Module java.base

Package java.util.concurrent

Class FutureTask<V>

java.lang.Object

java.util.concurrent.FutureTask<V>

Type Parameters:

V - The result type returned by this FutureTask's get methods

All Implemented Interfaces:

Runnable, Future<V>, RunnableFuture<V>

public class FutureTask<V>
extends Object
implements RunnableFuture<V>

A cancellable asynchronous computation. This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation. The result can only be retrieved when the computation has completed; the get methods will block if the computation has not yet completed. Once the computation has completed, the computation cannot be restarted or cancelled (unless the computation is invoked using runAndReset()).

A FutureTask can be used to wrap a Callable or Runnable object. Because FutureTask implements Runnable, a FutureTask can be submitted to an Executor for execution.

In addition to serving as a standalone class, this class provides protected functionality that may be useful when creating customized task classes.

Since:

1.5

Nested Class Summary

Nested classes/interfaces declared in interface java.util.concurrent.Future

Future.State

Constructor Summary

Constructors

Constructor	Description
FutureTask(Runnable runnable, V result)	Creates a FutureTask that will, upon running, execute the given Runnable, and arrange that get will return the given result on successful completion.
FutureTask(Callable<V> callable)	Creates a FutureTask that will, upon running, execute the given Callable.

Method Summary

Modifier and Type	Method	Description
boolean	cancel(boolean mayInterruptIfRunning)	Attempts to cancel execution of this task.
protected void	done()	Protected method invoked when this task transitions to state isDone (whether normally or via cancellation).
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.
void	run()	Sets this Future to the result of its computation unless it has been cancelled.
protected boolean	runAndReset()	Executes the computation without setting its result, and then resets this future to initial state, failing to do so if the computation encounters an exception or is cancelled.
protected void	set(V v)	Sets the result of this future to the given value unless this future has already been set or has been cancelled.
protected void	setException(Throwable t)	Causes this future to report an ExecutionException with the given throwable as its cause, unless this future has already been set or has been cancelled.
String	toString()	Returns a string representation of this FutureTask.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods declared in interface java.util.concurrent.Future

exceptionNow, resultNow, state

Constructor Details

FutureTask

public FutureTask(Callable<V> callable)

Creates a FutureTask that will, upon running, execute the given Callable.

Parameters:

callable - the callable task

Throws:

NullPointerException - if the callable is null

FutureTask

public FutureTask(Runnable runnable,
 V result)

Creates a FutureTask that will, upon running, execute the given Runnable, and arrange that get will return the given result on successful completion.

Parameters:

runnable - the runnable task

result - the result to return on successful completion. If you don't need a particular result, consider using constructions of the form: Future<?> f = new FutureTask<Void>(runnable, null)

Throws:

NullPointerException - if the runnable is null

Method Details

isCancelled

public boolean isCancelled()

Description copied from interface: Future

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

Specified by:

isCancelled in interface Future<V>

Returns:

true if this task was cancelled before it completed

isDone

public boolean isDone()

Description copied from interface: Future

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.

Specified by:

isDone in interface Future<V>

Returns:

true if this task completed

cancel

public boolean cancel(boolean mayInterruptIfRunning)

Description copied from interface: Future

Attempts to cancel execution of this task. This method has no effect if the task is already completed or cancelled, or could not be cancelled for some other reason. Otherwise, if 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 (when known by the implementation) is interrupted in an attempt to stop the task.

The return value from this method does not necessarily indicate whether the task is now cancelled; use Future.isCancelled().

Specified by:

cancel in interface Future<V>

Parameters:

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

Returns:

false if the task could not be cancelled, typically because it has already completed; true otherwise. If two or more threads cause a task to be cancelled, then at least one of them returns true. Implementations may provide stronger guarantees.

get

public V get()
      throws InterruptedException,
ExecutionException

Description copied from interface: Future

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

Specified by:

get in interface Future<V>

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

InterruptedException - if the current thread was interrupted while waiting

ExecutionException - if the computation threw an exception

get

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

Description copied from interface: Future

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

Specified by:

get in interface Future<V>

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

InterruptedException - if the current thread was interrupted while waiting

ExecutionException - if the computation threw an exception

TimeoutException - if the wait timed out

done

protected void done()

Protected method invoked when this task transitions to state isDone (whether normally or via cancellation). The default implementation does nothing. Subclasses may override this method to invoke completion callbacks or perform bookkeeping. Note that you can query status inside the implementation of this method to determine whether this task has been cancelled.

set

protected void set(V v)

Sets the result of this future to the given value unless this future has already been set or has been cancelled.

This method is invoked internally by the run() method upon successful completion of the computation.

Parameters:

v - the value

setException

protected void setException(Throwable t)

Causes this future to report an ExecutionException with the given throwable as its cause, unless this future has already been set or has been cancelled.

This method is invoked internally by the run() method upon failure of the computation.

Parameters:

t - the cause of failure

run

public void run()

Description copied from interface: RunnableFuture

Sets this Future to the result of its computation unless it has been cancelled.

Specified by:

run in interface Runnable

Specified by:

run in interface RunnableFuture<V>

runAndReset

protected boolean runAndReset()

Executes the computation without setting its result, and then resets this future to initial state, failing to do so if the computation encounters an exception or is cancelled. This is designed for use with tasks that intrinsically execute more than once.

Returns:

true if successfully run and reset

toString

public String toString()

Returns a string representation of this FutureTask.

Overrides:

toString in class Object

Implementation Requirements:

The default implementation returns a string identifying this FutureTask, as well as its completion state. The state, in brackets, contains one of the strings "Completed Normally", "Completed Exceptionally", "Cancelled", or "Not completed".

Returns:

a string representation of this FutureTask