1 /*
2 * Copyright (c) 1999, 2003, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package sun.misc;
27
28 import java.io.BufferedReader;
29 import java.io.IOException;
30 import java.io.InputStream;
31 import java.io.InputStreamReader;
32 import java.net.URL;
33 import java.util.ArrayList;
34 import java.util.Enumeration;
35 import java.util.Iterator;
36 import java.util.List;
37 import java.util.NoSuchElementException;
38 import java.util.Set;
39 import java.util.TreeSet;
40
41
42 /**
43 * A simple service-provider lookup mechanism. A <i>service</i> is a
44 * well-known set of interfaces and (usually abstract) classes. A <i>service
45 * provider</i> is a specific implementation of a service. The classes in a
46 * provider typically implement the interfaces and subclass the classes defined
47 * in the service itself. Service providers may be installed in an
48 * implementation of the Java platform in the form of extensions, that is, jar
49 * files placed into any of the usual extension directories. Providers may
50 * also be made available by adding them to the applet or application class
51 * path or by some other platform-specific means.
52 *
53 * <p> In this lookup mechanism a service is represented by an interface or an
54 * abstract class. (A concrete class may be used, but this is not
55 * recommended.) A provider of a given service contains one or more concrete
56 * classes that extend this <i>service class</i> with data and code specific to
57 * the provider. This <i>provider class</i> will typically not be the entire
58 * provider itself but rather a proxy that contains enough information to
59 * decide whether the provider is able to satisfy a particular request together
60 * with code that can create the actual provider on demand. The details of
61 * provider classes tend to be highly service-specific; no single class or
62 * interface could possibly unify them, so no such class has been defined. The
63 * only requirement enforced here is that provider classes must have a
64 * zero-argument constructor so that they may be instantiated during lookup.
65 *
66 * <p> A service provider identifies itself by placing a provider-configuration
67 * file in the resource directory <tt>META-INF/services</tt>. The file's name
68 * should consist of the fully-qualified name of the abstract service class.
69 * The file should contain a list of fully-qualified concrete provider-class
70 * names, one per line. Space and tab characters surrounding each name, as
71 * well as blank lines, are ignored. The comment character is <tt>'#'</tt>
72 * (<tt>0x23</tt>); on each line all characters following the first comment
73 * character are ignored. The file must be encoded in UTF-8.
74 *
75 * <p> If a particular concrete provider class is named in more than one
76 * configuration file, or is named in the same configuration file more than
77 * once, then the duplicates will be ignored. The configuration file naming a
78 * particular provider need not be in the same jar file or other distribution
79 * unit as the provider itself. The provider must be accessible from the same
80 * class loader that was initially queried to locate the configuration file;
81 * note that this is not necessarily the class loader that found the file.
82 *
83 * <p> <b>Example:</b> Suppose we have a service class named
84 * <tt>java.io.spi.CharCodec</tt>. It has two abstract methods:
85 *
86 * <pre>
87 * public abstract CharEncoder getEncoder(String encodingName);
88 * public abstract CharDecoder getDecoder(String encodingName);
89 * </pre>
90 *
91 * Each method returns an appropriate object or <tt>null</tt> if it cannot
92 * translate the given encoding. Typical <tt>CharCodec</tt> providers will
93 * support more than one encoding.
94 *
95 * <p> If <tt>sun.io.StandardCodec</tt> is a provider of the <tt>CharCodec</tt>
96 * service then its jar file would contain the file
97 * <tt>META-INF/services/java.io.spi.CharCodec</tt>. This file would contain
98 * the single line:
99 *
100 * <pre>
101 * sun.io.StandardCodec # Standard codecs for the platform
102 * </pre>
103 *
104 * To locate an encoder for a given encoding name, the internal I/O code would
105 * do something like this:
106 *
107 * <pre>
108 * CharEncoder getEncoder(String encodingName) {
109 * Iterator ps = Service.providers(CharCodec.class);
110 * while (ps.hasNext()) {
111 * CharCodec cc = (CharCodec)ps.next();
112 * CharEncoder ce = cc.getEncoder(encodingName);
113 * if (ce != null)
114 * return ce;
115 * }
116 * return null;
117 * }
118 * </pre>
119 *
120 * The provider-lookup mechanism always executes in the security context of the
121 * caller. Trusted system code should typically invoke the methods in this
122 * class from within a privileged security context.
123 *
124 * @author Mark Reinhold
125 * @since 1.3
126 */
127
128 public final class Service {
129
130 private static final String prefix = "META-INF/services/";
131
132 private Service() { }
133
134 private static void fail(Class service, String msg, Throwable cause)
135 throws ServiceConfigurationError
136 {
137 ServiceConfigurationError sce
138 = new ServiceConfigurationError(service.getName() + ": " + msg);
139 sce.initCause(cause);
140 throw sce;
141 }
142
143 private static void fail(Class service, String msg)
144 throws ServiceConfigurationError
145 {
146 throw new ServiceConfigurationError(service.getName() + ": " + msg);
147 }
148
149 private static void fail(Class service, URL u, int line, String msg)
150 throws ServiceConfigurationError
151 {
152 fail(service, u + ":" + line + ": " + msg);
153 }
154
155 /**
156 * Parse a single line from the given configuration file, adding the name
157 * on the line to both the names list and the returned set iff the name is
158 * not already a member of the returned set.
159 */
160 private static int parseLine(Class service, URL u, BufferedReader r, int lc,
161 List names, Set returned)
162 throws IOException, ServiceConfigurationError
163 {
164 String ln = r.readLine();
165 if (ln == null) {
166 return -1;
167 }
168 int ci = ln.indexOf('#');
169 if (ci >= 0) ln = ln.substring(0, ci);
170 ln = ln.trim();
171 int n = ln.length();
172 if (n != 0) {
173 if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
174 fail(service, u, lc, "Illegal configuration-file syntax");
175 int cp = ln.codePointAt(0);
176 if (!Character.isJavaIdentifierStart(cp))
177 fail(service, u, lc, "Illegal provider-class name: " + ln);
178 for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
179 cp = ln.codePointAt(i);
180 if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
181 fail(service, u, lc, "Illegal provider-class name: " + ln);
182 }
183 if (!returned.contains(ln)) {
184 names.add(ln);
185 returned.add(ln);
186 }
187 }
188 return lc + 1;
189 }
190
191 /**
192 * Parse the content of the given URL as a provider-configuration file.
193 *
194 * @param service
195 * The service class for which providers are being sought;
196 * used to construct error detail strings
197 *
198 * @param url
199 * The URL naming the configuration file to be parsed
200 *
201 * @param returned
202 * A Set containing the names of provider classes that have already
203 * been returned. This set will be updated to contain the names
204 * that will be yielded from the returned <tt>Iterator</tt>.
205 *
206 * @return A (possibly empty) <tt>Iterator</tt> that will yield the
207 * provider-class names in the given configuration file that are
208 * not yet members of the returned set
209 *
210 * @throws ServiceConfigurationError
211 * If an I/O error occurs while reading from the given URL, or
212 * if a configuration-file format error is detected
213 */
214 private static Iterator parse(Class service, URL u, Set returned)
215 throws ServiceConfigurationError
216 {
217 InputStream in = null;
218 BufferedReader r = null;
219 ArrayList names = new ArrayList();
220 try {
221 in = u.openStream();
222 r = new BufferedReader(new InputStreamReader(in, "utf-8"));
223 int lc = 1;
224 while ((lc = parseLine(service, u, r, lc, names, returned)) >= 0);
225 } catch (IOException x) {
226 fail(service, ": " + x);
227 } finally {
228 try {
229 if (r != null) r.close();
230 if (in != null) in.close();
231 } catch (IOException y) {
232 fail(service, ": " + y);
233 }
234 }
235 return names.iterator();
236 }
237
238
239 /**
240 * Private inner class implementing fully-lazy provider lookup
241 */
242 private static class LazyIterator implements Iterator {
243
244 Class service;
245 ClassLoader loader;
246 Enumeration configs = null;
247 Iterator pending = null;
248 Set returned = new TreeSet();
249 String nextName = null;
250
251 private LazyIterator(Class service, ClassLoader loader) {
252 this.service = service;
253 this.loader = loader;
254 }
255
256 public boolean hasNext() throws ServiceConfigurationError {
257 if (nextName != null) {
258 return true;
259 }
260 if (configs == null) {
261 try {
262 String fullName = prefix + service.getName();
263 if (loader == null)
264 configs = ClassLoader.getSystemResources(fullName);
265 else
266 configs = loader.getResources(fullName);
267 } catch (IOException x) {
268 fail(service, ": " + x);
269 }
270 }
271 while ((pending == null) || !pending.hasNext()) {
272 if (!configs.hasMoreElements()) {
273 return false;
274 }
275 pending = parse(service, (URL)configs.nextElement(), returned);
276 }
277 nextName = (String)pending.next();
278 return true;
279 }
280
281 public Object next() throws ServiceConfigurationError {
282 if (!hasNext()) {
283 throw new NoSuchElementException();
284 }
285 String cn = nextName;
286 nextName = null;
287 try {
288 return Class.forName(cn, true, loader).newInstance();
289 } catch (ClassNotFoundException x) {
290 fail(service,
291 "Provider " + cn + " not found");
292 } catch (Exception x) {
293 fail(service,
294 "Provider " + cn + " could not be instantiated: " + x,
295 x);
296 }
297 return null; /* This cannot happen */
298 }
299
300 public void remove() {
301 throw new UnsupportedOperationException();
302 }
303
304 }
305
306
307 /**
308 * Locates and incrementally instantiates the available providers of a
309 * given service using the given class loader.
310 *
311 * <p> This method transforms the name of the given service class into a
312 * provider-configuration filename as described above and then uses the
313 * <tt>getResources</tt> method of the given class loader to find all
314 * available files with that name. These files are then read and parsed to
315 * produce a list of provider-class names. The iterator that is returned
316 * uses the given class loader to lookup and then instantiate each element
317 * of the list.
318 *
319 * <p> Because it is possible for extensions to be installed into a running
320 * Java virtual machine, this method may return different results each time
321 * it is invoked. <p>
322 *
323 * @param service
324 * The service's abstract service class
325 *
326 * @param loader
327 * The class loader to be used to load provider-configuration files
328 * and instantiate provider classes, or <tt>null</tt> if the system
329 * class loader (or, failing that the bootstrap class loader) is to
330 * be used
331 *
332 * @return An <tt>Iterator</tt> that yields provider objects for the given
333 * service, in some arbitrary order. The iterator will throw a
334 * <tt>ServiceConfigurationError</tt> if a provider-configuration
335 * file violates the specified format or if a provider class cannot
336 * be found and instantiated.
337 *
338 * @throws ServiceConfigurationError
339 * If a provider-configuration file violates the specified format
340 * or names a provider class that cannot be found and instantiated
341 *
342 * @see #providers(java.lang.Class)
343 * @see #installedProviders(java.lang.Class)
344 */
345 public static Iterator providers(Class service, ClassLoader loader)
346 throws ServiceConfigurationError
347 {
348 return new LazyIterator(service, loader);
349 }
350
351
352 /**
353 * Locates and incrementally instantiates the available providers of a
354 * given service using the context class loader. This convenience method
355 * is equivalent to
356 *
357 * <pre>
358 * ClassLoader cl = Thread.currentThread().getContextClassLoader();
359 * return Service.providers(service, cl);
360 * </pre>
361 *
362 * @param service
363 * The service's abstract service class
364 *
365 * @return An <tt>Iterator</tt> that yields provider objects for the given
366 * service, in some arbitrary order. The iterator will throw a
367 * <tt>ServiceConfigurationError</tt> if a provider-configuration
368 * file violates the specified format or if a provider class cannot
369 * be found and instantiated.
370 *
371 * @throws ServiceConfigurationError
372 * If a provider-configuration file violates the specified format
373 * or names a provider class that cannot be found and instantiated
374 *
375 * @see #providers(java.lang.Class, java.lang.ClassLoader)
376 */
377 public static Iterator providers(Class service)
378 throws ServiceConfigurationError
379 {
380 ClassLoader cl = Thread.currentThread().getContextClassLoader();
381 return Service.providers(service, cl);
382 }
383
384
385 /**
386 * Locates and incrementally instantiates the available providers of a
387 * given service using the extension class loader. This convenience method
388 * simply locates the extension class loader, call it
389 * <tt>extClassLoader</tt>, and then does
390 *
391 * <pre>
392 * return Service.providers(service, extClassLoader);
393 * </pre>
394 *
395 * If the extension class loader cannot be found then the system class
396 * loader is used; if there is no system class loader then the bootstrap
397 * class loader is used.
398 *
399 * @param service
400 * The service's abstract service class
401 *
402 * @return An <tt>Iterator</tt> that yields provider objects for the given
403 * service, in some arbitrary order. The iterator will throw a
404 * <tt>ServiceConfigurationError</tt> if a provider-configuration
405 * file violates the specified format or if a provider class cannot
406 * be found and instantiated.
407 *
408 * @throws ServiceConfigurationError
409 * If a provider-configuration file violates the specified format
410 * or names a provider class that cannot be found and instantiated
411 *
412 * @see #providers(java.lang.Class, java.lang.ClassLoader)
413 */
414 public static Iterator installedProviders(Class service)
415 throws ServiceConfigurationError
416 {
417 ClassLoader cl = ClassLoader.getSystemClassLoader();
418 ClassLoader prev = null;
419 while (cl != null) {
420 prev = cl;
421 cl = cl.getParent();
422 }
423 return Service.providers(service, prev);
424 }
425
426 }