001/*
002 * Copyright (C) 2009 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 static com.google.common.base.Preconditions.checkNotNull;
018import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
019
020import com.google.common.annotations.Beta;
021import com.google.common.annotations.GwtIncompatible;
022import java.util.concurrent.Executor;
023import java.util.concurrent.Executors;
024import java.util.concurrent.Future;
025import java.util.concurrent.ThreadFactory;
026import java.util.concurrent.atomic.AtomicBoolean;
027import org.checkerframework.checker.nullness.qual.Nullable;
028
029/**
030 * Utilities necessary for working with libraries that supply plain {@link Future} instances. Note
031 * that, whenever possible, it is strongly preferred to modify those libraries to return {@code
032 * ListenableFuture} directly.
033 *
034 * @author Sven Mawson
035 * @since 10.0 (replacing {@code Futures.makeListenable}, which existed in 1.0)
036 */
037@Beta
038@GwtIncompatible
039@ElementTypesAreNonnullByDefault
040public final class JdkFutureAdapters {
041  /**
042   * Assigns a thread to the given {@link Future} to provide {@link ListenableFuture} functionality.
043   *
044   * <p><b>Warning:</b> If the input future does not already implement {@code ListenableFuture}, the
045   * returned future will emulate {@link ListenableFuture#addListener} by taking a thread from an
046   * internal, unbounded pool at the first call to {@code addListener} and holding it until the
047   * future is {@linkplain Future#isDone() done}.
048   *
049   * <p>Prefer to create {@code ListenableFuture} instances with {@link SettableFuture}, {@link
050   * MoreExecutors#listeningDecorator( java.util.concurrent.ExecutorService)}, {@link
051   * ListenableFutureTask}, {@link AbstractFuture}, and other utilities over creating plain {@code
052   * Future} instances to be upgraded to {@code ListenableFuture} after the fact.
053   */
054  public static <V extends @Nullable Object> ListenableFuture<V> listenInPoolThread(
055      Future<V> future) {
056    if (future instanceof ListenableFuture) {
057      return (ListenableFuture<V>) future;
058    }
059    return new ListenableFutureAdapter<>(future);
060  }
061
062  /**
063   * Submits a blocking task for the given {@link Future} to provide {@link ListenableFuture}
064   * functionality.
065   *
066   * <p><b>Warning:</b> If the input future does not already implement {@code ListenableFuture}, the
067   * returned future will emulate {@link ListenableFuture#addListener} by submitting a task to the
068   * given executor at the first call to {@code addListener}. The task must be started by the
069   * executor promptly, or else the returned {@code ListenableFuture} may fail to work. The task's
070   * execution consists of blocking until the input future is {@linkplain Future#isDone() done}, so
071   * each call to this method may claim and hold a thread for an arbitrary length of time. Use of
072   * bounded executors or other executors that may fail to execute a task promptly may result in
073   * deadlocks.
074   *
075   * <p>Prefer to create {@code ListenableFuture} instances with {@link SettableFuture}, {@link
076   * MoreExecutors#listeningDecorator( java.util.concurrent.ExecutorService)}, {@link
077   * ListenableFutureTask}, {@link AbstractFuture}, and other utilities over creating plain {@code
078   * Future} instances to be upgraded to {@code ListenableFuture} after the fact.
079   *
080   * @since 12.0
081   */
082  public static <V extends @Nullable Object> ListenableFuture<V> listenInPoolThread(
083      Future<V> future, Executor executor) {
084    checkNotNull(executor);
085    if (future instanceof ListenableFuture) {
086      return (ListenableFuture<V>) future;
087    }
088    return new ListenableFutureAdapter<>(future, executor);
089  }
090
091  /**
092   * An adapter to turn a {@link Future} into a {@link ListenableFuture}. This will wait on the
093   * future to finish, and when it completes, run the listeners. This implementation will wait on
094   * the source future indefinitely, so if the source future never completes, the adapter will never
095   * complete either.
096   *
097   * <p>If the delegate future is interrupted or throws an unexpected unchecked exception, the
098   * listeners will not be invoked.
099   */
100  private static class ListenableFutureAdapter<V extends @Nullable Object>
101      extends ForwardingFuture<V> implements ListenableFuture<V> {
102
103    private static final ThreadFactory threadFactory =
104        new ThreadFactoryBuilder()
105            .setDaemon(true)
106            .setNameFormat("ListenableFutureAdapter-thread-%d")
107            .build();
108    private static final Executor defaultAdapterExecutor =
109        Executors.newCachedThreadPool(threadFactory);
110
111    private final Executor adapterExecutor;
112
113    // The execution list to hold our listeners.
114    private final ExecutionList executionList = new ExecutionList();
115
116    // This allows us to only start up a thread waiting on the delegate future when the first
117    // listener is added.
118    private final AtomicBoolean hasListeners = new AtomicBoolean(false);
119
120    // The delegate future.
121    private final Future<V> delegate;
122
123    ListenableFutureAdapter(Future<V> delegate) {
124      this(delegate, defaultAdapterExecutor);
125    }
126
127    ListenableFutureAdapter(Future<V> delegate, Executor adapterExecutor) {
128      this.delegate = checkNotNull(delegate);
129      this.adapterExecutor = checkNotNull(adapterExecutor);
130    }
131
132    @Override
133    protected Future<V> delegate() {
134      return delegate;
135    }
136
137    @Override
138    public void addListener(Runnable listener, Executor exec) {
139      executionList.add(listener, exec);
140
141      // When a listener is first added, we run a task that will wait for the delegate to finish,
142      // and when it is done will run the listeners.
143      if (hasListeners.compareAndSet(false, true)) {
144        if (delegate.isDone()) {
145          // If the delegate is already done, run the execution list immediately on the current
146          // thread.
147          executionList.execute();
148          return;
149        }
150
151        // TODO(lukes): handle RejectedExecutionException
152        adapterExecutor.execute(
153            () -> {
154              try {
155                /*
156                 * Threads from our private pool are never interrupted. Threads from a
157                 * user-supplied executor might be, but... what can we do? This is another reason
158                 * to return a proper ListenableFuture instead of using listenInPoolThread.
159                 */
160                getUninterruptibly(delegate);
161              } catch (Throwable e) {
162                // ExecutionException / CancellationException / RuntimeException / Error
163                // The task is presumably done, run the listeners.
164              }
165              executionList.execute();
166            });
167      }
168    }
169  }
170
171  private JdkFutureAdapters() {}
172}