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Apache XML Security for Java supports XML-Signature Syntax and Processing, W3C Recommendation 12 February 2002, and XML Encryption Syntax and Processing, W3C Recommendation 10 December 2002. As of version 1.4, the library supports the standard Java API JSR-105: XML Digital Signature APIs.
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/**
 * 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.xml.security.utils;

import java.io.IOException;
import java.io.InputStream;
import java.lang.System.Logger;
import java.lang.System.Logger.Level;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;

/**
 * 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 static final Logger LOG = System.getLogger(ClassLoaderUtils.class.getName());

    private ClassLoaderUtils() {
    }

    /**
     * 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 of the resource to load
     * @param callingClass The Class object of the calling object
     */
    public static URL getResource(String resourceName, Class callingClass) {
        if (resourceName == null) {
            throw new NullPointerException();
        }
        URL url = Thread.currentThread().getContextClassLoader().getResource(resourceName);
        if (url == null && resourceName.charAt(0) == '/') {
            //certain classloaders need it without the leading /
            url =
                Thread.currentThread().getContextClassLoader().getResource(
                    resourceName.substring(1)
                );
        }

        ClassLoader cluClassloader = ClassLoaderUtils.class.getClassLoader();
        if (cluClassloader == null) {
            cluClassloader = ClassLoader.getSystemClassLoader();
        }
        if (url == null) {
            url = cluClassloader.getResource(resourceName);
        }
        if (url == null && resourceName.charAt(0) == '/') {
            //certain classloaders need it without the leading /
            url = cluClassloader.getResource(resourceName.substring(1));
        }

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

        return url;
    }

    /**
     * Load a given resources.  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 resource to load
     * @param callingClass The Class object of the calling object
     */
    public static List getResources(String resourceName, Class callingClass) {
        if (resourceName == null) {
            throw new NullPointerException();
        }
        List ret = new ArrayList<>();
        Enumeration urls = new Enumeration() {
            @Override
            public boolean hasMoreElements() {
                return false;
            }
            @Override
            public URL nextElement() {
                return null;
            }

        };
        try {
            urls = Thread.currentThread().getContextClassLoader().getResources(resourceName);
        } catch (IOException e) {
            LOG.log(Level.DEBUG, e.getMessage(), e);
            //ignore
        }
        if (!urls.hasMoreElements() && resourceName.charAt(0) == '/') {
            //certain classloaders need it without the leading /
            try {
                urls =
                    Thread.currentThread().getContextClassLoader().getResources(
                        resourceName.substring(1)
                    );
            } catch (IOException e) {
                LOG.log(Level.DEBUG, e.getMessage(), e);
                // ignore
            }
        }

        ClassLoader cluClassloader = ClassLoaderUtils.class.getClassLoader();
        if (cluClassloader == null) {
            cluClassloader = ClassLoader.getSystemClassLoader();
        }
        if (!urls.hasMoreElements()) {
            try {
                urls = cluClassloader.getResources(resourceName);
            } catch (IOException e) {
                LOG.log(Level.DEBUG, e.getMessage(), e);
                // ignore
            }
        }
        if (!urls.hasMoreElements() && resourceName.charAt(0) == '/') {
            //certain classloaders need it without the leading /
            try {
                urls = cluClassloader.getResources(resourceName.substring(1));
            } catch (IOException e) {
                LOG.log(Level.DEBUG, e.getMessage(), e);
                // ignore
            }
        }

        if (!urls.hasMoreElements()) {
            ClassLoader cl = callingClass.getClassLoader();

            if (cl != null) {
                try {
                    urls = cl.getResources(resourceName);
                } catch (IOException e) {
                    LOG.log(Level.DEBUG, e.getMessage(), e);
                    // ignore
                }
            }
        }

        if (!urls.hasMoreElements()) {
            URL url = callingClass.getResource(resourceName);
            if (url != null) {
                ret.add(url);
            }
        }
        while (urls.hasMoreElements()) {
            ret.add(urls.nextElement());
        }


        if (ret.isEmpty() && resourceName.charAt(0) != '/') {
            return getResources('/' + resourceName, callingClass);
        }
        return ret;
    }


    /**
     * 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) {
            LOG.log(Level.DEBUG, e.getMessage(), 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 {
            ClassLoader cl = Thread.currentThread().getContextClassLoader();

            if (cl != null) {
                return cl.loadClass(className);
            }
        } catch (ClassNotFoundException e) {
            LOG.log(Level.DEBUG, e.getMessage(), e);
            //ignore
        }
        return loadClass2(className, callingClass);
    }

    private static Class loadClass2(String className, Class callingClass)
        throws ClassNotFoundException {
        try {
            return Class.forName(className);
        } catch (ClassNotFoundException ex) {
            try {
                if (ClassLoaderUtils.class.getClassLoader() != null) {
                    return ClassLoaderUtils.class.getClassLoader().loadClass(className);
                }
            } catch (ClassNotFoundException exc) {
                if (callingClass != null && callingClass.getClassLoader() != null) {
                    return callingClass.getClassLoader().loadClass(className);
                }
            }
            LOG.log(Level.DEBUG, ex.getMessage(), ex);
            throw ex;
        }
    }
}