Interface Future
A Future represents the result of an asynchronous computation. Methods are provided to check if the computation is complete, to wait for its completion, and to retrieve the result of the computation. The result can only be retrieved using method get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the cancel method. Additional methods are provided to determine if the task completed normally or was cancelled. Once a computation has completed, the computation cannot be cancelled. If you would like to use a Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future and return null as a result of the underlying task.
Sample Usage (Note that the following classes are all made-up.)
interface ArchiveSearcher < String search(String target); >class App < ExecutorService executor = . ; ArchiveSearcher searcher = . ; void showSearch(String target) throws InterruptedException < Callabletask = () -> searcher.search(target); Future future = executor.submit(task); displayOtherThings(); // do other things while searching try < displayText(future.get()); // use future >catch (ExecutionException ex) < cleanup(); return; >> > The FutureTask class is an implementation of Future that implements Runnable , and so may be executed by an Executor . For example, the above construction with submit could be replaced by:
FutureTask future = new FutureTask<>(task); executor.execute(future); Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get() in another thread.
Java get futuretask concurrent util
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.
Constructor Summary
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
Protected method invoked when this task transitions to state isDone (whether normally or via cancellation).
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
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.
Sets the result of this future to the given value unless this future has already been set or has been cancelled.
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
Constructor Detail
FutureTask
FutureTask
Creates a FutureTask that will, upon running, execute the given Runnable , and arrange that get will return the given result on successful completion.
Method Detail
isCancelled
public boolean isCancelled()
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 .
cancel
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 Future.isDone() will always return true . Subsequent calls to Future.isCancelled() will always return true if this method returned true .
get
public V get() throws InterruptedException, ExecutionException
get
public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
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
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.
setException
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.
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.
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For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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Java get futuretask concurrent util
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.
Constructor Summary
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
Protected method invoked when this task transitions to state isDone (whether normally or via cancellation).
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
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.
Sets the result of this future to the given value unless this future has already been set or has been cancelled.
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.