Module java.base

Class FutureTask<V>

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

    Since:
    1.5
    Author:
    Doug Lea
    • Constructor Summary

      Constructors 
      Constructor Description
      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.
    • 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 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.
      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.
      String toString​()
      Returns a string representation of this FutureTask.
    • Constructor Detail

      • FutureTask

        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
      • 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.
        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
    • Method Detail

      • isCancelled

        public boolean isCancelled​()
        Description copied from interface: Future
        Returns true if this task was cancelled before it completed normally.
        Specified by:
        isCancelled in interface Future<V>
        Returns:
        true if this task was cancelled before it completed
      • isDone

        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.
        Specified by:
        isDone in interface Future<V>
        Returns:
        true if this task completed
      • cancel

        public boolean cancel​(boolean mayInterruptIfRunning)
        Description copied from interface: Future
        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.

        Specified by:
        cancel in interface Future<V>
        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
      • 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 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
      • 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.

        Parameters:
        t - the cause of failure
      • run

        public void run​()
        Description copied from interface: RunnableFuture
        Sets this Future to the result of its computation unless it has been cancelled.
        Specified by:
        run in interface Runnable
        Specified by:
        run in interface RunnableFuture<V>
      • 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
      • toString

        public String toString​()
        Returns a string representation of this FutureTask.
        Overrides:
        toString in class Object
        Returns:
        a string representation of this FutureTask
        Implementation Requirements:
        The default implementation returns a string identifying this FutureTask, as well as its completion state. The state, in brackets, contains one of the strings "Completed Normally", "Completed Exceptionally", "Cancelled", or "Not completed".