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