- java.lang.Object
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- java.util.concurrent.FutureTask<V>
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- Type Parameters:
V
- The result type returned by this FutureTask'sget
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 ofFuture
, 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; theget
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 usingrunAndReset()
).A
FutureTask
can be used to wrap aCallable
orRunnable
object. BecauseFutureTask
implementsRunnable
, aFutureTask
can be submitted to anExecutor
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
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Constructor Summary
Constructors Constructor Description FutureTask(Runnable runnable, V result)
Creates aFutureTask
that will, upon running, execute the givenRunnable
, and arrange thatget
will return the given result on successful completion.FutureTask(Callable<V> callable)
Creates aFutureTask
that will, upon running, execute the givenCallable
.
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Method Summary
All Methods Instance Methods Concrete Methods 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 stateisDone
(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()
Returnstrue
if this task was cancelled before it completed normally.boolean
isDone()
Returnstrue
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 anExecutionException
with the given throwable as its cause, unless this future has already been set or has been cancelled.
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Constructor Detail
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FutureTask
public FutureTask(Callable<V> callable)
Creates aFutureTask
that will, upon running, execute the givenCallable
.- Parameters:
callable
- the callable task- Throws:
NullPointerException
- if the callable is null
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FutureTask
public FutureTask(Runnable runnable, V result)
Creates aFutureTask
that will, upon running, execute the givenRunnable
, and arrange thatget
will return the given result on successful completion.- Parameters:
runnable
- the runnable taskresult
- 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
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Method Detail
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isCancelled
public boolean isCancelled()
Description copied from interface:Future
Returnstrue
if this task was cancelled before it completed normally.- Specified by:
isCancelled
in interfaceFuture<V>
- Returns:
true
if this task was cancelled before it completed
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isDone
public boolean isDone()
Description copied from interface:Future
Returnstrue
if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will returntrue
.
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cancel
public boolean cancel(boolean mayInterruptIfRunning)
Description copied from interface:Future
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 whencancel
is called, this task should never run. If the task has already started, then themayInterruptIfRunning
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 returntrue
. Subsequent calls toFuture.isCancelled()
will always returntrue
if this method returnedtrue
.- Specified by:
cancel
in interfaceFuture<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
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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 interfaceFuture<V>
- Returns:
- the computed result
- Throws:
CancellationException
- if the computation was cancelledInterruptedException
- if the current thread was interrupted while waitingExecutionException
- if the computation threw an exception
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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 interfaceFuture<V>
- Parameters:
timeout
- the maximum time to waitunit
- the time unit of the timeout argument- Returns:
- the computed result
- Throws:
CancellationException
- if the computation was cancelledInterruptedException
- if the current thread was interrupted while waitingExecutionException
- if the computation threw an exceptionTimeoutException
- if the wait timed out
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done
protected void done()
Protected method invoked when this task transitions to stateisDone
(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.
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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
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setException
protected void setException(Throwable t)
Causes this future to report anExecutionException
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
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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 interfaceRunnable
- Specified by:
run
in interfaceRunnableFuture<V>
- See Also:
Thread.run()
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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
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