V - The result type returned by this FutureTask's get methods
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.
Constructors
Creates a FutureTask that will, upon running, execute the given Runnable, and arrange that get will return the given result on successful completion.
Creates a FutureTask that will, upon running, execute the given Callable.
boolean
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
protected void
Protected method invoked when this task transitions to state isDone (whether normally or via cancellation).
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
Returns true if this task was cancelled before it completed normally.
boolean
Returns true if this task completed.
void
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 an
ExecutionException
with the given throwable as its cause, unless this future has already been set or has been cancelled.
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
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
public boolean isCancelled()
Description copied from interface:Future
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
public boolean isDone()
Description copied from interface:Future
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.
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 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 (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().
cancel in interface Future<V>
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
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 returns true. Implementations may provide stronger guarantees.
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.
This method is invoked internally by the 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 run() method upon failure of the computation.
t - the cause of failure
public void run()
Sets this Future to the result of its computation unless it has been cancelled.
run in interface Runnable
run in interface RunnableFuture<V>
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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