30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/licenses/publicdomain 34 */ 35 36 package java.util.concurrent; 37 38 /** 39 * A <tt>Future</tt> represents the result of an asynchronous 40 * computation. Methods are provided to check if the computation is 41 * complete, to wait for its completion, and to retrieve the result of 42 * the computation. The result can only be retrieved using method 43 * <tt>get</tt> when the computation has completed, blocking if 44 * necessary until it is ready. Cancellation is performed by the 45 * <tt>cancel</tt> method. Additional methods are provided to 46 * determine if the task completed normally or was cancelled. Once a 47 * computation has completed, the computation cannot be cancelled. 48 * If you would like to use a <tt>Future</tt> for the sake 49 * of cancellability but not provide a usable result, you can 50 * declare types of the form <tt>Future<?></tt> and 51 * return <tt>null</tt> as a result of the underlying task. 52 * 53 * <p> 54 * <b>Sample Usage</b> (Note that the following classes are all 55 * made-up.) <p> 56 * <pre> 57 * interface ArchiveSearcher { String search(String target); } 58 * class App { 59 * ExecutorService executor = ... 60 * ArchiveSearcher searcher = ... 61 * void showSearch(final String target) 62 * throws InterruptedException { 63 * Future<String> future 64 * = executor.submit(new Callable<String>() { 65 * public String call() { 66 * return searcher.search(target); 67 * }}); 68 * displayOtherThings(); // do other things while searching 69 * try { 70 * displayText(future.get()); // use future 71 * } catch (ExecutionException ex) { cleanup(); return; } 72 * } 73 * } 74 * </pre> 75 * 76 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that 77 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>. 78 * For example, the above construction with <tt>submit</tt> could be replaced by: 79 * <pre> 80 * FutureTask<String> future = 81 * new FutureTask<String>(new Callable<String>() { 82 * public String call() { 83 * return searcher.search(target); 84 * }}); 85 * executor.execute(future); 86 * </pre> 87 * 88 * <p>Memory consistency effects: Actions taken by the asynchronous computation 89 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 90 * actions following the corresponding {@code Future.get()} in another thread. 91 * 92 * @see FutureTask 93 * @see Executor 94 * @since 1.5 95 * @author Doug Lea 96 * @param <V> The result type returned by this Future's <tt>get</tt> method 97 */ 98 public interface Future<V> { 99 100 /** 101 * Attempts to cancel execution of this task. This attempt will 102 * fail if the task has already completed, has already been cancelled, 103 * or could not be cancelled for some other reason. If successful, 104 * and this task has not started when <tt>cancel</tt> is called, 105 * this task should never run. If the task has already started, 106 * then the <tt>mayInterruptIfRunning</tt> parameter determines | 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/licenses/publicdomain 34 */ 35 36 package java.util.concurrent; 37 38 /** 39 * A <tt>Future</tt> represents the result of an asynchronous 40 * computation. Methods are provided to check if the computation is 41 * complete, to wait for its completion, and to retrieve the result of 42 * the computation. The result can only be retrieved using method 43 * <tt>get</tt> when the computation has completed, blocking if 44 * necessary until it is ready. Cancellation is performed by the 45 * <tt>cancel</tt> method. Additional methods are provided to 46 * determine if the task completed normally or was cancelled. Once a 47 * computation has completed, the computation cannot be cancelled. 48 * If you would like to use a <tt>Future</tt> for the sake 49 * of cancellability but not provide a usable result, you can 50 * declare types of the form {@code Future<?>} and 51 * return <tt>null</tt> as a result of the underlying task. 52 * 53 * <p> 54 * <b>Sample Usage</b> (Note that the following classes are all 55 * made-up.) <p> 56 * <pre> {@code 57 * interface ArchiveSearcher { String search(String target); } 58 * class App { 59 * ExecutorService executor = ... 60 * ArchiveSearcher searcher = ... 61 * void showSearch(final String target) 62 * throws InterruptedException { 63 * Future<String> future 64 * = executor.submit(new Callable<String>() { 65 * public String call() { 66 * return searcher.search(target); 67 * }}); 68 * displayOtherThings(); // do other things while searching 69 * try { 70 * displayText(future.get()); // use future 71 * } catch (ExecutionException ex) { cleanup(); return; } 72 * } 73 * }}</pre> 74 * 75 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that 76 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>. 77 * For example, the above construction with <tt>submit</tt> could be replaced by: 78 * <pre> {@code 79 * FutureTask<String> future = 80 * new FutureTask<String>(new Callable<String>() { 81 * public String call() { 82 * return searcher.search(target); 83 * }}); 84 * executor.execute(future);}</pre> 85 * 86 * <p>Memory consistency effects: Actions taken by the asynchronous computation 87 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> 88 * actions following the corresponding {@code Future.get()} in another thread. 89 * 90 * @see FutureTask 91 * @see Executor 92 * @since 1.5 93 * @author Doug Lea 94 * @param <V> The result type returned by this Future's <tt>get</tt> method 95 */ 96 public interface Future<V> { 97 98 /** 99 * Attempts to cancel execution of this task. This attempt will 100 * fail if the task has already completed, has already been cancelled, 101 * or could not be cancelled for some other reason. If successful, 102 * and this task has not started when <tt>cancel</tt> is called, 103 * this task should never run. If the task has already started, 104 * then the <tt>mayInterruptIfRunning</tt> parameter determines |