java.util.concurrent

Interface Future<V>

Type Parameters:

V - The result type returned by this Future's get method

All Known Subinterfaces:

RunnableFuture<V>, RunnableScheduledFuture<V>, ScheduledFuture<V>

All Known Implementing Classes:

ForkJoinTask, FutureTask, RecursiveAction, RecursiveTask

public interface Future<V>

A Future represents the result of an asynchronous computation. Methods are provided to check if the computation is complete, to wait for its completion, and to retrieve the result of the computation. The result can only be retrieved using method get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the cancel method. Additional methods are provided to determine if the task completed normally or was cancelled. Once a computation has completed, the computation cannot be cancelled. If you would like to use a Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future<?> and return null as a result of the underlying task.

Sample Usage (Note that the following classes are all made-up.)

 
 interface ArchiveSearcher { String search(String target); }
 class App {
   ExecutorService executor = ...
   ArchiveSearcher searcher = ...
   void showSearch(final String target)
       throws InterruptedException {
     Future<String> future
       = executor.submit(new Callable<String>() {
         public String call() {
             return searcher.search(target);
         }});
     displayOtherThings(); // do other things while searching
     try {
       displayText(future.get()); // use future
     } catch (ExecutionException ex) { cleanup(); return; }
   }
 }

The FutureTask class is an implementation of Future that implements Runnable, and so may be executed by an Executor. For example, the above construction with submit could be replaced by:

 
     FutureTask<String> future =
       new FutureTask<String>(new Callable<String>() {
         public String call() {
           return searcher.search(target);
       }});
     executor.execute(future);

Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get() in another thread.

Since:

1.5

Author:

Doug Lea

See Also:

FutureTask, Executor

Method Summary

Modifier and Type	Method and Description
boolean	cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.
V	get() Waits if necessary for the computation to complete, and then retrieves its result.
V	get(long timeout, TimeUnit unit) Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
boolean	isCancelled() Returns true if this task was cancelled before it completed normally.
boolean	isDone() Returns true if this task completed.

Method Detail

cancel

boolean cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

After this method returns, subsequent calls to isDone() will always return true. Subsequent calls to isCancelled() will always return true if this method returned true.

Parameters:

mayInterruptIfRunning - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete

Returns:

false if the task could not be cancelled, typically because it has already completed normally; true otherwise

isCancelled

boolean isCancelled()

Returns true if this task was cancelled before it completed normally.

Returns:

true if this task was cancelled before it completed

isDone

boolean isDone()

Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.

Returns:

true if this task completed

get

V get()
throws InterruptedException,
      ExecutionException

Waits if necessary for the computation to complete, and then retrieves its result.

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

ExecutionException - if the computation threw an exception

InterruptedException - if the current thread was interrupted while waiting

get

V get(long timeout,
      TimeUnit unit)
throws InterruptedException,
      ExecutionException,
      TimeoutException

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

Parameters:

timeout - the maximum time to wait

unit - the time unit of the timeout argument

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

ExecutionException - if the computation threw an exception

InterruptedException - if the current thread was interrupted while waiting

TimeoutException - if the wait timed out