.charAt(0) != '/'))) { 
             return getResources('/' + resourceName, callingClass, aggregate);
         }

         return iterator;
     }

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

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

        if (url == null) {
            ClassLoader cl = callingClass.getClassLoader();

            if (cl != null) {
                url = cl.getResource(resourceName);
            }
        }

        if ((url == null) && (resourceName != null) && ((resourceName.length() == 0) || (resourceName.charAt(0) != '/'))) { 
            return getResource('/' + resourceName, callingClass);
        }

        return url;
    }

    /**
    * This is a convenience method to load a resource as a stream.
    *
    * 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.
    *
    * It will try to load the class in the following order:
    * <ul>
    *  <li>From Thread.currentThread().getContextClassLoader()
    *  <li>Using the basic Class.forName()
    *  <li>From ClassLoaderUtil.class.getClassLoader()
    *  <li>From the 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 ClassLoaderUtil.class.getClassLoader().loadClass(className);
                } catch (ClassNotFoundException exc) {
                    return callingClass.getClassLoader().loadClass(className);
                }
            }
        }
    }

    /**
     * Aggregates Enumeration instances into one iterator and filters out duplicates.  Always keeps one
     * ahead of the enumerator to protect against returning duplicates.
     */
    static class AggregateIterator<E> implements Iterator {

        LinkedList<Enumeration enums = new LinkedList>();
        Enumeration<E> cur = null;
        E next = null;
        Set<E> loaded = new HashSet();

        public AggregateIterator<E> addEnumeration(Enumeration e) {
            if (e.hasMoreElements()) {
                if (cur == null) {
                    cur = e;
                    next = e.nextElement();
                    loaded.add(next);
                } else {
                    enums.add(e);
                }
            }
            return this;
        }

        public boolean hasNext() {
            return (next != null);
        }

        public E next() {
            if (next != null) {
                E prev = next;
                next = loadNext();
                return prev;
            } else {
                throw new NoSuchElementException();
            }
        }

        private Enumeration<E> determineCurrentEnumeration() {
            if (cur != null && !cur.hasMoreElements()) {
                if (enums.size() > 0) {
                    cur = enums.removeLast();
                } else {
                    cur = null;
                }
            }
            return cur;
        }

        private E loadNext() {
            if (determineCurrentEnumeration() != null) {
                E tmp = cur.nextElement();
                int loadedSize = loaded.size();
                while (loaded.contains(tmp)) {
                    tmp = loadNext();
                    if (tmp == null || loaded.size() > loadedSize) {
                        break;
                    }
                }
                if (tmp != null) {
                    loaded.add(tmp);
                }
                return tmp;
            }
            return null;

        }

        public void remove() {
            throw new UnsupportedOperationException();
        }
    }
}