src/java/org/apache/xmlgraphics/util/ClasspathResource.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.
 */

/* $Id$ */

package org.apache.xmlgraphics.util;

import java.io.IOException;
import java.net.URL;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.Vector;
import java.util.jar.Attributes;
import java.util.jar.Manifest;

/**
 * A class to find resources in the classpath by their mime-type specified in
 * the MANIFEST.
 * <p>
 * This class searches for content entries in all META-INF/MANIFEST.MF files. It
 * will find files with a given Content-Type: attribute. This allows to add
 * arbitrary resources by content-type just by creating a JAR wrapper and adding
 * them to the classpath.
 * <p>
 * Example:<br>
 *
 * <pre>
 * Name: test.txt
 * Content-Type: text/plain
 * </pre>
 */
public final class ClasspathResource {

    /**
     * Actual Type: Map&lt;String,List&lt;URL&gt;&gt;.
     */
    private final Map contentMappings;

    private static final String MANIFEST_PATH = "META-INF/MANIFEST.MF";

    private static final String CONTENT_TYPE_KEY = "Content-Type";

    private static ClasspathResource classpathResource;

    private ClasspathResource() {
        contentMappings = new HashMap();
        loadManifests();
    }

    /**
     * Retrieve the singleton instance of this class.
     *
     * @return the ClassPathResource instance.
     */
    public static synchronized ClasspathResource getInstance() {
        if (classpathResource == null) {
            classpathResource = new ClasspathResource();
        }
        return classpathResource;
    }

    /* Actual return type: Set<ClassLoader> */
    private Set getClassLoadersForResources() {
        Set v = new HashSet();
        try {
            ClassLoader l = ClassLoader.getSystemClassLoader();
            if (l != null) {
                v.add(l);
            }
        } catch (SecurityException e) {
            // Ignore
        }
        try {
            ClassLoader l = Thread.currentThread().getContextClassLoader();
            if (l != null) {
                v.add(l);
            }
        } catch (SecurityException e) {
            // Ignore
        }
        try {
            ClassLoader l = ClasspathResource.class.getClassLoader();
            if (l != null) {
                v.add(l);
            }
        } catch (SecurityException e) {
            // Ignore
        }
        return v;
    }

    private void loadManifests() {
        Enumeration e;
        try {

            Iterator it = getClassLoadersForResources().iterator();
            while (it.hasNext()) {
                ClassLoader classLoader = (ClassLoader) it.next();

                e = classLoader.getResources(MANIFEST_PATH);

                while (e.hasMoreElements()) {
                    final URL u = (URL) e.nextElement();
                    try {
                        final Manifest manifest = new Manifest(u.openStream());
                        final Map entries = manifest.getEntries();
                        final Iterator entrysetiterator = entries.entrySet()
                                .iterator();
                        while (entrysetiterator.hasNext()) {
                            final Map.Entry entry = (Map.Entry) entrysetiterator
                                    .next();
                            final String name = (String) entry.getKey();
                            final Attributes attributes = (Attributes) entry
                                    .getValue();
                            final String contentType = attributes
                                    .getValue(CONTENT_TYPE_KEY);
                            if (contentType != null) {
                                addToMapping(contentType, name, classLoader);
                            }
                        }
                    } catch (IOException io) {
                        // TODO: Log.
                    }
                }
            }

        } catch (IOException io) {
            // TODO: Log.
        }
    }

    private void addToMapping(final String contentType, final String name,
            final ClassLoader classLoader) {
        List existingFiles = (List) contentMappings.get(contentType);
        if (existingFiles == null) {
            existingFiles = new Vector();
            contentMappings.put(contentType, existingFiles);
        }
        final URL url = classLoader.getResource(name);
        if (url != null) {
            existingFiles.add(url);
        }
    }

    /**
     * Retrieve a list of resources known to have the given mime-type.
     *
     * @param mimeType
     *            the mime-type to search for.
     * @return a List&lt;URL&gt;, guaranteed to be != null.
     */
    public List listResourcesOfMimeType(final String mimeType) {
        final List content = (List) contentMappings.get(mimeType);
        if (content == null) {
            return Collections.EMPTY_LIST;
        } else {
            return content;
        }
    }

}