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.jspecify.annotations.NullMarked;
022import org.jspecify.annotations.Nullable;
023
024/**
025 * A {@link Future} that accepts completion listeners. Each listener has an associated executor, and
026 * it is invoked using this executor once the future's computation is {@linkplain Future#isDone()
027 * complete}. If the computation has already completed when the listener is added, the listener will
028 * execute immediately.
029 *
030 * <p>See the Guava User Guide article on <a
031 * href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
032 * ListenableFuture}</a>.
033 *
034 * <p>This class is GWT-compatible.
035 *
036 * <h3>Purpose</h3>
037 *
038 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
039 * asynchronous operations. You can chain them together manually with calls to methods like {@link
040 * Futures#transform(ListenableFuture, com.google.common.base.Function, Executor) Futures.transform}
041 * (or {@link FluentFuture#transform(com.google.common.base.Function, Executor)
042 * FluentFuture.transform}), but you will often find it easier to use a framework. Frameworks
043 * automate the process, often adding features like monitoring, debugging, and cancellation.
044 * Examples of frameworks include:
045 *
046 * <ul>
047 *   <li><a href="https://dagger.dev/producers.html">Dagger Producers</a>
048 * </ul>
049 *
050 * <p>The main purpose of {@link #addListener addListener} is to support this chaining. You will
051 * rarely use it directly, in part because it does not provide direct access to the {@code Future}
052 * result. (If you want such access, you may prefer {@link Futures#addCallback
053 * Futures.addCallback}.) Still, direct {@code addListener} calls are occasionally useful:
054 *
055 * {@snippet :
056 * final String name = ...;
057 * inFlight.add(name);
058 * ListenableFuture<Result> future = service.query(name);
059 * future.addListener(new Runnable() {
060 *   public void run() {
061 *     processedCount.incrementAndGet();
062 *     inFlight.remove(name);
063 *     lastProcessed.set(name);
064 *     logger.info("Done with {0}", name);
065 *   }
066 * }, executor);
067 * }
068 *
069 * <h3>How to get an instance</h3>
070 *
071 * <p>We encourage you to return {@code ListenableFuture} from your methods so that your users can
072 * take advantage of the {@linkplain Futures utilities built atop the class}. The way that you will
073 * create {@code ListenableFuture} instances depends on how you currently create {@code Future}
074 * instances:
075 *
076 * <ul>
077 *   <li>If you receive them from an {@code java.util.concurrent.ExecutorService}, convert that
078 *       service to a {@link ListeningExecutorService}, usually by calling {@link
079 *       MoreExecutors#listeningDecorator(java.util.concurrent.ExecutorService)
080 *       MoreExecutors.listeningDecorator}.
081 *   <li>If you manually call {@link java.util.concurrent.FutureTask#set} or a similar method,
082 *       create a {@link SettableFuture} instead. (If your needs are more complex, you may prefer
083 *       {@link AbstractFuture}.)
084 * </ul>
085 *
086 * <p><b>Test doubles</b>: If you need a {@code ListenableFuture} for your test, try a {@link
087 * SettableFuture} or one of the methods in the {@link Futures#immediateFuture Futures.immediate*}
088 * family. <b>Avoid</b> creating a mock or stub {@code Future}. Mock and stub implementations are
089 * fragile because they assume that only certain methods will be called and because they often
090 * implement subtleties of the API improperly.
091 *
092 * <p><b>Custom implementation</b>: Avoid implementing {@code ListenableFuture} from scratch. If you
093 * can't get by with the standard implementations, prefer to derive a new {@code Future} instance
094 * with the methods in {@link Futures} or, if necessary, to extend {@link AbstractFuture}.
095 *
096 * <p>Occasionally, an API will return a plain {@code Future} and it will be impossible to change
097 * the return type. For this case, we provide a more expensive workaround in {@code
098 * JdkFutureAdapters}. However, when possible, it is more efficient and reliable to create a {@code
099 * ListenableFuture} directly.
100 *
101 * @author Sven Mawson
102 * @author Nishant Thakkar
103 * @since 1.0
104 */
105/*
106 * Some of the annotations below were added after we released our separate
107 * com.google.guava:listenablefuture:1.0 artifact. (For more on that artifact, see
108 * https://github.com/google/guava/releases/tag/v27.0) This means that the copy of ListenableFuture
109 * in com.google.guava:guava differs from the "frozen" copy in the listenablefuture artifact. This
110 * could in principle cause problems for some users. Still, we expect that the benefits of the
111 * nullness annotations in particular will outweigh the costs. (And it's worth noting that we have
112 * released multiple ListenableFuture.class files that are not byte-for-byte compatible even from
113 * the beginning, thanks to using different `-source -target` values for compiling our `-jre` and
114 * `-android` "flavors.")
115 *
116 * (We could consider releasing a listenablefuture:1.0.1 someday. But we would want to look into how
117 * that affects users, especially users of the Android Gradle Plugin, since the plugin developers
118 * put in a special hack for us: https://issuetracker.google.com/issues/131431257)
119 */
120@DoNotMock("Use the methods in Futures (like immediateFuture) or SettableFuture")
121@NullMarked
122public interface ListenableFuture<V extends @Nullable Object> extends Future<V> {
123  /**
124   * Registers a listener to be {@linkplain Executor#execute(Runnable) run} on the given executor.
125   * The listener will run when the {@code Future}'s computation is {@linkplain Future#isDone()
126   * complete} or, if the computation is already complete, immediately.
127   *
128   * <p>There is no guaranteed ordering of execution of listeners, but any listener added through
129   * this method is guaranteed to be called once the computation is complete.
130   *
131   * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
132   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
133   * thrown by {@linkplain MoreExecutors#directExecutor direct execution}) will be caught and
134   * logged.
135   *
136   * <p>Note: If your listener is lightweight -- and will not cause stack overflow by completing
137   * more futures or adding more {@code directExecutor()} listeners inline -- consider {@link
138   * MoreExecutors#directExecutor}. Otherwise, avoid it: See the warnings on the docs for {@code
139   * directExecutor}.
140   *
141   * <p>This is the most general listener interface. For common operations performed using
142   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
143   * Futures#addCallback addCallback()}.
144   *
145   * <p>Memory consistency effects: Actions in a thread prior to adding a listener <a
146   * href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5">
147   * <i>happen-before</i></a> its execution begins, perhaps in another thread.
148   *
149   * <p>Guava implementations of {@code ListenableFuture} promptly release references to listeners
150   * after executing them.
151   *
152   * @param listener the listener to run when the computation is complete
153   * @param executor the executor to run the listener in
154   * @throws RejectedExecutionException if we tried to execute the listener immediately but the
155   *     executor rejected it.
156   */
157  void addListener(Runnable listener, Executor executor);
158}