java.util.concurrent

Class FutureTask<V>

Implemented Interfaces:
Future<V>, Runnable, 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 method will block if the computation has not yet completed. Once the computation has completed, the computation cannot be restarted or cancelled.

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

Constructor Summary

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

Method Summary

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()
V
get(long timeout, TimeUnit unit)
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.

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

FutureTask

public FutureTask(Callable callable)
Creates a FutureTask that will upon running, execute the given Callable.
Parameters:
callable - the callable task
Throws:
NullPointerException - if 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<Object>(runnable, null)
Throws:
NullPointerException - if runnable is null

Method Details

cancel

public 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 Future.isDone() will always return true. Subsequent calls to Future.isCancelled() will always return true if this method returned true.

Specified by:
cancel in interface Future<V>
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

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.

get

public V get()
            throws InterruptedException,
                   ExecutionException
Specified by:
get in interface Future<V>

get

public V get(long timeout,
             TimeUnit unit)
            throws InterruptedException,
                   ExecutionException,
                   TimeoutException
Specified by:
get in interface Future<V>

isCancelled

public boolean isCancelled()
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()
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

run

public void run()
Sets this Future to the result of its computation unless it has been cancelled.
Specified by:
run in interface RunnableFuture<V>
run in interface Runnable

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

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