001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018import static com.google.common.base.Strings.isNullOrEmpty;
019import static com.google.common.base.Throwables.throwIfUnchecked;
020import static com.google.common.util.concurrent.Futures.getDone;
021import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
022import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;
023
024import com.google.common.annotations.Beta;
025import com.google.common.annotations.GwtCompatible;
026import com.google.common.base.Ascii;
027import com.google.errorprone.annotations.CanIgnoreReturnValue;
028import com.google.errorprone.annotations.DoNotMock;
029import com.google.errorprone.annotations.ForOverride;
030import com.google.j2objc.annotations.ReflectionSupport;
031import java.security.AccessController;
032import java.security.PrivilegedActionException;
033import java.security.PrivilegedExceptionAction;
034import java.util.concurrent.CancellationException;
035import java.util.concurrent.ExecutionException;
036import java.util.concurrent.Executor;
037import java.util.concurrent.Future;
038import java.util.concurrent.ScheduledFuture;
039import java.util.concurrent.TimeUnit;
040import java.util.concurrent.TimeoutException;
041import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
042import java.util.concurrent.locks.LockSupport;
043import java.util.logging.Level;
044import java.util.logging.Logger;
045import javax.annotation.Nullable;
046
047/**
048 * An abstract implementation of {@link ListenableFuture}, intended for advanced users only. More
049 * common ways to create a {@code ListenableFuture} include instantiating a {@link SettableFuture},
050 * submitting a task to a {@link ListeningExecutorService}, and deriving a {@code Future} from an
051 * existing one, typically using methods like {@link Futures#transform(ListenableFuture,
052 * com.google.common.base.Function, java.util.concurrent.Executor) Futures.transform} and {@link
053 * Futures#catching(ListenableFuture, Class, com.google.common.base.Function,
054 * java.util.concurrent.Executor) Futures.catching}.
055 *
056 * <p>This class implements all methods in {@code ListenableFuture}. Subclasses should provide a way
057 * to set the result of the computation through the protected methods {@link #set(Object)}, {@link
058 * #setFuture(ListenableFuture)} and {@link #setException(Throwable)}. Subclasses may also override
059 * {@link #afterDone()}, which will be invoked automatically when the future completes. Subclasses
060 * should rarely override other methods.
061 *
062 * @author Sven Mawson
063 * @author Luke Sandberg
064 * @since 1.0
065 */
066@SuppressWarnings("ShortCircuitBoolean") // we use non-short circuiting comparisons intentionally
067@DoNotMock("Use Futures.immediate*Future or SettableFuture")
068@GwtCompatible(emulated = true)
069@ReflectionSupport(value = ReflectionSupport.Level.FULL)
070public abstract class AbstractFuture<V> extends FluentFuture<V> {
071  // NOTE: Whenever both tests are cheap and functional, it's faster to use &, | instead of &&, ||
072
073  private static final boolean GENERATE_CANCELLATION_CAUSES =
074      Boolean.parseBoolean(
075          System.getProperty("guava.concurrent.generate_cancellation_cause", "false"));
076
077  /**
078   * A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring
079   * that {@link #get} calls exactly the implementation of {@link AbstractFuture#get}.
080   */
081  abstract static class TrustedFuture<V> extends AbstractFuture<V> {
082    @CanIgnoreReturnValue
083    @Override
084    public final V get() throws InterruptedException, ExecutionException {
085      return super.get();
086    }
087
088    @CanIgnoreReturnValue
089    @Override
090    public final V get(long timeout, TimeUnit unit)
091        throws InterruptedException, ExecutionException, TimeoutException {
092      return super.get(timeout, unit);
093    }
094
095    @Override
096    public final boolean isDone() {
097      return super.isDone();
098    }
099
100    @Override
101    public final boolean isCancelled() {
102      return super.isCancelled();
103    }
104
105    @Override
106    public final void addListener(Runnable listener, Executor executor) {
107      super.addListener(listener, executor);
108    }
109
110    @CanIgnoreReturnValue
111    @Override
112    public final boolean cancel(boolean mayInterruptIfRunning) {
113      return super.cancel(mayInterruptIfRunning);
114    }
115  }
116
117  // Logger to log exceptions caught when running listeners.
118  private static final Logger log = Logger.getLogger(AbstractFuture.class.getName());
119
120  // A heuristic for timed gets. If the remaining timeout is less than this, spin instead of
121  // blocking. This value is what AbstractQueuedSynchronizer uses.
122  private static final long SPIN_THRESHOLD_NANOS = 1000L;
123
124  private static final AtomicHelper ATOMIC_HELPER;
125
126  static {
127    AtomicHelper helper;
128    Throwable thrownUnsafeFailure = null;
129    Throwable thrownAtomicReferenceFieldUpdaterFailure = null;
130
131    try {
132      helper = new UnsafeAtomicHelper();
133    } catch (Throwable unsafeFailure) {
134      thrownUnsafeFailure = unsafeFailure;
135      // catch absolutely everything and fall through to our 'SafeAtomicHelper'
136      // The access control checks that ARFU does means the caller class has to be AbstractFuture
137      // instead of SafeAtomicHelper, so we annoyingly define these here
138      try {
139        helper =
140            new SafeAtomicHelper(
141                newUpdater(Waiter.class, Thread.class, "thread"),
142                newUpdater(Waiter.class, Waiter.class, "next"),
143                newUpdater(AbstractFuture.class, Waiter.class, "waiters"),
144                newUpdater(AbstractFuture.class, Listener.class, "listeners"),
145                newUpdater(AbstractFuture.class, Object.class, "value"));
146      } catch (Throwable atomicReferenceFieldUpdaterFailure) {
147        // Some Android 5.0.x Samsung devices have bugs in JDK reflection APIs that cause
148        // getDeclaredField to throw a NoSuchFieldException when the field is definitely there.
149        // For these users fallback to a suboptimal implementation, based on synchronized. This will
150        // be a definite performance hit to those users.
151        thrownAtomicReferenceFieldUpdaterFailure = atomicReferenceFieldUpdaterFailure;
152        helper = new SynchronizedHelper();
153      }
154    }
155    ATOMIC_HELPER = helper;
156
157    // Prevent rare disastrous classloading in first call to LockSupport.park.
158    // See: https://bugs.openjdk.java.net/browse/JDK-8074773
159    @SuppressWarnings("unused")
160    Class<?> ensureLoaded = LockSupport.class;
161
162    // Log after all static init is finished; if an installed logger uses any Futures methods, it
163    // shouldn't break in cases where reflection is missing/broken.
164    if (thrownAtomicReferenceFieldUpdaterFailure != null) {
165      log.log(Level.SEVERE, "UnsafeAtomicHelper is broken!", thrownUnsafeFailure);
166      log.log(
167          Level.SEVERE, "SafeAtomicHelper is broken!", thrownAtomicReferenceFieldUpdaterFailure);
168    }
169  }
170
171  /**
172   * Waiter links form a Treiber stack, in the {@link #waiters} field.
173   */
174  private static final class Waiter {
175    static final Waiter TOMBSTONE = new Waiter(false /* ignored param */);
176
177    @Nullable volatile Thread thread;
178    @Nullable volatile Waiter next;
179
180    /**
181     * Constructor for the TOMBSTONE, avoids use of ATOMIC_HELPER in case this class is loaded
182     * before the ATOMIC_HELPER. Apparently this is possible on some android platforms.
183     */
184    Waiter(boolean unused) {}
185
186    Waiter() {
187      // avoid volatile write, write is made visible by subsequent CAS on waiters field
188      ATOMIC_HELPER.putThread(this, Thread.currentThread());
189    }
190
191    // non-volatile write to the next field. Should be made visible by subsequent CAS on waiters
192    // field.
193    void setNext(Waiter next) {
194      ATOMIC_HELPER.putNext(this, next);
195    }
196
197    void unpark() {
198      // This is racy with removeWaiter. The consequence of the race is that we may spuriously call
199      // unpark even though the thread has already removed itself from the list. But even if we did
200      // use a CAS, that race would still exist (it would just be ever so slightly smaller).
201      Thread w = thread;
202      if (w != null) {
203        thread = null;
204        LockSupport.unpark(w);
205      }
206    }
207  }
208
209  /**
210   * Marks the given node as 'deleted' (null waiter) and then scans the list to unlink all deleted
211   * nodes. This is an O(n) operation in the common case (and O(n^2) in the worst), but we are saved
212   * by two things.
213   * <ul>
214   * <li>This is only called when a waiting thread times out or is interrupted. Both of which should
215   *     be rare.
216   * <li>The waiters list should be very short.
217   * </ul>
218   */
219  private void removeWaiter(Waiter node) {
220    node.thread = null; // mark as 'deleted'
221    restart:
222    while (true) {
223      Waiter pred = null;
224      Waiter curr = waiters;
225      if (curr == Waiter.TOMBSTONE) {
226        return; // give up if someone is calling complete
227      }
228      Waiter succ;
229      while (curr != null) {
230        succ = curr.next;
231        if (curr.thread != null) { // we aren't unlinking this node, update pred.
232          pred = curr;
233        } else if (pred != null) { // We are unlinking this node and it has a predecessor.
234          pred.next = succ;
235          if (pred.thread == null) { // We raced with another node that unlinked pred. Restart.
236            continue restart;
237          }
238        } else if (!ATOMIC_HELPER.casWaiters(this, curr, succ)) { // We are unlinking head
239          continue restart; // We raced with an add or complete
240        }
241        curr = succ;
242      }
243      break;
244    }
245  }
246
247  /** Listeners also form a stack through the {@link #listeners} field. */
248  private static final class Listener {
249    static final Listener TOMBSTONE = new Listener(null, null);
250    final Runnable task;
251    final Executor executor;
252
253    // writes to next are made visible by subsequent CAS's on the listeners field
254    @Nullable Listener next;
255
256    Listener(Runnable task, Executor executor) {
257      this.task = task;
258      this.executor = executor;
259    }
260  }
261
262  /** A special value to represent {@code null}. */
263  private static final Object NULL = new Object();
264
265  /** A special value to represent failure, when {@link #setException} is called successfully. */
266  private static final class Failure {
267    static final Failure FALLBACK_INSTANCE =
268        new Failure(
269            new Throwable("Failure occurred while trying to finish a future.") {
270              @Override
271              public synchronized Throwable fillInStackTrace() {
272                return this; // no stack trace
273              }
274            });
275    final Throwable exception;
276
277    Failure(Throwable exception) {
278      this.exception = checkNotNull(exception);
279    }
280  }
281
282  /** A special value to represent cancellation and the 'wasInterrupted' bit. */
283  private static final class Cancellation {
284    // constants to use when GENERATE_CANCELLATION_CAUSES = false
285    static final Cancellation CAUSELESS_INTERRUPTED;
286    static final Cancellation CAUSELESS_CANCELLED;
287
288    static {
289      if (GENERATE_CANCELLATION_CAUSES) {
290        CAUSELESS_CANCELLED = null;
291        CAUSELESS_INTERRUPTED = null;
292      } else {
293        CAUSELESS_CANCELLED = new Cancellation(false, null);
294        CAUSELESS_INTERRUPTED = new Cancellation(true, null);
295      }
296    }
297
298    final boolean wasInterrupted;
299    @Nullable final Throwable cause;
300
301    Cancellation(boolean wasInterrupted, @Nullable Throwable cause) {
302      this.wasInterrupted = wasInterrupted;
303      this.cause = cause;
304    }
305  }
306
307  /** A special value that encodes the 'setFuture' state. */
308  private static final class SetFuture<V> implements Runnable {
309    final AbstractFuture<V> owner;
310    final ListenableFuture<? extends V> future;
311
312    SetFuture(AbstractFuture<V> owner, ListenableFuture<? extends V> future) {
313      this.owner = owner;
314      this.future = future;
315    }
316
317    @Override
318    public void run() {
319      if (owner.value != this) {
320        // nothing to do, we must have been cancelled, don't bother inspecting the future.
321        return;
322      }
323      Object valueToSet = getFutureValue(future);
324      if (ATOMIC_HELPER.casValue(owner, this, valueToSet)) {
325        complete(owner);
326      }
327    }
328  }
329
330  // TODO(lukes): investigate using the @Contended annotation on these fields when jdk8 is
331  // available.
332  /**
333   * This field encodes the current state of the future.
334   *
335   * <p>The valid values are:
336   * <ul>
337   * <li>{@code null} initial state, nothing has happened.
338   * <li>{@link Cancellation} terminal state, {@code cancel} was called.
339   * <li>{@link Failure} terminal state, {@code setException} was called.
340   * <li>{@link SetFuture} intermediate state, {@code setFuture} was called.
341   * <li>{@link #NULL} terminal state, {@code set(null)} was called.
342   * <li>Any other non-null value, terminal state, {@code set} was called with a non-null argument.
343   * </ul>
344   */
345  private volatile Object value;
346
347  /** All listeners. */
348  private volatile Listener listeners;
349
350  /** All waiting threads. */
351  private volatile Waiter waiters;
352
353  /**
354   * Constructor for use by subclasses.
355   */
356  protected AbstractFuture() {}
357
358  // Gets and Timed Gets
359  //
360  // * Be responsive to interruption
361  // * Don't create Waiter nodes if you aren't going to park, this helps reduce contention on the
362  //   waiters field.
363  // * Future completion is defined by when #value becomes non-null/non SetFuture
364  // * Future completion can be observed if the waiters field contains a TOMBSTONE
365
366  // Timed Get
367  // There are a few design constraints to consider
368  // * We want to be responsive to small timeouts, unpark() has non trivial latency overheads (I
369  //   have observed 12 micros on 64 bit linux systems to wake up a parked thread). So if the
370  //   timeout is small we shouldn't park(). This needs to be traded off with the cpu overhead of
371  //   spinning, so we use SPIN_THRESHOLD_NANOS which is what AbstractQueuedSynchronizer uses for
372  //   similar purposes.
373  // * We want to behave reasonably for timeouts of 0
374  // * We are more responsive to completion than timeouts. This is because parkNanos depends on
375  //   system scheduling and as such we could either miss our deadline, or unpark() could be delayed
376  //   so that it looks like we timed out even though we didn't. For comparison FutureTask respects
377  //   completion preferably and AQS is non-deterministic (depends on where in the queue the waiter
378  //   is). If we wanted to be strict about it, we could store the unpark() time in the Waiter node
379  //   and we could use that to make a decision about whether or not we timed out prior to being
380  //   unparked.
381
382  /**
383   * {@inheritDoc}
384   *
385   * <p>The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
386   * current thread is interrupted during the call, even if the value is already available.
387   *
388   * @throws CancellationException {@inheritDoc}
389   */
390  @CanIgnoreReturnValue
391  @Override
392  public V get(long timeout, TimeUnit unit)
393      throws InterruptedException, TimeoutException, ExecutionException {
394    // NOTE: if timeout < 0, remainingNanos will be < 0 and we will fall into the while(true) loop
395    // at the bottom and throw a timeoutexception.
396    long remainingNanos = unit.toNanos(timeout); // we rely on the implicit null check on unit.
397    if (Thread.interrupted()) {
398      throw new InterruptedException();
399    }
400    Object localValue = value;
401    if (localValue != null & !(localValue instanceof SetFuture)) {
402      return getDoneValue(localValue);
403    }
404    // we delay calling nanoTime until we know we will need to either park or spin
405    final long endNanos = remainingNanos > 0 ? System.nanoTime() + remainingNanos : 0;
406    long_wait_loop:
407    if (remainingNanos >= SPIN_THRESHOLD_NANOS) {
408      Waiter oldHead = waiters;
409      if (oldHead != Waiter.TOMBSTONE) {
410        Waiter node = new Waiter();
411        do {
412          node.setNext(oldHead);
413          if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
414            while (true) {
415              LockSupport.parkNanos(this, remainingNanos);
416              // Check interruption first, if we woke up due to interruption we need to honor that.
417              if (Thread.interrupted()) {
418                removeWaiter(node);
419                throw new InterruptedException();
420              }
421
422              // Otherwise re-read and check doneness. If we loop then it must have been a spurious
423              // wakeup
424              localValue = value;
425              if (localValue != null & !(localValue instanceof SetFuture)) {
426                return getDoneValue(localValue);
427              }
428
429              // timed out?
430              remainingNanos = endNanos - System.nanoTime();
431              if (remainingNanos < SPIN_THRESHOLD_NANOS) {
432                // Remove the waiter, one way or another we are done parking this thread.
433                removeWaiter(node);
434                break long_wait_loop; // jump down to the busy wait loop
435              }
436            }
437          }
438          oldHead = waiters; // re-read and loop.
439        } while (oldHead != Waiter.TOMBSTONE);
440      }
441      // re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
442      // waiter.
443      return getDoneValue(value);
444    }
445    // If we get here then we have remainingNanos < SPIN_THRESHOLD_NANOS and there is no node on the
446    // waiters list
447    while (remainingNanos > 0) {
448      localValue = value;
449      if (localValue != null & !(localValue instanceof SetFuture)) {
450        return getDoneValue(localValue);
451      }
452      if (Thread.interrupted()) {
453        throw new InterruptedException();
454      }
455      remainingNanos = endNanos - System.nanoTime();
456    }
457
458    String futureToString = toString();
459    // It's confusing to see a completed future in a timeout message; if isDone() returns false,
460    // then we know it must have given a pending toString value earlier. If not, then the future
461    // completed after the timeout expired, and the message might be success.
462    if (isDone()) {
463      throw new TimeoutException(
464          "Waited " + timeout + " " + Ascii.toLowerCase(unit.toString())
465              + " but future completed as timeout expired");
466    }
467    throw new TimeoutException(
468        "Waited " + timeout + " " + Ascii.toLowerCase(unit.toString()) + " for " + futureToString);
469  }
470
471  /**
472   * {@inheritDoc}
473   *
474   * <p>The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
475   * current thread is interrupted during the call, even if the value is already available.
476   *
477   * @throws CancellationException {@inheritDoc}
478   */
479  @CanIgnoreReturnValue
480  @Override
481  public V get() throws InterruptedException, ExecutionException {
482    if (Thread.interrupted()) {
483      throw new InterruptedException();
484    }
485    Object localValue = value;
486    if (localValue != null & !(localValue instanceof SetFuture)) {
487      return getDoneValue(localValue);
488    }
489    Waiter oldHead = waiters;
490    if (oldHead != Waiter.TOMBSTONE) {
491      Waiter node = new Waiter();
492      do {
493        node.setNext(oldHead);
494        if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
495          // we are on the stack, now wait for completion.
496          while (true) {
497            LockSupport.park(this);
498            // Check interruption first, if we woke up due to interruption we need to honor that.
499            if (Thread.interrupted()) {
500              removeWaiter(node);
501              throw new InterruptedException();
502            }
503            // Otherwise re-read and check doneness. If we loop then it must have been a spurious
504            // wakeup
505            localValue = value;
506            if (localValue != null & !(localValue instanceof SetFuture)) {
507              return getDoneValue(localValue);
508            }
509          }
510        }
511        oldHead = waiters; // re-read and loop.
512      } while (oldHead != Waiter.TOMBSTONE);
513    }
514    // re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
515    // waiter.
516    return getDoneValue(value);
517  }
518
519  /**
520   * Unboxes {@code obj}. Assumes that obj is not {@code null} or a {@link SetFuture}.
521   */
522  private V getDoneValue(Object obj) throws ExecutionException {
523    // While this seems like it might be too branch-y, simple benchmarking proves it to be
524    // unmeasurable (comparing done AbstractFutures with immediateFuture)
525    if (obj instanceof Cancellation) {
526      throw cancellationExceptionWithCause("Task was cancelled.", ((Cancellation) obj).cause);
527    } else if (obj instanceof Failure) {
528      throw new ExecutionException(((Failure) obj).exception);
529    } else if (obj == NULL) {
530      return null;
531    } else {
532      @SuppressWarnings("unchecked") // this is the only other option
533      V asV = (V) obj;
534      return asV;
535    }
536  }
537
538  @Override
539  public boolean isDone() {
540    final Object localValue = value;
541    return localValue != null & !(localValue instanceof SetFuture);
542  }
543
544  @Override
545  public boolean isCancelled() {
546    final Object localValue = value;
547    return localValue instanceof Cancellation;
548  }
549
550  /**
551   * {@inheritDoc}
552   *
553   * <p>If a cancellation attempt succeeds on a {@code Future} that had previously been {@linkplain
554   * #setFuture set asynchronously}, then the cancellation will also be propagated to the delegate
555   * {@code Future} that was supplied in the {@code setFuture} call.
556   *
557   * <p>Rather than override this method to perform additional cancellation work or cleanup,
558   * subclasses should override {@link #afterDone}, consulting {@link #isCancelled} and {@link
559   * #wasInterrupted} as necessary. This ensures that the work is done even if the future is
560   * cancelled without a call to {@code cancel}, such as by calling {@code
561   * setFuture(cancelledFuture)}.
562   */
563  @CanIgnoreReturnValue
564  @Override
565  public boolean cancel(boolean mayInterruptIfRunning) {
566    Object localValue = value;
567    boolean rValue = false;
568    if (localValue == null | localValue instanceof SetFuture) {
569      // Try to delay allocating the exception. At this point we may still lose the CAS, but it is
570      // certainly less likely.
571      Object valueToSet =
572          GENERATE_CANCELLATION_CAUSES
573              ? new Cancellation(
574                  mayInterruptIfRunning, new CancellationException("Future.cancel() was called."))
575              : (mayInterruptIfRunning
576                  ? Cancellation.CAUSELESS_INTERRUPTED
577                  : Cancellation.CAUSELESS_CANCELLED);
578      AbstractFuture<?> abstractFuture = this;
579      while (true) {
580        if (ATOMIC_HELPER.casValue(abstractFuture, localValue, valueToSet)) {
581          rValue = true;
582          // We call interuptTask before calling complete(), which is consistent with
583          // FutureTask
584          if (mayInterruptIfRunning) {
585            abstractFuture.interruptTask();
586          }
587          complete(abstractFuture);
588          if (localValue instanceof SetFuture) {
589            // propagate cancellation to the future set in setfuture, this is racy, and we don't
590            // care if we are successful or not.
591            ListenableFuture<?> futureToPropagateTo = ((SetFuture) localValue).future;
592            if (futureToPropagateTo instanceof TrustedFuture) {
593              // If the future is a TrustedFuture then we specifically avoid calling cancel()
594              // this has 2 benefits
595              // 1. for long chains of futures strung together with setFuture we consume less stack
596              // 2. we avoid allocating Cancellation objects at every level of the cancellation
597              //    chain
598              // We can only do this for TrustedFuture, because TrustedFuture.cancel is final and
599              // does nothing but delegate to this method.
600              AbstractFuture<?> trusted = (AbstractFuture<?>) futureToPropagateTo;
601              localValue = trusted.value;
602              if (localValue == null | localValue instanceof SetFuture) {
603                abstractFuture = trusted;
604                continue;  // loop back up and try to complete the new future
605              }
606            } else {
607              // not a TrustedFuture, call cancel directly.
608              futureToPropagateTo.cancel(mayInterruptIfRunning);
609            }
610          }
611          break;
612        }
613        // obj changed, reread
614        localValue = abstractFuture.value;
615        if (!(localValue instanceof SetFuture)) {
616          // obj cannot be null at this point, because value can only change from null to non-null.
617          // So if value changed (and it did since we lost the CAS), then it cannot be null and
618          // since it isn't a SetFuture, then the future must be done and we should exit the loop
619          break;
620        }
621      }
622    }
623    return rValue;
624  }
625
626  /**
627   * Subclasses can override this method to implement interruption of the future's computation. The
628   * method is invoked automatically by a successful call to {@link #cancel(boolean) cancel(true)}.
629   *
630   * <p>The default implementation does nothing.
631   *
632   * <p>This method is likely to be deprecated. Prefer to override {@link #afterDone}, consulting
633   * {@link #wasInterrupted} to decide whether to interrupt your task.
634   *
635   * @since 10.0
636   */
637  protected void interruptTask() {}
638
639  /**
640   * Returns true if this future was cancelled with {@code mayInterruptIfRunning} set to {@code
641   * true}.
642   *
643   * @since 14.0
644   */
645  protected final boolean wasInterrupted() {
646    final Object localValue = value;
647    return (localValue instanceof Cancellation) && ((Cancellation) localValue).wasInterrupted;
648  }
649
650  /**
651   * {@inheritDoc}
652   *
653   * @since 10.0
654   */
655  @Override
656  public void addListener(Runnable listener, Executor executor) {
657    checkNotNull(listener, "Runnable was null.");
658    checkNotNull(executor, "Executor was null.");
659    Listener oldHead = listeners;
660    if (oldHead != Listener.TOMBSTONE) {
661      Listener newNode = new Listener(listener, executor);
662      do {
663        newNode.next = oldHead;
664        if (ATOMIC_HELPER.casListeners(this, oldHead, newNode)) {
665          return;
666        }
667        oldHead = listeners; // re-read
668      } while (oldHead != Listener.TOMBSTONE);
669    }
670    // If we get here then the Listener TOMBSTONE was set, which means the future is done, call
671    // the listener.
672    executeListener(listener, executor);
673  }
674
675  /**
676   * Sets the result of this {@code Future} unless this {@code Future} has already been cancelled or
677   * set (including {@linkplain #setFuture set asynchronously}). When a call to this method returns,
678   * the {@code Future} is guaranteed to be {@linkplain #isDone done} <b>only if</b> the call was
679   * accepted (in which case it returns {@code true}). If it returns {@code false}, the {@code
680   * Future} may have previously been set asynchronously, in which case its result may not be known
681   * yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
682   * method, only by a call to {@link #cancel}.
683   *
684   * @param value the value to be used as the result
685   * @return true if the attempt was accepted, completing the {@code Future}
686   */
687  @CanIgnoreReturnValue
688  protected boolean set(@Nullable V value) {
689    Object valueToSet = value == null ? NULL : value;
690    if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
691      complete(this);
692      return true;
693    }
694    return false;
695  }
696
697  /**
698   * Sets the failed result of this {@code Future} unless this {@code Future} has already been
699   * cancelled or set (including {@linkplain #setFuture set asynchronously}). When a call to this
700   * method returns, the {@code Future} is guaranteed to be {@linkplain #isDone done} <b>only if</b>
701   * the call was accepted (in which case it returns {@code true}). If it returns {@code false}, the
702   * {@code Future} may have previously been set asynchronously, in which case its result may not be
703   * known yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
704   * method, only by a call to {@link #cancel}.
705   *
706   * @param throwable the exception to be used as the failed result
707   * @return true if the attempt was accepted, completing the {@code Future}
708   */
709  @CanIgnoreReturnValue
710  protected boolean setException(Throwable throwable) {
711    Object valueToSet = new Failure(checkNotNull(throwable));
712    if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
713      complete(this);
714      return true;
715    }
716    return false;
717  }
718
719  /**
720   * Sets the result of this {@code Future} to match the supplied input {@code Future} once the
721   * supplied {@code Future} is done, unless this {@code Future} has already been cancelled or set
722   * (including "set asynchronously," defined below).
723   *
724   * <p>If the supplied future is {@linkplain #isDone done} when this method is called and the call
725   * is accepted, then this future is guaranteed to have been completed with the supplied future by
726   * the time this method returns. If the supplied future is not done and the call is accepted, then
727   * the future will be <i>set asynchronously</i>. Note that such a result, though not yet known,
728   * cannot be overridden by a call to a {@code set*} method, only by a call to {@link #cancel}.
729   *
730   * <p>If the call {@code setFuture(delegate)} is accepted and this {@code Future} is later
731   * cancelled, cancellation will be propagated to {@code delegate}. Additionally, any call to
732   * {@code setFuture} after any cancellation will propagate cancellation to the supplied {@code
733   * Future}.
734   *
735   * <p>Note that, even if the supplied future is cancelled and it causes this future to complete,
736   * it will never trigger interruption behavior. In particular, it will not cause this future to
737   * invoke the {@link #interruptTask} method, and the {@link #wasInterrupted} method will not
738   * return {@code true}.
739   *
740   * @param future the future to delegate to
741   * @return true if the attempt was accepted, indicating that the {@code Future} was not previously
742   *     cancelled or set.
743   * @since 19.0
744   */
745  @Beta
746  @CanIgnoreReturnValue
747  protected boolean setFuture(ListenableFuture<? extends V> future) {
748    checkNotNull(future);
749    Object localValue = value;
750    if (localValue == null) {
751      if (future.isDone()) {
752        Object value = getFutureValue(future);
753        if (ATOMIC_HELPER.casValue(this, null, value)) {
754          complete(this);
755          return true;
756        }
757        return false;
758      }
759      SetFuture valueToSet = new SetFuture<V>(this, future);
760      if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
761        // the listener is responsible for calling completeWithFuture, directExecutor is appropriate
762        // since all we are doing is unpacking a completed future which should be fast.
763        try {
764          future.addListener(valueToSet, directExecutor());
765        } catch (Throwable t) {
766          // addListener has thrown an exception! SetFuture.run can't throw any exceptions so this
767          // must have been caused by addListener itself. The most likely explanation is a
768          // misconfigured mock. Try to switch to Failure.
769          Failure failure;
770          try {
771            failure = new Failure(t);
772          } catch (Throwable oomMostLikely) {
773            failure = Failure.FALLBACK_INSTANCE;
774          }
775          // Note: The only way this CAS could fail is if cancel() has raced with us. That is ok.
776          boolean unused = ATOMIC_HELPER.casValue(this, valueToSet, failure);
777        }
778        return true;
779      }
780      localValue = value; // we lost the cas, fall through and maybe cancel
781    }
782    // The future has already been set to something. If it is cancellation we should cancel the
783    // incoming future.
784    if (localValue instanceof Cancellation) {
785      // we don't care if it fails, this is best-effort.
786      future.cancel(((Cancellation) localValue).wasInterrupted);
787    }
788    return false;
789  }
790
791  /**
792   * Returns a value that satisfies the contract of the {@link #value} field based on the state of
793   * given future.
794   *
795   * <p>This is approximately the inverse of {@link #getDoneValue(Object)}
796   */
797  private static Object getFutureValue(ListenableFuture<?> future) {
798    Object valueToSet;
799    if (future instanceof TrustedFuture) {
800      // Break encapsulation for TrustedFuture instances since we know that subclasses cannot
801      // override .get() (since it is final) and therefore this is equivalent to calling .get()
802      // and unpacking the exceptions like we do below (just much faster because it is a single
803      // field read instead of a read, several branches and possibly creating exceptions).
804      Object v = ((AbstractFuture<?>) future).value;
805      if (v instanceof Cancellation) {
806        // If the other future was interrupted, clear the interrupted bit while preserving the cause
807        // this will make it consistent with how non-trustedfutures work which cannot propagate the
808        // wasInterrupted bit
809        Cancellation c = (Cancellation) v;
810        if (c.wasInterrupted) {
811          v =
812              c.cause != null
813                  ? new Cancellation(/* wasInterrupted= */ false, c.cause)
814                  : Cancellation.CAUSELESS_CANCELLED;
815        }
816      }
817      return v;
818    } else {
819      // Otherwise calculate valueToSet by calling .get()
820      try {
821        Object v = getDone(future);
822        valueToSet = v == null ? NULL : v;
823      } catch (ExecutionException exception) {
824        valueToSet = new Failure(exception.getCause());
825      } catch (CancellationException cancellation) {
826        valueToSet = new Cancellation(false, cancellation);
827      } catch (Throwable t) {
828        valueToSet = new Failure(t);
829      }
830    }
831    return valueToSet;
832  }
833
834  /** Unblocks all threads and runs all listeners. */
835  private static void complete(AbstractFuture<?> future) {
836    Listener next = null;
837    outer: while (true) {
838      future.releaseWaiters();
839      // We call this before the listeners in order to avoid needing to manage a separate stack data
840      // structure for them.
841      // afterDone() should be generally fast and only used for cleanup work... but in theory can
842      // also be recursive and create StackOverflowErrors
843      future.afterDone();
844      // push the current set of listeners onto next
845      next = future.clearListeners(next);
846      future = null;
847      while (next != null) {
848        Listener curr = next;
849        next = next.next;
850        Runnable task = curr.task;
851        if (task instanceof SetFuture) {
852          SetFuture<?> setFuture = (SetFuture<?>) task;
853          // We unwind setFuture specifically to avoid StackOverflowErrors in the case of long
854          // chains of SetFutures
855          // Handling this special case is important because there is no way to pass an executor to
856          // setFuture, so a user couldn't break the chain by doing this themselves.  It is also
857          // potentially common if someone writes a recursive Futures.transformAsync transformer.
858          future = setFuture.owner;
859          if (future.value == setFuture) {
860            Object valueToSet = getFutureValue(setFuture.future);
861            if (ATOMIC_HELPER.casValue(future, setFuture, valueToSet)) {
862              continue outer;
863            }
864          }
865          // other wise the future we were trying to set is already done.
866        } else {
867          executeListener(task, curr.executor);
868        }
869      }
870      break;
871    }
872  }
873
874  /**
875   * Callback method that is called exactly once after the future is completed.
876   *
877   * <p>If {@link #interruptTask} is also run during completion, {@link #afterDone} runs after it.
878   *
879   * <p>The default implementation of this method in {@code AbstractFuture} does nothing. This is
880   * intended for very lightweight cleanup work, for example, timing statistics or clearing fields.
881   * If your task does anything heavier consider, just using a listener with an executor.
882   *
883   * @since 20.0
884   */
885  @Beta
886  @ForOverride
887  protected void afterDone() {}
888
889  /**
890   * Returns the exception that this {@code Future} completed with. This includes completion through
891   * a call to {@link #setException} or {@link #setFuture setFuture}{@code (failedFuture)} but not
892   * cancellation.
893   *
894   * @throws RuntimeException if the {@code Future} has not failed
895   */
896  final Throwable trustedGetException() {
897    return ((Failure) value).exception;
898  }
899
900  /**
901   * If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts)
902   * the given future (if available).
903   */
904  final void maybePropagateCancellationTo(@Nullable Future<?> related) {
905    if (related != null & isCancelled()) {
906      related.cancel(wasInterrupted());
907    }
908  }
909
910  /** Releases all threads in the {@link #waiters} list, and clears the list. */
911  private void releaseWaiters() {
912    Waiter head;
913    do {
914      head = waiters;
915    } while (!ATOMIC_HELPER.casWaiters(this, head, Waiter.TOMBSTONE));
916    for (Waiter currentWaiter = head;
917        currentWaiter != null;
918        currentWaiter = currentWaiter.next) {
919      currentWaiter.unpark();
920    }
921  }
922
923  /**
924   * Clears the {@link #listeners} list and prepends its contents to {@code onto}, least recently
925   * added first.
926   */
927  private Listener clearListeners(Listener onto) {
928    // We need to
929    // 1. atomically swap the listeners with TOMBSTONE, this is because addListener uses that to
930    //    to synchronize with us
931    // 2. reverse the linked list, because despite our rather clear contract, people depend on us
932    //    executing listeners in the order they were added
933    // 3. push all the items onto 'onto' and return the new head of the stack
934    Listener head;
935    do {
936      head = listeners;
937    } while (!ATOMIC_HELPER.casListeners(this, head, Listener.TOMBSTONE));
938    Listener reversedList = onto;
939    while (head != null) {
940      Listener tmp = head;
941      head = head.next;
942      tmp.next = reversedList;
943      reversedList = tmp;
944    }
945    return reversedList;
946  }
947
948  // TODO(user) move this up into FluentFuture, or parts as a default method on ListenableFuture?
949  @Override
950  public String toString() {
951    StringBuilder builder = new StringBuilder().append(super.toString()).append("[status=");
952    if (isCancelled()) {
953      builder.append("CANCELLED");
954    } else if (isDone()) {
955      addDoneString(builder);
956    } else {
957      String pendingDescription;
958      try {
959        pendingDescription = pendingToString();
960      } catch (RuntimeException e) {
961        // Don't call getMessage or toString() on the exception, in case the exception thrown by the
962        // subclass is implemented with bugs similar to the subclass.
963        pendingDescription = "Exception thrown from implementation: " + e.getClass();
964      }
965      // The future may complete during or before the call to getPendingToString, so we use null
966      // as a signal that we should try checking if the future is done again.
967      if (!isNullOrEmpty(pendingDescription)) {
968        builder.append("PENDING, info=[").append(pendingDescription).append("]");
969      } else if (isDone()) {
970        addDoneString(builder);
971      } else {
972        builder.append("PENDING");
973      }
974    }
975    return builder.append("]").toString();
976  }
977
978  /**
979   * Provide a human-readable explanation of why this future has not yet completed.
980   *
981   * @return null if an explanation cannot be provided because the future is done.
982   * @since 23.0
983   */
984  @Nullable
985  protected String pendingToString() {
986    Object localValue = value;
987    if (localValue instanceof SetFuture) {
988      return "setFuture=[" + ((SetFuture) localValue).future + "]";
989    } else if (this instanceof ScheduledFuture) {
990      return "remaining delay=["
991          + ((ScheduledFuture) this).getDelay(TimeUnit.MILLISECONDS)
992          + " ms]";
993    }
994    return null;
995  }
996
997  private void addDoneString(StringBuilder builder) {
998    try {
999      V value = getDone(this);
1000      builder.append("SUCCESS, result=[").append(value).append("]");
1001    } catch (ExecutionException e) {
1002      builder.append("FAILURE, cause=[").append(e.getCause()).append("]");
1003    } catch (CancellationException e) {
1004      builder.append("CANCELLED"); // shouldn't be reachable
1005    } catch (RuntimeException e) {
1006      builder.append("UNKNOWN, cause=[").append(e.getClass()).append(" thrown from get()]");
1007    }
1008  }
1009
1010  /**
1011   * Submits the given runnable to the given {@link Executor} catching and logging all
1012   * {@linkplain RuntimeException runtime exceptions} thrown by the executor.
1013   */
1014  private static void executeListener(Runnable runnable, Executor executor) {
1015    try {
1016      executor.execute(runnable);
1017    } catch (RuntimeException e) {
1018      // Log it and keep going -- bad runnable and/or executor. Don't punish the other runnables if
1019      // we're given a bad one. We only catch RuntimeException because we want Errors to propagate
1020      // up.
1021      log.log(
1022          Level.SEVERE,
1023          "RuntimeException while executing runnable " + runnable + " with executor " + executor,
1024          e);
1025    }
1026  }
1027
1028  private abstract static class AtomicHelper {
1029    /** Non volatile write of the thread to the {@link Waiter#thread} field. */
1030    abstract void putThread(Waiter waiter, Thread newValue);
1031
1032    /** Non volatile write of the waiter to the {@link Waiter#next} field. */
1033    abstract void putNext(Waiter waiter, Waiter newValue);
1034
1035    /** Performs a CAS operation on the {@link #waiters} field. */
1036    abstract boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update);
1037
1038    /** Performs a CAS operation on the {@link #listeners} field. */
1039    abstract boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update);
1040
1041    /** Performs a CAS operation on the {@link #value} field. */
1042    abstract boolean casValue(AbstractFuture<?> future, Object expect, Object update);
1043  }
1044
1045  /**
1046   * {@link AtomicHelper} based on {@link sun.misc.Unsafe}.
1047   *
1048   * <p>Static initialization of this class will fail if the {@link sun.misc.Unsafe} object cannot
1049   * be accessed.
1050   */
1051  private static final class UnsafeAtomicHelper extends AtomicHelper {
1052    static final sun.misc.Unsafe UNSAFE;
1053    static final long LISTENERS_OFFSET;
1054    static final long WAITERS_OFFSET;
1055    static final long VALUE_OFFSET;
1056    static final long WAITER_THREAD_OFFSET;
1057    static final long WAITER_NEXT_OFFSET;
1058
1059    static {
1060      sun.misc.Unsafe unsafe = null;
1061      try {
1062        unsafe = sun.misc.Unsafe.getUnsafe();
1063      } catch (SecurityException tryReflectionInstead) {
1064        try {
1065          unsafe =
1066              AccessController.doPrivileged(
1067                  new PrivilegedExceptionAction<sun.misc.Unsafe>() {
1068                    @Override
1069                    public sun.misc.Unsafe run() throws Exception {
1070                      Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
1071                      for (java.lang.reflect.Field f : k.getDeclaredFields()) {
1072                        f.setAccessible(true);
1073                        Object x = f.get(null);
1074                        if (k.isInstance(x)) {
1075                          return k.cast(x);
1076                        }
1077                      }
1078                      throw new NoSuchFieldError("the Unsafe");
1079                    }
1080                  });
1081        } catch (PrivilegedActionException e) {
1082          throw new RuntimeException("Could not initialize intrinsics", e.getCause());
1083        }
1084      }
1085      try {
1086        Class<?> abstractFuture = AbstractFuture.class;
1087        WAITERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("waiters"));
1088        LISTENERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("listeners"));
1089        VALUE_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("value"));
1090        WAITER_THREAD_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("thread"));
1091        WAITER_NEXT_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("next"));
1092        UNSAFE = unsafe;
1093      } catch (Exception e) {
1094        throwIfUnchecked(e);
1095        throw new RuntimeException(e);
1096      }
1097    }
1098
1099    @Override
1100    void putThread(Waiter waiter, Thread newValue) {
1101      UNSAFE.putObject(waiter, WAITER_THREAD_OFFSET, newValue);
1102    }
1103
1104    @Override
1105    void putNext(Waiter waiter, Waiter newValue) {
1106      UNSAFE.putObject(waiter, WAITER_NEXT_OFFSET, newValue);
1107    }
1108
1109    /** Performs a CAS operation on the {@link #waiters} field. */
1110    @Override
1111    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
1112      return UNSAFE.compareAndSwapObject(future, WAITERS_OFFSET, expect, update);
1113    }
1114
1115    /** Performs a CAS operation on the {@link #listeners} field. */
1116    @Override
1117    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
1118      return UNSAFE.compareAndSwapObject(future, LISTENERS_OFFSET, expect, update);
1119    }
1120
1121    /** Performs a CAS operation on the {@link #value} field. */
1122    @Override
1123    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
1124      return UNSAFE.compareAndSwapObject(future, VALUE_OFFSET, expect, update);
1125    }
1126  }
1127
1128  /** {@link AtomicHelper} based on {@link AtomicReferenceFieldUpdater}. */
1129  private static final class SafeAtomicHelper extends AtomicHelper {
1130    final AtomicReferenceFieldUpdater<Waiter, Thread> waiterThreadUpdater;
1131    final AtomicReferenceFieldUpdater<Waiter, Waiter> waiterNextUpdater;
1132    final AtomicReferenceFieldUpdater<AbstractFuture, Waiter> waitersUpdater;
1133    final AtomicReferenceFieldUpdater<AbstractFuture, Listener> listenersUpdater;
1134    final AtomicReferenceFieldUpdater<AbstractFuture, Object> valueUpdater;
1135
1136    SafeAtomicHelper(
1137        AtomicReferenceFieldUpdater<Waiter, Thread> waiterThreadUpdater,
1138        AtomicReferenceFieldUpdater<Waiter, Waiter> waiterNextUpdater,
1139        AtomicReferenceFieldUpdater<AbstractFuture, Waiter> waitersUpdater,
1140        AtomicReferenceFieldUpdater<AbstractFuture, Listener> listenersUpdater,
1141        AtomicReferenceFieldUpdater<AbstractFuture, Object> valueUpdater) {
1142      this.waiterThreadUpdater = waiterThreadUpdater;
1143      this.waiterNextUpdater = waiterNextUpdater;
1144      this.waitersUpdater = waitersUpdater;
1145      this.listenersUpdater = listenersUpdater;
1146      this.valueUpdater = valueUpdater;
1147    }
1148
1149    @Override
1150    void putThread(Waiter waiter, Thread newValue) {
1151      waiterThreadUpdater.lazySet(waiter, newValue);
1152    }
1153
1154    @Override
1155    void putNext(Waiter waiter, Waiter newValue) {
1156      waiterNextUpdater.lazySet(waiter, newValue);
1157    }
1158
1159    @Override
1160    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
1161      return waitersUpdater.compareAndSet(future, expect, update);
1162    }
1163
1164    @Override
1165    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
1166      return listenersUpdater.compareAndSet(future, expect, update);
1167    }
1168
1169    @Override
1170    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
1171      return valueUpdater.compareAndSet(future, expect, update);
1172    }
1173  }
1174
1175  /**
1176   * {@link AtomicHelper} based on {@code synchronized} and volatile writes.
1177   *
1178   * <p>This is an implementation of last resort for when certain basic VM features are broken (like
1179   * AtomicReferenceFieldUpdater).
1180   */
1181  private static final class SynchronizedHelper extends AtomicHelper {
1182    @Override
1183    void putThread(Waiter waiter, Thread newValue) {
1184      waiter.thread = newValue;
1185    }
1186
1187    @Override
1188    void putNext(Waiter waiter, Waiter newValue) {
1189      waiter.next = newValue;
1190    }
1191
1192    @Override
1193    boolean casWaiters(AbstractFuture<?> future, Waiter expect, Waiter update) {
1194      synchronized (future) {
1195        if (future.waiters == expect) {
1196          future.waiters = update;
1197          return true;
1198        }
1199        return false;
1200      }
1201    }
1202
1203    @Override
1204    boolean casListeners(AbstractFuture<?> future, Listener expect, Listener update) {
1205      synchronized (future) {
1206        if (future.listeners == expect) {
1207          future.listeners = update;
1208          return true;
1209        }
1210        return false;
1211      }
1212    }
1213
1214    @Override
1215    boolean casValue(AbstractFuture<?> future, Object expect, Object update) {
1216      synchronized (future) {
1217        if (future.value == expect) {
1218          future.value = update;
1219          return true;
1220        }
1221        return false;
1222      }
1223    }
1224  }
1225
1226  private static CancellationException cancellationExceptionWithCause(
1227      @Nullable String message, @Nullable Throwable cause) {
1228    CancellationException exception = new CancellationException(message);
1229    exception.initCause(cause);
1230    return exception;
1231  }
1232}