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Java example source code file (TimeoutFuture.java)

This example Java source code file (TimeoutFuture.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

annotation, fire, future, gwtincompatible, listenablefuture, nullable, override, runnable, scheduledexecutorservice, threading, threads, timeoutexception, timeoutfuture, timeunit

The TimeoutFuture.java Java example source code

/*
 * Copyright (C) 2006 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.util.concurrent.MoreExecutors.directExecutor;

import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Preconditions;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

import javax.annotation.Nullable;

/**
 * Implementation of {@code Futures#withTimeout}.
 *
 * <p>Future that delegates to another but will finish early (via a {@link TimeoutException} wrapped
 * in an {@link ExecutionException}) if the specified duration expires. The delegate future is
 * interrupted and cancelled if it times out.
 */
@GwtIncompatible
final class TimeoutFuture<V> extends AbstractFuture.TrustedFuture {
  static <V> ListenableFuture create(
      ListenableFuture<V> delegate,
      long time,
      TimeUnit unit,
      ScheduledExecutorService scheduledExecutor) {
    TimeoutFuture<V> result = new TimeoutFuture(delegate);
    TimeoutFuture.Fire<V> fire = new TimeoutFuture.Fire(result);
    result.timer = scheduledExecutor.schedule(fire, time, unit);
    delegate.addListener(fire, directExecutor());
    return result;
  }

  /*
   * Memory visibility of these fields. There are two cases to consider.
   *
   * 1. visibility of the writes to these fields to Fire.run:
   *
   * The initial write to delegateRef is made definitely visible via the semantics of
   * addListener/SES.schedule. The later racy write in cancel() is not guaranteed to be observed,
   * however that is fine since the correctness is based on the atomic state in our base class. The
   * initial write to timer is never definitely visible to Fire.run since it is assigned after
   * SES.schedule is called. Therefore Fire.run has to check for null. However, it should be visible
   * if Fire.run is called by delegate.addListener since addListener is called after the assignment
   * to timer, and importantly this is the main situation in which we need to be able to see the
   * write.
   *
   * 2. visibility of the writes to cancel:
   *
   * Since these fields are non-final that means that TimeoutFuture is not being 'safely published',
   * thus a motivated caller may be able to expose the reference to another thread that would then
   * call cancel() and be unable to cancel the delegate.
   * There are a number of ways to solve this, none of which are very pretty, and it is currently
   * believed to be a purely theoretical problem (since the other actions should supply sufficient
   * write-barriers).
   */

  @Nullable private ListenableFuture<V> delegateRef;
  @Nullable private Future<?> timer;

  private TimeoutFuture(ListenableFuture<V> delegate) {
    this.delegateRef = Preconditions.checkNotNull(delegate);
  }

  /** A runnable that is called when the delegate or the timer completes. */
  private static final class Fire<V> implements Runnable {
    @Nullable TimeoutFuture<V> timeoutFutureRef;

    Fire(TimeoutFuture<V> timeoutFuture) {
      this.timeoutFutureRef = timeoutFuture;
    }

    @Override
    public void run() {
      // If either of these reads return null then we must be after a successful cancel or another
      // call to this method.
      TimeoutFuture<V> timeoutFuture = timeoutFutureRef;
      if (timeoutFuture == null) {
        return;
      }
      ListenableFuture<V> delegate = timeoutFuture.delegateRef;
      if (delegate == null) {
        return;
      }

      /*
       * If we're about to complete the TimeoutFuture, we want to release our reference to it.
       * Otherwise, we'll pin it (and its result) in memory until the timeout task is GCed. (The
       * need to clear our reference to the TimeoutFuture is the reason we use a *static* nested
       * class with a manual reference back to the "containing" class.)
       *
       * This has the nice-ish side effect of limiting reentrancy: run() calls
       * timeoutFuture.setException() calls run(). That reentrancy would already be harmless, since
       * timeoutFuture can be set (and delegate cancelled) only once. (And "set only once" is
       * important for other reasons: run() can still be invoked concurrently in different threads,
       * even with the above null checks.)
       */
      timeoutFutureRef = null;
      if (delegate.isDone()) {
        timeoutFuture.setFuture(delegate);
      } else {
        try {
          // TODO(lukes): this stack trace is particularly useless (all it does is point at the
          // scheduledexecutorservice thread), consider eliminating it altogether?
          timeoutFuture.setException(new TimeoutException("Future timed out: " + delegate));
        } finally {
          delegate.cancel(true);
        }
      }
    }
  }

  @Override
  protected void afterDone() {
    maybePropagateCancellation(delegateRef);

    Future<?> localTimer = timer;
    // Try to cancel the timer as an optimization.
    // timer may be null if this call to run was by the timer task since there is no happens-before
    // edge between the assignment to timer and an execution of the timer task.
    if (localTimer != null) {
      localTimer.cancel(false);
    }

    delegateRef = null;
    timer = null;
  }
}

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