1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 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/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent; 37 38 /** 39 * A {@link CompletionService} that uses a supplied {@link Executor} 40 * to execute tasks. This class arranges that submitted tasks are, 41 * upon completion, placed on a queue accessible using {@code take}. 42 * The class is lightweight enough to be suitable for transient use 43 * when processing groups of tasks. 44 * 45 * <p> 46 * 47 * <b>Usage Examples.</b> 48 * 49 * Suppose you have a set of solvers for a certain problem, each 50 * returning a value of some type {@code Result}, and would like to 51 * run them concurrently, processing the results of each of them that 52 * return a non-null value, in some method {@code use(Result r)}. You 53 * could write this as: 54 * 55 * <pre> {@code 56 * void solve(Executor e, 57 * Collection<Callable<Result>> solvers) 58 * throws InterruptedException, ExecutionException { 59 * CompletionService<Result> cs 60 * = new ExecutorCompletionService<>(e); 61 * solvers.forEach(cs::submit); 62 * for (int i = solvers.size(); i > 0; i--) { 63 * Result r = cs.take().get(); 64 * if (r != null) 65 * use(r); 66 * } 67 * }}</pre> 68 * 69 * Suppose instead that you would like to use the first non-null result 70 * of the set of tasks, ignoring any that encounter exceptions, 71 * and cancelling all other tasks when the first one is ready: 72 * 73 * <pre> {@code 74 * void solve(Executor e, 75 * Collection<Callable<Result>> solvers) 76 * throws InterruptedException { 77 * CompletionService<Result> cs 78 * = new ExecutorCompletionService<>(e); 79 * int n = solvers.size(); 80 * List<Future<Result>> futures = new ArrayList<>(n); 81 * Result result = null; 82 * try { 83 * solvers.forEach(solver -> futures.add(cs.submit(solver))); 84 * for (int i = n; i > 0; i--) { 85 * try { 86 * Result r = cs.take().get(); 87 * if (r != null) { 88 * result = r; 89 * break; 90 * } 91 * } catch (ExecutionException ignore) {} 92 * } 93 * } finally { 94 * futures.forEach(future -> future.cancel(true)); 95 * } 96 * 97 * if (result != null) 98 * use(result); 99 * }}</pre> 100 * 101 * @since 1.5 102 */ 103 public class ExecutorCompletionService<V> implements CompletionService<V> { 104 private final Executor executor; 105 private final AbstractExecutorService aes; 106 private final BlockingQueue<Future<V>> completionQueue; 107 108 /** 109 * FutureTask extension to enqueue upon completion. 110 */ 111 private static class QueueingFuture<V> extends FutureTask<Void> { QueueingFuture(RunnableFuture<V> task, BlockingQueue<Future<V>> completionQueue)112 QueueingFuture(RunnableFuture<V> task, 113 BlockingQueue<Future<V>> completionQueue) { 114 super(task, null); 115 this.task = task; 116 this.completionQueue = completionQueue; 117 } 118 private final Future<V> task; 119 private final BlockingQueue<Future<V>> completionQueue; done()120 protected void done() { completionQueue.add(task); } 121 } 122 newTaskFor(Callable<V> task)123 private RunnableFuture<V> newTaskFor(Callable<V> task) { 124 if (aes == null) 125 return new FutureTask<V>(task); 126 else 127 return aes.newTaskFor(task); 128 } 129 newTaskFor(Runnable task, V result)130 private RunnableFuture<V> newTaskFor(Runnable task, V result) { 131 if (aes == null) 132 return new FutureTask<V>(task, result); 133 else 134 return aes.newTaskFor(task, result); 135 } 136 137 /** 138 * Creates an ExecutorCompletionService using the supplied 139 * executor for base task execution and a 140 * {@link LinkedBlockingQueue} as a completion queue. 141 * 142 * @param executor the executor to use 143 * @throws NullPointerException if executor is {@code null} 144 */ ExecutorCompletionService(Executor executor)145 public ExecutorCompletionService(Executor executor) { 146 if (executor == null) 147 throw new NullPointerException(); 148 this.executor = executor; 149 this.aes = (executor instanceof AbstractExecutorService) ? 150 (AbstractExecutorService) executor : null; 151 this.completionQueue = new LinkedBlockingQueue<Future<V>>(); 152 } 153 154 /** 155 * Creates an ExecutorCompletionService using the supplied 156 * executor for base task execution and the supplied queue as its 157 * completion queue. 158 * 159 * @param executor the executor to use 160 * @param completionQueue the queue to use as the completion queue 161 * normally one dedicated for use by this service. This 162 * queue is treated as unbounded -- failed attempted 163 * {@code Queue.add} operations for completed tasks cause 164 * them not to be retrievable. 165 * @throws NullPointerException if executor or completionQueue are {@code null} 166 */ ExecutorCompletionService(Executor executor, BlockingQueue<Future<V>> completionQueue)167 public ExecutorCompletionService(Executor executor, 168 BlockingQueue<Future<V>> completionQueue) { 169 if (executor == null || completionQueue == null) 170 throw new NullPointerException(); 171 this.executor = executor; 172 this.aes = (executor instanceof AbstractExecutorService) ? 173 (AbstractExecutorService) executor : null; 174 this.completionQueue = completionQueue; 175 } 176 177 /** 178 * @throws RejectedExecutionException {@inheritDoc} 179 * @throws NullPointerException {@inheritDoc} 180 */ submit(Callable<V> task)181 public Future<V> submit(Callable<V> task) { 182 if (task == null) throw new NullPointerException(); 183 RunnableFuture<V> f = newTaskFor(task); 184 executor.execute(new QueueingFuture<V>(f, completionQueue)); 185 return f; 186 } 187 188 /** 189 * @throws RejectedExecutionException {@inheritDoc} 190 * @throws NullPointerException {@inheritDoc} 191 */ submit(Runnable task, V result)192 public Future<V> submit(Runnable task, V result) { 193 if (task == null) throw new NullPointerException(); 194 RunnableFuture<V> f = newTaskFor(task, result); 195 executor.execute(new QueueingFuture<V>(f, completionQueue)); 196 return f; 197 } 198 take()199 public Future<V> take() throws InterruptedException { 200 return completionQueue.take(); 201 } 202 poll()203 public Future<V> poll() { 204 return completionQueue.poll(); 205 } 206 poll(long timeout, TimeUnit unit)207 public Future<V> poll(long timeout, TimeUnit unit) 208 throws InterruptedException { 209 return completionQueue.poll(timeout, unit); 210 } 211 212 } 213