Class FutureTask<V>
- Type Parameters:
V
- The result type returned by this FutureTask'sget
methods
- All Implemented Interfaces:
Runnable
,Future<V>
,RunnableFuture<V>
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
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Constructor Summary
ConstructorDescriptionFutureTask
(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
. -
Method Summary
Modifier and TypeMethodDescriptionboolean
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).Returns the exception thrown by the task, without waiting.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.boolean
Returnstrue
if this task was cancelled before it completed normally.boolean
isDone()
Returnstrue
if this task completed.Returns the computed result, without waiting.void
run()
Sets this Future to the result of its computation unless it has been cancelled.protected boolean
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
Sets the result of this future to the given value unless this future has already been set or has been cancelled.protected void
Causes this future to report anExecutionException
with the given throwable as its cause, unless this future has already been set or has been cancelled.state()
Returns the computation state.toString()
Returns a string representation of this FutureTask.
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Constructor Details
-
FutureTask
Creates aFutureTask
that will, upon running, execute the givenCallable
.- Parameters:
callable
- the callable task- Throws:
NullPointerException
- if the callable is null
-
FutureTask
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 Details
-
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
-
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
. -
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 whencancel
is called, this task should never run. If the task has already started, then themayInterruptIfRunning
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 interfaceFuture<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 returnstrue
. Implementations may provide stronger guarantees.
-
get
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
-
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
-
resultNow
Description copied from interface:Future
Returns the computed result, without waiting.This method is for cases where the caller knows that the task has already completed successfully, for example when filtering a stream of Future objects for the successful tasks and using a mapping operation to obtain a stream of results.
results = futures.stream() .filter(f -> f.state() == Future.State.SUCCESS) .map(Future::resultNow) .toList();
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exceptionNow
Description copied from interface:Future
Returns the exception thrown by the task, without waiting.This method is for cases where the caller knows that the task has already completed with an exception.
- Specified by:
exceptionNow
in interfaceFuture<V>
- Returns:
- the exception thrown by the task
- Since:
- 19
-
state
Description copied from interface:Future
Returns the computation state. -
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
-
setException
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
-
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>
-
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
Returns a string representation of this FutureTask.- Overrides:
toString
in 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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