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>.</a> 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>,
77 * <font style="font-size:smaller;">NUMBER SIGN</font>); on
78 * each line all characters following the first comment character are ignored.
79 * The file must be encoded in UTF-8.
80 *
81 * <p> If a particular concrete provider class is named in more than one
82 * configuration file, or is named in the same configuration file more than
83 * once, then the duplicates are ignored. The configuration file naming a
84 * particular provider need not be in the same jar file or other distribution
85 * unit as the provider itself. The provider must be accessible from the same
86 * class loader that was initially queried to locate the configuration file;
87 * note that this is not necessarily the class loader from which the file was
88 * actually loaded.
89 *
90 * <p> Providers are located and instantiated lazily, that is, on demand. A
91 * service loader maintains a cache of the providers that have been loaded so
92 * far. Each invocation of the {@link #iterator iterator} method returns an
93 * iterator that first yields all of the elements of the cache, in
94 * instantiation order, and then lazily locates and instantiates any remaining
95 * providers, adding each one to the cache in turn. The cache can be cleared
96 * via the {@link #reload reload} method.
97 *
98 * <p> Service loaders always execute in the security context of the caller.
99 * Trusted system code should typically invoke the methods in this class, and
100 * the methods of the iterators which they return, from within a privileged
101 * security context.
102 *
103 * <p> Instances of this class are not safe for use by multiple concurrent
104 * threads.
105 *
106 * <p> Unless otherwise specified, passing a <tt>null</tt> argument to any
107 * method in this class will cause a {@link NullPointerException} to be thrown.
108 *
109 *
110 * <p><span style="font-weight: bold; padding-right: 1em">Example</span>
111 * Suppose we have a service type <tt>com.example.CodecSet</tt> which is
112 * intended to represent sets of encoder/decoder pairs for some protocol. In
113 * this case it is an abstract class with two abstract methods:
114 *
115 * <blockquote><pre>
116 * public abstract Encoder getEncoder(String encodingName);
117 * public abstract Decoder getDecoder(String encodingName);</pre></blockquote>
118 *
119 * Each method returns an appropriate object or <tt>null</tt> if the provider
120 * does not support the given encoding. Typical providers support more than
121 * one encoding.
122 *
123 * <p> If <tt>com.example.impl.StandardCodecs</tt> is an implementation of the
124 * <tt>CodecSet</tt> service then its jar file also contains a file named
125 *
126 * <blockquote><pre>
127 * META-INF/services/com.example.CodecSet</pre></blockquote>
128 *
129 * <p> This file contains the single line:
130 *
131 * <blockquote><pre>
132 * com.example.impl.StandardCodecs # Standard codecs</pre></blockquote>
133 *
134 * <p> The <tt>CodecSet</tt> class creates and saves a single service instance
135 * at initialization:
136 *
137 * <blockquote><pre>
138 * private static ServiceLoader<CodecSet> codecSetLoader
139 * = ServiceLoader.load(CodecSet.class);</pre></blockquote>
140 *
141 * <p> To locate an encoder for a given encoding name it defines a static
142 * factory method which iterates through the known and available providers,
143 * returning only when it has located a suitable encoder or has run out of
144 * providers.
145 *
146 * <blockquote><pre>
147 * public static Encoder getEncoder(String encodingName) {
148 * for (CodecSet cp : codecSetLoader) {
149 * Encoder enc = cp.getEncoder(encodingName);
150 * if (enc != null)
151 * return enc;
152 * }
153 * return null;
154 * }</pre></blockquote>
155 *
156 * <p> A <tt>getDecoder</tt> method is defined similarly.
157 *
158 *
159 * <p><span style="font-weight: bold; padding-right: 1em">Usage Note</span> If
160 * the class path of a class loader that is used for provider loading includes
161 * remote network URLs then those URLs will be dereferenced in the process of
162 * searching for provider-configuration files.
163 *
164 * <p> This activity is normal, although it may cause puzzling entries to be
165 * created in web-server logs. If a web server is not configured correctly,
166 * however, then this activity may cause the provider-loading algorithm to fail
167 * spuriously.
168 *
169 * <p> A web server should return an HTTP 404 (Not Found) response when a
170 * requested resource does not exist. Sometimes, however, web servers are
171 * erroneously configured to return an HTTP 200 (OK) response along with a
172 * helpful HTML error page in such cases. This will cause a {@link
173 * ServiceConfigurationError} to be thrown when this class attempts to parse
174 * the HTML page as a provider-configuration file. The best solution to this
175 * problem is to fix the misconfigured web server to return the correct
176 * response code (HTTP 404) along with the HTML error page.
177 *
178 * @param <S>
179 * The type of the service to be loaded by this loader
180 *
181 * @author Mark Reinhold
182 * @since 1.6
183 */
184
185 public final class ServiceLoader<S>
186 implements Iterable<S>
187 {
188
189 private static final String PREFIX = "META-INF/services/";
190
191 // The class or interface representing the service being loaded
192 private final Class<S> service;
193
194 // The class loader used to locate, load, and instantiate providers
195 private final ClassLoader loader;
196
197 // The access control context taken when the ServiceLoader is created
198 private final AccessControlContext acc;
199
200 // Cached providers, in instantiation order
201 private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
202
203 // The current lazy-lookup iterator
204 private LazyIterator lookupIterator;
205
206 /**
207 * Clear this loader's provider cache so that all providers will be
208 * reloaded.
209 *
210 * <p> After invoking this method, subsequent invocations of the {@link
211 * #iterator() iterator} method will lazily look up and instantiate
212 * providers from scratch, just as is done by a newly-created loader.
213 *
214 * <p> This method is intended for use in situations in which new providers
215 * can be installed into a running Java virtual machine.
216 */
217 public void reload() {
218 providers.clear();
219 lookupIterator = new LazyIterator(service, loader);
220 }
221
222 private ServiceLoader(Class<S> svc, ClassLoader cl) {
223 service = Objects.requireNonNull(svc, "Service interface cannot be null");
224 loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
225 acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
226 reload();
227 }
228
229 private static void fail(Class<?> service, String msg, Throwable cause)
230 throws ServiceConfigurationError
231 {
232 throw new ServiceConfigurationError(service.getName() + ": " + msg,
233 cause);
234 }
235
236 private static void fail(Class<?> service, String msg)
237 throws ServiceConfigurationError
238 {
239 throw new ServiceConfigurationError(service.getName() + ": " + msg);
240 }
241
242 private static void fail(Class<?> service, URL u, int line, String msg)
243 throws ServiceConfigurationError
244 {
245 fail(service, u + ":" + line + ": " + msg);
246 }
247
248 // Parse a single line from the given configuration file, adding the name
249 // on the line to the names list.
250 //
251 private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,
252 List<String> names)
253 throws IOException, ServiceConfigurationError
254 {
255 String ln = r.readLine();
256 if (ln == null) {
257 return -1;
258 }
259 int ci = ln.indexOf('#');
260 if (ci >= 0) ln = ln.substring(0, ci);
261 ln = ln.trim();
262 int n = ln.length();
263 if (n != 0) {
264 if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0))
265 fail(service, u, lc, "Illegal configuration-file syntax");
266 int cp = ln.codePointAt(0);
267 if (!Character.isJavaIdentifierStart(cp))
268 fail(service, u, lc, "Illegal provider-class name: " + ln);
269 for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) {
270 cp = ln.codePointAt(i);
271 if (!Character.isJavaIdentifierPart(cp) && (cp != '.'))
272 fail(service, u, lc, "Illegal provider-class name: " + ln);
273 }
274 if (!providers.containsKey(ln) && !names.contains(ln))
275 names.add(ln);
276 }
277 return lc + 1;
278 }
279
280 // Parse the content of the given URL as a provider-configuration file.
281 //
282 // @param service
283 // The service type for which providers are being sought;
284 // used to construct error detail strings
285 //
286 // @param u
287 // The URL naming the configuration file to be parsed
288 //
289 // @return A (possibly empty) iterator that will yield the provider-class
290 // names in the given configuration file that are not yet members
291 // of the returned set
292 //
293 // @throws ServiceConfigurationError
294 // If an I/O error occurs while reading from the given URL, or
295 // if a configuration-file format error is detected
296 //
297 private Iterator<String> parse(Class<?> service, URL u)
298 throws ServiceConfigurationError
299 {
300 InputStream in = null;
301 BufferedReader r = null;
302 ArrayList<String> names = new ArrayList<>();
303 try {
304 in = u.openStream();
305 r = new BufferedReader(new InputStreamReader(in, "utf-8"));
306 int lc = 1;
307 while ((lc = parseLine(service, u, r, lc, names)) >= 0);
308 } catch (IOException x) {
309 fail(service, "Error reading configuration file", x);
310 } finally {
311 try {
312 if (r != null) r.close();
313 if (in != null) in.close();
314 } catch (IOException y) {
315 fail(service, "Error closing configuration file", y);
316 }
317 }
318 return names.iterator();
319 }
320
321 // Private inner class implementing fully-lazy provider lookup
322 //
323 private class LazyIterator
324 implements Iterator<S>
325 {
326
327 Class<S> service;
328 ClassLoader loader;
329 Enumeration<URL> configs = null;
330 Iterator<String> pending = null;
331 String nextName = null;
332
333 private LazyIterator(Class<S> service, ClassLoader loader) {
334 this.service = service;
335 this.loader = loader;
336 }
337
338 private boolean hasNextService() {
339 if (nextName != null) {
340 return true;
341 }
342 if (configs == null) {
343 try {
344 String fullName = PREFIX + service.getName();
345 if (loader == null)
346 configs = ClassLoader.getSystemResources(fullName);
347 else
348 configs = loader.getResources(fullName);
349 } catch (IOException x) {
350 fail(service, "Error locating configuration files", x);
351 }
352 }
353 while ((pending == null) || !pending.hasNext()) {
354 if (!configs.hasMoreElements()) {
355 return false;
356 }
357 pending = parse(service, configs.nextElement());
358 }
359 nextName = pending.next();
360 return true;
361 }
362
363 private S nextService() {
364 if (!hasNextService())
365 throw new NoSuchElementException();
366 String cn = nextName;
367 nextName = null;
368 Class<?> c = null;
369 try {
370 c = Class.forName(cn, false, loader);
371 } catch (ClassNotFoundException x) {
372 fail(service,
373 "Provider " + cn + " not found");
374 }
375 if (!service.isAssignableFrom(c)) {
376 fail(service,
377 "Provider " + cn + " not a subtype");
378 }
379 try {
380 S p = service.cast(c.newInstance());
381 providers.put(cn, p);
382 return p;
383 } catch (Throwable x) {
384 fail(service,
385 "Provider " + cn + " could not be instantiated: " + x,
386 x);
387 }
388 throw new Error(); // This cannot happen
389 }
390
391 public boolean hasNext() {
392 if (acc == null) {
393 return hasNextService();
394 } else {
395 PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
396 public Boolean run() { return hasNextService(); }
397 };
398 return AccessController.doPrivileged(action, acc);
399 }
400 }
401
402 public S next() {
403 if (acc == null) {
404 return nextService();
405 } else {
406 PrivilegedAction<S> action = new PrivilegedAction<S>() {
407 public S run() { return nextService(); }
408 };
409 return AccessController.doPrivileged(action, acc);
410 }
411 }
412
413 public void remove() {
414 throw new UnsupportedOperationException();
415 }
416
417 }
418
419 /**
420 * Lazily loads the available providers of this loader's service.
421 *
422 * <p> The iterator returned by this method first yields all of the
423 * elements of the provider cache, in instantiation order. It then lazily
424 * loads and instantiates any remaining providers, adding each one to the
425 * cache in turn.
426 *
427 * <p> To achieve laziness the actual work of parsing the available
428 * provider-configuration files and instantiating providers must be done by
429 * the iterator itself. Its {@link java.util.Iterator#hasNext hasNext} and
430 * {@link java.util.Iterator#next next} methods can therefore throw a
431 * {@link ServiceConfigurationError} if a provider-configuration file
432 * violates the specified format, or if it names a provider class that
433 * cannot be found and instantiated, or if the result of instantiating the
434 * class is not assignable to the service type, or if any other kind of
435 * exception or error is thrown as the next provider is located and
436 * instantiated. To write robust code it is only necessary to catch {@link
437 * ServiceConfigurationError} when using a service iterator.
438 *
439 * <p> If such an error is thrown then subsequent invocations of the
440 * iterator will make a best effort to locate and instantiate the next
441 * available provider, but in general such recovery cannot be guaranteed.
442 *
443 * <blockquote style="font-size: smaller; line-height: 1.2"><span
444 * style="padding-right: 1em; font-weight: bold">Design Note</span>
445 * Throwing an error in these cases may seem extreme. The rationale for
446 * this behavior is that a malformed provider-configuration file, like a
447 * malformed class file, indicates a serious problem with the way the Java
448 * virtual machine is configured or is being used. As such it is
449 * preferable to throw an error rather than try to recover or, even worse,
450 * fail silently.</blockquote>
451 *
452 * <p> The iterator returned by this method does not support removal.
453 * Invoking its {@link java.util.Iterator#remove() remove} method will
454 * cause an {@link UnsupportedOperationException} to be thrown.
455 *
456 * @return An iterator that lazily loads providers for this loader's
457 * service
458 */
459 public Iterator<S> iterator() {
460 return new Iterator<S>() {
461
462 Iterator<Map.Entry<String,S>> knownProviders
463 = providers.entrySet().iterator();
464
465 public boolean hasNext() {
466 if (knownProviders.hasNext())
467 return true;
468 return lookupIterator.hasNext();
469 }
470
471 public S next() {
472 if (knownProviders.hasNext())
473 return knownProviders.next().getValue();
474 return lookupIterator.next();
475 }
476
477 public void remove() {
478 throw new UnsupportedOperationException();
479 }
480
481 };
482 }
483
484 /**
485 * Creates a new service loader for the given service type and class
486 * loader.
487 *
488 * @param <S> the class of the service type
489 *
490 * @param service
491 * The interface or abstract class representing the service
492 *
493 * @param loader
494 * The class loader to be used to load provider-configuration files
495 * and provider classes, or <tt>null</tt> if the system class
496 * loader (or, failing that, the bootstrap class loader) is to be
497 * used
498 *
499 * @return A new service loader
500 */
501 public static <S> ServiceLoader<S> load(Class<S> service,
502 ClassLoader loader)
503 {
504 return new ServiceLoader<>(service, loader);
505 }
506
507 /**
508 * Creates a new service loader for the given service type, using the
509 * current thread's {@linkplain java.lang.Thread#getContextClassLoader
510 * context class loader}.
511 *
512 * <p> An invocation of this convenience method of the form
513 *
514 * <blockquote><pre>
515 * ServiceLoader.load(<i>service</i>)</pre></blockquote>
516 *
517 * is equivalent to
518 *
519 * <blockquote><pre>
520 * ServiceLoader.load(<i>service</i>,
521 * Thread.currentThread().getContextClassLoader())</pre></blockquote>
522 *
523 * @param <S> the class of the service type
524 *
525 * @param service
526 * The interface or abstract class representing the service
527 *
528 * @return A new service loader
529 */
530 public static <S> ServiceLoader<S> load(Class<S> service) {
531 ClassLoader cl = Thread.currentThread().getContextClassLoader();
532 return ServiceLoader.load(service, cl);
533 }
534
535 /**
536 * Creates a new service loader for the given service type, using the
537 * extension class loader.
538 *
539 * <p> This convenience method simply locates the extension class loader,
540 * call it <tt><i>extClassLoader</i></tt>, and then returns
541 *
542 * <blockquote><pre>
543 * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote>
544 *
545 * <p> If the extension class loader cannot be found then the system class
546 * loader is used; if there is no system class loader then the bootstrap
547 * class loader is used.
548 *
549 * <p> This method is intended for use when only installed providers are
550 * desired. The resulting service will only find and load providers that
551 * have been installed into the current Java virtual machine; providers on
552 * the application's class path will be ignored.
553 *
554 * @param <S> the class of the service type
555 *
556 * @param service
557 * The interface or abstract class representing the service
558 *
559 * @return A new service loader
560 */
561 public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
562 ClassLoader cl = ClassLoader.getSystemClassLoader();
563 ClassLoader prev = null;
564 while (cl != null) {
565 prev = cl;
566 cl = cl.getParent();
567 }
568 return ServiceLoader.load(service, prev);
569 }
570
571 /**
572 * Returns a string describing this service.
573 *
574 * @return A descriptive string
575 */
576 public String toString() {
577 return "java.util.ServiceLoader[" + service.getName() + "]";
578 }
579
580 }