java.util.concurrent
Class FutureTask<V>
- Future<V>, Runnable, 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.
FutureTask(Callable callable) - Creates a FutureTask that will upon running, execute the
given Callable.
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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.
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boolean | cancel(boolean mayInterruptIfRunning) - Attempts to cancel execution of this task.
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protected void | done() - Protected method invoked when this task transitions to state
isDone (whether normally or via cancellation).
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V | get()
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V | get(long timeout, TimeUnit unit)
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boolean | isCancelled() - Returns true if this task was cancelled before it completed
normally.
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boolean | isDone() - Returns true if this task completed.
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void | run() - Sets this Future to the result of its computation
unless it has been cancelled.
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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.
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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.
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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.
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clone , equals , extends Object> getClass , finalize , hashCode , notify , notifyAll , toString , wait , wait , wait |
FutureTask
public FutureTask(Callable callable)
Creates a FutureTask that will upon running, execute the
given Callable.
callable
- the callable task
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.
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<Object>(runnable, null)
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.
- cancel in interface Future<V>
mayInterruptIfRunning
- true if the thread executing this
task should be interrupted; otherwise, in-progress tasks are allowed
to complete
- 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.
isCancelled
public boolean isCancelled()
Returns true if this task was cancelled before it completed
normally.
- isCancelled in interface Future<V>
- 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.
- isDone in interface Future<V>
- true if this task completed
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.
- 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.
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.