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/*
* 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 com.google.common.annotations.Beta;
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;
/**
* Utilities necessary for working with libraries that supply plain {@link
* Future} instances. Note that, whenver 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
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> 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
* 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> 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 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;
}
adapterExecutor.execute(new Runnable() {
@Override
public void run() {
try {
delegate.get();
} catch (Error e) {
throw e;
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
// Threads from our private pool are never interrupted.
throw new AssertionError(e);
} catch (Throwable e) {
// ExecutionException / CancellationException / RuntimeException
// The task is done, run the listeners.
}
executionList.execute();
}
});
}
}
}
private JdkFutureAdapters() {}
}
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