001/*
002 * Copyright (C) 2006 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.Preconditions.checkState;
019import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
020import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
021
022import com.google.common.annotations.Beta;
023import com.google.common.annotations.GwtCompatible;
024import com.google.common.annotations.GwtIncompatible;
025import com.google.common.base.Function;
026import com.google.common.base.MoreObjects;
027import com.google.common.base.Preconditions;
028import com.google.common.collect.ImmutableList;
029import com.google.common.util.concurrent.CollectionFuture.ListFuture;
030import com.google.common.util.concurrent.ImmediateFuture.ImmediateCancelledFuture;
031import com.google.common.util.concurrent.ImmediateFuture.ImmediateFailedCheckedFuture;
032import com.google.common.util.concurrent.ImmediateFuture.ImmediateFailedFuture;
033import com.google.common.util.concurrent.ImmediateFuture.ImmediateSuccessfulCheckedFuture;
034import com.google.common.util.concurrent.ImmediateFuture.ImmediateSuccessfulFuture;
035import com.google.errorprone.annotations.CanIgnoreReturnValue;
036import java.util.Collection;
037import java.util.List;
038import java.util.concurrent.Callable;
039import java.util.concurrent.CancellationException;
040import java.util.concurrent.ExecutionException;
041import java.util.concurrent.Executor;
042import java.util.concurrent.Future;
043import java.util.concurrent.ScheduledExecutorService;
044import java.util.concurrent.TimeUnit;
045import java.util.concurrent.TimeoutException;
046import java.util.concurrent.atomic.AtomicInteger;
047import javax.annotation.Nullable;
048
049/**
050 * Static utility methods pertaining to the {@link Future} interface.
051 *
052 * <p>Many of these methods use the {@link ListenableFuture} API; consult the Guava User Guide
053 * article on <a href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
054 * ListenableFuture}</a>.
055 *
056 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
057 * asynchronous operations. You can chain them together manually with calls to methods like {@link
058 * Futures#transform(ListenableFuture, Function, Executor) Futures.transform}, but you will often
059 * find it easier to use a framework. Frameworks automate the process, often adding features like
060 * monitoring, debugging, and cancellation. Examples of frameworks include:
061 *
062 * <ul>
063 *   <li><a href="http://google.github.io/dagger/producers.html">Dagger Producers</a>
064 * </ul>
065 *
066 * <p>If you do chain your operations manually, you may want to use {@link FluentFuture}.
067 *
068 * @author Kevin Bourrillion
069 * @author Nishant Thakkar
070 * @author Sven Mawson
071 * @since 1.0
072 */
073@Beta
074@GwtCompatible(emulated = true)
075public final class Futures extends GwtFuturesCatchingSpecialization {
076
077  // A note on memory visibility.
078  // Many of the utilities in this class (transform, withFallback, withTimeout, asList, combine)
079  // have two requirements that significantly complicate their design.
080  // 1. Cancellation should propagate from the returned future to the input future(s).
081  // 2. The returned futures shouldn't unnecessarily 'pin' their inputs after completion.
082  //
083  // A consequence of these requirements is that the delegate futures cannot be stored in
084  // final fields.
085  //
086  // For simplicity the rest of this description will discuss Futures.catching since it is the
087  // simplest instance, though very similar descriptions apply to many other classes in this file.
088  //
089  // In the constructor of AbstractCatchingFuture, the delegate future is assigned to a field
090  // 'inputFuture'. That field is non-final and non-volatile. There are 2 places where the
091  // 'inputFuture' field is read and where we will have to consider visibility of the write
092  // operation in the constructor.
093  //
094  // 1. In the listener that performs the callback. In this case it is fine since inputFuture is
095  //    assigned prior to calling addListener, and addListener happens-before any invocation of the
096  //    listener. Notably, this means that 'volatile' is unnecessary to make 'inputFuture' visible
097  //    to the listener.
098  //
099  // 2. In done() where we may propagate cancellation to the input. In this case it is _not_ fine.
100  //    There is currently nothing that enforces that the write to inputFuture in the constructor is
101  //    visible to done(). This is because there is no happens before edge between the write and a
102  //    (hypothetical) unsafe read by our caller. Note: adding 'volatile' does not fix this issue,
103  //    it would just add an edge such that if done() observed non-null, then it would also
104  //    definitely observe all earlier writes, but we still have no guarantee that done() would see
105  //    the inital write (just stronger guarantees if it does).
106  //
107  // See: http://cs.oswego.edu/pipermail/concurrency-interest/2015-January/013800.html
108  // For a (long) discussion about this specific issue and the general futility of life.
109  //
110  // For the time being we are OK with the problem discussed above since it requires a caller to
111  // introduce a very specific kind of data-race. And given the other operations performed by these
112  // methods that involve volatile read/write operations, in practice there is no issue. Also, the
113  // way in such a visibility issue would surface is most likely as a failure of cancel() to
114  // propagate to the input. Cancellation propagation is fundamentally racy so this is fine.
115  //
116  // Future versions of the JMM may revise safe construction semantics in such a way that we can
117  // safely publish these objects and we won't need this whole discussion.
118  // TODO(user,lukes): consider adding volatile to all these fields since in current known JVMs
119  // that should resolve the issue. This comes at the cost of adding more write barriers to the
120  // implementations.
121
122  private Futures() {}
123
124  /**
125   * Creates a {@link CheckedFuture} out of a normal {@link ListenableFuture} and a {@link Function}
126   * that maps from {@link Exception} instances into the appropriate checked type.
127   *
128   * <p><b>Warning:</b> We recommend against using {@code CheckedFuture} in new projects. {@code
129   * CheckedFuture} is difficult to build libraries atop. {@code CheckedFuture} ports of methods
130   * like {@link Futures#transformAsync} have historically had bugs, and some of these bugs are
131   * necessary, unavoidable consequences of the {@code CheckedFuture} API. Additionally, {@code
132   * CheckedFuture} encourages users to take exceptions from one thread and rethrow them in another,
133   * producing confusing stack traces.
134   *
135   * <p>The given mapping function will be applied to an {@link InterruptedException}, a {@link
136   * CancellationException}, or an {@link ExecutionException}. See {@link Future#get()} for details
137   * on the exceptions thrown.
138   *
139   * @since 9.0 (source-compatible since 1.0)
140   * @deprecated {@link CheckedFuture} cannot properly support the chained operations that are the
141   *     primary goal of {@link ListenableFuture}. {@code CheckedFuture} also encourages users to
142   *     rethrow exceptions from one thread in another thread, producing misleading stack traces.
143   *     Additionally, it has a surprising policy about which exceptions to map and which to leave
144   *     untouched. Guava users who want a {@code CheckedFuture} can fork the classes for their own
145   *     use, possibly specializing them to the particular exception type they use. We recommend
146   *     that most people use {@code ListenableFuture} and perform any exception wrapping
147   *     themselves. This method is scheduled for removal from Guava in February 2018.
148   */
149  @Deprecated
150  @GwtIncompatible // TODO
151  public static <V, X extends Exception> CheckedFuture<V, X> makeChecked(
152      ListenableFuture<V> future, Function<? super Exception, X> mapper) {
153    return new MappingCheckedFuture<>(checkNotNull(future), mapper);
154  }
155
156  /**
157   * Creates a {@code ListenableFuture} which has its value set immediately upon construction. The
158   * getters just return the value. This {@code Future} can't be canceled or timed out and its
159   * {@code isDone()} method always returns {@code true}.
160   */
161  public static <V> ListenableFuture<V> immediateFuture(@Nullable V value) {
162    if (value == null) {
163      // This cast is safe because null is assignable to V for all V (i.e. it is covariant)
164      @SuppressWarnings({"unchecked", "rawtypes"})
165      ListenableFuture<V> typedNull = (ListenableFuture) ImmediateSuccessfulFuture.NULL;
166      return typedNull;
167    }
168    return new ImmediateSuccessfulFuture<V>(value);
169  }
170
171  /**
172   * Returns a {@code CheckedFuture} which has its value set immediately upon construction.
173   *
174   * <p>The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
175   * returns {@code true}. Calling {@code get()} or {@code checkedGet()} will immediately return the
176   * provided value.
177   *
178   * @deprecated {@link CheckedFuture} cannot properly support the chained operations that are the
179   *     primary goal of {@link ListenableFuture}. {@code CheckedFuture} also encourages users to
180   *     rethrow exceptions from one thread in another thread, producing misleading stack traces.
181   *     Additionally, it has a surprising policy about which exceptions to map and which to leave
182   *     untouched. Guava users who want a {@code CheckedFuture} can fork the classes for their own
183   *     use, possibly specializing them to the particular exception type they use. We recommend
184   *     that most people use {@code ListenableFuture} and perform any exception wrapping
185   *     themselves. This method is scheduled for removal from Guava in February 2018.
186   */
187  @Deprecated
188  @GwtIncompatible // TODO
189  public static <V, X extends Exception> CheckedFuture<V, X> immediateCheckedFuture(
190      @Nullable V value) {
191    return new ImmediateSuccessfulCheckedFuture<>(value);
192  }
193
194  /**
195   * Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
196   *
197   * <p>The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
198   * returns {@code true}. Calling {@code get()} will immediately throw the provided {@code
199   * Throwable} wrapped in an {@code ExecutionException}.
200   */
201  public static <V> ListenableFuture<V> immediateFailedFuture(Throwable throwable) {
202    checkNotNull(throwable);
203    return new ImmediateFailedFuture<V>(throwable);
204  }
205
206  /**
207   * Creates a {@code ListenableFuture} which is cancelled immediately upon construction, so that
208   * {@code isCancelled()} always returns {@code true}.
209   *
210   * @since 14.0
211   */
212  public static <V> ListenableFuture<V> immediateCancelledFuture() {
213    return new ImmediateCancelledFuture<V>();
214  }
215
216  /**
217   * Returns a {@code CheckedFuture} which has an exception set immediately upon construction.
218   *
219   * <p>The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
220   * returns {@code true}. Calling {@code get()} will immediately throw the provided {@code
221   * Exception} wrapped in an {@code ExecutionException}, and calling {@code checkedGet()} will
222   * throw the provided exception itself.
223   *
224   * @deprecated {@link CheckedFuture} cannot properly support the chained operations that are the
225   *     primary goal of {@link ListenableFuture}. {@code CheckedFuture} also encourages users to
226   *     rethrow exceptions from one thread in another thread, producing misleading stack traces.
227   *     Additionally, it has a surprising policy about which exceptions to map and which to leave
228   *     untouched. Guava users who want a {@code CheckedFuture} can fork the classes for their own
229   *     use, possibly specializing them to the particular exception type they use. We recommend
230   *     that most people use {@code ListenableFuture} and perform any exception wrapping
231   *     themselves. This method is scheduled for removal from Guava in February 2018.
232   */
233  @Deprecated
234  @GwtIncompatible // TODO
235  public static <V, X extends Exception> CheckedFuture<V, X> immediateFailedCheckedFuture(
236      X exception) {
237    checkNotNull(exception);
238    return new ImmediateFailedCheckedFuture<>(exception);
239  }
240
241  /**
242   * Executes {@code callable} on the specified {@code executor}, returning a {@code Future}.
243   *
244   * @throws RejectedExecutionException if the task cannot be scheduled for execution
245   * @since 23.0
246   */
247  public static <O> ListenableFuture<O> submitAsync(AsyncCallable<O> callable, Executor executor) {
248    TrustedListenableFutureTask<O> task = TrustedListenableFutureTask.create(callable);
249    executor.execute(task);
250    return task;
251  }
252
253  /**
254   * Schedules {@code callable} on the specified {@code executor}, returning a {@code Future}.
255   *
256   * @throws RejectedExecutionException if the task cannot be scheduled for execution
257   * @since 23.0
258   */
259  @GwtIncompatible // java.util.concurrent.ScheduledExecutorService
260  public static <O> ListenableFuture<O> scheduleAsync(
261      AsyncCallable<O> callable,
262      long delay,
263      TimeUnit timeUnit,
264      ScheduledExecutorService executorService) {
265    TrustedListenableFutureTask<O> task = TrustedListenableFutureTask.create(callable);
266    final Future<?> scheduled = executorService.schedule(task, delay, timeUnit);
267    task.addListener(
268        new Runnable() {
269          @Override
270          public void run() {
271            // Don't want to interrupt twice
272            scheduled.cancel(false);
273          }
274        },
275        directExecutor());
276    return task;
277  }
278
279  /**
280   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
281   * primary input fails with the given {@code exceptionType}, from the result provided by the
282   * {@code fallback}. {@link Function#apply} is not invoked until the primary input has failed, so
283   * if the primary input succeeds, it is never invoked. If, during the invocation of {@code
284   * fallback}, an exception is thrown, this exception is used as the result of the output {@code
285   * Future}.
286   *
287   * <p>Usage example:
288   *
289   * <pre>{@code
290   * ListenableFuture<Integer> fetchCounterFuture = ...;
291   *
292   * // Falling back to a zero counter in case an exception happens when
293   * // processing the RPC to fetch counters.
294   * ListenableFuture<Integer> faultTolerantFuture = Futures.catching(
295   *     fetchCounterFuture, FetchException.class, x -> 0);
296   * }</pre>
297   *
298   * <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
299   * choice in some cases. See the discussion in the {@link ListenableFuture#addListener
300   * ListenableFuture.addListener} documentation. All its warnings about heavyweight listeners are
301   * also applicable to heavyweight functions passed to this method.
302   *
303   * @param input the primary input {@code Future}
304   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
305   *     type is matched against the input's exception. "The input's exception" means the cause of
306   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
307   *     different kind of exception, that exception itself. To avoid hiding bugs and other
308   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
309   *     Throwable.class} in particular.
310   * @param fallback the {@link Function} to be called if {@code input} fails with the expected
311   *     exception type. The function's argument is the input's exception. "The input's exception"
312   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
313   *     {@code get()} throws a different kind of exception, that exception itself.
314   * @since 19.0
315   * @deprecated Use {@linkplain #catching(ListenableFuture, Class, Function, Executor) the overload
316   *     that requires an executor}. For identical behavior, pass {@link
317   *     MoreExecutors#directExecutor}, but consider whether another executor would be safer, as
318   *     discussed in the {@link ListenableFuture#addListener ListenableFuture.addListener}
319   *     documentation. This method is scheduled to be removed in April 2018.
320   */
321  @Deprecated
322  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
323  public static <V, X extends Throwable> ListenableFuture<V> catching(
324      ListenableFuture<? extends V> input,
325      Class<X> exceptionType,
326      Function<? super X, ? extends V> fallback) {
327    return AbstractCatchingFuture.create(input, exceptionType, fallback, directExecutor());
328  }
329
330  /**
331   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
332   * primary input fails with the given {@code exceptionType}, from the result provided by the
333   * {@code fallback}. {@link Function#apply} is not invoked until the primary input has failed, so
334   * if the primary input succeeds, it is never invoked. If, during the invocation of {@code
335   * fallback}, an exception is thrown, this exception is used as the result of the output {@code
336   * Future}.
337   *
338   * <p>Usage example:
339   *
340   * <pre>{@code
341   * ListenableFuture<Integer> fetchCounterFuture = ...;
342   *
343   * // Falling back to a zero counter in case an exception happens when
344   * // processing the RPC to fetch counters.
345   * ListenableFuture<Integer> faultTolerantFuture = Futures.catching(
346   *     fetchCounterFuture, FetchException.class, x -> 0, directExecutor());
347   * }</pre>
348   *
349   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
350   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
351   * documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
352   * functions passed to this method.
353   *
354   * @param input the primary input {@code Future}
355   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
356   *     type is matched against the input's exception. "The input's exception" means the cause of
357   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
358   *     different kind of exception, that exception itself. To avoid hiding bugs and other
359   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
360   *     Throwable.class} in particular.
361   * @param fallback the {@link Function} to be called if {@code input} fails with the expected
362   *     exception type. The function's argument is the input's exception. "The input's exception"
363   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
364   *     {@code get()} throws a different kind of exception, that exception itself.
365   * @param executor the executor that runs {@code fallback} if {@code input} fails
366   * @since 19.0
367   */
368  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
369  public static <V, X extends Throwable> ListenableFuture<V> catching(
370      ListenableFuture<? extends V> input,
371      Class<X> exceptionType,
372      Function<? super X, ? extends V> fallback,
373      Executor executor) {
374    return AbstractCatchingFuture.create(input, exceptionType, fallback, executor);
375  }
376
377  /**
378   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
379   * primary input fails with the given {@code exceptionType}, from the result provided by the
380   * {@code fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has
381   * failed, so if the primary input succeeds, it is never invoked. If, during the invocation of
382   * {@code fallback}, an exception is thrown, this exception is used as the result of the output
383   * {@code Future}.
384   *
385   * <p>Usage examples:
386   *
387   * <pre>{@code
388   * ListenableFuture<Integer> fetchCounterFuture = ...;
389   *
390   * // Falling back to a zero counter in case an exception happens when
391   * // processing the RPC to fetch counters.
392   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
393   *     fetchCounterFuture, FetchException.class, x -> immediateFuture(0));
394   * }</pre>
395   *
396   * <p>The fallback can also choose to propagate the original exception when desired:
397   *
398   * <pre>{@code
399   * ListenableFuture<Integer> fetchCounterFuture = ...;
400   *
401   * // Falling back to a zero counter only in case the exception was a
402   * // TimeoutException.
403   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
404   *     fetchCounterFuture,
405   *     FetchException.class,
406   *     e -> {
407   *       if (omitDataOnFetchFailure) {
408   *         return immediateFuture(0);
409   *       }
410   *       throw e;
411   *     });
412   * }</pre>
413   *
414   * <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
415   * choice in some cases. See the discussion in the {@link ListenableFuture#addListener
416   * ListenableFuture.addListener} documentation. All its warnings about heavyweight listeners are
417   * also applicable to heavyweight functions passed to this method. (Specifically, {@code
418   * directExecutor} functions should avoid heavyweight operations inside {@code
419   * AsyncFunction.apply}. Any heavyweight operations should occur in other threads responsible for
420   * completing the returned {@code Future}.)
421   *
422   * @param input the primary input {@code Future}
423   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
424   *     type is matched against the input's exception. "The input's exception" means the cause of
425   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
426   *     different kind of exception, that exception itself. To avoid hiding bugs and other
427   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
428   *     Throwable.class} in particular.
429   * @param fallback the {@link AsyncFunction} to be called if {@code input} fails with the expected
430   *     exception type. The function's argument is the input's exception. "The input's exception"
431   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
432   *     {@code get()} throws a different kind of exception, that exception itself.
433   * @since 19.0 (similar functionality in 14.0 as {@code withFallback})
434   * @deprecated Use {@linkplain #catchingAsync(ListenableFuture, Class, AsyncFunction, Executor)
435   *     the overload that requires an executor}. For identical behavior, pass {@link
436   *     MoreExecutors#directExecutor}, but consider whether another executor would be safer, as
437   *     discussed in the {@link ListenableFuture#addListener ListenableFuture.addListener}
438   *     documentation. This method is scheduled to be removed in April 2018.
439   */
440  @CanIgnoreReturnValue // TODO(kak): @CheckReturnValue
441  @Deprecated
442  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
443  public static <V, X extends Throwable> ListenableFuture<V> catchingAsync(
444      ListenableFuture<? extends V> input,
445      Class<X> exceptionType,
446      AsyncFunction<? super X, ? extends V> fallback) {
447    return AbstractCatchingFuture.create(input, exceptionType, fallback, directExecutor());
448  }
449
450  /**
451   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
452   * primary input fails with the given {@code exceptionType}, from the result provided by the
453   * {@code fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has
454   * failed, so if the primary input succeeds, it is never invoked. If, during the invocation of
455   * {@code fallback}, an exception is thrown, this exception is used as the result of the output
456   * {@code Future}.
457   *
458   * <p>Usage examples:
459   *
460   * <pre>{@code
461   * ListenableFuture<Integer> fetchCounterFuture = ...;
462   *
463   * // Falling back to a zero counter in case an exception happens when
464   * // processing the RPC to fetch counters.
465   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
466   *     fetchCounterFuture, FetchException.class, x -> immediateFuture(0), directExecutor());
467   * }</pre>
468   *
469   * <p>The fallback can also choose to propagate the original exception when desired:
470   *
471   * <pre>{@code
472   * ListenableFuture<Integer> fetchCounterFuture = ...;
473   *
474   * // Falling back to a zero counter only in case the exception was a
475   * // TimeoutException.
476   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
477   *     fetchCounterFuture,
478   *     FetchException.class,
479   *     e -> {
480   *       if (omitDataOnFetchFailure) {
481   *         return immediateFuture(0);
482   *       }
483   *       throw e;
484   *     },
485   *     directExecutor());
486   * }</pre>
487   *
488   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
489   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
490   * documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
491   * functions passed to this method. (Specifically, {@code directExecutor} functions should avoid
492   * heavyweight operations inside {@code AsyncFunction.apply}. Any heavyweight operations should
493   * occur in other threads responsible for completing the returned {@code Future}.)
494   *
495   * @param input the primary input {@code Future}
496   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
497   *     type is matched against the input's exception. "The input's exception" means the cause of
498   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
499   *     different kind of exception, that exception itself. To avoid hiding bugs and other
500   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
501   *     Throwable.class} in particular.
502   * @param fallback the {@link AsyncFunction} to be called if {@code input} fails with the expected
503   *     exception type. The function's argument is the input's exception. "The input's exception"
504   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
505   *     {@code get()} throws a different kind of exception, that exception itself.
506   * @param executor the executor that runs {@code fallback} if {@code input} fails
507   * @since 19.0 (similar functionality in 14.0 as {@code withFallback})
508   */
509  @CanIgnoreReturnValue // TODO(kak): @CheckReturnValue
510  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
511  public static <V, X extends Throwable> ListenableFuture<V> catchingAsync(
512      ListenableFuture<? extends V> input,
513      Class<X> exceptionType,
514      AsyncFunction<? super X, ? extends V> fallback,
515      Executor executor) {
516    return AbstractCatchingFuture.create(input, exceptionType, fallback, executor);
517  }
518
519  /**
520   * Returns a future that delegates to another but will finish early (via a {@link
521   * TimeoutException} wrapped in an {@link ExecutionException}) if the specified duration expires.
522   *
523   * <p>The delegate future is interrupted and cancelled if it times out.
524   *
525   * @param delegate The future to delegate to.
526   * @param time when to timeout the future
527   * @param unit the time unit of the time parameter
528   * @param scheduledExecutor The executor service to enforce the timeout.
529   * @since 19.0
530   */
531  @GwtIncompatible // java.util.concurrent.ScheduledExecutorService
532  public static <V> ListenableFuture<V> withTimeout(
533      ListenableFuture<V> delegate,
534      long time,
535      TimeUnit unit,
536      ScheduledExecutorService scheduledExecutor) {
537    return TimeoutFuture.create(delegate, time, unit, scheduledExecutor);
538  }
539
540  /**
541   * Returns a new {@code Future} whose result is asynchronously derived from the result of the
542   * given {@code Future}. If the given {@code Future} fails, the returned {@code Future} fails with
543   * the same exception (and the function is not invoked).
544   *
545   * <p>More precisely, the returned {@code Future} takes its result from a {@code Future} produced
546   * by applying the given {@code AsyncFunction} to the result of the original {@code Future}.
547   * Example usage:
548   *
549   * <pre>{@code
550   * ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
551   * ListenableFuture<QueryResult> queryFuture =
552   *     transformAsync(rowKeyFuture, dataService::readFuture);
553   * }</pre>
554   *
555   * <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
556   * choice in some cases. See the discussion in the {@link ListenableFuture#addListener
557   * ListenableFuture.addListener} documentation. All its warnings about heavyweight listeners are
558   * also applicable to heavyweight functions passed to this method. (Specifically, {@code
559   * directExecutor} functions should avoid heavyweight operations inside {@code
560   * AsyncFunction.apply}. Any heavyweight operations should occur in other threads responsible for
561   * completing the returned {@code Future}.)
562   *
563   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
564   * input future and that of the future returned by the function. That is, if the returned {@code
565   * Future} is cancelled, it will attempt to cancel the other two, and if either of the other two
566   * is cancelled, the returned {@code Future} will receive a callback in which it will attempt to
567   * cancel itself.
568   *
569   * @param input The future to transform
570   * @param function A function to transform the result of the input future to the result of the
571   *     output future
572   * @return A future that holds result of the function (if the input succeeded) or the original
573   *     input's failure (if not)
574   * @since 19.0 (in 11.0 as {@code transform})
575   * @deprecated Use {@linkplain #transformAsync(ListenableFuture, AsyncFunction, Executor) the
576   *     overload that requires an executor}. For identical behavior, pass {@link
577   *     MoreExecutors#directExecutor}, but consider whether another executor would be safer, as
578   *     discussed in the {@link ListenableFuture#addListener ListenableFuture.addListener}
579   *     documentation. This method is scheduled to be removed in April 2018.
580   */
581  @Deprecated
582  public static <I, O> ListenableFuture<O> transformAsync(
583      ListenableFuture<I> input, AsyncFunction<? super I, ? extends O> function) {
584    return AbstractTransformFuture.create(input, function, directExecutor());
585  }
586
587  /**
588   * Returns a new {@code Future} whose result is asynchronously derived from the result of the
589   * given {@code Future}. If the given {@code Future} fails, the returned {@code Future} fails with
590   * the same exception (and the function is not invoked).
591   *
592   * <p>More precisely, the returned {@code Future} takes its result from a {@code Future} produced
593   * by applying the given {@code AsyncFunction} to the result of the original {@code Future}.
594   * Example usage:
595   *
596   * <pre>{@code
597   * ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
598   * ListenableFuture<QueryResult> queryFuture =
599   *     transformAsync(rowKeyFuture, dataService::readFuture, executor);
600   * }</pre>
601   *
602   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
603   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
604   * documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
605   * functions passed to this method. (Specifically, {@code directExecutor} functions should avoid
606   * heavyweight operations inside {@code AsyncFunction.apply}. Any heavyweight operations should
607   * occur in other threads responsible for completing the returned {@code Future}.)
608   *
609   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
610   * input future and that of the future returned by the chain function. That is, if the returned
611   * {@code Future} is cancelled, it will attempt to cancel the other two, and if either of the
612   * other two is cancelled, the returned {@code Future} will receive a callback in which it will
613   * attempt to cancel itself.
614   *
615   * @param input The future to transform
616   * @param function A function to transform the result of the input future to the result of the
617   *     output future
618   * @param executor Executor to run the function in.
619   * @return A future that holds result of the function (if the input succeeded) or the original
620   *     input's failure (if not)
621   * @since 19.0 (in 11.0 as {@code transform})
622   */
623  public static <I, O> ListenableFuture<O> transformAsync(
624      ListenableFuture<I> input,
625      AsyncFunction<? super I, ? extends O> function,
626      Executor executor) {
627    return AbstractTransformFuture.create(input, function, executor);
628  }
629
630  /**
631   * Returns a new {@code Future} whose result is derived from the result of the given {@code
632   * Future}. If {@code input} fails, the returned {@code Future} fails with the same exception (and
633   * the function is not invoked). Example usage:
634   *
635   * <pre>{@code
636   * ListenableFuture<QueryResult> queryFuture = ...;
637   * ListenableFuture<List<Row>> rowsFuture =
638   *     transform(queryFuture, QueryResult::getRows);
639   * }</pre>
640   *
641   * <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
642   * choice in some cases. See the discussion in the {@link ListenableFuture#addListener
643   * ListenableFuture.addListener} documentation. All its warnings about heavyweight listeners are
644   * also applicable to heavyweight functions passed to this method.
645   *
646   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
647   * input future. That is, if the returned {@code Future} is cancelled, it will attempt to cancel
648   * the input, and if the input is cancelled, the returned {@code Future} will receive a callback
649   * in which it will attempt to cancel itself.
650   *
651   * <p>An example use of this method is to convert a serializable object returned from an RPC into
652   * a POJO.
653   *
654   * @param input The future to transform
655   * @param function A Function to transform the results of the provided future to the results of
656   *     the returned future. This will be run in the thread that notifies input it is complete.
657   * @return A future that holds result of the transformation.
658   * @since 9.0 (in 1.0 as {@code compose})
659   * @deprecated Use {@linkplain #transform(ListenableFuture, Function, Executor) the overload that
660   *     requires an executor}. For identical behavior, pass {@link MoreExecutors#directExecutor},
661   *     but consider whether another executor would be safer, as discussed in the {@link
662   *     ListenableFuture#addListener ListenableFuture.addListener} documentation. This method is
663   *     scheduled to be removed in April 2018.
664   */
665  @Deprecated
666  public static <I, O> ListenableFuture<O> transform(
667      ListenableFuture<I> input, Function<? super I, ? extends O> function) {
668    return AbstractTransformFuture.create(input, function, directExecutor());
669  }
670
671  /**
672   * Returns a new {@code Future} whose result is derived from the result of the given {@code
673   * Future}. If {@code input} fails, the returned {@code Future} fails with the same exception (and
674   * the function is not invoked). Example usage:
675   *
676   * <pre>{@code
677   * ListenableFuture<QueryResult> queryFuture = ...;
678   * ListenableFuture<List<Row>> rowsFuture =
679   *     transform(queryFuture, QueryResult::getRows, executor);
680   * }</pre>
681   *
682   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
683   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
684   * documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
685   * functions passed to this method.
686   *
687   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
688   * input future. That is, if the returned {@code Future} is cancelled, it will attempt to cancel
689   * the input, and if the input is cancelled, the returned {@code Future} will receive a callback
690   * in which it will attempt to cancel itself.
691   *
692   * <p>An example use of this method is to convert a serializable object returned from an RPC into
693   * a POJO.
694   *
695   * @param input The future to transform
696   * @param function A Function to transform the results of the provided future to the results of
697   *     the returned future.
698   * @param executor Executor to run the function in.
699   * @return A future that holds result of the transformation.
700   * @since 9.0 (in 2.0 as {@code compose})
701   */
702  public static <I, O> ListenableFuture<O> transform(
703      ListenableFuture<I> input, Function<? super I, ? extends O> function, Executor executor) {
704    return AbstractTransformFuture.create(input, function, executor);
705  }
706
707  /**
708   * Like {@link #transform(ListenableFuture, Function, Executor)} except that the transformation
709   * {@code function} is invoked on each call to {@link Future#get() get()} on the returned future.
710   *
711   * <p>The returned {@code Future} reflects the input's cancellation state directly, and any
712   * attempt to cancel the returned Future is likewise passed through to the input Future.
713   *
714   * <p>Note that calls to {@linkplain Future#get(long, TimeUnit) timed get} only apply the timeout
715   * to the execution of the underlying {@code Future}, <em>not</em> to the execution of the
716   * transformation function.
717   *
718   * <p>The primary audience of this method is callers of {@code transform} who don't have a {@code
719   * ListenableFuture} available and do not mind repeated, lazy function evaluation.
720   *
721   * @param input The future to transform
722   * @param function A Function to transform the results of the provided future to the results of
723   *     the returned future.
724   * @return A future that returns the result of the transformation.
725   * @since 10.0
726   */
727  @GwtIncompatible // TODO
728  public static <I, O> Future<O> lazyTransform(
729      final Future<I> input, final Function<? super I, ? extends O> function) {
730    checkNotNull(input);
731    checkNotNull(function);
732    return new Future<O>() {
733
734      @Override
735      public boolean cancel(boolean mayInterruptIfRunning) {
736        return input.cancel(mayInterruptIfRunning);
737      }
738
739      @Override
740      public boolean isCancelled() {
741        return input.isCancelled();
742      }
743
744      @Override
745      public boolean isDone() {
746        return input.isDone();
747      }
748
749      @Override
750      public O get() throws InterruptedException, ExecutionException {
751        return applyTransformation(input.get());
752      }
753
754      @Override
755      public O get(long timeout, TimeUnit unit)
756          throws InterruptedException, ExecutionException, TimeoutException {
757        return applyTransformation(input.get(timeout, unit));
758      }
759
760      private O applyTransformation(I input) throws ExecutionException {
761        try {
762          return function.apply(input);
763        } catch (Throwable t) {
764          throw new ExecutionException(t);
765        }
766      }
767    };
768  }
769
770  /**
771   * Returns a new {@code ListenableFuture} whose result is the product of calling {@code get()} on
772   * the {@code Future} nested within the given {@code Future}, effectively chaining the futures one
773   * after the other. Example:
774   *
775   * <pre>{@code
776   * SettableFuture<ListenableFuture<String>> nested = SettableFuture.create();
777   * ListenableFuture<String> dereferenced = dereference(nested);
778   * }</pre>
779   *
780   * <p>Most users will not need this method. To create a {@code Future} that completes with the
781   * result of another {@code Future}, create a {@link SettableFuture}, and call {@link
782   * SettableFuture#setFuture setFuture(otherFuture)} on it.
783   *
784   * <p>{@code dereference} has the same cancellation and execution semantics as {@link
785   * #transformAsync(ListenableFuture, AsyncFunction, Executor)}, in that the returned {@code
786   * Future} attempts to keep its cancellation state in sync with both the input {@code Future} and
787   * the nested {@code Future}. The transformation is very lightweight and therefore takes place in
788   * the same thread (either the thread that called {@code dereference}, or the thread in which the
789   * dereferenced future completes).
790   *
791   * @deprecated Use {@link #submitAsync(AsyncCallable, Executor)} or {@link
792   *     SettableFuture#setFuture} instead. Or, if you're dereferencing the output of {@link
793   *     #transform} or {@link #catching}, switch to {@link #transformAsync} or {@link
794   *     #catchingAsync} (and likewise for similar APIs). If the cancellation of this method's
795   *     output future races with completion of the outer input future, cancellation may not be
796   *     propagated to the inner input future. This method is scheduled to be removed in January
797   *     2018.
798   * @param nested The nested future to transform.
799   * @return A future that holds result of the inner future.
800   * @since 13.0
801   */
802  @Deprecated
803  @SuppressWarnings({"rawtypes", "unchecked"})
804  public static <V> ListenableFuture<V> dereference(
805      ListenableFuture<? extends ListenableFuture<? extends V>> nested) {
806    return transformAsync(
807        (ListenableFuture) nested, (AsyncFunction) DEREFERENCER, directExecutor());
808  }
809
810  /**
811   * Helper {@code Function} for {@link #dereference}.
812   */
813  private static final AsyncFunction<ListenableFuture<Object>, Object> DEREFERENCER =
814      new AsyncFunction<ListenableFuture<Object>, Object>() {
815        @Override
816        public ListenableFuture<Object> apply(ListenableFuture<Object> input) {
817          return input;
818        }
819      };
820
821  /**
822   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
823   * input futures, if all succeed.
824   *
825   * <p>The list of results is in the same order as the input list.
826   *
827   * <p>Canceling this future will attempt to cancel all the component futures, and if any of the
828   * provided futures fails or is canceled, this one is, too.
829   *
830   * @param futures futures to combine
831   * @return a future that provides a list of the results of the component futures
832   * @since 10.0
833   */
834  @Beta
835  @SafeVarargs
836  public static <V> ListenableFuture<List<V>> allAsList(ListenableFuture<? extends V>... futures) {
837    return new ListFuture<V>(ImmutableList.copyOf(futures), true);
838  }
839
840  /**
841   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
842   * input futures, if all succeed.
843   *
844   * <p>The list of results is in the same order as the input list.
845   *
846   * <p>Canceling this future will attempt to cancel all the component futures, and if any of the
847   * provided futures fails or is canceled, this one is, too.
848   *
849   * @param futures futures to combine
850   * @return a future that provides a list of the results of the component futures
851   * @since 10.0
852   */
853  @Beta
854  public static <V> ListenableFuture<List<V>> allAsList(
855      Iterable<? extends ListenableFuture<? extends V>> futures) {
856    return new ListFuture<V>(ImmutableList.copyOf(futures), true);
857  }
858
859  /**
860   * Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
861   * successful.
862   *
863   * @since 20.0
864   */
865  @SafeVarargs
866  public static <V> FutureCombiner<V> whenAllComplete(ListenableFuture<? extends V>... futures) {
867    return new FutureCombiner<V>(false, ImmutableList.copyOf(futures));
868  }
869
870  /**
871   * Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
872   * successful.
873   *
874   * @since 20.0
875   */
876  public static <V> FutureCombiner<V> whenAllComplete(
877      Iterable<? extends ListenableFuture<? extends V>> futures) {
878    return new FutureCombiner<V>(false, ImmutableList.copyOf(futures));
879  }
880
881  /**
882   * Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
883   *
884   * <p>If any input fails, the returned future fails immediately.
885   *
886   * @since 20.0
887   */
888  @SafeVarargs
889  public static <V> FutureCombiner<V> whenAllSucceed(ListenableFuture<? extends V>... futures) {
890    return new FutureCombiner<V>(true, ImmutableList.copyOf(futures));
891  }
892
893  /**
894   * Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
895   *
896   * <p>If any input fails, the returned future fails immediately.
897   *
898   * @since 20.0
899   */
900  public static <V> FutureCombiner<V> whenAllSucceed(
901      Iterable<? extends ListenableFuture<? extends V>> futures) {
902    return new FutureCombiner<V>(true, ImmutableList.copyOf(futures));
903  }
904
905  /**
906   * A helper to create a new {@code ListenableFuture} whose result is generated from a combination
907   * of input futures.
908   *
909   * <p>See {@link #whenAllComplete} and {@link #whenAllSucceed} for how to instantiate this class.
910   *
911   * <p>Example:
912   *
913   * <pre>   {@code
914   *   final ListenableFuture<Instant> loginDateFuture =
915   *       loginService.findLastLoginDate(username);
916   *   final ListenableFuture<List<String>> recentCommandsFuture =
917   *       recentCommandsService.findRecentCommands(username);
918   *   Callable<UsageHistory> usageComputation =
919   *       new Callable<UsageHistory>() {
920   *         public UsageHistory call() throws Exception {
921   *           return new UsageHistory(
922   *               username, loginDateFuture.get(), recentCommandsFuture.get());
923   *         }
924   *       };
925   *   ListenableFuture<UsageHistory> usageFuture =
926   *       Futures.whenAllSucceed(loginDateFuture, recentCommandsFuture)
927   *           .call(usageComputation, executor);}</pre>
928   *
929   * @since 20.0
930   */
931  @Beta
932  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing, especially if we provide run(Runnable)
933  @GwtCompatible
934  public static final class FutureCombiner<V> {
935    private final boolean allMustSucceed;
936    private final ImmutableList<ListenableFuture<? extends V>> futures;
937
938    private FutureCombiner(
939        boolean allMustSucceed, ImmutableList<ListenableFuture<? extends V>> futures) {
940      this.allMustSucceed = allMustSucceed;
941      this.futures = futures;
942    }
943
944    /**
945     * Creates the {@link ListenableFuture} which will return the result of calling {@link
946     * AsyncCallable#call} in {@code combiner} when all futures complete, using the specified {@code
947     * executor}.
948     *
949     * <p>If the combiner throws a {@code CancellationException}, the returned future will be
950     * cancelled.
951     *
952     * <p>If the combiner throws an {@code ExecutionException}, the cause of the thrown {@code
953     * ExecutionException} will be extracted and returned as the cause of the new {@code
954     * ExecutionException} that gets thrown by the returned combined future.
955     *
956     * <p>Canceling this future will attempt to cancel all the component futures.
957     */
958    public <C> ListenableFuture<C> callAsync(AsyncCallable<C> combiner, Executor executor) {
959      return new CombinedFuture<C>(futures, allMustSucceed, executor, combiner);
960    }
961
962    /**
963     * Like {@link #callAsync(AsyncCallable, Executor)} but using {@linkplain
964     * MoreExecutors#directExecutor direct executor}.
965     *
966     * @deprecated Use {@linkplain #callAsync(AsyncCallable, Executor) the overload that requires an
967     *     executor}. For identical behavior, pass {@link MoreExecutors#directExecutor}, but
968     *     consider whether another executor would be safer, as discussed in the {@link
969     *     ListenableFuture#addListener ListenableFuture.addListener} documentation. This method is
970     *     scheduled to be removed in April 2018.
971     */
972    @Deprecated
973    public <C> ListenableFuture<C> callAsync(AsyncCallable<C> combiner) {
974      return callAsync(combiner, directExecutor());
975    }
976
977    /**
978     * Creates the {@link ListenableFuture} which will return the result of calling {@link
979     * Callable#call} in {@code combiner} when all futures complete, using the specified {@code
980     * executor}.
981     *
982     * <p>If the combiner throws a {@code CancellationException}, the returned future will be
983     * cancelled.
984     *
985     * <p>If the combiner throws an {@code ExecutionException}, the cause of the thrown {@code
986     * ExecutionException} will be extracted and returned as the cause of the new {@code
987     * ExecutionException} that gets thrown by the returned combined future.
988     *
989     * <p>Canceling this future will attempt to cancel all the component futures.
990     */
991    @CanIgnoreReturnValue
992    public <C> ListenableFuture<C> call(Callable<C> combiner, Executor executor) {
993      return new CombinedFuture<C>(futures, allMustSucceed, executor, combiner);
994    }
995
996    /**
997     * Like {@link #call(Callable, Executor)} but using {@linkplain MoreExecutors#directExecutor
998     * direct executor}.
999     *
1000     * @deprecated Use {@linkplain #call(Callable, Executor) the overload that requires an
1001     *     executor}. For identical behavior, pass {@link MoreExecutors#directExecutor}, but
1002     *     consider whether another executor would be safer, as discussed in the {@link
1003     *     ListenableFuture#addListener ListenableFuture.addListener} documentation. This method is
1004     *     scheduled to be removed in April 2018.
1005     */
1006    @CanIgnoreReturnValue
1007    @Deprecated
1008    public <C> ListenableFuture<C> call(Callable<C> combiner) {
1009      return call(combiner, directExecutor());
1010    }
1011
1012    /*
1013     * TODO(cpovirk): Evaluate demand for a run(Runnable) version. Would it allow us to remove
1014     * @CanIgnoreReturnValue from the call() methods above?
1015     * https://github.com/google/guava/issues/2371
1016     */
1017  }
1018
1019  /**
1020   * Returns a {@code ListenableFuture} whose result is set from the supplied future when it
1021   * completes. Cancelling the supplied future will also cancel the returned future, but cancelling
1022   * the returned future will have no effect on the supplied future.
1023   *
1024   * @since 15.0
1025   */
1026  public static <V> ListenableFuture<V> nonCancellationPropagating(ListenableFuture<V> future) {
1027    if (future.isDone()) {
1028      return future;
1029    }
1030    NonCancellationPropagatingFuture<V> output = new NonCancellationPropagatingFuture<>(future);
1031    future.addListener(output, directExecutor());
1032    return output;
1033  }
1034
1035  /** A wrapped future that does not propagate cancellation to its delegate. */
1036  private static final class NonCancellationPropagatingFuture<V>
1037      extends AbstractFuture.TrustedFuture<V> implements Runnable {
1038    private ListenableFuture<V> delegate;
1039
1040    NonCancellationPropagatingFuture(final ListenableFuture<V> delegate) {
1041      this.delegate = delegate;
1042    }
1043
1044    @Override
1045    public void run() {
1046      // This prevents cancellation from propagating because we don't assign delegate until
1047      // delegate is already done, so calling cancel() on it is a no-op.
1048      ListenableFuture<V> localDelegate = delegate;
1049      if (localDelegate != null) {
1050        setFuture(localDelegate);
1051      }
1052    }
1053
1054    @Override
1055    protected String pendingToString() {
1056      ListenableFuture<V> localDelegate = delegate;
1057      if (localDelegate != null) {
1058        return "delegate=[" + localDelegate + "]";
1059      }
1060      return null;
1061    }
1062
1063    @Override
1064    protected void afterDone() {
1065      delegate = null;
1066    }
1067  }
1068
1069  /**
1070   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
1071   * successful input futures. The list of results is in the same order as the input list, and if
1072   * any of the provided futures fails or is canceled, its corresponding position will contain
1073   * {@code null} (which is indistinguishable from the future having a successful value of {@code
1074   * null}).
1075   *
1076   * <p>Canceling this future will attempt to cancel all the component futures.
1077   *
1078   * @param futures futures to combine
1079   * @return a future that provides a list of the results of the component futures
1080   * @since 10.0
1081   */
1082  @Beta
1083  @SafeVarargs
1084  public static <V> ListenableFuture<List<V>> successfulAsList(
1085      ListenableFuture<? extends V>... futures) {
1086    return new ListFuture<V>(ImmutableList.copyOf(futures), false);
1087  }
1088
1089  /**
1090   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
1091   * successful input futures. The list of results is in the same order as the input list, and if
1092   * any of the provided futures fails or is canceled, its corresponding position will contain
1093   * {@code null} (which is indistinguishable from the future having a successful value of {@code
1094   * null}).
1095   *
1096   * <p>Canceling this future will attempt to cancel all the component futures.
1097   *
1098   * @param futures futures to combine
1099   * @return a future that provides a list of the results of the component futures
1100   * @since 10.0
1101   */
1102  @Beta
1103  public static <V> ListenableFuture<List<V>> successfulAsList(
1104      Iterable<? extends ListenableFuture<? extends V>> futures) {
1105    return new ListFuture<V>(ImmutableList.copyOf(futures), false);
1106  }
1107
1108  /**
1109   * Returns a list of delegate futures that correspond to the futures received in the order that
1110   * they complete. Delegate futures return the same value or throw the same exception as the
1111   * corresponding input future returns/throws.
1112   *
1113   * <p>"In the order that they complete" means, for practical purposes, about what you would
1114   * expect, but there are some subtleties. First, we do guarantee that, if the output future at
1115   * index n is done, the output future at index n-1 is also done. (But as usual with futures, some
1116   * listeners for future n may complete before some for future n-1.) However, it is possible, if
1117   * one input completes with result X and another later with result Y, for Y to come before X in
1118   * the output future list. (Such races are impossible to solve without global synchronization of
1119   * all future completions. And they should have little practical impact.)
1120   *
1121   * <p>Cancelling a delegate future propagates to input futures once all the delegates complete,
1122   * either from cancellation or because an input future has completed. If N futures are passed in,
1123   * and M delegates are cancelled, the remaining M input futures will be cancelled once N - M of
1124   * the input futures complete. If all the delegates are cancelled, all the input futures will be
1125   * too.
1126   *
1127   * @since 17.0
1128   */
1129  @Beta
1130  public static <T> ImmutableList<ListenableFuture<T>> inCompletionOrder(
1131      Iterable<? extends ListenableFuture<? extends T>> futures) {
1132    // Can't use Iterables.toArray because it's not gwt compatible
1133    final Collection<ListenableFuture<? extends T>> collection;
1134    if (futures instanceof Collection) {
1135      collection = (Collection<ListenableFuture<? extends T>>) futures;
1136    } else {
1137      collection = ImmutableList.copyOf(futures);
1138    }
1139    @SuppressWarnings("unchecked")
1140    ListenableFuture<? extends T>[] copy =
1141        (ListenableFuture<? extends T>[])
1142            collection.toArray(new ListenableFuture[collection.size()]);
1143    final InCompletionOrderState<T> state = new InCompletionOrderState<>(copy);
1144    ImmutableList.Builder<AbstractFuture<T>> delegatesBuilder = ImmutableList.builder();
1145    for (int i = 0; i < copy.length; i++) {
1146      delegatesBuilder.add(new InCompletionOrderFuture<T>(state));
1147    }
1148
1149    final ImmutableList<AbstractFuture<T>> delegates = delegatesBuilder.build();
1150    for (int i = 0; i < copy.length; i++) {
1151      final int localI = i;
1152      copy[i].addListener(
1153          new Runnable() {
1154            @Override
1155            public void run() {
1156              state.recordInputCompletion(delegates, localI);
1157            }
1158          },
1159          directExecutor());
1160    }
1161
1162    @SuppressWarnings("unchecked")
1163    ImmutableList<ListenableFuture<T>> delegatesCast = (ImmutableList) delegates;
1164    return delegatesCast;
1165  }
1166
1167  // This can't be a TrustedFuture, because TrustedFuture has clever optimizations that
1168  // mean cancel won't be called if this Future is passed into setFuture, and then
1169  // cancelled.
1170  private static final class InCompletionOrderFuture<T> extends AbstractFuture<T> {
1171    private InCompletionOrderState<T> state;
1172
1173    private InCompletionOrderFuture(InCompletionOrderState<T> state) {
1174      this.state = state;
1175    }
1176
1177    @Override
1178    public boolean cancel(boolean interruptIfRunning) {
1179      InCompletionOrderState<T> localState = state;
1180      if (super.cancel(interruptIfRunning)) {
1181        localState.recordOutputCancellation(interruptIfRunning);
1182        return true;
1183      }
1184      return false;
1185    }
1186
1187    @Override
1188    protected void afterDone() {
1189      state = null;
1190    }
1191
1192    @Override
1193    protected String pendingToString() {
1194      InCompletionOrderState<T> localState = state;
1195      if (localState != null) {
1196        // Don't print the actual array! We don't want inCompletionOrder(list).toString() to have
1197        // quadratic output.
1198        return "inputCount=["
1199            + localState.inputFutures.length
1200            + "], remaining=["
1201            + localState.incompleteOutputCount.get()
1202            + "]";
1203      }
1204      return null;
1205    }
1206  }
1207
1208  private static final class InCompletionOrderState<T> {
1209    // A happens-before edge between the writes of these fields and their reads exists, because
1210    // in order to read these fields, the corresponding write to incompleteOutputCount must have
1211    // been read.
1212    private boolean wasCancelled = false;
1213    private boolean shouldInterrupt = true;
1214    private final AtomicInteger incompleteOutputCount;
1215    private final ListenableFuture<? extends T>[] inputFutures;
1216    private volatile int delegateIndex = 0;
1217
1218    private InCompletionOrderState(ListenableFuture<? extends T>[] inputFutures) {
1219      this.inputFutures = inputFutures;
1220      incompleteOutputCount = new AtomicInteger(inputFutures.length);
1221    }
1222
1223    private void recordOutputCancellation(boolean interruptIfRunning) {
1224      wasCancelled = true;
1225      // If all the futures were cancelled with interruption, cancel the input futures
1226      // with interruption; otherwise cancel without
1227      if (!interruptIfRunning) {
1228        shouldInterrupt = false;
1229      }
1230      recordCompletion();
1231    }
1232
1233    private void recordInputCompletion(
1234        ImmutableList<AbstractFuture<T>> delegates, int inputFutureIndex) {
1235      ListenableFuture<? extends T> inputFuture = inputFutures[inputFutureIndex];
1236      // Null out our reference to this future, so it can be GCed
1237      inputFutures[inputFutureIndex] = null;
1238      for (int i = delegateIndex; i < delegates.size(); i++) {
1239        if (delegates.get(i).setFuture(inputFuture)) {
1240          recordCompletion();
1241          // this is technically unnecessary, but should speed up later accesses
1242          delegateIndex = i + 1;
1243          return;
1244        }
1245      }
1246      // If all the delegates were complete, no reason for the next listener to have to
1247      // go through the whole list. Avoids O(n^2) behavior when the entire output list is
1248      // cancelled.
1249      delegateIndex = delegates.size();
1250    }
1251
1252    private void recordCompletion() {
1253      if (incompleteOutputCount.decrementAndGet() == 0 && wasCancelled) {
1254        for (ListenableFuture<?> toCancel : inputFutures) {
1255          if (toCancel != null) {
1256            toCancel.cancel(shouldInterrupt);
1257          }
1258        }
1259      }
1260    }
1261  }
1262
1263  /**
1264   * Registers separate success and failure callbacks to be run when the {@code Future}'s
1265   * computation is {@linkplain java.util.concurrent.Future#isDone() complete} or, if the
1266   * computation is already complete, immediately.
1267   *
1268   * <p>There is no guaranteed ordering of execution of callbacks, but any callback added through
1269   * this method is guaranteed to be called once the computation is complete.
1270   *
1271   * <p>Example:
1272   *
1273   * <pre>{@code
1274   * ListenableFuture<QueryResult> future = ...;
1275   * addCallback(future,
1276   *     new FutureCallback<QueryResult>() {
1277   *       public void onSuccess(QueryResult result) {
1278   *         storeInCache(result);
1279   *       }
1280   *       public void onFailure(Throwable t) {
1281   *         reportError(t);
1282   *       }
1283   *     });
1284   * }</pre>
1285   *
1286   * <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
1287   * choice in some cases. See the discussion in the {@link ListenableFuture#addListener
1288   * ListenableFuture.addListener} documentation. All its warnings about heavyweight listeners are
1289   * also applicable to heavyweight callbacks passed to this method.
1290   *
1291   * <p>For a more general interface to attach a completion listener to a {@code Future}, see {@link
1292   * ListenableFuture#addListener addListener}.
1293   *
1294   * @param future The future attach the callback to.
1295   * @param callback The callback to invoke when {@code future} is completed.
1296   * @since 10.0
1297   * @deprecated Use {@linkplain #addCallback(ListenableFuture, FutureCallback, Executor) the
1298   *     overload that requires an executor}. For identical behavior, pass {@link
1299   *     MoreExecutors#directExecutor}, but consider whether another executor would be safer, as
1300   *     discussed in the {@link ListenableFuture#addListener ListenableFuture.addListener}
1301   *     documentation. This method is scheduled to be removed in April 2018.
1302   */
1303  @Deprecated
1304  public static <V> void addCallback(
1305      ListenableFuture<V> future, FutureCallback<? super V> callback) {
1306    addCallback(future, callback, directExecutor());
1307  }
1308
1309  /**
1310   * Registers separate success and failure callbacks to be run when the {@code Future}'s
1311   * computation is {@linkplain java.util.concurrent.Future#isDone() complete} or, if the
1312   * computation is already complete, immediately.
1313   *
1314   * <p>The callback is run on {@code executor}. There is no guaranteed ordering of execution of
1315   * callbacks, but any callback added through this method is guaranteed to be called once the
1316   * computation is complete.
1317   *
1318   * <p>Example:
1319   *
1320   * <pre>{@code
1321   * ListenableFuture<QueryResult> future = ...;
1322   * Executor e = ...
1323   * addCallback(future,
1324   *     new FutureCallback<QueryResult>() {
1325   *       public void onSuccess(QueryResult result) {
1326   *         storeInCache(result);
1327   *       }
1328   *       public void onFailure(Throwable t) {
1329   *         reportError(t);
1330   *       }
1331   *     }, e);
1332   * }</pre>
1333   *
1334   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
1335   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
1336   * documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
1337   * callbacks passed to this method.
1338   *
1339   * <p>For a more general interface to attach a completion listener to a {@code Future}, see {@link
1340   * ListenableFuture#addListener addListener}.
1341   *
1342   * @param future The future attach the callback to.
1343   * @param callback The callback to invoke when {@code future} is completed.
1344   * @param executor The executor to run {@code callback} when the future completes.
1345   * @since 10.0
1346   */
1347  public static <V> void addCallback(
1348      final ListenableFuture<V> future,
1349      final FutureCallback<? super V> callback,
1350      Executor executor) {
1351    Preconditions.checkNotNull(callback);
1352    future.addListener(new CallbackListener<V>(future, callback), executor);
1353  }
1354
1355  /** See {@link #addCallback(ListenableFuture, FutureCallback, Executor)} for behavioral notes. */
1356  private static final class CallbackListener<V> implements Runnable {
1357    final Future<V> future;
1358    final FutureCallback<? super V> callback;
1359
1360    CallbackListener(Future<V> future, FutureCallback<? super V> callback) {
1361      this.future = future;
1362      this.callback = callback;
1363    }
1364
1365    @Override
1366    public void run() {
1367      final V value;
1368      try {
1369        value = getDone(future);
1370      } catch (ExecutionException e) {
1371        callback.onFailure(e.getCause());
1372        return;
1373      } catch (RuntimeException | Error e) {
1374        callback.onFailure(e);
1375        return;
1376      }
1377      callback.onSuccess(value);
1378    }
1379
1380    @Override
1381    public String toString() {
1382      return MoreObjects.toStringHelper(this).addValue(callback).toString();
1383    }
1384  }
1385
1386  /**
1387   * Returns the result of the input {@code Future}, which must have already completed.
1388   *
1389   * <p>The benefits of this method are twofold. First, the name "getDone" suggests to readers that
1390   * the {@code Future} is already done. Second, if buggy code calls {@code getDone} on a {@code
1391   * Future} that is still pending, the program will throw instead of block. This can be important
1392   * for APIs like {@link #whenAllComplete whenAllComplete(...)}{@code .}{@link
1393   * FutureCombiner#call(Callable, Executor) call(...)}, where it is easy to use a new input from
1394   * the {@code call} implementation but forget to add it to the arguments of {@code
1395   * whenAllComplete}.
1396   *
1397   * <p>If you are looking for a method to determine whether a given {@code Future} is done, use the
1398   * instance method {@link Future#isDone()}.
1399   *
1400   * @throws ExecutionException if the {@code Future} failed with an exception
1401   * @throws CancellationException if the {@code Future} was cancelled
1402   * @throws IllegalStateException if the {@code Future} is not done
1403   * @since 20.0
1404   */
1405  @CanIgnoreReturnValue
1406  // TODO(cpovirk): Consider calling getDone() in our own code.
1407  public static <V> V getDone(Future<V> future) throws ExecutionException {
1408    /*
1409     * We throw IllegalStateException, since the call could succeed later. Perhaps we "should" throw
1410     * IllegalArgumentException, since the call could succeed with a different argument. Those
1411     * exceptions' docs suggest that either is acceptable. Google's Java Practices page recommends
1412     * IllegalArgumentException here, in part to keep its recommendation simple: Static methods
1413     * should throw IllegalStateException only when they use static state.
1414     *
1415     *
1416     * Why do we deviate here? The answer: We want for fluentFuture.getDone() to throw the same
1417     * exception as Futures.getDone(fluentFuture).
1418     */
1419    checkState(future.isDone(), "Future was expected to be done: %s", future);
1420    return getUninterruptibly(future);
1421  }
1422
1423  /**
1424   * Returns the result of {@link Future#get()}, converting most exceptions to a new instance of the
1425   * given checked exception type. This reduces boilerplate for a common use of {@code Future} in
1426   * which it is unnecessary to programmatically distinguish between exception types or to extract
1427   * other information from the exception instance.
1428   *
1429   * <p>Exceptions from {@code Future.get} are treated as follows:
1430   * <ul>
1431   * <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@code X} if the cause is
1432   *     a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
1433   *     RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
1434   * <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
1435   *     interrupt).
1436   * <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
1437   *     RuntimeException} (though {@code get} implementations are discouraged from throwing such
1438   *     exceptions).
1439   * </ul>
1440   *
1441   * <p>The overall principle is to continue to treat every checked exception as a checked
1442   * exception, every unchecked exception as an unchecked exception, and every error as an error. In
1443   * addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
1444   * new stack trace matches that of the current thread.
1445   *
1446   * <p>Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
1447   * that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
1448   * (preferring constructors with at least one {@code String}) and calling the constructor via
1449   * reflection. If the exception did not already have a cause, one is set by calling {@link
1450   * Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
1451   * IllegalArgumentException} is thrown.
1452   *
1453   * @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
1454   *     whose cause is not itself a checked exception
1455   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
1456   *     {@code RuntimeException} as its cause
1457   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1458   *     Error} as its cause
1459   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1460   * @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
1461   *     does not have a suitable constructor
1462   * @since 19.0 (in 10.0 as {@code get})
1463   */
1464  @CanIgnoreReturnValue
1465  @GwtIncompatible // reflection
1466  public static <V, X extends Exception> V getChecked(Future<V> future, Class<X> exceptionClass)
1467      throws X {
1468    return FuturesGetChecked.getChecked(future, exceptionClass);
1469  }
1470
1471  /**
1472   * Returns the result of {@link Future#get(long, TimeUnit)}, converting most exceptions to a new
1473   * instance of the given checked exception type. This reduces boilerplate for a common use of
1474   * {@code Future} in which it is unnecessary to programmatically distinguish between exception
1475   * types or to extract other information from the exception instance.
1476   *
1477   * <p>Exceptions from {@code Future.get} are treated as follows:
1478   * <ul>
1479   * <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@code X} if the cause is
1480   *     a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
1481   *     RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
1482   * <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
1483   *     interrupt).
1484   * <li>Any {@link TimeoutException} is wrapped in an {@code X}.
1485   * <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
1486   *     RuntimeException} (though {@code get} implementations are discouraged from throwing such
1487   *     exceptions).
1488   * </ul>
1489   *
1490   * <p>The overall principle is to continue to treat every checked exception as a checked
1491   * exception, every unchecked exception as an unchecked exception, and every error as an error. In
1492   * addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
1493   * new stack trace matches that of the current thread.
1494   *
1495   * <p>Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
1496   * that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
1497   * (preferring constructors with at least one {@code String}) and calling the constructor via
1498   * reflection. If the exception did not already have a cause, one is set by calling {@link
1499   * Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
1500   * IllegalArgumentException} is thrown.
1501   *
1502   * @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
1503   *     whose cause is not itself a checked exception
1504   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
1505   *     {@code RuntimeException} as its cause
1506   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1507   *     Error} as its cause
1508   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1509   * @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
1510   *     does not have a suitable constructor
1511   * @since 19.0 (in 10.0 as {@code get} and with different parameter order)
1512   */
1513  @CanIgnoreReturnValue
1514  @GwtIncompatible // reflection
1515  public static <V, X extends Exception> V getChecked(
1516      Future<V> future, Class<X> exceptionClass, long timeout, TimeUnit unit) throws X {
1517    return FuturesGetChecked.getChecked(future, exceptionClass, timeout, unit);
1518  }
1519
1520  /**
1521   * Returns the result of calling {@link Future#get()} uninterruptibly on a task known not to throw
1522   * a checked exception. This makes {@code Future} more suitable for lightweight, fast-running
1523   * tasks that, barring bugs in the code, will not fail. This gives it exception-handling behavior
1524   * similar to that of {@code ForkJoinTask.join}.
1525   *
1526   * <p>Exceptions from {@code Future.get} are treated as follows:
1527   * <ul>
1528   * <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@link
1529   *     UncheckedExecutionException} (if the cause is an {@code Exception}) or {@link
1530   *     ExecutionError} (if the cause is an {@code Error}).
1531   * <li>Any {@link InterruptedException} causes a retry of the {@code get} call. The interrupt is
1532   *     restored before {@code getUnchecked} returns.
1533   * <li>Any {@link CancellationException} is propagated untouched. So is any other {@link
1534   *     RuntimeException} ({@code get} implementations are discouraged from throwing such
1535   *     exceptions).
1536   * </ul>
1537   *
1538   * <p>The overall principle is to eliminate all checked exceptions: to loop to avoid {@code
1539   * InterruptedException}, to pass through {@code CancellationException}, and to wrap any exception
1540   * from the underlying computation in an {@code UncheckedExecutionException} or {@code
1541   * ExecutionError}.
1542   *
1543   * <p>For an uninterruptible {@code get} that preserves other exceptions, see {@link
1544   * Uninterruptibles#getUninterruptibly(Future)}.
1545   *
1546   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with an
1547   *     {@code Exception} as its cause
1548   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1549   *     Error} as its cause
1550   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1551   * @since 10.0
1552   */
1553  @CanIgnoreReturnValue
1554  @GwtIncompatible // TODO
1555  public static <V> V getUnchecked(Future<V> future) {
1556    checkNotNull(future);
1557    try {
1558      return getUninterruptibly(future);
1559    } catch (ExecutionException e) {
1560      wrapAndThrowUnchecked(e.getCause());
1561      throw new AssertionError();
1562    }
1563  }
1564
1565  @GwtIncompatible // TODO
1566  private static void wrapAndThrowUnchecked(Throwable cause) {
1567    if (cause instanceof Error) {
1568      throw new ExecutionError((Error) cause);
1569    }
1570    /*
1571     * It's an Exception. (Or it's a non-Error, non-Exception Throwable. From my survey of such
1572     * classes, I believe that most users intended to extend Exception, so we'll treat it like an
1573     * Exception.)
1574     */
1575    throw new UncheckedExecutionException(cause);
1576  }
1577
1578  /*
1579   * Arguably we don't need a timed getUnchecked because any operation slow enough to require a
1580   * timeout is heavyweight enough to throw a checked exception and therefore be inappropriate to
1581   * use with getUnchecked. Further, it's not clear that converting the checked TimeoutException to
1582   * a RuntimeException -- especially to an UncheckedExecutionException, since it wasn't thrown by
1583   * the computation -- makes sense, and if we don't convert it, the user still has to write a
1584   * try-catch block.
1585   *
1586   * If you think you would use this method, let us know. You might also also look into the
1587   * Fork-Join framework: http://docs.oracle.com/javase/tutorial/essential/concurrency/forkjoin.html
1588   */
1589
1590  /**
1591   * A checked future that uses a function to map from exceptions to the appropriate checked type.
1592   */
1593  @GwtIncompatible // TODO
1594  private static class MappingCheckedFuture<V, X extends Exception>
1595      extends AbstractCheckedFuture<V, X> {
1596
1597    final Function<? super Exception, X> mapper;
1598
1599    MappingCheckedFuture(ListenableFuture<V> delegate, Function<? super Exception, X> mapper) {
1600      super(delegate);
1601
1602      this.mapper = checkNotNull(mapper);
1603    }
1604
1605    @Override
1606    protected X mapException(Exception e) {
1607      return mapper.apply(e);
1608    }
1609  }
1610}