java.lang.Object
java.util.concurrent.FutureTask<V>
- Type Parameters:
V- The result type returned by this FutureTask'sgetmethods
- All Implemented Interfaces:
Runnable,Future<V>,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
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Constructor Summary
ConstructorsConstructorDescriptionFutureTask(Runnable runnable, V result) Creates aFutureTaskthat will, upon running, execute the givenRunnable, and arrange thatgetwill return the given result on successful completion.FutureTask(Callable<V> callable) Creates aFutureTaskthat will, upon running, execute the givenCallable. -
Method Summary
Modifier and TypeMethodDescriptionbooleancancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.protected voiddone()Protected method invoked when this task transitions to stateisDone(whether normally or via cancellation).get()Waits if necessary for the computation to complete, and then retrieves its result.Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.booleanReturnstrueif this task was cancelled before it completed normally.booleanisDone()Returnstrueif this task completed.voidrun()Sets this Future to the result of its computation unless it has been cancelled.protected booleanExecutes 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 voidSets the result of this future to the given value unless this future has already been set or has been cancelled.protected voidCauses this future to report anExecutionExceptionwith the given throwable as its cause, unless this future has already been set or has been cancelled.toString()Returns a string representation of this FutureTask.
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Constructor Details
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FutureTask
Creates aFutureTaskthat will, upon running, execute the givenCallable.- Parameters:
callable- the callable task- Throws:
NullPointerException- if the callable is null
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FutureTask
Creates aFutureTaskthat will, upon running, execute the givenRunnable, and arrange thatgetwill 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 Details
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isCancelled
public boolean isCancelled()Description copied from interface:FutureReturnstrueif this task was cancelled before it completed normally.- Specified by:
isCancelledin interfaceFuture<V>- Returns:
trueif this task was cancelled before it completed
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isDone
public boolean isDone()Description copied from interface:FutureReturnstrueif this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will returntrue. -
cancel
public boolean cancel(boolean mayInterruptIfRunning) Description copied from interface:FutureAttempts 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 whencancelis called, this task should never run. If the task has already started, then themayInterruptIfRunningparameter 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:
cancelin interfaceFuture<V>- Parameters:
mayInterruptIfRunning-trueif 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:
falseif the task could not be cancelled, typically because it has already completed;trueotherwise. If two or more threads cause a task to be cancelled, then at least one of them returnstrue. Implementations may provide stronger guarantees.
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get
Description copied from interface:FutureWaits if necessary for the computation to complete, and then retrieves its result.- Specified by:
getin 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:FutureWaits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.- Specified by:
getin 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. -
set
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
Causes this future to report anExecutionExceptionwith 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:RunnableFutureSets this Future to the result of its computation unless it has been cancelled.- Specified by:
runin interfaceRunnable- Specified by:
runin interfaceRunnableFuture<V>- See Also:
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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:
trueif successfully run and reset
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toString
Returns a string representation of this FutureTask.- Overrides:
toStringin classObject- 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
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