1 /*
2 * Copyright (c) 2005, 2012, 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.security.AccessController;
34 import java.security.AccessControlContext;
35 import java.security.PrivilegedAction;
36 import java.util.ArrayList;
37 import java.util.Enumeration;
38 import java.util.Iterator;
39 import java.util.List;
40 import java.util.NoSuchElementException;
41
42
43 /**
44 * A simple service-provider loading facility.
45 *
46 * <p> A <i>service</i> is a well-known set of interfaces and (usually
47 * abstract) classes. A <i>service provider</i> is a specific implementation
48 * of a service. The classes in a provider typically implement the interfaces
49 * and subclass the classes defined in the service itself. Service providers
50 * can be installed in an implementation of the Java platform in the form of
51 * extensions, that is, jar files placed into any of the usual extension
52 * directories. Providers can also be made available by adding them to the
53 * application's class path or by some other platform-specific means.
54 *
55 * <p> For the purpose of loading, a service is represented by a single type,
56 * that is, a single interface or abstract class. (A concrete class can be
57 * used, but this is not recommended.) A provider of a given service contains
58 * one or more concrete classes that extend this <i>service type</i> with data
59 * and code specific to the provider. The <i>provider class</i> is typically
60 * not the entire provider itself but rather a proxy which contains enough
61 * information to decide whether the provider is able to satisfy a particular
62 * request together with code that can create the actual provider on demand.
63 * The details of provider classes tend to be highly service-specific; no
64 * single class or interface could possibly unify them, so no such type is
65 * defined here. The only requirement enforced by this facility is that
66 * provider classes must have a zero-argument constructor so that they can be
67 * instantiated during loading.
68 *
69 * <p><a name="format"> A service provider is identified by placing a
70 * <i>provider-configuration file</i> in the resource directory
71 * <tt>META-INF/services</tt>. The file's name is the fully-qualified <a
72 * href="../lang/ClassLoader.html#name">binary name</a> of the service's type.
73 * The file contains a list of fully-qualified binary names of concrete
74 * provider classes, one per line. Space and tab characters surrounding each
75 * name, as well as blank lines, are ignored. The comment character is
76 * <tt>'#'</tt> (<tt>'\u0023'</tt>, <font size="-1">NUMBER SIGN</font>); on
77 * each line all characters following the first comment character are ignored.
78 * The file must be encoded in UTF-8.
79 *
80 * <p> If a particular concrete provider class is named in more than one
81 * configuration file, or is named in the same configuration file more than
82 * once, then the duplicates are ignored. The configuration file naming a
83 * particular provider need not be in the same jar file or other distribution
84 * unit as the provider itself. The provider must be accessible from the same
85 * class loader that was initially queried to locate the configuration file;
86 * note that this is not necessarily the class loader from which the file was
87 * actually loaded.
88 *
89 * <p> Providers are located and instantiated lazily, that is, on demand. A
90 * service loader maintains a cache of the providers that have been loaded so
91 * far. Each invocation of the {@link #iterator iterator} method returns an
92 * iterator that first yields all of the elements of the cache, in
93 * instantiation order, and then lazily locates and instantiates any remaining
94 * providers, adding each one to the cache in turn. The cache can be cleared
95 * via the {@link #reload reload} method.
96 *
97 * <p> Service loaders always execute in the security context of the caller.
98 * Trusted system code should typically invoke the methods in this class, and
99 * the methods of the iterators which they return, from within a privileged
100 * security context.
101 *
102 * <p> Instances of this class are not safe for use by multiple concurrent
103 * threads.
104 *
105 * <p> Unless otherwise specified, passing a <tt>null</tt> argument to any
106 * method in this class will cause a {@link NullPointerException} to be thrown.
107 *
108 *
109 * <p><span style="font-weight: bold; padding-right: 1em">Example</span>
110 * Suppose we have a service type <tt>com.example.CodecSet</tt> which is
111 * intended to represent sets of encoder/decoder pairs for some protocol. In
112 * this case it is an abstract class with two abstract methods:
113 *
114 * <blockquote><pre>
115 * public abstract Encoder getEncoder(String encodingName);
116 * public abstract Decoder getDecoder(String encodingName);</pre></blockquote>
117 *
118 * Each method returns an appropriate object or <tt>null</tt> if the provider
119 * does not support the given encoding. Typical providers support more than
120 * one encoding.
121 *
122 * <p> If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the
123 * <tt>CodecSet</tt> service then its jar file also contains a file named
124 *
125 * <blockquote><pre>
126 * META-INF/services/com.example.CodecSet</pre></blockquote>
127 *
128 * <p> This file contains the single line:
129 *
130 * <blockquote><pre>
131 * com.example.impl.StandardCodecs # Standard codecs</pre></blockquote>
132 *
133 * <p> The <tt>CodecSet</tt> class creates and saves a single service instance
134 * at initialization:
135 *
136 * <blockquote><pre>
137 * private static ServiceLoader<CodecSet> codecSetLoader
138 * = ServiceLoader.load(CodecSet.class);</pre></blockquote>
139 *
140 * <p> To locate an encoder for a given encoding name it defines a static
141 * factory method which iterates through the known and available providers,
142 * returning only when it has located a suitable encoder or has run out of
143 * providers.
144 *
145 * <blockquote><pre>
146 * public static Encoder getEncoder(String encodingName) {
147 * for (CodecSet cp : codecSetLoader) {
148 * Encoder enc = cp.getEncoder(encodingName);
149 * if (enc != null)
150 * return enc;
151 * }
152 * return null;
153 * }</pre></blockquote>
154 *
155 * <p> A <tt>getDecoder</tt> method is defined similarly.
156 *
157 *
158 * <p><span style="font-weight: bold; padding-right: 1em">Usage Note</span> If
159 * the class path of a class loader that is used for provider loading includes
160 * remote network URLs then those URLs will be dereferenced in the process of
161 * searching for provider-configuration files.
162 *
163 * <p> This activity is normal, although it may cause puzzling entries to be
164 * created in web-server logs. If a web server is not configured correctly,
165 * however, then this activity may cause the provider-loading algorithm to fail
166 * spuriously.
167 *
168 * <p> A web server should return an HTTP 404 (Not Found) response when a
169 * requested resource does not exist. Sometimes, however, web servers are
170 * erroneously configured to return an HTTP 200 (OK) response along with a
171 * helpful HTML error page in such cases. This will cause a {@link
172 * ServiceConfigurationError} to be thrown when this class attempts to parse
173 * the HTML page as a provider-configuration file. The best solution to this
174 * problem is to fix the misconfigured web server to return the correct
175 * response code (HTTP 404) along with the HTML error page.
176 *
177 * @param <S>
178 * The type of the service to be loaded by this loader
179 *
180 * @author Mark Reinhold
181 * @since 1.6
182 */
183
184 public final class ServiceLoader<S>
185 implements Iterable<S>
186 {
187
188 private static final String PREFIX = "META-INF/services/";
189
190 // The class or interface representing the service being loaded
191 private final Class<S> service;
192
193 // The class loader used to locate, load, and instantiate providers
194 private final ClassLoader loader;
195
196 // The access control context taken when the ServiceLoader is created
197 private final AccessControlContext acc;
198
199 // Cached providers, in instantiation order
200 private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
201
202 // The current lazy-lookup iterator
203 private LazyIterator lookupIterator;
204
205 /**
206 * Clear this loader's provider cache so that all providers will be
207 * reloaded.
208 *
209 * <p> After invoking this method, subsequent invocations of the {@link
210 * #iterator() iterator} method will lazily look up and instantiate
211 * providers from scratch, just as is done by a newly-created loader.
212 *
213 * <p> This method is intended for use in situations in which new providers
214 * can be installed into a running Java virtual machine.
215 */
216 public void reload() {
217 providers.clear();
218 lookupIterator = new LazyIterator(service, loader);
219 }
220
221 private ServiceLoader(Class<S> svc, ClassLoader cl) {
222 service = Objects.requireNonNull(svc, "Service interface cannot be null");
223 loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
224 acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
225 reload();
226 }
227
228 private static void fail(Class<?> service, String msg, Throwable cause)
229 throws ServiceConfigurationError
230 {
231 throw new ServiceConfigurationError(service.getName() + ": " + msg,
232 cause);
233 }
234
235 private static void fail(Class<?> service, String msg)
236 throws ServiceConfigurationError
237 {
238 throw new ServiceConfigurationError(service.getName() + ": " + msg);
239 }
240
241 private static void fail(Class<?> service, URL u, int line, String msg)
242 throws ServiceConfigurationError
243 {
244 fail(service, u + ":" + line + ": " + msg);
245 }
246
247 // Parse a single line from the given configuration file, adding the name
248 // on the line to the names list.
249 //
250 private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,
251 List<String> names)
252 throws IOException, ServiceConfigurationError
253 {
254 String ln = r.readLine();
255 if (ln == null) {
256 return -1;
257 }
258 int ci = ln.indexOf('#');
259 if (ci >= 0) ln = ln.substring(0, ci);
260 ln = ln.trim();
261 int n = ln.length();
262 if (n != 0) {
263 if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
264 fail(service, u, lc, "Illegal configuration-file syntax");
265 int cp = ln.codePointAt(0);
266 if (!Character.isJavaIdentifierStart(cp))
267 fail(service, u, lc, "Illegal provider-class name: " + ln);
268 for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
269 cp = ln.codePointAt(i);
270 if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
271 fail(service, u, lc, "Illegal provider-class name: " + ln);
272 }
273 if (!providers.containsKey(ln) && !names.contains(ln))
274 names.add(ln);
275 }
276 return lc + 1;
277 }
278
279 // Parse the content of the given URL as a provider-configuration file.
280 //
281 // @param service
282 // The service type for which providers are being sought;
283 // used to construct error detail strings
284 //
285 // @param u
286 // The URL naming the configuration file to be parsed
287 //
288 // @return A (possibly empty) iterator that will yield the provider-class
289 // names in the given configuration file that are not yet members
290 // of the returned set
291 //
292 // @throws ServiceConfigurationError
293 // If an I/O error occurs while reading from the given URL, or
294 // if a configuration-file format error is detected
295 //
296 private Iterator<String> parse(Class<?> service, URL u)
297 throws ServiceConfigurationError
298 {
299 InputStream in = null;
300 BufferedReader r = null;
301 ArrayList<String> names = new ArrayList<>();
302 try {
303 in = u.openStream();
304 r = new BufferedReader(new InputStreamReader(in, "utf-8"));
305 int lc = 1;
306 while ((lc = parseLine(service, u, r, lc, names)) >= 0);
307 } catch (IOException x) {
308 fail(service, "Error reading configuration file", x);
309 } finally {
310 try {
311 if (r != null) r.close();
312 if (in != null) in.close();
313 } catch (IOException y) {
314 fail(service, "Error closing configuration file", y);
315 }
316 }
317 return names.iterator();
318 }
319
320 // Private inner class implementing fully-lazy provider lookup
321 //
322 private class LazyIterator
323 implements Iterator<S>
324 {
325
326 Class<S> service;
327 ClassLoader loader;
328 Enumeration<URL> configs = null;
329 Iterator<String> pending = null;
330 String nextName = null;
331
332 private LazyIterator(Class<S> service, ClassLoader loader) {
333 this.service = service;
334 this.loader = loader;
335 }
336
337 private boolean hasNextService() {
338 if (nextName != null) {
339 return true;
340 }
341 if (configs == null) {
342 try {
343 String fullName = PREFIX + service.getName();
344 if (loader == null)
345 configs = ClassLoader.getSystemResources(fullName);
346 else
347 configs = loader.getResources(fullName);
348 } catch (IOException x) {
349 fail(service, "Error locating configuration files", x);
350 }
351 }
352 while ((pending == null) || !pending.hasNext()) {
353 if (!configs.hasMoreElements()) {
354 return false;
355 }
356 pending = parse(service, configs.nextElement());
357 }
358 nextName = pending.next();
359 return true;
360 }
361
362 private S nextService() {
363 if (!hasNextService())
364 throw new NoSuchElementException();
365 String cn = nextName;
366 nextName = null;
367 Class<?> c = null;
368 try {
369 c = Class.forName(cn, false, loader);
370 } catch (ClassNotFoundException x) {
371 fail(service,
372 "Provider " + cn + " not found");
373 }
374 if (!service.isAssignableFrom(c)) {
375 fail(service,
376 "Provider " + cn + " not a subtype");
377 }
378 try {
379 S p = service.cast(c.newInstance());
380 providers.put(cn, p);
381 return p;
382 } catch (Throwable x) {
383 fail(service,
384 "Provider " + cn + " could not be instantiated: " + x,
385 x);
386 }
387 throw new Error(); // This cannot happen
388 }
389
390 public boolean hasNext() {
391 if (acc == null) {
392 return hasNextService();
393 } else {
394 PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
395 public Boolean run() { return hasNextService(); }
396 };
397 return AccessController.doPrivileged(action, acc);
398 }
399 }
400
401 public S next() {
402 if (acc == null) {
403 return nextService();
404 } else {
405 PrivilegedAction<S> action = new PrivilegedAction<S>() {
406 public S run() { return nextService(); }
407 };
408 return AccessController.doPrivileged(action, acc);
409 }
410 }
411
412 public void remove() {
413 throw new UnsupportedOperationException();
414 }
415
416 }
417
418 /**
419 * Lazily loads the available providers of this loader's service.
420 *
421 * <p> The iterator returned by this method first yields all of the
422 * elements of the provider cache, in instantiation order. It then lazily
423 * loads and instantiates any remaining providers, adding each one to the
424 * cache in turn.
425 *
426 * <p> To achieve laziness the actual work of parsing the available
427 * provider-configuration files and instantiating providers must be done by
428 * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and
429 * {@link java.util.Iterator#next next} methods can therefore throw a
430 * {@link ServiceConfigurationError} if a provider-configuration file
431 * violates the specified format, or if it names a provider class that
432 * cannot be found and instantiated, or if the result of instantiating the
433 * class is not assignable to the service type, or if any other kind of
434 * exception or error is thrown as the next provider is located and
435 * instantiated. To write robust code it is only necessary to catch {@link
436 * ServiceConfigurationError} when using a service iterator.
437 *
438 * <p> If such an error is thrown then subsequent invocations of the
439 * iterator will make a best effort to locate and instantiate the next
440 * available provider, but in general such recovery cannot be guaranteed.
441 *
442 * <blockquote style="font-size: smaller; line-height: 1.2"><span
443 * style="padding-right: 1em; font-weight: bold">Design Note</span>
444 * Throwing an error in these cases may seem extreme. The rationale for
445 * this behavior is that a malformed provider-configuration file, like a
446 * malformed class file, indicates a serious problem with the way the Java
447 * virtual machine is configured or is being used. As such it is
448 * preferable to throw an error rather than try to recover or, even worse,
449 * fail silently.</blockquote>
450 *
451 * <p> The iterator returned by this method does not support removal.
452 * Invoking its {@link java.util.Iterator#remove() remove} method will
453 * cause an {@link UnsupportedOperationException} to be thrown.
454 *
455 * @return An iterator that lazily loads providers for this loader's
456 * service
457 */
458 public Iterator<S> iterator() {
459 return new Iterator<S>() {
460
461 Iterator<Map.Entry<String,S>> knownProviders
462 = providers.entrySet().iterator();
463
464 public boolean hasNext() {
465 if (knownProviders.hasNext())
466 return true;
467 return lookupIterator.hasNext();
468 }
469
470 public S next() {
471 if (knownProviders.hasNext())
472 return knownProviders.next().getValue();
473 return lookupIterator.next();
474 }
475
476 public void remove() {
477 throw new UnsupportedOperationException();
478 }
479
480 };
481 }
482
483 /**
484 * Creates a new service loader for the given service type and class
485 * loader.
486 *
487 * @param service
488 * The interface or abstract class representing the service
489 *
490 * @param loader
491 * The class loader to be used to load provider-configuration files
492 * and provider classes, or <tt>null</tt> if the system class
493 * loader (or, failing that, the bootstrap class loader) is to be
494 * used
495 *
496 * @return A new service loader
497 */
498 public static <S> ServiceLoader<S> load(Class<S> service,
499 ClassLoader loader)
500 {
501 return new ServiceLoader<>(service, loader);
502 }
503
504 /**
505 * Creates a new service loader for the given service type, using the
506 * current thread's {@linkplain java.lang.Thread#getContextClassLoader
507 * context class loader}.
508 *
509 * <p> An invocation of this convenience method of the form
510 *
511 * <blockquote><pre>
512 * ServiceLoader.load(<i>service</i>)</pre></blockquote>
513 *
514 * is equivalent to
515 *
516 * <blockquote><pre>
517 * ServiceLoader.load(<i>service</i>,
518 * Thread.currentThread().getContextClassLoader())</pre></blockquote>
519 *
520 * @param service
521 * The interface or abstract class representing the service
522 *
523 * @return A new service loader
524 */
525 public static <S> ServiceLoader<S> load(Class<S> service) {
526 ClassLoader cl = Thread.currentThread().getContextClassLoader();
527 return ServiceLoader.load(service, cl);
528 }
529
530 /**
531 * Creates a new service loader for the given service type, using the
532 * extension class loader.
533 *
534 * <p> This convenience method simply locates the extension class loader,
535 * call it <tt><i>extClassLoader</i></tt>, and then returns
536 *
537 * <blockquote><pre>
538 * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote>
539 *
540 * <p> If the extension class loader cannot be found then the system class
541 * loader is used; if there is no system class loader then the bootstrap
542 * class loader is used.
543 *
544 * <p> This method is intended for use when only installed providers are
545 * desired. The resulting service will only find and load providers that
546 * have been installed into the current Java virtual machine; providers on
547 * the application's class path will be ignored.
548 *
549 * @param service
550 * The interface or abstract class representing the service
551 *
552 * @return A new service loader
553 */
554 public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
555 ClassLoader cl = ClassLoader.getSystemClassLoader();
556 ClassLoader prev = null;
557 while (cl != null) {
558 prev = cl;
559 cl = cl.getParent();
560 }
561 return ServiceLoader.load(service, prev);
562 }
563
564 /**
565 * Returns a string describing this service.
566 *
567 * @return A descriptive string
568 */
569 public String toString() {
570 return "java.util.ServiceLoader[" + service.getName() + "]";
571 }
572
573 }