Interface ListenableFuture<V>

All Superinterfaces:
All Known Implementing Classes:
AbstractFuture, SettableFuture

public interface ListenableFuture<V> extends 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.


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 = ...;
   ListenableFuture<Result> future = service.query(name);
   future.addListener(new Runnable() {
     public void run() {
       lastProcessed.set(name);"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:

  • If they are returned from an ExecutorService, convert that service to a ListeningExecutorService, usually by calling MoreExecutors.listeningDecorator. (Custom executors may find it more convenient to use ListenableFutureTask directly.)
  • If they are manually filled in by a call to FutureTask.set(V) or a similar method, create a SettableFuture instead. (Users with more complex needs may prefer AbstractFuture.)

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.

Sven Mawson, Nishant Thakkar
  • Method Details

    • addListener

      void addListener(Runnable listener, 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(). For heavier listeners, directExecutor() carries some caveats. For example, the listener may run on an unpredictable or undesirable thread:

      • If this Future is done at the time addListener is called, addListener will execute the listener inline.
      • If this Future is not yet done, addListener will schedule the listener to be run by the thread that completes this Future, which may be an internal system thread such as an RPC network thread.

      Also note that, regardless of which thread executes the directExecutor() listener, all other registered but unexecuted listeners are prevented from running during its execution, even if those listeners are to run in other executors.

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

      listener - the listener to run when the computation is complete
      executor - the executor to run the listener in
      NullPointerException - if the executor or listener was null
      RejectedExecutionException - if we tried to execute the listener immediately but the executor rejected it.