FutureTask

AI-generated Key Takeaways

• FutureTask represents a cancellable asynchronous computation that implements RunnableFuture.

• It can wrap a Callable or Runnable for execution by an Executor.

• Provides methods to start, cancel, query completion status, and retrieve the computation result.

• get methods will block until the computation is completed.

• Offers protected methods for creating custom task classes.

public class FutureTask 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.

Public Constructor Summary

	FutureTask(Callable<V> callable) Creates a FutureTask that will, upon running, execute the given Callable.
	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.

Public Method Summary

boolean	cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.
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.
V	get() Waits if necessary for the computation to complete, and then retrieves its result.
boolean	isCancelled() Returns true if this task was cancelled before it completed normally.
boolean	isDone() Returns true if this task completed.
void	run() Sets this Future to the result of its computation unless it has been cancelled.

Protected Method Summary

void	done() Protected method invoked when this task transitions to state isDone (whether normally or via cancellation).
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.
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.
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.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.util.concurrent.RunnableFuture

abstract void

run() Sets this Future to the result of its computation unless it has been cancelled.

From interface java.lang.Runnable

abstract void

run() When an object implementing interface Runnable is used to create a thread, starting the thread causes the object's run method to be called in that separately executing thread.

From interface java.util.concurrent.Future

abstract boolean	cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.
abstract 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.
abstract V	get() Waits if necessary for the computation to complete, and then retrieves its result.
abstract boolean	isCancelled() Returns true if this task was cancelled before it completed normally.
abstract boolean	isDone() Returns true if this task completed.

Public Constructors

public FutureTask (Callable<V> callable)

Creates a FutureTask that will, upon running, execute the given Callable.

Parameters

callable	the callable task

Throws

NullPointerException	if the callable is null

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.

Parameters

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)

Throws

NullPointerException	if the runnable is null

Public Methods

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 isDone() will always return true. Subsequent calls to isCancelled() will always return true if this method returned true.

Parameters

mayInterruptIfRunning	true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete

Returns

• false if the task could not be cancelled, typically because it has already completed normally; true otherwise

public 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.

Parameters

timeout	the maximum time to wait
unit	the time unit of the timeout argument

Returns

• the computed result

Throws

CancellationException
InterruptedException
ExecutionException
TimeoutException

public V get ()

Waits if necessary for the computation to complete, and then retrieves its result.

Returns

• the computed result

Throws

CancellationException
InterruptedException
ExecutionException

public boolean isCancelled ()

Returns true if this task was cancelled before it completed normally.

Returns

• true if this task was cancelled before it completed

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.

Returns

• true if this task completed

public void run ()

Sets this Future to the result of its computation unless it has been cancelled.

Protected Methods

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.

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

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.

Parameters

v	the value

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.

Parameters

t	the cause of failure