1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.discovery.tools;
18
19 import java.util.Enumeration;
20 import java.util.NoSuchElementException;
21
22 import org.apache.commons.discovery.ResourceClass;
23 import org.apache.commons.discovery.ResourceClassIterator;
24 import org.apache.commons.discovery.ResourceNameIterator;
25 import org.apache.commons.discovery.resource.ClassLoaders;
26 import org.apache.commons.discovery.resource.classes.DiscoverClasses;
27 import org.apache.commons.discovery.resource.names.DiscoverServiceNames;
28
29 /**
30 * [this was ServiceDiscovery12... the 1.1 versus 1.2 issue
31 * has been abstracted to org.apache.commons.discover.jdk.JDKHooks]
32 *
33 * <p>Implement the JDK1.3 'Service Provider' specification.
34 * ( http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html )
35 * </p>
36 *
37 * This class supports any VM, including JDK1.1, via
38 * org.apache.commons.discover.jdk.JDKHooks.
39 *
40 * The caller will first configure the discoverer by adding ( in the desired
41 * order ) all the places to look for the META-INF/services. Currently
42 * we support loaders.
43 *
44 * The findResources() method will check every loader.
45 */
46 public class Service {
47
48 /**
49 * Construct a new service discoverer
50 */
51 protected Service() {
52 }
53
54 /**
55 * as described in
56 * sun/jdk1.3.1/docs/guide/jar/jar.html#Service Provider,
57 * Except this uses <code>Enumeration</code>
58 * instead of <code>Interator</code>.
59 *
60 * @param <T> Service Provider Interface type
61 * @param <S> Any type extends the SPI type
62 * @param spiClass Service Provider Interface Class
63 * @return Enumeration of class instances ({@code S})
64 */
65 public static <T, S extends T> Enumeration<S> providers(Class<T> spiClass) {
66 return providers(new SPInterface<T>(spiClass), null);
67 }
68
69 /**
70 * This version lets you specify constructor arguments..
71 *
72 * @param <T> Service Provider Interface type
73 * @param <S> Any type extends the SPI type
74 * @param spi SPI to look for and load.
75 * @param loaders loaders to use in search.
76 * If <code>null</code> then use ClassLoaders.getAppLoaders().
77 * @return Enumeration of class instances ({@code S})
78 */
79 public static <T, S extends T> Enumeration<S> providers(final SPInterface<T> spi,
80 ClassLoaders loaders) {
81 if (loaders == null) {
82 loaders = ClassLoaders.getAppLoaders(spi.getSPClass(),
83 Service.class,
84 true);
85 }
86
87 ResourceNameIterator servicesIter =
88 (new DiscoverServiceNames(loaders)).findResourceNames(spi.getSPName());
89
90 final ResourceClassIterator<T> services =
91 (new DiscoverClasses<T>(loaders)).findResourceClasses(servicesIter);
92
93 return new Enumeration<S>() {
94
95 private S object = getNextClassInstance();
96
97 public boolean hasMoreElements() {
98 return object != null;
99 }
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 }