util/concurrent/AbstractFuture.java

/*
 * Copyright (C) 2007 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 java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

import javax.annotation.Nullable;

/**
 * An abstract implementation of the {@link ListenableFuture} interface. This
 * class is preferable to {@link java.util.concurrent.FutureTask} for two
 * reasons: It implements {@code ListenableFuture}, and it does not implement
 * {@code Runnable}. (If you want a {@code Runnable} implementation of {@code
 * ListenableFuture}, create a {@link ListenableFutureTask}, or submit your
 * tasks to a {@link ListeningExecutorService}.)
 *
 * <p>This class implements all methods in {@code ListenableFuture}.
 * Subclasses should provide a way to set the result of the computation through
 * the protected methods {@link #set(Object)} and
 * {@link #setException(Throwable)}. Subclasses may also override {@link
 * #interruptTask()}, which will be invoked automatically if a call to {@link
 * #cancel(boolean) cancel(true)} succeeds in canceling the future.
 *
 * <p>{@code AbstractFuture} uses an {@link AbstractQueuedSynchronizer} to deal
 * with concurrency issues and guarantee thread safety.
 *
 * <p>The state changing methods all return a boolean indicating success or
 * failure in changing the future's state.  Valid states are running,
 * completed, failed, or cancelled.
 *
 * <p>This class uses an {@link ExecutionList} to guarantee that all registered
 * listeners will be executed, either when the future finishes or, for listeners
 * that are added after the future completes, immediately.
 * {@code Runnable}-{@code Executor} pairs are stored in the execution list but
 * are not necessarily executed in the order in which they were added.  (If a
 * listener is added after the Future is complete, it will be executed
 * immediately, even if earlier listeners have not been executed. Additionally,
 * executors need not guarantee FIFO execution, or different listeners may run
 * in different executors.)
 *
 * @author Sven Mawson
 * @since 1.0
 */
public abstract class AbstractFuture<V> implements ListenableFuture<V> {

  /** Synchronization control for AbstractFutures. */
  private final Sync<V> sync = new Sync<V>();

  // The execution list to hold our executors.
  private final ExecutionList executionList = new ExecutionList();

  /*
   * Improve the documentation of when InterruptedException is thrown. Our
   * behavior matches the JDK's, but the JDK's documentation is misleading.
   */
  /**
   * {@inheritDoc}
   *
   * <p>The default {@link AbstractFuture} implementation throws {@code
   * InterruptedException} if the current thread is interrupted before or during
   * the call, even if the value is already available.
   *
   * @throws InterruptedException if the current thread was interrupted before
   *     or during the call (optional but recommended).
   * @throws CancellationException {@inheritDoc}
   */
  @Override
  public V get(long timeout, TimeUnit unit) throws InterruptedException,
      TimeoutException, ExecutionException {
    return sync.get(unit.toNanos(timeout));
  }

  /*
   * Improve the documentation of when InterruptedException is thrown. Our
   * behavior matches the JDK's, but the JDK's documentation is misleading.
   */
  /**
   * {@inheritDoc}
   *
   * <p>The default {@link AbstractFuture} implementation throws {@code
   * InterruptedException} if the current thread is interrupted before or during
   * the call, even if the value is already available.
   *
   * @throws InterruptedException if the current thread was interrupted before
   *     or during the call (optional but recommended).
   * @throws CancellationException {@inheritDoc}
   */
  @Override
  public V get() throws InterruptedException, ExecutionException {
    return sync.get();
  }

  @Override
  public boolean isDone() {
    return sync.isDone();
  }

  @Override
  public boolean isCancelled() {
    return sync.isCancelled();
  }

  @Override
  public boolean cancel(boolean mayInterruptIfRunning) {
    if (!sync.cancel()) {
      return false;
    }
    executionList.execute();
    if (mayInterruptIfRunning) {
      interruptTask();
    }
    return true;
  }

  /**
   * Subclasses can override this method to implement interruption of the
   * future's computation. The method is invoked automatically by a successful
   * call to {@link #cancel(boolean) cancel(true)}.
   *
   * <p>The default implementation does nothing.
   *
   * @since 10.0
   */
  protected void interruptTask() {
  }

  /**
   * {@inheritDoc}
   *
   * @since 10.0
   */
  @Override
  public void addListener(Runnable listener, Executor exec) {
    executionList.add(listener, exec);
  }

  /**
   * Subclasses should invoke this method to set the result of the computation
   * to {@code value}.  This will set the state of the future to
   * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
   * state was successfully changed.
   *
   * @param value the value that was the result of the task.
   * @return true if the state was successfully changed.
   */
  protected boolean set(@Nullable V value) {
    boolean result = sync.set(value);
    if (result) {
      executionList.execute();
    }
    return result;
  }

  /**
   * Subclasses should invoke this method to set the result of the computation
   * to an error, {@code throwable}.  This will set the state of the future to
   * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
   * state was successfully changed.
   *
   * @param throwable the exception that the task failed with.
   * @return true if the state was successfully changed.
   * @throws Error if the throwable was an {@link Error}.
   */
  protected boolean setException(Throwable throwable) {
    boolean result = sync.setException(checkNotNull(throwable));
    if (result) {
      executionList.execute();
    }

    // If it's an Error, we want to make sure it reaches the top of the
    // call stack, so we rethrow it.
    if (throwable instanceof Error) {
      throw (Error) throwable;
    }
    return result;
  }

  /**
   * <p>Following the contract of {@link AbstractQueuedSynchronizer} we create a
   * private subclass to hold the synchronizer.  This synchronizer is used to
   * implement the blocking and waiting calls as well as to handle state changes
   * in a thread-safe manner.  The current state of the future is held in the
   * Sync state, and the lock is released whenever the state changes to either
   * {@link #COMPLETED} or {@link #CANCELLED}.
   *
   * <p>To avoid races between threads doing release and acquire, we transition
   * to the final state in two steps.  One thread will successfully CAS from
   * RUNNING to COMPLETING, that thread will then set the result of the
   * computation, and only then transition to COMPLETED or CANCELLED.
   *
   * <p>We don't use the integer argument passed between acquire methods so we
   * pass around a -1 everywhere.
   */
  static final class Sync<V> extends AbstractQueuedSynchronizer {

    private static final long serialVersionUID = 0L;

    /* Valid states. */
    static final int RUNNING = 0;
    static final int COMPLETING = 1;
    static final int COMPLETED = 2;
    static final int CANCELLED = 4;

    private V value;
    private Throwable exception;

    /*
     * Acquisition succeeds if the future is done, otherwise it fails.
     */
    @Override
    protected int tryAcquireShared(int ignored) {
      if (isDone()) {
        return 1;
      }
      return -1;
    }

    /*
     * We always allow a release to go through, this means the state has been
     * successfully changed and the result is available.
     */
    @Override
    protected boolean tryReleaseShared(int finalState) {
      setState(finalState);
      return true;
    }

    /**
     * Blocks until the task is complete or the timeout expires.  Throws a
     * {@link TimeoutException} if the timer expires, otherwise behaves like
     * {@link #get()}.
     */
    V get(long nanos) throws TimeoutException, CancellationException,
        ExecutionException, InterruptedException {

      // Attempt to acquire the shared lock with a timeout.
      if (!tryAcquireSharedNanos(-1, nanos)) {
        throw new TimeoutException("Timeout waiting for task.");
      }

      return getValue();
    }

    /**
     * Blocks until {@link #complete(Object, Throwable, int)} has been
     * successfully called.  Throws a {@link CancellationException} if the task
     * was cancelled, or a {@link ExecutionException} if the task completed with
     * an error.
     */
    V get() throws CancellationException, ExecutionException,
        InterruptedException {

      // Acquire the shared lock allowing interruption.
      acquireSharedInterruptibly(-1);
      return getValue();
    }

    /**
     * Implementation of the actual value retrieval.  Will return the value
     * on success, an exception on failure, a cancellation on cancellation, or
     * an illegal state if the synchronizer is in an invalid state.
     */
    private V getValue() throws CancellationException, ExecutionException {
      int state = getState();
      switch (state) {
        case COMPLETED:
          if (exception != null) {
            throw new ExecutionException(exception);
          } else {
            return value;
          }

        case CANCELLED:
          throw new CancellationException("Task was cancelled.");

        default:
          throw new IllegalStateException(
              "Error, synchronizer in invalid state: " + state);
      }
    }

    /**
     * Checks if the state is {@link #COMPLETED} or {@link #CANCELLED}.
     */
    boolean isDone() {
      return (getState() & (COMPLETED | CANCELLED)) != 0;
    }

    /**
     * Checks if the state is {@link #CANCELLED}.
     */
    boolean isCancelled() {
      return getState() == CANCELLED;
    }

    /**
     * Transition to the COMPLETED state and set the value.
     */
    boolean set(@Nullable V v) {
      return complete(v, null, COMPLETED);
    }

    /**
     * Transition to the COMPLETED state and set the exception.
     */
    boolean setException(Throwable t) {
      return complete(null, t, COMPLETED);
    }

    /**
     * Transition to the CANCELLED state.
     */
    boolean cancel() {
      return complete(null, null, CANCELLED);
    }

    /**
     * Implementation of completing a task.  Either {@code v} or {@code t} will
     * be set but not both.  The {@code finalState} is the state to change to
     * from {@link #RUNNING}.  If the state is not in the RUNNING state we
     * return {@code false} after waiting for the state to be set to a valid
     * final state ({@link #COMPLETED} or {@link #CANCELLED}).
     *
     * @param v the value to set as the result of the computation.
     * @param t the exception to set as the result of the computation.
     * @param finalState the state to transition to.
     */
    private boolean complete(@Nullable V v, @Nullable Throwable t,
        int finalState) {
      boolean doCompletion = compareAndSetState(RUNNING, COMPLETING);
      if (doCompletion) {
        // If this thread successfully transitioned to COMPLETING, set the value
        // and exception and then release to the final state.
        this.value = v;
        this.exception = t;
        releaseShared(finalState);
      } else if (getState() == COMPLETING) {
        // If some other thread is currently completing the future, block until
        // they are done so we can guarantee completion.
        acquireShared(-1);
      }
      return doCompletion;
    }
  }
}