1 /* 2 * Copyright (c) 2005, 2013, 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", 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 * @implNote When adding providers to the cache, the {@link #iterator 457 * Iterator} processes resources in the order that the {@link 458 * java.lang.ClassLoader#getResources(java.lang.String) 459 * ClassLoader.getResources(String)} method finds the service configuration 460 * files. 461 * 462 * @return An iterator that lazily loads providers for this loader's 463 * service 464 */ 465 public Iterator<S> iterator() { 466 return new Iterator<S>() { 467 468 Iterator<Map.Entry<String,S>> knownProviders 469 = providers.entrySet().iterator(); 470 471 public boolean hasNext() { 472 if (knownProviders.hasNext()) 473 return true; 474 return lookupIterator.hasNext(); 475 } 476 477 public S next() { 478 if (knownProviders.hasNext()) 479 return knownProviders.next().getValue(); 480 return lookupIterator.next(); 481 } 482 483 public void remove() { 484 throw new UnsupportedOperationException(); 485 } 486 487 }; 488 } 489 490 /** 491 * Creates a new service loader for the given service type and class 492 * loader. 493 * 494 * @param <S> the class of the service type 495 * 496 * @param service 497 * The interface or abstract class representing the service 498 * 499 * @param loader 500 * The class loader to be used to load provider-configuration files 501 * and provider classes, or <tt>null</tt> if the system class 502 * loader (or, failing that, the bootstrap class loader) is to be 503 * used 504 * 505 * @return A new service loader 506 */ 507 public static <S> ServiceLoader<S> load(Class<S> service, 508 ClassLoader loader) 509 { 510 return new ServiceLoader<>(service, loader); 511 } 512 513 /** 514 * Creates a new service loader for the given service type, using the 515 * current thread's {@linkplain java.lang.Thread#getContextClassLoader 516 * context class loader}. 517 * 518 * <p> An invocation of this convenience method of the form 519 * 520 * <blockquote><pre> 521 * ServiceLoader.load(<i>service</i>)</pre></blockquote> 522 * 523 * is equivalent to 524 * 525 * <blockquote><pre> 526 * ServiceLoader.load(<i>service</i>, 527 * Thread.currentThread().getContextClassLoader())</pre></blockquote> 528 * 529 * @param <S> the class of the service type 530 * 531 * @param service 532 * The interface or abstract class representing the service 533 * 534 * @return A new service loader 535 */ 536 public static <S> ServiceLoader<S> load(Class<S> service) { 537 ClassLoader cl = Thread.currentThread().getContextClassLoader(); 538 return ServiceLoader.load(service, cl); 539 } 540 541 /** 542 * Creates a new service loader for the given service type, using the 543 * extension class loader. 544 * 545 * <p> This convenience method simply locates the extension class loader, 546 * call it <tt><i>extClassLoader</i></tt>, and then returns 547 * 548 * <blockquote><pre> 549 * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote> 550 * 551 * <p> If the extension class loader cannot be found then the system class 552 * loader is used; if there is no system class loader then the bootstrap 553 * class loader is used. 554 * 555 * <p> This method is intended for use when only installed providers are 556 * desired. The resulting service will only find and load providers that 557 * have been installed into the current Java virtual machine; providers on 558 * the application's class path will be ignored. 559 * 560 * @param <S> the class of the service type 561 * 562 * @param service 563 * The interface or abstract class representing the service 564 * 565 * @return A new service loader 566 */ 567 public static <S> ServiceLoader<S> loadInstalled(Class<S> service) { 568 ClassLoader cl = ClassLoader.getSystemClassLoader(); 569 ClassLoader prev = null; 570 while (cl != null) { 571 prev = cl; 572 cl = cl.getParent(); 573 } 574 return ServiceLoader.load(service, prev); 575 } 576 577 /** 578 * Returns a string describing this service. 579 * 580 * @return A descriptive string 581 */ 582 public String toString() { 583 return "java.util.ServiceLoader[" + service.getName() + "]"; 584 } 585 586 }