001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.discovery;
018
019import java.net.URL;
020import java.security.AccessController;
021import java.security.PrivilegedAction;
022
023import org.apache.commons.logging.Log;
024import org.apache.commons.logging.LogFactory;
025
026/**
027 * 'Resource' located by discovery.
028 * Naming of methods becomes a real pain ('getClass()')
029 * so I've patterned this after ClassLoader...
030 *
031 * I think it works well as it will give users a point-of-reference.
032 *
033 * @param <T> The SPI type
034 */
035public class ResourceClass<T> extends Resource {
036
037    private static Log log = LogFactory.getLog(ResourceClass.class);
038
039    /**
040     * Sets the {@code Log} for this class.
041     *
042     * @param _log This class {@code Log}
043     * @deprecated This method is not thread-safe
044     */
045    @Deprecated
046    public static void setLog(Log _log) {
047        log = _log;
048    }
049
050    protected Class<? extends T> resourceClass;
051
052    /**
053     * Create a new {@code Resource} class located by discovery.
054     *
055     * @param <S> Any type extends T
056     * @param resourceClass The resource class has to be located
057     * @param resource The resource URL has to be located
058     */
059    public <S extends T> ResourceClass(Class<S> resourceClass, URL resource) {
060        super(resourceClass.getName(), resource, resourceClass.getClassLoader());
061        this.resourceClass = resourceClass;
062    }
063
064    /**
065     * Create a new {@code Resource} class located by discovery.
066     *
067     * @param resourceName The resource class name has to be located
068     * @param resource The resource URL has to be located
069     * @param loader The class loaders holder
070     */
071    public ResourceClass(String resourceName, URL resource, ClassLoader loader) {
072        super(resourceName, resource, loader);
073    }
074
075    /**
076     * Get the value of resourceClass.
077     * Loading the class does NOT guarentee that the class can be
078     * instantiated.  Go figure.
079     * The class can be instantiated when the class is linked/resolved,
080     * and all dependencies are resolved.
081     * Various JDKs do this at different times, so beware:
082     * java.lang.NoClassDefFoundError when
083     * calling Class.getDeclaredMethod() (JDK14),
084     * java.lang.reflect.InvocationTargetException
085     * (wrapping java.lang.NoClassDefFoundError) when calling
086     * java.lang.newInstance (JDK13),
087     * and who knows what else..
088     *
089     * @param <S> Any type extends T
090     *
091     * @return value of resourceClass.
092     */
093    public <S extends T> Class<S> loadClass() {
094        if (resourceClass == null  &&  getClassLoader() != null) {
095            if (log.isDebugEnabled()) {
096                log.debug("loadClass: Loading class '" + getName() + "' with " + getClassLoader());
097            }
098
099            resourceClass = AccessController.doPrivileged(
100                new PrivilegedAction<Class<? extends T>>() {
101                    public Class<? extends T> run() {
102                        try {
103                            @SuppressWarnings("unchecked") // this can raise a ClassCastException at runtime
104                            Class<S> returned = (Class<S>) getClassLoader().loadClass(getName());
105                            return returned;
106                        } catch (ClassNotFoundException e) {
107                            return null;
108                        }
109                    }
110                });
111        }
112
113        @SuppressWarnings("unchecked") // this is assumed by default, see the ctor
114        Class<S> returned = (Class<S>) resourceClass;
115        return returned;
116    }
117
118    /**
119     * {@inheritDoc}
120     */
121    @Override
122    public String toString() {
123        return "ResourceClass[" + getName() +  ", " + getResource() + ", " + getClassLoader() + "]";
124    }
125
126}