V - The result type returned by this FutureTask's get methods
Runnable, Future<V>, 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
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.
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.
FutureTask(Callable<V> callable)
Creates a FutureTask that will, upon running, execute the given Callable.
protected void done()
Protected method invoked when this task transitions to state isDone (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.
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.
String toString()
Returns a string representation of this FutureTask.
Creates a FutureTask that will, upon running, execute the given Callable.
callable - the callable task
NullPointerException - if the callable is null
(
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 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<Void>(runnable, null)
NullPointerException - if the runnable is null
Future
Waits if necessary for the computation to complete, and then retrieves its result.
get in interface Future<V>
CancellationException - if the computation was cancelled
InterruptedException - if the current thread was interrupted while waiting
ExecutionException - if the computation threw an exception
Future
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
get in interface Future<V>
timeout - the maximum time to wait
unit - the time unit of the timeout argument
CancellationException - if the computation was cancelled
InterruptedException - if the current thread was interrupted while waiting
ExecutionException - if the computation threw an exception
TimeoutException - if the wait timed out
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.
(
V v)This method is invoked internally by the RunnableFuture.run() method upon successful completion of the computation.
v - the value
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 RunnableFuture.run() method upon failure of the computation.
t - the cause of failure
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
Returns a string representation of this FutureTask.
toString in class Object
"Completed Normally", "Completed Exceptionally", "Cancelled", or "Not completed".
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