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.common.annotations.GwtCompatible;
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 * <h3>Purpose</h3>
033 *
034 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
035 * asynchronous operations. You can chain them together manually with calls to methods like {@link
036 * Futures#transform(ListenableFuture, com.google.common.base.Function, Executor)
037 * Futures.transform}, but you will often find it easier to use a framework. Frameworks automate the
038 * process, often adding features like monitoring, debugging, and cancellation. Examples of
039 * frameworks include:
040 *
041 * <ul>
042 *   <li><a href="http://google.github.io/dagger/producers.html">Dagger Producers</a>
043 * </ul>
044 *
045 * <p>The main purpose of {@link #addListener addListener} is to support this chaining. You will
046 * rarely use it directly, in part because it does not provide direct access to the {@code Future}
047 * result. (If you want such access, you may prefer {@link Futures#addCallback
048 * Futures.addCallback}.) Still, direct {@code addListener} calls are occasionally useful:
049 *
050 * <pre>{@code
051 * final String name = ...;
052 * inFlight.add(name);
053 * ListenableFuture<Result> future = service.query(name);
054 * future.addListener(new Runnable() {
055 *   public void run() {
056 *     processedCount.incrementAndGet();
057 *     inFlight.remove(name);
058 *     lastProcessed.set(name);
059 *     logger.info("Done with {0}", name);
060 *   }
061 * }, executor);
062 * }</pre>
063 *
064 * <h3>How to get an instance</h3>
065 *
066 * <p>We encourage you to return {@code ListenableFuture} from your methods so that your users can
067 * take advantage of the {@linkplain Futures utilities built atop the class}. The way that you will
068 * create {@code ListenableFuture} instances depends on how you currently create {@code Future}
069 * instances:
070 *
071 * <ul>
072 *   <li>If you receive them from an {@code java.util.concurrent.ExecutorService}, convert that
073 *       service to a {@link ListeningExecutorService}, usually by calling {@link
074 *       MoreExecutors#listeningDecorator(java.util.concurrent.ExecutorService)
075 *       MoreExecutors.listeningDecorator}.
076 *   <li>If you manually call {@link java.util.concurrent.FutureTask#set} or a similar method,
077 *       create a {@link SettableFuture} instead. (If your needs are more complex, you may prefer
078 *       {@link AbstractFuture}.)
079 * </ul>
080 *
081 * <p><b>Test doubles</b>: If you need a {@code ListenableFuture} for your test, try a {@link
082 * SettableFuture} or one of the methods in the {@link Futures#immediateFuture Futures.immediate*}
083 * family. <b>Avoid</b> creating a mock or stub {@code Future}. Mock and stub implementations are
084 * fragile because they assume that only certain methods will be called and because they often
085 * implement subtleties of the API improperly.
086 *
087 * <p><b>Custom implementation</b>: Avoid implementing {@code ListenableFuture} from scratch. If you
088 * can't get by with the standard implementations, prefer to derive a new {@code Future} instance
089 * with the methods in {@link Futures} or, if necessary, to extend {@link AbstractFuture}.
090 *
091 * <p>Occasionally, an API will return a plain {@code Future} and it will be impossible to change
092 * the return type. For this case, we provide a more expensive workaround in {@code
093 * JdkFutureAdapters}. However, when possible, it is more efficient and reliable to create a {@code
094 * ListenableFuture} directly.
095 *
096 * @author Sven Mawson
097 * @author Nishant Thakkar
098 * @since 1.0
099 */
100@GwtCompatible
101public interface ListenableFuture<V> extends Future<V> {
102  /**
103   * Registers a listener to be {@linkplain Executor#execute(Runnable) run} on the given executor.
104   * The listener will run when the {@code Future}'s computation is {@linkplain Future#isDone()
105   * complete} or, if the computation is already complete, immediately.
106   *
107   * <p>There is no guaranteed ordering of execution of listeners, but any listener added through
108   * this method is guaranteed to be called once the computation is complete.
109   *
110   * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
111   * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
112   * thrown by {@linkplain MoreExecutors#directExecutor direct execution}) will be caught and
113   * logged.
114   *
115   * <p>Note: For fast, lightweight listeners that would be safe to execute in any thread, consider
116   * {@link MoreExecutors#directExecutor}. Otherwise, avoid it. Heavyweight {@code directExecutor}
117   * listeners can cause problems, and these problems can be difficult to reproduce because they
118   * depend on timing. For example:
119   *
120   * <ul>
121   *   <li>The listener may be executed by the caller of {@code addListener}. That caller may be a
122   *       UI thread or other latency-sensitive thread. This can harm UI responsiveness.
123   *   <li>The listener may be executed by the thread that completes this {@code Future}. That
124   *       thread may be an internal system thread such as an RPC network thread. Blocking that
125   *       thread may stall progress of the whole system. It may even cause a deadlock.
126   *   <li>The listener may delay other listeners, even listeners that are not themselves {@code
127   *       directExecutor} listeners.
128   * </ul>
129   *
130   * <p>This is the most general listener interface. For common operations performed using
131   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
132   * Futures#addCallback addCallback()}.
133   *
134   * <p>Memory consistency effects: Actions in a thread prior to adding a listener <a
135   * href="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5">
136   * <i>happen-before</i></a> its execution begins, perhaps in another thread.
137   *
138   * @param listener the listener to run when the computation is complete
139   * @param executor the executor to run the listener in
140   * @throws RejectedExecutionException if we tried to execute the listener immediately but the
141   *     executor rejected it.
142   */
143  void addListener(Runnable listener, Executor executor);
144}