FutureTask (Java Platform SE 8 ) (original) (raw)

• java.lang.Object

• • java.util.concurrent.FutureTask

• Type Parameters:
  V - The result type returned by this FutureTask's get methods
  All Implemented Interfaces:
  Runnable, Future, RunnableFuture

public class FutureTask
extends Object
implements RunnableFuture
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 usingrunAndReset()).
A FutureTask can be used to wrap a Callable orRunnable object. Because FutureTask implementsRunnable, a FutureTask can be submitted to anExecutor for execution.
In addition to serving as a standalone class, this class providesprotected functionality that may be useful when creating customized task classes.
Since:
1.5

• • Constructor Summary

  Constructors

  Constructor	Description
  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.

  • Method Summary

  All Methods Instance Methods Concrete Methods

  Modifier and Type	Method	Description
  boolean	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).
  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.
  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 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.

     * ### Methods inherited from class java.lang.[Object](../../../java/lang/Object.html "class in java.lang")  
     `[clone](../../../java/lang/Object.html#clone--), [equals](../../../java/lang/Object.html#equals-java.lang.Object-), [finalize](../../../java/lang/Object.html#finalize--), [getClass](../../../java/lang/Object.html#getClass--), [hashCode](../../../java/lang/Object.html#hashCode--), [notify](../../../java/lang/Object.html#notify--), [notifyAll](../../../java/lang/Object.html#notifyAll--), [toString](../../../java/lang/Object.html#toString--), [wait](../../../java/lang/Object.html#wait--), [wait](../../../java/lang/Object.html#wait-long-), [wait](../../../java/lang/Object.html#wait-long-int-)`

• • Constructor Detail

     * #### FutureTask  
     public FutureTask([Callable](../../../java/util/concurrent/Callable.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")> callable)  
     Creates a `FutureTask` that will, upon running, execute the given `Callable`.  
     Parameters:  
     `callable` \- the callable task  
     Throws:  
     `[NullPointerException](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the callable is null  
     * #### FutureTask  
     public FutureTask([Runnable](../../../java/lang/Runnable.html "interface in java.lang") runnable,  
                       [V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask") 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](../../../java/lang/NullPointerException.html "class in java.lang")` \- if the runnable is null  

  • Method Detail

    * #### isCancelled  
    public boolean isCancelled()  
    Description copied from interface: `[Future](../../../java/util/concurrent/Future.html#isCancelled--)`  
    Returns `true` if this task was cancelled before it completed normally.  
    Specified by:  
    `[isCancelled](../../../java/util/concurrent/Future.html#isCancelled--)` in interface `[Future](../../../java/util/concurrent/Future.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")>`  
    Returns:  
    `true` if this task was cancelled before it completed  
    * #### isDone  
    public boolean isDone()  
    Description copied from interface: `[Future](../../../java/util/concurrent/Future.html#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`.  
    Specified by:  
    `[isDone](../../../java/util/concurrent/Future.html#isDone--)` in interface `[Future](../../../java/util/concurrent/Future.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")>`  
    Returns:  
    `true` if this task completed  
    * #### cancel  
    public boolean cancel(boolean mayInterruptIfRunning)  
    Description copied from interface: `[Future](../../../java/util/concurrent/Future.html#cancel-boolean-)`  
    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()](../../../java/util/concurrent/Future.html#isDone--) will always return `true`. Subsequent calls to [Future.isCancelled()](../../../java/util/concurrent/Future.html#isCancelled--) will always return `true` if this method returned `true`.  
    Specified by:  
    `[cancel](../../../java/util/concurrent/Future.html#cancel-boolean-)` in interface `[Future](../../../java/util/concurrent/Future.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")>`  
    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  
    * #### get  
    public [V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask") get()  
          throws [InterruptedException](../../../java/lang/InterruptedException.html "class in java.lang"),  
                 [ExecutionException](../../../java/util/concurrent/ExecutionException.html "class in java.util.concurrent")  
    Description copied from interface: `[Future](../../../java/util/concurrent/Future.html#get--)`  
    Waits if necessary for the computation to complete, and then retrieves its result.  
    Specified by:  
    `[get](../../../java/util/concurrent/Future.html#get--)` in interface `[Future](../../../java/util/concurrent/Future.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")>`  
    Returns:  
    the computed result  
    Throws:  
    `[CancellationException](../../../java/util/concurrent/CancellationException.html "class in java.util.concurrent")` \- if the computation was cancelled  
    `[InterruptedException](../../../java/lang/InterruptedException.html "class in java.lang")` \- if the current thread was interrupted while waiting  
    `[ExecutionException](../../../java/util/concurrent/ExecutionException.html "class in java.util.concurrent")` \- if the computation threw an exception  
    * #### get  
    public [V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask") get(long timeout,  
                 [TimeUnit](../../../java/util/concurrent/TimeUnit.html "enum in java.util.concurrent") unit)  
          throws [InterruptedException](../../../java/lang/InterruptedException.html "class in java.lang"),  
                 [ExecutionException](../../../java/util/concurrent/ExecutionException.html "class in java.util.concurrent"),  
                 [TimeoutException](../../../java/util/concurrent/TimeoutException.html "class in java.util.concurrent")  
    Description copied from interface: `[Future](../../../java/util/concurrent/Future.html#get-long-java.util.concurrent.TimeUnit-)`  
    Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.  
    Specified by:  
    `[get](../../../java/util/concurrent/Future.html#get-long-java.util.concurrent.TimeUnit-)` in interface `[Future](../../../java/util/concurrent/Future.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")>`  
    Parameters:  
    `timeout` \- the maximum time to wait  
    `unit` \- the time unit of the timeout argument  
    Returns:  
    the computed result  
    Throws:  
    `[CancellationException](../../../java/util/concurrent/CancellationException.html "class in java.util.concurrent")` \- if the computation was cancelled  
    `[InterruptedException](../../../java/lang/InterruptedException.html "class in java.lang")` \- if the current thread was interrupted while waiting  
    `[ExecutionException](../../../java/util/concurrent/ExecutionException.html "class in java.util.concurrent")` \- if the computation threw an exception  
    `[TimeoutException](../../../java/util/concurrent/TimeoutException.html "class in java.util.concurrent")` \- if the wait timed out  
    * #### 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.  
    * #### set  
    protected void set([V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask") 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()](../../../java/util/concurrent/FutureTask.html#run--) method upon successful completion of the computation.  
    Parameters:  
    `v` \- the value  
    * #### setException  
    protected void setException([Throwable](../../../java/lang/Throwable.html "class in java.lang") t)  
    Causes this future to report an [ExecutionException](../../../java/util/concurrent/ExecutionException.html "class in java.util.concurrent") 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()](../../../java/util/concurrent/FutureTask.html#run--) method upon failure of the computation.  
    Parameters:  
    `t` \- the cause of failure  
    * #### run  
    public void run()  
    Sets this Future to the result of its computation unless it has been cancelled.  
    Specified by:  
    `[run](../../../java/lang/Runnable.html#run--)` in interface `[Runnable](../../../java/lang/Runnable.html "interface in java.lang")`  
    Specified by:  
    `[run](../../../java/util/concurrent/RunnableFuture.html#run--)` in interface `[RunnableFuture](../../../java/util/concurrent/RunnableFuture.html "interface in java.util.concurrent")<[V](../../../java/util/concurrent/FutureTask.html "type parameter in FutureTask")>`  
    See Also:  
    [Thread.run()](../../../java/lang/Thread.html#run--)  
    * #### 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

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