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jeremybenn |
/*
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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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package java.util.concurrent;
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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)
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* throws InterruptedException {
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* Future<String> future
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* = executor.submit(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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* 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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*
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* <p>Memory consistency effects: Actions taken by the asynchronous computation
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* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
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* actions following the corresponding {@code Future.get()} in another thread.
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*
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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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* Attempts to cancel execution of this task. This attempt will
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* fail if the task has already completed, has 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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* <p>After this method returns, subsequent calls to {@link #isDone} will
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* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
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* will always return <tt>true</tt> if this method returned <tt>true</tt>.
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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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* 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 this task was cancelled before it completed
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*/
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boolean isCancelled();
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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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* 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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* 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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}
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