001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.util.concurrent;
018
019import com.google.common.annotations.Beta;
020
021import java.util.concurrent.Executor;
022import java.util.concurrent.Future;
023import java.util.concurrent.RejectedExecutionException;
024
025/**
026 * A {@link Future} that accepts completion listeners.  Each listener has an
027 * associated executor, and is invoked using this executor once the future's
028 * computation is {@linkplain Future#isDone() complete}.  If the computation has
029 * already completed when the listener is added, the listener will execute
030 * immediately.
031 *
032 * <p>Common {@code ListenableFuture} implementations include {@link
033 * SettableFuture} and the futures returned by a {@link
034 * ListeningExecutorService} (typically {@link ListenableFutureTask}
035 * instances).
036 *
037 * <p>Usage:
038 * <pre>   {@code
039 *   final ListenableFuture<?> future = myService.async(myRequest);
040 *   future.addListener(new Runnable() {
041 *     public void run() {
042 *       System.out.println("Operation Complete.");
043 *       try {
044 *         System.out.println("Result: " + future.get());
045 *       } catch (Exception e) {
046 *         System.out.println("Error: " + e.message());
047 *       }
048 *     }
049 *   }, executor);}</pre>
050 *
051 * @author Sven Mawson
052 * @author Nishant Thakkar
053 * @since 1
054 */
055@Beta
056public interface ListenableFuture<V> extends Future<V> {
057  /**
058   * Registers a listener to be {@linkplain Executor#execute(Runnable) run} on
059   * the given executor.  The listener will run when the {@code Future}'s
060   * computation is {@linkplain Future#isDone() complete} or, if the computation
061   * is already complete, immediately.
062   *
063   * <p>There is no guaranteed ordering of execution of listeners, but any
064   * listener added through this method is guaranteed to be called once the
065   * computation is complete.
066   *
067   * <p>Listeners cannot throw checked exceptions and should not throw {@code
068   * RuntimeException} unless their executors are prepared to handle it.
069   * Listeners that will execute in {@link MoreExecutors#sameThreadExecutor}
070   * should take special care, since they may run during the call to {@code
071   * addListener} or during the call that sets the future's value.
072   *
073   * @param listener the listener to run when the computation is complete
074   * @param executor the executor to run the listener in
075   * @throws NullPointerException if the executor or listener was null
076   * @throws RejectedExecutionException if we tried to execute the listener
077   *         immediately but the executor rejected it.
078   */
079  void addListener(Runnable listener, Executor executor);
080}