001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.util.concurrent;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020
021import java.util.concurrent.CancellationException;
022import java.util.concurrent.ExecutionException;
023import java.util.concurrent.Executor;
024import java.util.concurrent.TimeUnit;
025import java.util.concurrent.TimeoutException;
026import java.util.concurrent.locks.AbstractQueuedSynchronizer;
027
028import javax.annotation.Nullable;
029
030/**
031 * An abstract implementation of the {@link ListenableFuture} interface. This
032 * class is preferable to {@link java.util.concurrent.FutureTask} for two
033 * reasons: It implements {@code ListenableFuture}, and it does not implement
034 * {@code Runnable}. (If you want a {@code Runnable} implementation of {@code
035 * ListenableFuture}, create a {@link ListenableFutureTask}, or submit your
036 * tasks to a {@link ListeningExecutorService}.)
037 *
038 * <p>This class implements all methods in {@code ListenableFuture}.
039 * Subclasses should provide a way to set the result of the computation through
040 * the protected methods {@link #set(Object)} and
041 * {@link #setException(Throwable)}. Subclasses may also override {@link
042 * #interruptTask()}, which will be invoked automatically if a call to {@link
043 * #cancel(boolean) cancel(true)} succeeds in canceling the future.
044 *
045 * <p>{@code AbstractFuture} uses an {@link AbstractQueuedSynchronizer} to deal
046 * with concurrency issues and guarantee thread safety.
047 *
048 * <p>The state changing methods all return a boolean indicating success or
049 * failure in changing the future's state.  Valid states are running,
050 * completed, failed, or cancelled.
051 *
052 * <p>This class uses an {@link ExecutionList} to guarantee that all registered
053 * listeners will be executed, either when the future finishes or, for listeners
054 * that are added after the future completes, immediately.
055 * {@code Runnable}-{@code Executor} pairs are stored in the execution list but
056 * are not necessarily executed in the order in which they were added.  (If a
057 * listener is added after the Future is complete, it will be executed
058 * immediately, even if earlier listeners have not been executed. Additionally,
059 * executors need not guarantee FIFO execution, or different listeners may run
060 * in different executors.)
061 *
062 * @author Sven Mawson
063 * @since 1.0
064 */
065public abstract class AbstractFuture<V> implements ListenableFuture<V> {
066
067  /** Synchronization control for AbstractFutures. */
068  private final Sync<V> sync = new Sync<V>();
069
070  // The execution list to hold our executors.
071  private final ExecutionList executionList = new ExecutionList();
072
073  /**
074   * Constructor for use by subclasses.
075   */
076  protected AbstractFuture() {}
077
078  /*
079   * Improve the documentation of when InterruptedException is thrown. Our
080   * behavior matches the JDK's, but the JDK's documentation is misleading.
081   */
082  /**
083   * {@inheritDoc}
084   *
085   * <p>The default {@link AbstractFuture} implementation throws {@code
086   * InterruptedException} if the current thread is interrupted before or during
087   * the call, even if the value is already available.
088   *
089   * @throws InterruptedException if the current thread was interrupted before
090   *     or during the call (optional but recommended).
091   * @throws CancellationException {@inheritDoc}
092   */
093  @Override
094  public V get(long timeout, TimeUnit unit) throws InterruptedException,
095      TimeoutException, ExecutionException {
096    return sync.get(unit.toNanos(timeout));
097  }
098
099  /*
100   * Improve the documentation of when InterruptedException is thrown. Our
101   * behavior matches the JDK's, but the JDK's documentation is misleading.
102   */
103  /**
104   * {@inheritDoc}
105   *
106   * <p>The default {@link AbstractFuture} implementation throws {@code
107   * InterruptedException} if the current thread is interrupted before or during
108   * the call, even if the value is already available.
109   *
110   * @throws InterruptedException if the current thread was interrupted before
111   *     or during the call (optional but recommended).
112   * @throws CancellationException {@inheritDoc}
113   */
114  @Override
115  public V get() throws InterruptedException, ExecutionException {
116    return sync.get();
117  }
118
119  @Override
120  public boolean isDone() {
121    return sync.isDone();
122  }
123
124  @Override
125  public boolean isCancelled() {
126    return sync.isCancelled();
127  }
128
129  @Override
130  public boolean cancel(boolean mayInterruptIfRunning) {
131    if (!sync.cancel()) {
132      return false;
133    }
134    executionList.execute();
135    if (mayInterruptIfRunning) {
136      interruptTask();
137    }
138    return true;
139  }
140
141  /**
142   * Subclasses can override this method to implement interruption of the
143   * future's computation. The method is invoked automatically by a successful
144   * call to {@link #cancel(boolean) cancel(true)}.
145   *
146   * <p>The default implementation does nothing.
147   *
148   * @since 10.0
149   */
150  protected void interruptTask() {
151  }
152
153  /**
154   * {@inheritDoc}
155   *
156   * @since 10.0
157   */
158  @Override
159  public void addListener(Runnable listener, Executor exec) {
160    executionList.add(listener, exec);
161  }
162
163  /**
164   * Subclasses should invoke this method to set the result of the computation
165   * to {@code value}.  This will set the state of the future to
166   * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
167   * state was successfully changed.
168   *
169   * @param value the value that was the result of the task.
170   * @return true if the state was successfully changed.
171   */
172  protected boolean set(@Nullable V value) {
173    boolean result = sync.set(value);
174    if (result) {
175      executionList.execute();
176    }
177    return result;
178  }
179
180  /**
181   * Subclasses should invoke this method to set the result of the computation
182   * to an error, {@code throwable}.  This will set the state of the future to
183   * {@link AbstractFuture.Sync#COMPLETED} and invoke the listeners if the
184   * state was successfully changed.
185   *
186   * @param throwable the exception that the task failed with.
187   * @return true if the state was successfully changed.
188   * @throws Error if the throwable was an {@link Error}.
189   */
190  protected boolean setException(Throwable throwable) {
191    boolean result = sync.setException(checkNotNull(throwable));
192    if (result) {
193      executionList.execute();
194    }
195
196    // If it's an Error, we want to make sure it reaches the top of the
197    // call stack, so we rethrow it.
198    if (throwable instanceof Error) {
199      throw (Error) throwable;
200    }
201    return result;
202  }
203
204  /**
205   * <p>Following the contract of {@link AbstractQueuedSynchronizer} we create a
206   * private subclass to hold the synchronizer.  This synchronizer is used to
207   * implement the blocking and waiting calls as well as to handle state changes
208   * in a thread-safe manner.  The current state of the future is held in the
209   * Sync state, and the lock is released whenever the state changes to either
210   * {@link #COMPLETED} or {@link #CANCELLED}.
211   *
212   * <p>To avoid races between threads doing release and acquire, we transition
213   * to the final state in two steps.  One thread will successfully CAS from
214   * RUNNING to COMPLETING, that thread will then set the result of the
215   * computation, and only then transition to COMPLETED or CANCELLED.
216   *
217   * <p>We don't use the integer argument passed between acquire methods so we
218   * pass around a -1 everywhere.
219   */
220  static final class Sync<V> extends AbstractQueuedSynchronizer {
221
222    private static final long serialVersionUID = 0L;
223
224    /* Valid states. */
225    static final int RUNNING = 0;
226    static final int COMPLETING = 1;
227    static final int COMPLETED = 2;
228    static final int CANCELLED = 4;
229
230    private V value;
231    private Throwable exception;
232
233    /*
234     * Acquisition succeeds if the future is done, otherwise it fails.
235     */
236    @Override
237    protected int tryAcquireShared(int ignored) {
238      if (isDone()) {
239        return 1;
240      }
241      return -1;
242    }
243
244    /*
245     * We always allow a release to go through, this means the state has been
246     * successfully changed and the result is available.
247     */
248    @Override
249    protected boolean tryReleaseShared(int finalState) {
250      setState(finalState);
251      return true;
252    }
253
254    /**
255     * Blocks until the task is complete or the timeout expires.  Throws a
256     * {@link TimeoutException} if the timer expires, otherwise behaves like
257     * {@link #get()}.
258     */
259    V get(long nanos) throws TimeoutException, CancellationException,
260        ExecutionException, InterruptedException {
261
262      // Attempt to acquire the shared lock with a timeout.
263      if (!tryAcquireSharedNanos(-1, nanos)) {
264        throw new TimeoutException("Timeout waiting for task.");
265      }
266
267      return getValue();
268    }
269
270    /**
271     * Blocks until {@link #complete(Object, Throwable, int)} has been
272     * successfully called.  Throws a {@link CancellationException} if the task
273     * was cancelled, or a {@link ExecutionException} if the task completed with
274     * an error.
275     */
276    V get() throws CancellationException, ExecutionException,
277        InterruptedException {
278
279      // Acquire the shared lock allowing interruption.
280      acquireSharedInterruptibly(-1);
281      return getValue();
282    }
283
284    /**
285     * Implementation of the actual value retrieval.  Will return the value
286     * on success, an exception on failure, a cancellation on cancellation, or
287     * an illegal state if the synchronizer is in an invalid state.
288     */
289    private V getValue() throws CancellationException, ExecutionException {
290      int state = getState();
291      switch (state) {
292        case COMPLETED:
293          if (exception != null) {
294            throw new ExecutionException(exception);
295          } else {
296            return value;
297          }
298
299        case CANCELLED:
300          throw new CancellationException("Task was cancelled.");
301
302        default:
303          throw new IllegalStateException(
304              "Error, synchronizer in invalid state: " + state);
305      }
306    }
307
308    /**
309     * Checks if the state is {@link #COMPLETED} or {@link #CANCELLED}.
310     */
311    boolean isDone() {
312      return (getState() & (COMPLETED | CANCELLED)) != 0;
313    }
314
315    /**
316     * Checks if the state is {@link #CANCELLED}.
317     */
318    boolean isCancelled() {
319      return getState() == CANCELLED;
320    }
321
322    /**
323     * Transition to the COMPLETED state and set the value.
324     */
325    boolean set(@Nullable V v) {
326      return complete(v, null, COMPLETED);
327    }
328
329    /**
330     * Transition to the COMPLETED state and set the exception.
331     */
332    boolean setException(Throwable t) {
333      return complete(null, t, COMPLETED);
334    }
335
336    /**
337     * Transition to the CANCELLED state.
338     */
339    boolean cancel() {
340      return complete(null, null, CANCELLED);
341    }
342
343    /**
344     * Implementation of completing a task.  Either {@code v} or {@code t} will
345     * be set but not both.  The {@code finalState} is the state to change to
346     * from {@link #RUNNING}.  If the state is not in the RUNNING state we
347     * return {@code false} after waiting for the state to be set to a valid
348     * final state ({@link #COMPLETED} or {@link #CANCELLED}).
349     *
350     * @param v the value to set as the result of the computation.
351     * @param t the exception to set as the result of the computation.
352     * @param finalState the state to transition to.
353     */
354    private boolean complete(@Nullable V v, @Nullable Throwable t,
355        int finalState) {
356      boolean doCompletion = compareAndSetState(RUNNING, COMPLETING);
357      if (doCompletion) {
358        // If this thread successfully transitioned to COMPLETING, set the value
359        // and exception and then release to the final state.
360        this.value = v;
361        this.exception = t;
362        releaseShared(finalState);
363      } else if (getState() == COMPLETING) {
364        // If some other thread is currently completing the future, block until
365        // they are done so we can guarantee completion.
366        acquireShared(-1);
367      }
368      return doCompletion;
369    }
370  }
371}