/*
 * 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 $Author$
 * @version $Revision$
 */
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<E> {

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

        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;
        }

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

        @Override
        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;

        }

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