001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import com.google.errorprone.annotations.DoNotMock;
018import java.util.concurrent.Executor;
019import java.util.concurrent.Future;
020import java.util.concurrent.RejectedExecutionException;
021
022/**
023 * A {@link Future} that accepts completion listeners. Each listener has an associated executor, and
024 * it is invoked using this executor once the future's computation is {@linkplain Future#isDone()
025 * complete}. If the computation has already completed when the listener is added, the listener will
026 * execute immediately.
027 *
028 * <p>See the Guava User Guide article on <a
029 * href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
030 * ListenableFuture}</a>.
031 *
032 * <p>This class is GWT-compatible.
033 *
034 * <h3>Purpose</h3>
035 *
036 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
037 * asynchronous operations. You can chain them together manually with calls to methods like {@link
038 * Futures#transform(ListenableFuture, com.google.common.base.Function, Executor)
039 * Futures.transform}, but you will often find it easier to use a framework. Frameworks automate the
040 * process, often adding features like monitoring, debugging, and cancellation. Examples of
041 * frameworks include:
042 *
043 * <ul>
044 *   <li><a href="http://dagger.dev/producers.html">Dagger Producers</a>
045 * </ul>
046 *
047 * <p>The main purpose of {@link #addListener addListener} is to support this chaining. You will
048 * rarely use it directly, in part because it does not provide direct access to the {@code Future}
049 * result. (If you want such access, you may prefer {@link Futures#addCallback
050 * Futures.addCallback}.) Still, direct {@code addListener} calls are occasionally useful:
051 *
052 * <pre>{@code
053 * final String name = ...;
054 * inFlight.add(name);
055 * ListenableFuture<Result> future = service.query(name);
056 * future.addListener(new Runnable() {
057 *   public void run() {
058 *     processedCount.incrementAndGet();
059 *     inFlight.remove(name);
060 *     lastProcessed.set(name);
061 *     logger.info("Done with {0}", name);
062 *   }
063 * }, executor);
064 * }</pre>
065 *
066 * <h3>How to get an instance</h3>
067 *
068 * <p>We encourage you to return {@code ListenableFuture} from your methods so that your users can
069 * take advantage of the {@linkplain Futures utilities built atop the class}. The way that you will
070 * create {@code ListenableFuture} instances depends on how you currently create {@code Future}
071 * instances:
072 *
073 * <ul>
074 *   <li>If you receive them from an {@code java.util.concurrent.ExecutorService}, convert that
075 *       service to a {@link ListeningExecutorService}, usually by calling {@link
076 *       MoreExecutors#listeningDecorator(java.util.concurrent.ExecutorService)
077 *       MoreExecutors.listeningDecorator}.
078 *   <li>If you manually call {@link java.util.concurrent.FutureTask#set} or a similar method,
079 *       create a {@link SettableFuture} instead. (If your needs are more complex, you may prefer
080 *       {@link AbstractFuture}.)
081 * </ul>
082 *
083 * <p><b>Test doubles</b>: If you need a {@code ListenableFuture} for your test, try a {@link
084 * SettableFuture} or one of the methods in the {@link Futures#immediateFuture Futures.immediate*}
085 * family. <b>Avoid</b> creating a mock or stub {@code Future}. Mock and stub implementations are
086 * fragile because they assume that only certain methods will be called and because they often
087 * implement subtleties of the API improperly.
088 *
089 * <p><b>Custom implementation</b>: Avoid implementing {@code ListenableFuture} from scratch. If you
090 * can't get by with the standard implementations, prefer to derive a new {@code Future} instance
091 * with the methods in {@link Futures} or, if necessary, to extend {@link AbstractFuture}.
092 *
093 * <p>Occasionally, an API will return a plain {@code Future} and it will be impossible to change
094 * the return type. For this case, we provide a more expensive workaround in {@code
095 * JdkFutureAdapters}. However, when possible, it is more efficient and reliable to create a {@code
096 * ListenableFuture} directly.
097 *
098 * @author Sven Mawson
099 * @author Nishant Thakkar
100 * @since 1.0
101 */
102@DoNotMock("Use the methods in Futures (like immediateFuture) or SettableFuture")
103public interface ListenableFuture<V> extends Future<V> {
104  /**
105   * Registers a listener to be {@linkplain Executor#execute(Runnable) run} on the given executor.
106   * The listener will run when the {@code Future}'s computation is {@linkplain Future#isDone()
107   * complete} or, if the computation is already complete, immediately.
108   *
109   * <p>There is no guaranteed ordering of execution of listeners, but any listener added through
110   * this method is guaranteed to be called once the computation is complete.
111   *
112   * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
113   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
114   * thrown by {@linkplain MoreExecutors#directExecutor direct execution}) will be caught and
115   * logged.
116   *
117   * <p>Note: For fast, lightweight listeners that would be safe to execute in any thread, consider
118   * {@link MoreExecutors#directExecutor}. Otherwise, avoid it. Heavyweight {@code directExecutor}
119   * listeners can cause problems, and these problems can be difficult to reproduce because they
120   * depend on timing. For example:
121   *
122   * <ul>
123   *   <li>The listener may be executed by the caller of {@code addListener}. That caller may be a
124   *       UI thread or other latency-sensitive thread. This can harm UI responsiveness.
125   *   <li>The listener may be executed by the thread that completes this {@code Future}. That
126   *       thread may be an internal system thread such as an RPC network thread. Blocking that
127   *       thread may stall progress of the whole system. It may even cause a deadlock.
128   *   <li>The listener may delay other listeners, even listeners that are not themselves {@code
129   *       directExecutor} listeners.
130   * </ul>
131   *
132   * <p>This is the most general listener interface. For common operations performed using
133   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
134   * Futures#addCallback addCallback()}.
135   *
136   * <p>Memory consistency effects: Actions in a thread prior to adding a listener <a
137   * href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5">
138   * <i>happen-before</i></a> its execution begins, perhaps in another thread.
139   *
140   * <p>Guava implementations of {@code ListenableFuture} promptly release references to listeners
141   * after executing them.
142   *
143   * @param listener the listener to run when the computation is complete
144   * @param executor the executor to run the listener in
145   * @throws RejectedExecutionException if we tried to execute the listener immediately but the
146   *     executor rejected it.
147   */
148  void addListener(Runnable listener, Executor executor);
149}