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 */
017 package org.apache.commons.discovery.tools;
018
019 import java.util.Enumeration;
020 import java.util.NoSuchElementException;
021
022 import org.apache.commons.discovery.ResourceClass;
023 import org.apache.commons.discovery.ResourceClassIterator;
024 import org.apache.commons.discovery.ResourceNameIterator;
025 import org.apache.commons.discovery.resource.ClassLoaders;
026 import org.apache.commons.discovery.resource.classes.DiscoverClasses;
027 import org.apache.commons.discovery.resource.names.DiscoverServiceNames;
028
029 /**
030 * [this was ServiceDiscovery12... the 1.1 versus 1.2 issue
031 * has been abstracted to org.apache.commons.discover.jdk.JDKHooks]
032 *
033 * <p>Implement the JDK1.3 'Service Provider' specification.
034 * ( http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html )
035 * </p>
036 *
037 * This class supports any VM, including JDK1.1, via
038 * org.apache.commons.discover.jdk.JDKHooks.
039 *
040 * The caller will first configure the discoverer by adding ( in the desired
041 * order ) all the places to look for the META-INF/services. Currently
042 * we support loaders.
043 *
044 * The findResources() method will check every loader.
045 */
046 public class Service {
047
048 /**
049 * Construct a new service discoverer
050 */
051 protected Service() {
052 }
053
054 /**
055 * as described in
056 * sun/jdk1.3.1/docs/guide/jar/jar.html#Service Provider,
057 * Except this uses <code>Enumeration</code>
058 * instead of <code>Interator</code>.
059 *
060 * @param <T> Service Provider Interface type
061 * @param <S> Any type extends the SPI type
062 * @param spiClass Service Provider Interface Class
063 * @return Enumeration of class instances ({@code S})
064 */
065 public static <T, S extends T> Enumeration<S> providers(Class<T> spiClass) {
066 return providers(new SPInterface<T>(spiClass), null);
067 }
068
069 /**
070 * This version lets you specify constructor arguments..
071 *
072 * @param <T> Service Provider Interface type
073 * @param <S> Any type extends the SPI type
074 * @param spi SPI to look for and load.
075 * @param loaders loaders to use in search.
076 * If <code>null</code> then use ClassLoaders.getAppLoaders().
077 * @return Enumeration of class instances ({@code S})
078 */
079 public static <T, S extends T> Enumeration<S> providers(final SPInterface<T> spi,
080 ClassLoaders loaders) {
081 if (loaders == null) {
082 loaders = ClassLoaders.getAppLoaders(spi.getSPClass(),
083 Service.class,
084 true);
085 }
086
087 ResourceNameIterator servicesIter =
088 (new DiscoverServiceNames(loaders)).findResourceNames(spi.getSPName());
089
090 final ResourceClassIterator<T> services =
091 (new DiscoverClasses<T>(loaders)).findResourceClasses(servicesIter);
092
093 return new Enumeration<S>() {
094
095 private S object = getNextClassInstance();
096
097 public boolean hasMoreElements() {
098 return object != null;
099 }
100
101 public S nextElement() {
102 if (object == null) {
103 throw new NoSuchElementException();
104 }
105
106 S obj = object;
107 object = getNextClassInstance();
108 return obj;
109 }
110
111 private S getNextClassInstance() {
112 while (services.hasNext()) {
113 ResourceClass<S> info = services.nextResourceClass();
114 try {
115 return spi.newInstance(info.loadClass());
116 } catch (Exception e) {
117 // ignore
118 } catch (LinkageError le) {
119 // ignore
120 }
121 }
122 return null;
123 }
124 };
125 }
126
127 }