Interface ListenableFuture<V>

  • All Superinterfaces:
    java.util.concurrent.Future<V>
    All Known Subinterfaces:
    CheckedFuture<V,​X>, ListenableScheduledFuture<V>
    All Known Implementing Classes:
    AbstractCheckedFuture, AbstractFuture, ForwardingCheckedFuture, ForwardingCheckedFuture.SimpleForwardingCheckedFuture, ForwardingListenableFuture, ForwardingListenableFuture.SimpleForwardingListenableFuture, ListenableFutureTask, SettableFuture

    @GwtCompatible
    public interface ListenableFuture<V>
    extends java.util.concurrent.Future<V>
    A Future that accepts completion listeners. Each listener has an associated executor, and it is invoked using this executor once the future's computation is complete. If the computation has already completed when the listener is added, the listener will execute immediately.

    See the Guava User Guide article on ListenableFuture.

    Purpose

    Most commonly, ListenableFuture is used as an input to another derived Future, as in Futures.allAsList. Many such methods are impossible to implement efficiently without listener support.

    It is possible to call addListener directly, but this is uncommon because the Runnable interface does not provide direct access to the Future result. (Users who want such access may prefer Futures.addCallback.) Still, direct addListener calls are occasionally useful:

       
       final String name = ...;
       inFlight.add(name);
       ListenableFuture<Result> future = service.query(name);
       future.addListener(new Runnable() {
         public void run() {
           processedCount.incrementAndGet();
           inFlight.remove(name);
           lastProcessed.set(name);
           logger.info("Done with {0}", name);
         }
       }, executor);

    How to get an instance

    Developers are encouraged to return ListenableFuture from their methods so that users can take advantages of the utilities built atop the class. The way that they will create ListenableFuture instances depends on how they currently create Future instances:

    Occasionally, an API will return a plain Future and it will be impossible to change the return type. For this case, we provide a more expensive workaround in JdkFutureAdapters. However, when possible, it is more efficient and reliable to create a ListenableFuture directly.

    Since:
    1.0
    Author:
    Sven Mawson, Nishant Thakkar
    • Method Summary

      All Methods Instance Methods Abstract Methods 
      Modifier and Type Method Description
      void addListener​(java.lang.Runnable listener, java.util.concurrent.Executor executor)
      Registers a listener to be run on the given executor.
      • Methods inherited from interface java.util.concurrent.Future

        cancel, get, get, isCancelled, isDone
    • Method Detail

      • addListener

        void addListener​(java.lang.Runnable listener,
                         java.util.concurrent.Executor executor)
        Registers a listener to be run on the given executor. The listener will run when the Future's computation is complete or, if the computation is already complete, immediately.

        There is no guaranteed ordering of execution of listeners, but any listener added through this method is guaranteed to be called once the computation is complete.

        Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown during Executor.execute (e.g., a RejectedExecutionException or an exception thrown by direct execution) will be caught and logged.

        Note: For fast, lightweight listeners that would be safe to execute in any thread, consider MoreExecutors.directExecutor(). Otherwise, avoid it. Heavyweight directExecutor listeners can cause problems, and these problems can be difficult to reproduce because they depend on timing. For example:

        • The listener may be executed by the caller of addListener. That caller may be a UI thread or other latency-sensitive thread. This can harm UI responsiveness.
        • The listener may be executed by the thread that completes this Future. That thread may be an internal system thread such as an RPC network thread. Blocking that thread may stall progress of the whole system. It may even cause a deadlock.
        • The listener may delay other listeners, even listeners that are not themselves directExecutor listeners.

        This is the most general listener interface. For common operations performed using listeners, see Futures. For a simplified but general listener interface, see addCallback().

        Memory consistency effects: Actions in a thread prior to adding a listener happen-before its execution begins, perhaps in another thread.

        Parameters:
        listener - the listener to run when the computation is complete
        executor - the executor to run the listener in
        Throws:
        java.lang.NullPointerException - if the executor or listener was null
        java.util.concurrent.RejectedExecutionException - if we tried to execute the listener immediately but the executor rejected it.