1 /*
2 * Copyright (c) 2005, 2006, 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 java.util;
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
39
40 /**
41 * A simple service-provider loading facility.
42 *
43 * <p> A <i>service</i> is a well-known set of interfaces and (usually
44 * abstract) classes. A <i>service provider</i> is a specific implementation
45 * of a service. The classes in a provider typically implement the interfaces
46 * and subclass the classes defined in the service itself. Service providers
47 * can be installed in an implementation of the Java platform in the form of
48 * extensions, that is, jar files placed into any of the usual extension
49 * directories. Providers can also be made available by adding them to the
50 * application's class path or by some other platform-specific means.
51 *
52 * <p> For the purpose of loading, a service is represented by a single type,
53 * that is, a single interface or abstract class. (A concrete class can be
54 * used, but this is not recommended.) A provider of a given service contains
55 * one or more concrete classes that extend this <i>service type</i> with data
56 * and code specific to the provider. The <i>provider class</i> is typically
57 * not the entire provider itself but rather a proxy which contains enough
58 * information to decide whether the provider is able to satisfy a particular
59 * request together with code that can create the actual provider on demand.
60 * The details of provider classes tend to be highly service-specific; no
61 * single class or interface could possibly unify them, so no such type is
62 * defined here. The only requirement enforced by this facility is that
63 * provider classes must have a zero-argument constructor so that they can be
64 * instantiated during loading.
65 *
66 * <p><a name="format"> A service provider is identified by placing a
67 * <i>provider-configuration file</i> in the resource directory
68 * <tt>META-INF/services</tt>. The file's name is the fully-qualified <a
69 * href="../lang/ClassLoader.html#name">binary name</a> of the service's type.
70 * The file contains a list of fully-qualified binary names of concrete
71 * provider classes, one per line. Space and tab characters surrounding each
72 * name, as well as blank lines, are ignored. The comment character is
73 * <tt>'#'</tt> (<tt>'\u0023'</tt>, <font size="-1">NUMBER SIGN</font>); on
74 * each line all characters following the first comment character are ignored.
75 * The file must be encoded in UTF-8.
76 *
77 * <p> If a particular concrete provider class is named in more than one
78 * configuration file, or is named in the same configuration file more than
79 * once, then the duplicates are ignored. The configuration file naming a
80 * particular provider need not be in the same jar file or other distribution
81 * unit as the provider itself. The provider must be accessible from the same
82 * class loader that was initially queried to locate the configuration file;
83 * note that this is not necessarily the class loader from which the file was
84 * actually loaded.
85 *
86 * <p> Providers are located and instantiated lazily, that is, on demand. A
87 * service loader maintains a cache of the providers that have been loaded so
88 * far. Each invocation of the {@link #iterator iterator} method returns an
89 * iterator that first yields all of the elements of the cache, in
90 * instantiation order, and then lazily locates and instantiates any remaining
91 * providers, adding each one to the cache in turn. The cache can be cleared
92 * via the {@link #reload reload} method.
93 *
94 * <p> Service loaders always execute in the security context of the caller.
95 * Trusted system code should typically invoke the methods in this class, and
96 * the methods of the iterators which they return, from within a privileged
97 * security context.
98 *
99 * <p> Instances of this class are not safe for use by multiple concurrent
100 * threads.
101 *
102 * <p> Unless otherwise specified, passing a <tt>null</tt> argument to any
103 * method in this class will cause a {@link NullPointerException} to be thrown.
104 *
105 *
106 * <p><span style="font-weight: bold; padding-right: 1em">Example</span>
107 * Suppose we have a service type <tt>com.example.CodecSet</tt> which is
108 * intended to represent sets of encoder/decoder pairs for some protocol. In
109 * this case it is an abstract class with two abstract methods:
110 *
111 * <blockquote><pre>
112 * public abstract Encoder getEncoder(String encodingName);
113 * public abstract Decoder getDecoder(String encodingName);</pre></blockquote>
114 *
115 * Each method returns an appropriate object or <tt>null</tt> if the provider
116 * does not support the given encoding. Typical providers support more than
117 * one encoding.
118 *
119 * <p> If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the
120 * <tt>CodecSet</tt> service then its jar file also contains a file named
121 *
122 * <blockquote><pre>
123 * META-INF/services/com.example.CodecSet</pre></blockquote>
124 *
125 * <p> This file contains the single line:
126 *
127 * <blockquote><pre>
128 * com.example.impl.StandardCodecs # Standard codecs</pre></blockquote>
129 *
130 * <p> The <tt>CodecSet</tt> class creates and saves a single service instance
131 * at initialization:
132 *
133 * <blockquote><pre>
134 * private static ServiceLoader<CodecSet> codecSetLoader
135 * = ServiceLoader.load(CodecSet.class);</pre></blockquote>
136 *
137 * <p> To locate an encoder for a given encoding name it defines a static
138 * factory method which iterates through the known and available providers,
139 * returning only when it has located a suitable encoder or has run out of
140 * providers.
141 *
142 * <blockquote><pre>
143 * public static Encoder getEncoder(String encodingName) {
144 * for (CodecSet cp : codecSetLoader) {
145 * Encoder enc = cp.getEncoder(encodingName);
146 * if (enc != null)
147 * return enc;
148 * }
149 * return null;
150 * }</pre></blockquote>
151 *
152 * <p> A <tt>getDecoder</tt> method is defined similarly.
153 *
154 *
155 * <p><span style="font-weight: bold; padding-right: 1em">Usage Note</span> If
156 * the class path of a class loader that is used for provider loading includes
157 * remote network URLs then those URLs will be dereferenced in the process of
158 * searching for provider-configuration files.
159 *
160 * <p> This activity is normal, although it may cause puzzling entries to be
161 * created in web-server logs. If a web server is not configured correctly,
162 * however, then this activity may cause the provider-loading algorithm to fail
163 * spuriously.
164 *
165 * <p> A web server should return an HTTP 404 (Not Found) response when a
166 * requested resource does not exist. Sometimes, however, web servers are
167 * erroneously configured to return an HTTP 200 (OK) response along with a
168 * helpful HTML error page in such cases. This will cause a {@link
169 * ServiceConfigurationError} to be thrown when this class attempts to parse
170 * the HTML page as a provider-configuration file. The best solution to this
171 * problem is to fix the misconfigured web server to return the correct
172 * response code (HTTP 404) along with the HTML error page.
173 *
174 * @param <S>
175 * The type of the service to be loaded by this loader
176 *
177 * @author Mark Reinhold
178 * @since 1.6
179 */
180
181 public final class ServiceLoader<S>
182 implements Iterable<S>
183 {
184
185 private static final String PREFIX = "META-INF/services/";
186
187 // The class or interface representing the service being loaded
188 private Class<S> service;
189
190 // The class loader used to locate, load, and instantiate providers
191 private ClassLoader loader;
192
193 // Cached providers, in instantiation order
194 private LinkedHashMap<String,S> providers = new LinkedHashMap<String,S>();
195
196 // The current lazy-lookup iterator
197 private LazyIterator lookupIterator;
198
199 /**
200 * Clear this loader's provider cache so that all providers will be
201 * reloaded.
202 *
203 * <p> After invoking this method, subsequent invocations of the {@link
204 * #iterator() iterator} method will lazily look up and instantiate
205 * providers from scratch, just as is done by a newly-created loader.
206 *
207 * <p> This method is intended for use in situations in which new providers
208 * can be installed into a running Java virtual machine.
209 */
210 public void reload() {
211 providers.clear();
212 lookupIterator = new LazyIterator(service, loader);
213 }
214
215 private ServiceLoader(Class<S> svc, ClassLoader cl) {
216 service = svc;
217 loader = cl;
218 reload();
219 }
220
221 private static void fail(Class service, String msg, Throwable cause)
222 throws ServiceConfigurationError
223 {
224 throw new ServiceConfigurationError(service.getName() + ": " + msg,
225 cause);
226 }
227
228 private static void fail(Class service, String msg)
229 throws ServiceConfigurationError
230 {
231 throw new ServiceConfigurationError(service.getName() + ": " + msg);
232 }
233
234 private static void fail(Class service, URL u, int line, String msg)
235 throws ServiceConfigurationError
236 {
237 fail(service, u + ":" + line + ": " + msg);
238 }
239
240 // Parse a single line from the given configuration file, adding the name
241 // on the line to the names list.
242 //
243 private int parseLine(Class service, URL u, BufferedReader r, int lc,
244 List<String> names)
245 throws IOException, ServiceConfigurationError
246 {
247 String ln = r.readLine();
248 if (ln == null) {
249 return -1;
250 }
251 int ci = ln.indexOf('#');
252 if (ci >= 0) ln = ln.substring(0, ci);
253 ln = ln.trim();
254 int n = ln.length();
255 if (n != 0) {
256 if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
257 fail(service, u, lc, "Illegal configuration-file syntax");
258 int cp = ln.codePointAt(0);
259 if (!Character.isJavaIdentifierStart(cp))
260 fail(service, u, lc, "Illegal provider-class name: " + ln);
261 for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
262 cp = ln.codePointAt(i);
263 if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
264 fail(service, u, lc, "Illegal provider-class name: " + ln);
265 }
266 if (!providers.containsKey(ln) && !names.contains(ln))
267 names.add(ln);
268 }
269 return lc + 1;
270 }
271
272 // Parse the content of the given URL as a provider-configuration file.
273 //
274 // @param service
275 // The service type for which providers are being sought;
276 // used to construct error detail strings
277 //
278 // @param u
279 // The URL naming the configuration file to be parsed
280 //
281 // @return A (possibly empty) iterator that will yield the provider-class
282 // names in the given configuration file that are not yet members
283 // of the returned set
284 //
285 // @throws ServiceConfigurationError
286 // If an I/O error occurs while reading from the given URL, or
287 // if a configuration-file format error is detected
288 //
289 private Iterator<String> parse(Class service, URL u)
290 throws ServiceConfigurationError
291 {
292 InputStream in = null;
293 BufferedReader r = null;
294 ArrayList<String> names = new ArrayList<String>();
295 try {
296 in = u.openStream();
297 r = new BufferedReader(new InputStreamReader(in, "utf-8"));
298 int lc = 1;
299 while ((lc = parseLine(service, u, r, lc, names)) >= 0);
300 } catch (IOException x) {
301 fail(service, "Error reading configuration file", x);
302 } finally {
303 try {
304 if (r != null) r.close();
305 if (in != null) in.close();
306 } catch (IOException y) {
307 fail(service, "Error closing configuration file", y);
308 }
309 }
310 return names.iterator();
311 }
312
313 // Private inner class implementing fully-lazy provider lookup
314 //
315 private class LazyIterator
316 implements Iterator<S>
317 {
318
319 Class<S> service;
320 ClassLoader loader;
321 Enumeration<URL> configs = null;
322 Iterator<String> pending = null;
323 String nextName = null;
324
325 private LazyIterator(Class<S> service, ClassLoader loader) {
326 this.service = service;
327 this.loader = loader;
328 }
329
330 public boolean hasNext() {
331 if (nextName != null) {
332 return true;
333 }
334 if (configs == null) {
335 try {
336 String fullName = PREFIX + service.getName();
337 if (loader == null)
338 configs = ClassLoader.getSystemResources(fullName);
339 else
340 configs = loader.getResources(fullName);
341 } catch (IOException x) {
342 fail(service, "Error locating configuration files", x);
343 }
344 }
345 while ((pending == null) || !pending.hasNext()) {
346 if (!configs.hasMoreElements()) {
347 return false;
348 }
349 pending = parse(service, configs.nextElement());
350 }
351 nextName = pending.next();
352 return true;
353 }
354
355 public S next() {
356 if (!hasNext()) {
357 throw new NoSuchElementException();
358 }
359 String cn = nextName;
360 nextName = null;
361 try {
362 S p = service.cast(Class.forName(cn, true, loader)
363 .newInstance());
364 providers.put(cn, p);
365 return p;
366 } catch (ClassNotFoundException x) {
367 fail(service,
368 "Provider " + cn + " not found");
369 } catch (Throwable x) {
370 fail(service,
371 "Provider " + cn + " could not be instantiated: " + x,
372 x);
373 }
374 throw new Error(); // This cannot happen
375 }
376
377 public void remove() {
378 throw new UnsupportedOperationException();
379 }
380
381 }
382
383 /**
384 * Lazily loads the available providers of this loader's service.
385 *
386 * <p> The iterator returned by this method first yields all of the
387 * elements of the provider cache, in instantiation order. It then lazily
388 * loads and instantiates any remaining providers, adding each one to the
389 * cache in turn.
390 *
391 * <p> To achieve laziness the actual work of parsing the available
392 * provider-configuration files and instantiating providers must be done by
393 * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and
394 * {@link java.util.Iterator#next next} methods can therefore throw a
395 * {@link ServiceConfigurationError} if a provider-configuration file
396 * violates the specified format, or if it names a provider class that
397 * cannot be found and instantiated, or if the result of instantiating the
398 * class is not assignable to the service type, or if any other kind of
399 * exception or error is thrown as the next provider is located and
400 * instantiated. To write robust code it is only necessary to catch {@link
401 * ServiceConfigurationError} when using a service iterator.
402 *
403 * <p> If such an error is thrown then subsequent invocations of the
404 * iterator will make a best effort to locate and instantiate the next
405 * available provider, but in general such recovery cannot be guaranteed.
406 *
407 * <blockquote style="font-size: smaller; line-height: 1.2"><span
408 * style="padding-right: 1em; font-weight: bold">Design Note</span>
409 * Throwing an error in these cases may seem extreme. The rationale for
410 * this behavior is that a malformed provider-configuration file, like a
411 * malformed class file, indicates a serious problem with the way the Java
412 * virtual machine is configured or is being used. As such it is
413 * preferable to throw an error rather than try to recover or, even worse,
414 * fail silently.</blockquote>
415 *
416 * <p> The iterator returned by this method does not support removal.
417 * Invoking its {@link java.util.Iterator#remove() remove} method will
418 * cause an {@link UnsupportedOperationException} to be thrown.
419 *
420 * @return An iterator that lazily loads providers for this loader's
421 * service
422 */
423 public Iterator<S> iterator() {
424 return new Iterator<S>() {
425
426 Iterator<Map.Entry<String,S>> knownProviders
427 = providers.entrySet().iterator();
428
429 public boolean hasNext() {
430 if (knownProviders.hasNext())
431 return true;
432 return lookupIterator.hasNext();
433 }
434
435 public S next() {
436 if (knownProviders.hasNext())
437 return knownProviders.next().getValue();
438 return lookupIterator.next();
439 }
440
441 public void remove() {
442 throw new UnsupportedOperationException();
443 }
444
445 };
446 }
447
448 /**
449 * Creates a new service loader for the given service type and class
450 * loader.
451 *
452 * @param service
453 * The interface or abstract class representing the service
454 *
455 * @param loader
456 * The class loader to be used to load provider-configuration files
457 * and provider classes, or <tt>null</tt> if the system class
458 * loader (or, failing that, the bootstrap class loader) is to be
459 * used
460 *
461 * @return A new service loader
462 */
463 public static <S> ServiceLoader<S> load(Class<S> service,
464 ClassLoader loader)
465 {
466 return new ServiceLoader<S>(service, loader);
467 }
468
469 /**
470 * Creates a new service loader for the given service type, using the
471 * current thread's {@linkplain java.lang.Thread#getContextClassLoader
472 * context class loader}.
473 *
474 * <p> An invocation of this convenience method of the form
475 *
476 * <blockquote><pre>
477 * ServiceLoader.load(<i>service</i>)</pre></blockquote>
478 *
479 * is equivalent to
480 *
481 * <blockquote><pre>
482 * ServiceLoader.load(<i>service</i>,
483 * Thread.currentThread().getContextClassLoader())</pre></blockquote>
484 *
485 * @param service
486 * The interface or abstract class representing the service
487 *
488 * @return A new service loader
489 */
490 public static <S> ServiceLoader<S> load(Class<S> service) {
491 ClassLoader cl = Thread.currentThread().getContextClassLoader();
492 return ServiceLoader.load(service, cl);
493 }
494
495 /**
496 * Creates a new service loader for the given service type, using the
497 * extension class loader.
498 *
499 * <p> This convenience method simply locates the extension class loader,
500 * call it <tt><i>extClassLoader</i></tt>, and then returns
501 *
502 * <blockquote><pre>
503 * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote>
504 *
505 * <p> If the extension class loader cannot be found then the system class
506 * loader is used; if there is no system class loader then the bootstrap
507 * class loader is used.
508 *
509 * <p> This method is intended for use when only installed providers are
510 * desired. The resulting service will only find and load providers that
511 * have been installed into the current Java virtual machine; providers on
512 * the application's class path will be ignored.
513 *
514 * @param service
515 * The interface or abstract class representing the service
516 *
517 * @return A new service loader
518 */
519 public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
520 ClassLoader cl = ClassLoader.getSystemClassLoader();
521 ClassLoader prev = null;
522 while (cl != null) {
523 prev = cl;
524 cl = cl.getParent();
525 }
526 return ServiceLoader.load(service, prev);
527 }
528
529 /**
530 * Returns a string describing this service.
531 *
532 * @return A descriptive string
533 */
534 public String toString() {
535 return "java.util.ServiceLoader[" + service.getName() + "]";
536 }
537
538 }