servicefinder.java

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/*
 *
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 * 
 * Copyright 1997-2007 Sun Microsystems, Inc. All rights reserved.
 * 
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  You
 * may not use this file except in compliance with the License. You can obtain
 * a copy of the License at https://jersey.dev.java.net/CDDL+GPL.html
 * or jersey/legal/LICENSE.txt.  See the License for the specific
 * language governing permissions and limitations under the License.
 * 
 * When distributing the software, include this License Header Notice in each
 * file and include the License file at jersey/legal/LICENSE.txt.
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 * as provided by Sun in the GPL Version 2 section of the License file that
 * accompanied this code.  If applicable, add the following below the License
 * Header, with the fields enclosed by brackets [] replaced by your own
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 * only the GPL Version 2, indicate your decision by adding "[Contributor]
 * elects to include this software in this distribution under the [CDDL or GPL
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 * either the CDDL, the GPL Version 2 or to extend the choice of license to
 * its licensees as provided above.  However, if you add GPL Version 2 code
 * and therefore, elected the GPL Version 2 license, then the option applies
 * only if the new code is made subject to such option by the copyright
 * holder.
 */

package com.sun.jersey.spi.service;

import com.sun.jersey.spi.SpiMessages;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Array;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.TreeSet;
import java.util.logging.Level;
import java.util.logging.Logger;


/**
 * A simple service-provider lookup mechanism.  A <i>service</i> is a
 * well-known set of interfaces and (usually abstract) classes.  A <i>service
 * provider</i> is a specific implementation of a service.  The classes in a
 * provider typically implement the interfaces and subclass the classes defined
 * in the service itself.  Service providers may be installed in an
 * implementation of the Java platform in the form of extensions, that is, jar
 * files placed into any of the usual extension directories.  Providers may
 * also be made available by adding them to the applet or application class
 * path or by some other platform-specific means.
 * <p/>
 * <p> In this lookup mechanism a service is represented by an interface or an
 * abstract class.  (A concrete class may be used, but this is not
 * recommended.)  A provider of a given service contains one or more concrete
 * classes that extend this <i>service class</i> with data and code specific to
 * the provider.  This <i>provider class</i> will typically not be the entire
 * provider itself but rather a proxy that contains enough information to
 * decide whether the provider is able to satisfy a particular request together
 * with code that can create the actual provider on demand.  The details of
 * provider classes tend to be highly service-specific; no single class or
 * interface could possibly unify them, so no such class has been defined.  The
 * only requirement enforced here is that provider classes must have a
 * zero-argument constructor so that they may be instantiated during lookup.
 * <p/>
 * <p> A service provider identifies itself by placing a provider-configuration
 * file in the resource directory <tt>META-INF/services</tt>.  The file's name
 * should consist of the fully-qualified name of the abstract service class.
 * The file should contain a list of fully-qualified concrete provider-class
 * names, one per line.  Space and tab characters surrounding each name, as
 * well as blank lines, are ignored.  The comment character is <tt>'#'</tt>
 * (<tt>0x23</tt>); on each line all characters following the first comment
 * character are ignored.  The file must be encoded in UTF-8.
 * <p/>
 * <p> If a particular concrete provider class is named in more than one
 * configuration file, or is named in the same configuration file more than
 * once, then the duplicates will be ignored.  The configuration file naming a
 * particular provider need not be in the same jar file or other distribution
 * unit as the provider itself.  The provider must be accessible from the same
 * class loader that was initially queried to locate the configuration file;
 * note that this is not necessarily the class loader that found the file.
 * <p/>
 * <p> <b>Example:</b> Suppose we have a service class named
 * <tt>java.io.spi.CharCodec</tt>.  It has two abstract methods:
 * <p/>
 * <pre>
 *   public abstract CharEncoder getEncoder(String encodingName);
 *   public abstract CharDecoder getDecoder(String encodingName);
 * </pre>
 * <p/>
 * Each method returns an appropriate object or <tt>null</tt> if it cannot
 * translate the given encoding.  Typical <tt>CharCodec</tt> providers will
 * support more than one encoding.
 * <p/>
 * <p> If <tt>sun.io.StandardCodec</tt> is a provider of the <tt>CharCodec</tt>
 * service then its jar file would contain the file
 * <tt>META-INF/services/java.io.spi.CharCodec</tt>.  This file would contain
 * the single line:
 * <p/>
 * <pre>
 *   sun.io.StandardCodec    # Standard codecs for the platform
 * </pre>
 * <p/>
 * To locate an codec for a given encoding name, the internal I/O code would
 * do something like this:
 * <p/>
 * <pre>
 *   CharEncoder getEncoder(String encodingName) {
 *       for( CharCodec cc : ServiceFinder.find(CharCodec.class) ) {
 *           CharEncoder ce = cc.getEncoder(encodingName);
 *           if (ce != null)
 *               return ce;
 *       }
 *       return null;
 *   }
 * </pre>
 * <p/>
 * The provider-lookup mechanism always executes in the security context of the
 * caller.  Trusted system code should typically invoke the methods in this
 * class from within a privileged security context.
 *
 * @author Mark Reinhold
 * @version 1.11, 03/12/19
 * @since 1.3
 */
public final class ServiceFinder<T> implements Iterable<T> {
    private static final Logger LOGGER = Logger.getLogger(ServiceFinder.class.getName());
    
    private static final String PREFIX = "META-INF/services/";
    
    private static final ComponentProvider DEFAULT_COMPONENT_PROVIDER = new ComponentProvider() {
        public <T> T getInstance(Scope scope, Class<T> c) 
                throws InstantiationException, IllegalAccessException {
            return c.newInstance();
        }

        public <T> T getInstance(Scope scope, Constructor<T> contructor, Object[] parameters) 
                throws InstantiationException, IllegalArgumentException, 
                IllegalAccessException, InvocationTargetException {
            throw new UnsupportedOperationException("");
        }

        public <T> T getInstance(ComponentContext cc, Scope scope, Class<T> c) 
                throws InstantiationException, IllegalAccessException {
            return getInstance(scope, c);
        }
        
        public <T> T getInjectableInstance(T instance) {
            return instance;
        }
        
        public void inject(Object instance) {
            throw new UnsupportedOperationException("");
        }
    };
            
    private final Class<T> serviceClass;
    private final ClassLoader classLoader;
    private final boolean ignoreOnClassNotFound;
    private final ComponentProvider componentProvider;
    
    
    /**
     * Locates and incrementally instantiates the available providers of a
     * given service using the given class loader.
     * <p/>
     * <p> This method transforms the name of the given service class into a
     * provider-configuration filename as described above and then uses the
     * <tt>getResources</tt> method of the given class loader to find all
     * available files with that name.  These files are then read and parsed to
     * produce a list of provider-class names.  The iterator that is returned
     * uses the given class loader to lookup and then instantiate each element
     * of the list.
     * <p/>
     * <p> Because it is possible for extensions to be installed into a running
     * Java virtual machine, this method may return different results each time
     * it is invoked. <p>
     * @param service The service's abstract service class
     * @param loader The class loader to be used to load provider-configuration files
     *                and instantiate provider classes, or <tt>null</tt> if the system
     *                class loader (or, failing that the bootstrap class loader) is to
     *                be used
     * @throws ServiceConfigurationError If a provider-configuration file violates the specified format
     *                                   or names a provider class that cannot be found and instantiated
     * @see #find(Class)
     * @return the service finder
     */
    public static <T> ServiceFinder<T> find(Class<T> service, ClassLoader loader) 
            throws ServiceConfigurationError {
        return find(service, 
                loader, 
                false, 
                DEFAULT_COMPONENT_PROVIDER);
    }
    
    /**
     * Locates and incrementally instantiates the available providers of a
     * given service using the given class loader.
     * <p/>
     * <p> This method transforms the name of the given service class into a
     * provider-configuration filename as described above and then uses the
     * <tt>getResources</tt> method of the given class loader to find all
     * available files with that name.  These files are then read and parsed to
     * produce a list of provider-class names.  The iterator that is returned
     * uses the given class loader to lookup and then instantiate each element
     * of the list.
     * <p/>
     * <p> Because it is possible for extensions to be installed into a running
     * Java virtual machine, this method may return different results each time
     * it is invoked. <p>
     * @param service The service's abstract service class