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;
021import org.checkerframework.checker.nullness.qual.Nullable;
022
023/**
024 * A {@link Future} that accepts completion listeners. Each listener has an associated executor, and
025 * it is invoked using this executor once the future's computation is {@linkplain Future#isDone()
026 * complete}. If the computation has already completed when the listener is added, the listener will
027 * execute immediately.
028 *
029 * <p>See the Guava User Guide article on <a
030 * href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
031 * ListenableFuture}</a>.
032 *
033 * <p>This class is GWT-compatible.
034 *
035 * <h3>Purpose</h3>
036 *
037 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
038 * asynchronous operations. You can chain them together manually with calls to methods like {@link
039 * Futures#transform(ListenableFuture, com.google.common.base.Function, Executor) Futures.transform}
040 * (or {@link FluentFuture#transform(com.google.common.base.Function, Executor)
041 * FluentFuture.transform}), but you will often find it easier to use a framework. Frameworks
042 * automate the process, often adding features like monitoring, debugging, and cancellation.
043 * Examples of frameworks include:
044 *
045 * <ul>
046 *   <li><a href="https://dagger.dev/producers.html">Dagger Producers</a>
047 * </ul>
048 *
049 * <p>The main purpose of {@link #addListener addListener} is to support this chaining. You will
050 * rarely use it directly, in part because it does not provide direct access to the {@code Future}
051 * result. (If you want such access, you may prefer {@link Futures#addCallback
052 * Futures.addCallback}.) Still, direct {@code addListener} calls are occasionally useful:
053 *
054 * <pre>{@code
055 * final String name = ...;
056 * inFlight.add(name);
057 * ListenableFuture<Result> future = service.query(name);
058 * future.addListener(new Runnable() {
059 *   public void run() {
060 *     processedCount.incrementAndGet();
061 *     inFlight.remove(name);
062 *     lastProcessed.set(name);
063 *     logger.info("Done with {0}", name);
064 *   }
065 * }, executor);
066 * }</pre>
067 *
068 * <h3>How to get an instance</h3>
069 *
070 * <p>We encourage you to return {@code ListenableFuture} from your methods so that your users can
071 * take advantage of the {@linkplain Futures utilities built atop the class}. The way that you will
072 * create {@code ListenableFuture} instances depends on how you currently create {@code Future}
073 * instances:
074 *
075 * <ul>
076 *   <li>If you receive them from an {@code java.util.concurrent.ExecutorService}, convert that
077 *       service to a {@link ListeningExecutorService}, usually by calling {@link
078 *       MoreExecutors#listeningDecorator(java.util.concurrent.ExecutorService)
079 *       MoreExecutors.listeningDecorator}.
080 *   <li>If you manually call {@link java.util.concurrent.FutureTask#set} or a similar method,
081 *       create a {@link SettableFuture} instead. (If your needs are more complex, you may prefer
082 *       {@link AbstractFuture}.)
083 * </ul>
084 *
085 * <p><b>Test doubles</b>: If you need a {@code ListenableFuture} for your test, try a {@link
086 * SettableFuture} or one of the methods in the {@link Futures#immediateFuture Futures.immediate*}
087 * family. <b>Avoid</b> creating a mock or stub {@code Future}. Mock and stub implementations are
088 * fragile because they assume that only certain methods will be called and because they often
089 * implement subtleties of the API improperly.
090 *
091 * <p><b>Custom implementation</b>: Avoid implementing {@code ListenableFuture} from scratch. If you
092 * can't get by with the standard implementations, prefer to derive a new {@code Future} instance
093 * with the methods in {@link Futures} or, if necessary, to extend {@link AbstractFuture}.
094 *
095 * <p>Occasionally, an API will return a plain {@code Future} and it will be impossible to change
096 * the return type. For this case, we provide a more expensive workaround in {@code
097 * JdkFutureAdapters}. However, when possible, it is more efficient and reliable to create a {@code
098 * ListenableFuture} directly.
099 *
100 * @author Sven Mawson
101 * @author Nishant Thakkar
102 * @since 1.0
103 */
104/*
105 * Some of the annotations below were added after we released our separate
106 * com.google.guava:listenablefuture:1.0 artifact. (For more on that artifact, see
107 * https://github.com/google/guava/releases/tag/v27.0) This means that the copy of ListenableFuture
108 * in com.google.guava:guava differs from the "frozen" copy in the listenablefuture artifact. This
109 * could in principle cause problems for some users. Still, we expect that the benefits of the
110 * nullness annotations in particular will outweigh the costs. (And it's worth noting that we have
111 * released multiple ListenableFuture.class files that are not byte-for-byte compatible even from
112 * the beginning, thanks to using different `-source -target` values for compiling our `-jre` and
113 * `-android` "flavors.")
114 *
115 * (We could consider releasing a listenablefuture:1.0.1 someday. But we would want to look into how
116 * that affects users, especially users of the Android Gradle Plugin, since the plugin developers
117 * put in a special hack for us: https://issuetracker.google.com/issues/131431257)
118 */
119@DoNotMock("Use the methods in Futures (like immediateFuture) or SettableFuture")
120@ElementTypesAreNonnullByDefault
121public interface ListenableFuture<V extends @Nullable Object> extends Future<V> {
122  /**
123   * Registers a listener to be {@linkplain Executor#execute(Runnable) run} on the given executor.
124   * The listener will run when the {@code Future}'s computation is {@linkplain Future#isDone()
125   * complete} or, if the computation is already complete, immediately.
126   *
127   * <p>There is no guaranteed ordering of execution of listeners, but any listener added through
128   * this method is guaranteed to be called once the computation is complete.
129   *
130   * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
131   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
132   * thrown by {@linkplain MoreExecutors#directExecutor direct execution}) will be caught and
133   * logged.
134   *
135   * <p>Note: If your listener is lightweight -- and will not cause stack overflow by completing
136   * more futures or adding more {@code directExecutor()} listeners inline -- consider {@link
137   * MoreExecutors#directExecutor}. Otherwise, avoid it: See the warnings on the docs for {@code
138   * directExecutor}.
139   *
140   * <p>This is the most general listener interface. For common operations performed using
141   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
142   * Futures#addCallback addCallback()}.
143   *
144   * <p>Memory consistency effects: Actions in a thread prior to adding a listener <a
145   * href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5">
146   * <i>happen-before</i></a> its execution begins, perhaps in another thread.
147   *
148   * <p>Guava implementations of {@code ListenableFuture} promptly release references to listeners
149   * after executing them.
150   *
151   * @param listener the listener to run when the computation is complete
152   * @param executor the executor to run the listener in
153   * @throws RejectedExecutionException if we tried to execute the listener immediately but the
154   *     executor rejected it.
155   */
156  void addListener(Runnable listener, Executor executor);
157}