/*
 * 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;

Utilities necessary for working with libraries that supply plain Future instances. Note that, whenever possible, it is strongly preferred to modify those libraries to return 
ListenableFuture directly. 
Author:
Sven Mawson
Since:
10.0 (replacing Futures.makeListenable, which existed in 1.0)/**
 * 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
public final class JdkFutureAdapters {
  
Assigns a thread to the given Future to provide ListenableFuture functionality. 
Warning: If the input future does not already implement ListenableFuture, the returned future will emulate ListenableFuture.addListener by taking a thread from an internal, unbounded pool at the first call to addListener and holding it until the future is done. 
Prefer to create ListenableFuture instances with SettableFuture, MoreExecutors.listeningDecorator(ExecutorService), ListenableFutureTask, AbstractFuture, and other utilities over creating plain 
Future instances to be upgraded to ListenableFuture after the fact. /**
   * 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 Future to provide ListenableFuture functionality. 
Warning: If the input future does not already implement ListenableFuture, the returned future will emulate ListenableFuture.addListener by submitting a task to the given executor at the first call to addListener. The task must be started by the executor promptly, or else the returned ListenableFuture may fail to work. The task's execution consists of blocking until the input future is 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. 
Prefer to create ListenableFuture instances with SettableFuture, MoreExecutors.listeningDecorator(ExecutorService), ListenableFutureTask, AbstractFuture, and other utilities over creating plain 
Future instances to be upgraded to ListenableFuture after the fact. 
Since:
12.0/**
   * 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> 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 Future into a 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. 
If the delegate future is interrupted or throws an unexpected unchecked exception, the
listeners will not be invoked.
/**
   * 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;
        }

        // 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() {}
}