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}