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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, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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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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* A <tt>Future</tt> represents the result of an asynchronous |
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* computation. Methods are provided to check if the computation is |
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* complete, to wait for its completion, and to retrieve the result of |
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* the computation. The result can only be retrieved using method |
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* <tt>get</tt> when the computation has completed, blocking if |
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* necessary until it is ready. Cancellation is performed by the |
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* <tt>cancel</tt> method. Additional methods are provided to |
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* determine if the task completed normally or was cancelled. Once a |
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* computation has completed, the computation cannot be cancelled. |
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* If you would like to use a <tt>Future</tt> for the sake |
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* of cancellability but not provide a usable result, you can |
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* declare types of the form <tt>Future<?></tt> and |
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* return <tt>null</tt> as a result of the underlying task. |
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* |
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* <p> |
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* <b>Sample Usage</b> (Note that the following classes are all |
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* made-up.) <p> |
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* <pre> |
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* interface ArchiveSearcher { String search(String target); } |
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* class App { |
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* ExecutorService executor = ... |
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* ArchiveSearcher searcher = ... |
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* void showSearch(final String target) throws InterruptedException { |
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* Future<String> future = executor.submit(new Callable<String>() { |
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* public String call() { return searcher.search(target); } |
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* }); |
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* displayOtherThings(); // do other things while searching |
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* try { |
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* displayText(future.get()); // use future |
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* } catch (ExecutionException ex) { cleanup(); return; } |
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* } |
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* } |
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* </pre> |
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* |
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* The {@link FutureTask} class is an implementation of <tt>Future</tt> that |
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* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>. |
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* For example, the above construction with <tt>submit</tt> could be replaced by: |
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* <pre> |
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* FutureTask<String> future = |
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* new FutureTask<String>(new Callable<String>() { |
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* public String call() { |
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* return searcher.search(target); |
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* }}); |
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* executor.execute(future); |
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* </pre> |
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* @see FutureTask |
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* @see Executor |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <V> The result type returned by this Future's <tt>get</tt> method |
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*/ |
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public interface Future<V> { |
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|
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/** |
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* Attempts to cancel execution of this task. This attempt will |
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* fail if the task has already completed, already been cancelled, |
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* or could not be cancelled for some other reason. If successful, |
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* and this task has not started when <tt>cancel</tt> is called, |
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* this task should never run. If the task has already started, |
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* then the <tt>mayInterruptIfRunning</tt> parameter determines |
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* whether the thread executing this task should be interrupted in |
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* an attempt to stop the task. |
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* |
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* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this |
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* task should be interrupted; otherwise, in-progress tasks are allowed |
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* to complete |
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* @return <tt>false</tt> if the task could not be cancelled, |
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* typically because it has already completed normally; |
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* <tt>true</tt> otherwise |
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*/ |
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boolean cancel(boolean mayInterruptIfRunning); |
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|
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/** |
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* Returns <tt>true</tt> if this task was cancelled before it completed |
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* normally. |
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* |
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* @return <tt>true</tt> if task was cancelled before it completed |
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*/ |
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boolean isCancelled(); |
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|
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/** |
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* Returns <tt>true</tt> if this task completed. |
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* |
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* Completion may be due to normal termination, an exception, or |
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* cancellation -- in all of these cases, this method will return |
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* <tt>true</tt>. |
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* |
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* @return <tt>true</tt> if this task completed. |
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*/ |
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boolean isDone(); |
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|
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/** |
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* Waits if necessary for the computation to complete, and then |
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* retrieves its result. |
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* |
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* @return the computed result |
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* @throws CancellationException if the computation was cancelled |
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* @throws ExecutionException if the computation threw an |
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* exception |
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* @throws InterruptedException if the current thread was interrupted |
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* while waiting |
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*/ |
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V get() throws InterruptedException, ExecutionException; |
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|
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/** |
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* Waits if necessary for at most the given time for the computation |
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* to complete, and then retrieves its result, if available. |
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* |
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* @param timeout the maximum time to wait |
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* @param unit the time unit of the timeout argument |
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* @return the computed result |
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* @throws CancellationException if the computation was cancelled |
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* @throws ExecutionException if the computation threw an |
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* exception |
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* @throws InterruptedException if the current thread was interrupted |
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* while waiting |
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* @throws TimeoutException if the wait timed out |
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*/ |
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V get(long timeout, TimeUnit unit) |
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throws InterruptedException, ExecutionException, TimeoutException; |
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} |
