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   * Copyright (C) 2007 The Guava Authors
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
A java.util.concurrent.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: 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
 public interface ListenableFuture<V> extends Future<V> {
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 MoreExecutors.sameThreadExecutor()) will be caught and logged.

Note: For fast, lightweight listeners that would be safe to execute in any thread, consider MoreExecutors.sameThreadExecutor(). For heavier listeners, sameThreadExecutor() carries some caveats. For example, the listener may run on an unpredictable or undesirable thread:

  • If the input Future is done at the time addListener is called, addListener will execute the listener inline.
  • If the input Future is not yet done, addListener will schedule the listener to be run by the thread that completes the input Future, which may be an internal system thread such as an RPC network thread.
Also note that, regardless of which thread executes the 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 Futures.addCallback(,

listener the listener to run when the computation is complete
executor the executor to run the listener in
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.
  void addListener(Runnable listenerExecutor executor);
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