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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain. Use, modify, and |
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* redistribute this code in any way without acknowledgement. |
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*/ |
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|
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package java.util.concurrent; |
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|
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|
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/** |
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* A {@link CompletionService} that uses a supplied {@link Executor} |
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* to execute tasks. This class arranges that submitted tasks are, |
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* upon completion, placed on a queue accessible using <tt>take</tt>. |
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* The class is lightweight enough to be suitable for transient use |
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* when processing groups of tasks. |
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* |
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* <p> |
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* |
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* <b>Usage Examples.</b> |
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* Suppose you have a set of solvers for a certain problem (each returning |
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* a value of some type <tt>Result</tt>), |
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* and would like to run them concurrently, using the results of each of them |
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* that return a non-null value. You could write this as: |
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* |
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* <pre> |
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* void solve(Executor e, Collection<Callable<Result>> solvers) |
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* throws InterruptedException, ExecutionException { |
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* CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e); |
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* for (Callable<Result> s : solvers) |
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* ecs.submit(s); |
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* int n = solvers.size(); |
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* for (int i = 0; i < n; ++i) { |
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* Result r = ecs.take().get(); |
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* if (r != null) |
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* use(r); |
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* } |
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* } |
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* </pre> |
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* |
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* Suppose instead that you would like to use the first non-null result |
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* of the set of tasks, ignoring any of those that encounter exceptions, |
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* and cancelling all other tasks when the first one is ready: |
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* |
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* <pre> |
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* void solve(Executor e, Collection<Callable<Result>> solvers) |
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* throws InterruptedException { |
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* CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e); |
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* int n = solvers.size(); |
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* List<Future<Result>> futures = new ArrayList<Future<Result>>(n); |
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* Result result = null; |
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* try { |
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* for (Callable<Result> s : solvers) |
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* futures.add(ecs.submit(s)); |
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* for (int i = 0; i < n; ++i) { |
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* try { |
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* Result r = ecs.take().get(); |
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* if (r != null) { |
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* result = r; |
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* break; |
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* } |
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* } catch(ExecutionException ignore) {} |
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* } |
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* } |
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* finally { |
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* for (Future<Result> f : futures) |
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* f.cancel(true); |
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* } |
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* |
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* if (result != null) |
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* use(result); |
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* } |
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* </pre> |
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*/ |
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public class ExecutorCompletionService<V> implements CompletionService<V> { |
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private final Executor executor; |
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private final BlockingQueue<Future<V>> completionQueue; |
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|
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/** |
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* FutureTask extension to enqueue upon completion |
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*/ |
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private class QueueingFuture extends FutureTask<V> { |
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QueueingFuture(Callable<V> c) { super(c); } |
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QueueingFuture(Runnable t, V r) { super(t, r); } |
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protected void done() { completionQueue.add(this); } |
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} |
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|
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/** |
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* Creates an ExecutorCompletionService using the supplied |
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* executor for base task execution and a |
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* {@link LinkedBlockingQueue} as a completion queue. |
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* @param executor the executor to use |
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8 @throws NullPointerException if executor is <tt>null</tt> |
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*/ |
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public ExecutorCompletionService(Executor executor) { |
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if (executor == null) |
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throw new NullPointerException(); |
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this.executor = executor; |
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this.completionQueue = new LinkedBlockingQueue<Future<V>>(); |
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} |
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|
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/** |
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* Creates an ExecutorCompletionService using the supplied |
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* executor for base task execution and the supplied queue as its |
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* completion queue. |
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* @param executor the executor to use |
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* @param completionQueue the queue to use as the completion queue |
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* normally one dedicated for use by this service |
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8 @throws NullPointerException if executor or completionQueue are <tt>null</tt> |
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*/ |
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public ExecutorCompletionService(Executor executor, |
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BlockingQueue<Future<V>> completionQueue) { |
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if (executor == null || completionQueue == null) |
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throw new NullPointerException(); |
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this.executor = executor; |
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this.completionQueue = completionQueue; |
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} |
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|
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public Future<V> submit(Callable<V> task) { |
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if (task == null) throw new NullPointerException(); |
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QueueingFuture f = new QueueingFuture(task); |
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executor.execute(f); |
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return f; |
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} |
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|
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public Future<V> submit(Runnable task, V result) { |
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if (task == null) throw new NullPointerException(); |
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QueueingFuture f = new QueueingFuture(task, result); |
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executor.execute(f); |
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return f; |
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} |
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|
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public Future<V> take() throws InterruptedException { |
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return completionQueue.take(); |
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} |
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|
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public Future<V> poll() { |
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return completionQueue.poll(); |
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} |
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public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException { |
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return completionQueue.poll(timeout, unit); |
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} |
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|
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} |
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