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XWork is an command-pattern framework that is used to power WebWork as well as other applications. XWork provides an Inversion of Control container, a powerful expression language, data type conversion, validation, and pluggable configuration.
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/*
 * Copyright (c) 2002-2003 by OpenSymphony
 * All rights reserved.
 */
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: hani $
 * @version $Revision: 117 $
 */
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):
     * 
     *  From Thread.currentThread().getContextClassLoader()
     *  
From ClassLoaderUtil.class.getClassLoader()
     *  
callingClass.getClassLoader()
     * 
     *
     * @param resourceName The name of the resources to load
     * @param callingClass The Class object of the calling object
     */
     public static Iterator getResources(String resourceName, Class callingClass, boolean aggregate) throws IOException {

         AggregateIterator 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):
    * 
    *  From Thread.currentThread().getContextClassLoader()
    *  
From ClassLoaderUtil.class.getClassLoader()
    *  
callingClass.getClassLoader()
    * 
    *
    * @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:
    * 
    *  From Thread.currentThread().getContextClassLoader()
    *  
Using the basic Class.forName()
    *  
From ClassLoaderUtil.class.getClassLoader()
    *  
From the callingClass.getClassLoader()
    * 
    *
    * @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 implements Iterator {

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

        public AggregateIterator 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 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();
        }
    }
}