Java tutorial
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See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; /** * A {@link CompletionService} that uses a supplied {@link Executor} * to execute tasks. This class arranges that submitted tasks are, * upon completion, placed on a queue accessible using {@code take}. * The class is lightweight enough to be suitable for transient use * when processing groups of tasks. * * <p> * * <b>Usage Examples.</b> * * Suppose you have a set of solvers for a certain problem, each * returning a value of some type {@code Result}, and would like to * run them concurrently, processing the results of each of them that * return a non-null value, in some method {@code use(Result r)}. You * could write this as: * * <pre> {@code * void solve(Executor e, * Collection<Callable<Result>> solvers) * throws InterruptedException, ExecutionException { * CompletionService<Result> cs * = new ExecutorCompletionService<>(e); * solvers.forEach(cs::submit); * for (int i = solvers.size(); i > 0; i--) { * Result r = cs.take().get(); * if (r != null) * use(r); * } * }}</pre> * * Suppose instead that you would like to use the first non-null result * of the set of tasks, ignoring any that encounter exceptions, * and cancelling all other tasks when the first one is ready: * * <pre> {@code * void solve(Executor e, * Collection<Callable<Result>> solvers) * throws InterruptedException { * CompletionService<Result> cs * = new ExecutorCompletionService<>(e); * int n = solvers.size(); * List<Future<Result>> futures = new ArrayList<>(n); * Result result = null; * try { * solvers.forEach(solver -> futures.add(cs.submit(solver))); * for (int i = n; i > 0; i--) { * try { * Result r = cs.take().get(); * if (r != null) { * result = r; * break; * } * } catch (ExecutionException ignore) {} * } * } finally { * futures.forEach(future -> future.cancel(true)); * } * * if (result != null) * use(result); * }}</pre> * * @since 1.5 */ public class ExecutorCompletionService<V> implements CompletionService<V> { private final Executor executor; private final AbstractExecutorService aes; private final BlockingQueue<Future<V>> completionQueue; /** * FutureTask extension to enqueue upon completion. */ private static class QueueingFuture<V> extends FutureTask<Void> { QueueingFuture(RunnableFuture<V> task, BlockingQueue<Future<V>> completionQueue) { super(task, null); this.task = task; this.completionQueue = completionQueue; } private final Future<V> task; private final BlockingQueue<Future<V>> completionQueue; protected void done() { completionQueue.add(task); } } private RunnableFuture<V> newTaskFor(Callable<V> task) { if (aes == null) return new FutureTask<V>(task); else return aes.newTaskFor(task); } private RunnableFuture<V> newTaskFor(Runnable task, V result) { if (aes == null) return new FutureTask<V>(task, result); else return aes.newTaskFor(task, result); } /** * Creates an ExecutorCompletionService using the supplied * executor for base task execution and a * {@link LinkedBlockingQueue} as a completion queue. * * @param executor the executor to use * @throws NullPointerException if executor is {@code null} */ public ExecutorCompletionService(Executor executor) { if (executor == null) throw new NullPointerException(); this.executor = executor; this.aes = (executor instanceof AbstractExecutorService) ? (AbstractExecutorService) executor : null; this.completionQueue = new LinkedBlockingQueue<Future<V>>(); } /** * Creates an ExecutorCompletionService using the supplied * executor for base task execution and the supplied queue as its * completion queue. * * @param executor the executor to use * @param completionQueue the queue to use as the completion queue * normally one dedicated for use by this service. This * queue is treated as unbounded -- failed attempted * {@code Queue.add} operations for completed tasks cause * them not to be retrievable. * @throws NullPointerException if executor or completionQueue are {@code null} */ public ExecutorCompletionService(Executor executor, BlockingQueue<Future<V>> completionQueue) { if (executor == null || completionQueue == null) throw new NullPointerException(); this.executor = executor; this.aes = (executor instanceof AbstractExecutorService) ? (AbstractExecutorService) executor : null; this.completionQueue = completionQueue; } /** * @throws RejectedExecutionException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public Future<V> submit(Callable<V> task) { if (task == null) throw new NullPointerException(); RunnableFuture<V> f = newTaskFor(task); executor.execute(new QueueingFuture<V>(f, completionQueue)); return f; } /** * @throws RejectedExecutionException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public Future<V> submit(Runnable task, V result) { if (task == null) throw new NullPointerException(); RunnableFuture<V> f = newTaskFor(task, result); executor.execute(new QueueingFuture<V>(f, completionQueue)); return f; } public Future<V> take() throws InterruptedException { return completionQueue.take(); } public Future<V> poll() { return completionQueue.poll(); } public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException { return completionQueue.poll(timeout, unit); } }