/**
 * 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.cxf.common.classloader;

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. Do not
 * touch this unless you're a grizzled classloading guru veteran who is going to
 * verify any change on 6 different application servers.
 */
public final class ClassLoaderUtils {
    
    private ClassLoaderUtils() {
    }
    
    /**
     * Load a given resource. <p/> 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 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 = ClassLoaderUtils.class.getClassLoader().getResource(resourceName);
        }

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

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

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

        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 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 {
            ClassLoader cl = Thread.currentThread().getContextClassLoader();

            if (cl != null) {
                return cl.loadClass(className);
            }
            
            return loadClass2(className, callingClass);
        } catch (ClassNotFoundException e) {
            return loadClass2(className, callingClass);
        }
    }

    private static Class loadClass2(String className, Class callingClass) throws ClassNotFoundException {
        try {
            return Class.forName(className);
        } catch (ClassNotFoundException ex) {
            try {
                return ClassLoaderUtils.class.getClassLoader().loadClass(className);
            } catch (ClassNotFoundException exc) {
                return callingClass.getClassLoader().loadClass(className);
            }
        }
    }
}
