/*
* 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/licenses/publicdomain
*/
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 <tt>take</tt>.
* 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 <tt>Result</tt>, and would like to
* run them concurrently, processing the results of each of them that
* return a non-null value, in some method <tt>use(Result r)</tt>. You
* could write this as:
*
* <pre>
* void solve(Executor e, Collection<Callable<Result>> solvers)
* throws InterruptedException, ExecutionException {
* CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e);
* for (Callable<Result> s : solvers)
* ecs.submit(s);
* int n = solvers.size();
* for (int i = 0; i < n; ++i) {
* Result r = ecs.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>
* void solve(Executor e, Collection<Callable<Result>> solvers)
* throws InterruptedException {
* CompletionService<Result> ecs = new ExecutorCompletionService<Result>(e);
* int n = solvers.size();
* List<Future<Result>> futures = new ArrayList<Future<Result>>(n);
* Result result = null;
* try {
* for (Callable<Result> s : solvers)
* futures.add(ecs.submit(s));
* for (int i = 0; i < n; ++i) {
* try {
* Result r = ecs.take().get();
* if (r != null) {
* result = r;
* break;
* }
* } catch(ExecutionException ignore) {}
* }
* }
* finally {
* for (Future<Result> f : futures)
* f.cancel(true);
* }
*
* if (result != null)
* use(result);
* }
* </pre>
*/
public class ExecutorCompletionService<V> implements CompletionService<V> {
private final Executor executor;
private final BlockingQueue<Future<V>> completionQueue;
/**
* FutureTask extension to enqueue upon completion
*/
private class QueueingFuture extends FutureTask<V> {
QueueingFuture(Callable<V> c) { super(c); }
QueueingFuture(Runnable t, V r) { super(t, r); }
protected void done() { completionQueue.add(this); }
}
/**
* 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 <tt>null</tt>
*/
public ExecutorCompletionService(Executor executor) {
if (executor == null)
throw new NullPointerException();
this.executor = executor;
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
* @throws NullPointerException if executor or completionQueue are <tt>null</tt>
*/
public ExecutorCompletionService(Executor executor,
BlockingQueue<Future<V>> completionQueue) {
if (executor == null || completionQueue == null)
throw new NullPointerException();
this.executor = executor;
this.completionQueue = completionQueue;
}
public Future<V> submit(Callable<V> task) {
if (task == null) throw new NullPointerException();
QueueingFuture f = new QueueingFuture(task);
executor.execute(f);
return f;
}
public Future<V> submit(Runnable task, V result) {
if (task == null) throw new NullPointerException();
QueueingFuture f = new QueueingFuture(task, result);
executor.execute(f);
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);
}
}
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