JdkFutureAdapters.java
/*
* Copyright (C) 2009 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
* in compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License
* is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
* or implied. See the License for the specific language governing permissions and limitations under
* the License.
*/
package com.google.common.util.concurrent;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicBoolean;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* Utilities necessary for working with libraries that supply plain {@link Future} instances. Note
* that, whenever possible, it is strongly preferred to modify those libraries to return {@code
* ListenableFuture} directly.
*
* @author Sven Mawson
* @since 10.0 (replacing {@code Futures.makeListenable}, which existed in 1.0)
*/
@Beta
@GwtIncompatible
@ElementTypesAreNonnullByDefault
public final class JdkFutureAdapters {
/**
* Assigns a thread to the given {@link Future} to provide {@link ListenableFuture} functionality.
*
* <p><b>Warning:</b> If the input future does not already implement {@code ListenableFuture}, the
* returned future will emulate {@link ListenableFuture#addListener} by taking a thread from an
* internal, unbounded pool at the first call to {@code addListener} and holding it until the
* future is {@linkplain Future#isDone() done}.
*
* <p>Prefer to create {@code ListenableFuture} instances with {@link SettableFuture}, {@link
* MoreExecutors#listeningDecorator( java.util.concurrent.ExecutorService)}, {@link
* ListenableFutureTask}, {@link AbstractFuture}, and other utilities over creating plain {@code
* Future} instances to be upgraded to {@code ListenableFuture} after the fact.
*/
public static <V extends @Nullable Object> ListenableFuture<V> listenInPoolThread(
Future<V> future) {
if (future instanceof ListenableFuture) {
return (ListenableFuture<V>) future;
}
return new ListenableFutureAdapter<V>(future);
}
/**
* Submits a blocking task for the given {@link Future} to provide {@link ListenableFuture}
* functionality.
*
* <p><b>Warning:</b> If the input future does not already implement {@code ListenableFuture}, the
* returned future will emulate {@link ListenableFuture#addListener} by submitting a task to the
* given executor at the first call to {@code addListener}. The task must be started by the
* executor promptly, or else the returned {@code ListenableFuture} may fail to work. The task's
* execution consists of blocking until the input future is {@linkplain Future#isDone() done}, so
* each call to this method may claim and hold a thread for an arbitrary length of time. Use of
* bounded executors or other executors that may fail to execute a task promptly may result in
* deadlocks.
*
* <p>Prefer to create {@code ListenableFuture} instances with {@link SettableFuture}, {@link
* MoreExecutors#listeningDecorator( java.util.concurrent.ExecutorService)}, {@link
* ListenableFutureTask}, {@link AbstractFuture}, and other utilities over creating plain {@code
* Future} instances to be upgraded to {@code ListenableFuture} after the fact.
*
* @since 12.0
*/
public static <V extends @Nullable Object> ListenableFuture<V> listenInPoolThread(
Future<V> future, Executor executor) {
checkNotNull(executor);
if (future instanceof ListenableFuture) {
return (ListenableFuture<V>) future;
}
return new ListenableFutureAdapter<V>(future, executor);
}
/**
* An adapter to turn a {@link Future} into a {@link ListenableFuture}. This will wait on the
* future to finish, and when it completes, run the listeners. This implementation will wait on
* the source future indefinitely, so if the source future never completes, the adapter will never
* complete either.
*
* <p>If the delegate future is interrupted or throws an unexpected unchecked exception, the
* listeners will not be invoked.
*/
private static class ListenableFutureAdapter<V extends @Nullable Object>
extends ForwardingFuture<V> implements ListenableFuture<V> {
private static final ThreadFactory threadFactory =
new ThreadFactoryBuilder()
.setDaemon(true)
.setNameFormat("ListenableFutureAdapter-thread-%d")
.build();
private static final Executor defaultAdapterExecutor =
Executors.newCachedThreadPool(threadFactory);
private final Executor adapterExecutor;
// The execution list to hold our listeners.
private final ExecutionList executionList = new ExecutionList();
// This allows us to only start up a thread waiting on the delegate future when the first
// listener is added.
private final AtomicBoolean hasListeners = new AtomicBoolean(false);
// The delegate future.
private final Future<V> delegate;
ListenableFutureAdapter(Future<V> delegate) {
this(delegate, defaultAdapterExecutor);
}
ListenableFutureAdapter(Future<V> delegate, Executor adapterExecutor) {
this.delegate = checkNotNull(delegate);
this.adapterExecutor = checkNotNull(adapterExecutor);
}
@Override
protected Future<V> delegate() {
return delegate;
}
@Override
public void addListener(Runnable listener, Executor exec) {
executionList.add(listener, exec);
// When a listener is first added, we run a task that will wait for the delegate to finish,
// and when it is done will run the listeners.
if (hasListeners.compareAndSet(false, true)) {
if (delegate.isDone()) {
// If the delegate is already done, run the execution list immediately on the current
// thread.
executionList.execute();
return;
}
// TODO(lukes): handle RejectedExecutionException
adapterExecutor.execute(
new Runnable() {
@Override
public void run() {
try {
/*
* Threads from our private pool are never interrupted. Threads from a
* user-supplied executor might be, but... what can we do? This is another reason
* to return a proper ListenableFuture instead of using listenInPoolThread.
*/
getUninterruptibly(delegate);
} catch (Throwable e) {
// ExecutionException / CancellationException / RuntimeException / Error
// The task is presumably done, run the listeners.
}
executionList.execute();
}
});
}
}
}
private JdkFutureAdapters() {}
}