core/src/main/java/com/opensymphony/xwork2/util/ClassLoaderUtil.java

/*
 * 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 com.opensymphony.xwork2.util;

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


/**
 * <p>
 * This class is extremely useful for loading resources and classes in a fault tolerant manner
 * that works across different applications servers.
 * </p>
 *
 * <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!
 * </p>
 */
public class ClassLoaderUtil {

    /**
     * <p>
     * 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.
     * </p>
     *
     * <p>
     * This method will try to load the resources using the following methods (in order):
     * </p>
     *
     * <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
     * @param aggregate aggregate
     *
     * @return all resources found
     *
     * @throws IOException in case of IO errors
     */
     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.
     * <p>
     * This method will try to load the resource using the following methods (in order):
     * </p>
     *
     * <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
     *
     * @return the resource
     */
    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
    * @return resource as a stream
    */
    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;
        }
    }

    /**
     * <p>
     * Load a class with a given name.
     * </p>
     *
     * <p>
     * It will try to load the class in the following order:
     * </p>
     *
     * <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
     * @return class with for the given name
     * @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);
                }
            }
        }
    }

    /**
     * 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.
     *
     * @param cl the class loader
     */
    public static void printClassLoader(ClassLoader cl) {
        System.out.println("ClassLoaderUtils.printClassLoader(cl = " + cl + ")");

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



    /**
     * 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<E> {

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

        public AggregateIterator<E> addEnumeration(Enumeration<E> 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();
        }
    }
}