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.Internal.toNanosSaturated;
020import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
021import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
022
023import com.google.common.annotations.Beta;
024import com.google.common.annotations.GwtCompatible;
025import com.google.common.annotations.GwtIncompatible;
026import com.google.common.base.Function;
027import com.google.common.base.MoreObjects;
028import com.google.common.base.Preconditions;
029import com.google.common.collect.ImmutableList;
030import com.google.common.util.concurrent.CollectionFuture.ListFuture;
031import com.google.common.util.concurrent.ImmediateFuture.ImmediateCancelledFuture;
032import com.google.common.util.concurrent.ImmediateFuture.ImmediateFailedFuture;
033import com.google.common.util.concurrent.internal.InternalFutureFailureAccess;
034import com.google.common.util.concurrent.internal.InternalFutures;
035import com.google.errorprone.annotations.CanIgnoreReturnValue;
036import java.time.Duration;
037import java.util.Collection;
038import java.util.List;
039import java.util.concurrent.Callable;
040import java.util.concurrent.CancellationException;
041import java.util.concurrent.ExecutionException;
042import java.util.concurrent.Executor;
043import java.util.concurrent.Future;
044import java.util.concurrent.RejectedExecutionException;
045import java.util.concurrent.ScheduledExecutorService;
046import java.util.concurrent.TimeUnit;
047import java.util.concurrent.TimeoutException;
048import java.util.concurrent.atomic.AtomicInteger;
049import org.checkerframework.checker.nullness.qual.Nullable;
050
051/**
052 * Static utility methods pertaining to the {@link Future} interface.
053 *
054 * <p>Many of these methods use the {@link ListenableFuture} API; consult the Guava User Guide
055 * article on <a href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
056 * ListenableFuture}</a>.
057 *
058 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
059 * asynchronous operations. You can chain them together manually with calls to methods like {@link
060 * Futures#transform(ListenableFuture, Function, Executor) Futures.transform}, but you will often
061 * find it easier to use a framework. Frameworks automate the process, often adding features like
062 * monitoring, debugging, and cancellation. Examples of frameworks include:
063 *
064 * <ul>
065 *   <li><a href="https://dagger.dev/producers.html">Dagger Producers</a>
066 * </ul>
067 *
068 * <p>If you do chain your operations manually, you may want to use {@link FluentFuture}.
069 *
070 * @author Kevin Bourrillion
071 * @author Nishant Thakkar
072 * @author Sven Mawson
073 * @since 1.0
074 */
075@GwtCompatible(emulated = true)
076public final class Futures extends GwtFuturesCatchingSpecialization {
077
078  // A note on memory visibility.
079  // Many of the utilities in this class (transform, withFallback, withTimeout, asList, combine)
080  // have two requirements that significantly complicate their design.
081  // 1. Cancellation should propagate from the returned future to the input future(s).
082  // 2. The returned futures shouldn't unnecessarily 'pin' their inputs after completion.
083  //
084  // A consequence of these requirements is that the delegate futures cannot be stored in
085  // final fields.
086  //
087  // For simplicity the rest of this description will discuss Futures.catching since it is the
088  // simplest instance, though very similar descriptions apply to many other classes in this file.
089  //
090  // In the constructor of AbstractCatchingFuture, the delegate future is assigned to a field
091  // 'inputFuture'. That field is non-final and non-volatile. There are 2 places where the
092  // 'inputFuture' field is read and where we will have to consider visibility of the write
093  // operation in the constructor.
094  //
095  // 1. In the listener that performs the callback. In this case it is fine since inputFuture is
096  //    assigned prior to calling addListener, and addListener happens-before any invocation of the
097  //    listener. Notably, this means that 'volatile' is unnecessary to make 'inputFuture' visible
098  //    to the listener.
099  //
100  // 2. In done() where we may propagate cancellation to the input. In this case it is _not_ fine.
101  //    There is currently nothing that enforces that the write to inputFuture in the constructor is
102  //    visible to done(). This is because there is no happens before edge between the write and a
103  //    (hypothetical) unsafe read by our caller. Note: adding 'volatile' does not fix this issue,
104  //    it would just add an edge such that if done() observed non-null, then it would also
105  //    definitely observe all earlier writes, but we still have no guarantee that done() would see
106  //    the inital write (just stronger guarantees if it does).
107  //
108  // See: http://cs.oswego.edu/pipermail/concurrency-interest/2015-January/013800.html
109  // For a (long) discussion about this specific issue and the general futility of life.
110  //
111  // For the time being we are OK with the problem discussed above since it requires a caller to
112  // introduce a very specific kind of data-race. And given the other operations performed by these
113  // methods that involve volatile read/write operations, in practice there is no issue. Also, the
114  // way in such a visibility issue would surface is most likely as a failure of cancel() to
115  // propagate to the input. Cancellation propagation is fundamentally racy so this is fine.
116  //
117  // Future versions of the JMM may revise safe construction semantics in such a way that we can
118  // safely publish these objects and we won't need this whole discussion.
119  // TODO(user,lukes): consider adding volatile to all these fields since in current known JVMs
120  // that should resolve the issue. This comes at the cost of adding more write barriers to the
121  // implementations.
122
123  private Futures() {}
124
125  /**
126   * Creates a {@code ListenableFuture} which has its value set immediately upon construction. The
127   * getters just return the value. This {@code Future} can't be canceled or timed out and its
128   * {@code isDone()} method always returns {@code true}.
129   */
130  public static <V> ListenableFuture<V> immediateFuture(@Nullable V value) {
131    if (value == null) {
132      // This cast is safe because null is assignable to V for all V (i.e. it is bivariant)
133      @SuppressWarnings("unchecked")
134      ListenableFuture<V> typedNull = (ListenableFuture<V>) ImmediateFuture.NULL;
135      return typedNull;
136    }
137    return new ImmediateFuture<>(value);
138  }
139
140  /**
141   * Returns a successful {@code ListenableFuture<Void>}. This method is equivalent to {@code
142   * immediateFuture(null)} except that it is restricted to produce futures of type {@code Void}.
143   *
144   * @since 29.0
145   */
146  @SuppressWarnings("unchecked")
147  public static ListenableFuture<Void> immediateVoidFuture() {
148    return (ListenableFuture<Void>) ImmediateFuture.NULL;
149  }
150
151  /**
152   * Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
153   *
154   * <p>The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
155   * returns {@code true}. Calling {@code get()} will immediately throw the provided {@code
156   * Throwable} wrapped in an {@code ExecutionException}.
157   */
158  public static <V> ListenableFuture<V> immediateFailedFuture(Throwable throwable) {
159    checkNotNull(throwable);
160    return new ImmediateFailedFuture<V>(throwable);
161  }
162
163  /**
164   * Creates a {@code ListenableFuture} which is cancelled immediately upon construction, so that
165   * {@code isCancelled()} always returns {@code true}.
166   *
167   * @since 14.0
168   */
169  public static <V> ListenableFuture<V> immediateCancelledFuture() {
170    return new ImmediateCancelledFuture<V>();
171  }
172
173  /**
174   * Executes {@code callable} on the specified {@code executor}, returning a {@code Future}.
175   *
176   * @throws RejectedExecutionException if the task cannot be scheduled for execution
177   * @since 28.2
178   */
179  @Beta
180  public static <O> ListenableFuture<O> submit(Callable<O> callable, Executor executor) {
181    TrustedListenableFutureTask<O> task = TrustedListenableFutureTask.create(callable);
182    executor.execute(task);
183    return task;
184  }
185
186  /**
187   * Executes {@code runnable} on the specified {@code executor}, returning a {@code Future} that
188   * will complete after execution.
189   *
190   * @throws RejectedExecutionException if the task cannot be scheduled for execution
191   * @since 28.2
192   */
193  @Beta
194  public static ListenableFuture<Void> submit(Runnable runnable, Executor executor) {
195    TrustedListenableFutureTask<Void> task = TrustedListenableFutureTask.create(runnable, null);
196    executor.execute(task);
197    return task;
198  }
199
200  /**
201   * Executes {@code callable} on the specified {@code executor}, returning a {@code Future}.
202   *
203   * @throws RejectedExecutionException if the task cannot be scheduled for execution
204   * @since 23.0
205   */
206  @Beta
207  public static <O> ListenableFuture<O> submitAsync(AsyncCallable<O> callable, Executor executor) {
208    TrustedListenableFutureTask<O> task = TrustedListenableFutureTask.create(callable);
209    executor.execute(task);
210    return task;
211  }
212
213  /**
214   * Schedules {@code callable} on the specified {@code executor}, returning a {@code Future}.
215   *
216   * @throws RejectedExecutionException if the task cannot be scheduled for execution
217   * @since 28.0
218   */
219  @Beta
220  @GwtIncompatible // java.util.concurrent.ScheduledExecutorService
221  public static <O> ListenableFuture<O> scheduleAsync(
222      AsyncCallable<O> callable, Duration delay, ScheduledExecutorService executorService) {
223    return scheduleAsync(callable, toNanosSaturated(delay), TimeUnit.NANOSECONDS, executorService);
224  }
225
226  /**
227   * Schedules {@code callable} on the specified {@code executor}, returning a {@code Future}.
228   *
229   * @throws RejectedExecutionException if the task cannot be scheduled for execution
230   * @since 23.0
231   */
232  @Beta
233  @GwtIncompatible // java.util.concurrent.ScheduledExecutorService
234  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
235  public static <O> ListenableFuture<O> scheduleAsync(
236      AsyncCallable<O> callable,
237      long delay,
238      TimeUnit timeUnit,
239      ScheduledExecutorService executorService) {
240    TrustedListenableFutureTask<O> task = TrustedListenableFutureTask.create(callable);
241    final Future<?> scheduled = executorService.schedule(task, delay, timeUnit);
242    task.addListener(
243        new Runnable() {
244          @Override
245          public void run() {
246            // Don't want to interrupt twice
247            scheduled.cancel(false);
248          }
249        },
250        directExecutor());
251    return task;
252  }
253
254  /**
255   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
256   * primary input fails with the given {@code exceptionType}, from the result provided by the
257   * {@code fallback}. {@link Function#apply} is not invoked until the primary input has failed, so
258   * if the primary input succeeds, it is never invoked. If, during the invocation of {@code
259   * fallback}, an exception is thrown, this exception is used as the result of the output {@code
260   * Future}.
261   *
262   * <p>Usage example:
263   *
264   * <pre>{@code
265   * ListenableFuture<Integer> fetchCounterFuture = ...;
266   *
267   * // Falling back to a zero counter in case an exception happens when
268   * // processing the RPC to fetch counters.
269   * ListenableFuture<Integer> faultTolerantFuture = Futures.catching(
270   *     fetchCounterFuture, FetchException.class, x -> 0, directExecutor());
271   * }</pre>
272   *
273   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
274   * the warnings the {@link MoreExecutors#directExecutor} documentation.
275   *
276   * @param input the primary input {@code Future}
277   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
278   *     type is matched against the input's exception. "The input's exception" means the cause of
279   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
280   *     different kind of exception, that exception itself. To avoid hiding bugs and other
281   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
282   *     Throwable.class} in particular.
283   * @param fallback the {@link Function} to be called if {@code input} fails with the expected
284   *     exception type. The function's argument is the input's exception. "The input's exception"
285   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
286   *     {@code get()} throws a different kind of exception, that exception itself.
287   * @param executor the executor that runs {@code fallback} if {@code input} fails
288   * @since 19.0
289   */
290  @Beta
291  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
292  public static <V, X extends Throwable> ListenableFuture<V> catching(
293      ListenableFuture<? extends V> input,
294      Class<X> exceptionType,
295      Function<? super X, ? extends V> fallback,
296      Executor executor) {
297    return AbstractCatchingFuture.create(input, exceptionType, fallback, executor);
298  }
299
300  /**
301   * Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
302   * primary input fails with the given {@code exceptionType}, from the result provided by the
303   * {@code fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has
304   * failed, so if the primary input succeeds, it is never invoked. If, during the invocation of
305   * {@code fallback}, an exception is thrown, this exception is used as the result of the output
306   * {@code Future}.
307   *
308   * <p>Usage examples:
309   *
310   * <pre>{@code
311   * ListenableFuture<Integer> fetchCounterFuture = ...;
312   *
313   * // Falling back to a zero counter in case an exception happens when
314   * // processing the RPC to fetch counters.
315   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
316   *     fetchCounterFuture, FetchException.class, x -> immediateFuture(0), directExecutor());
317   * }</pre>
318   *
319   * <p>The fallback can also choose to propagate the original exception when desired:
320   *
321   * <pre>{@code
322   * ListenableFuture<Integer> fetchCounterFuture = ...;
323   *
324   * // Falling back to a zero counter only in case the exception was a
325   * // TimeoutException.
326   * ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
327   *     fetchCounterFuture,
328   *     FetchException.class,
329   *     e -> {
330   *       if (omitDataOnFetchFailure) {
331   *         return immediateFuture(0);
332   *       }
333   *       throw e;
334   *     },
335   *     directExecutor());
336   * }</pre>
337   *
338   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
339   * the warnings the {@link MoreExecutors#directExecutor} documentation.
340   *
341   * @param input the primary input {@code Future}
342   * @param exceptionType the exception type that triggers use of {@code fallback}. The exception
343   *     type is matched against the input's exception. "The input's exception" means the cause of
344   *     the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
345   *     different kind of exception, that exception itself. To avoid hiding bugs and other
346   *     unrecoverable errors, callers should prefer more specific types, avoiding {@code
347   *     Throwable.class} in particular.
348   * @param fallback the {@link AsyncFunction} to be called if {@code input} fails with the expected
349   *     exception type. The function's argument is the input's exception. "The input's exception"
350   *     means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
351   *     {@code get()} throws a different kind of exception, that exception itself.
352   * @param executor the executor that runs {@code fallback} if {@code input} fails
353   * @since 19.0 (similar functionality in 14.0 as {@code withFallback})
354   */
355  @Beta
356  @Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
357  public static <V, X extends Throwable> ListenableFuture<V> catchingAsync(
358      ListenableFuture<? extends V> input,
359      Class<X> exceptionType,
360      AsyncFunction<? super X, ? extends V> fallback,
361      Executor executor) {
362    return AbstractCatchingFuture.create(input, exceptionType, fallback, executor);
363  }
364
365  /**
366   * Returns a future that delegates to another but will finish early (via a {@link
367   * TimeoutException} wrapped in an {@link ExecutionException}) if the specified duration expires.
368   *
369   * <p>The delegate future is interrupted and cancelled if it times out.
370   *
371   * @param delegate The future to delegate to.
372   * @param time when to timeout the future
373   * @param scheduledExecutor The executor service to enforce the timeout.
374   * @since 28.0
375   */
376  @Beta
377  @GwtIncompatible // java.util.concurrent.ScheduledExecutorService
378  public static <V> ListenableFuture<V> withTimeout(
379      ListenableFuture<V> delegate, Duration time, ScheduledExecutorService scheduledExecutor) {
380    return withTimeout(delegate, toNanosSaturated(time), TimeUnit.NANOSECONDS, scheduledExecutor);
381  }
382
383  /**
384   * Returns a future that delegates to another but will finish early (via a {@link
385   * TimeoutException} wrapped in an {@link ExecutionException}) if the specified duration expires.
386   *
387   * <p>The delegate future is interrupted and cancelled if it times out.
388   *
389   * @param delegate The future to delegate to.
390   * @param time when to timeout the future
391   * @param unit the time unit of the time parameter
392   * @param scheduledExecutor The executor service to enforce the timeout.
393   * @since 19.0
394   */
395  @Beta
396  @GwtIncompatible // java.util.concurrent.ScheduledExecutorService
397  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
398  public static <V> ListenableFuture<V> withTimeout(
399      ListenableFuture<V> delegate,
400      long time,
401      TimeUnit unit,
402      ScheduledExecutorService scheduledExecutor) {
403    if (delegate.isDone()) {
404      return delegate;
405    }
406    return TimeoutFuture.create(delegate, time, unit, scheduledExecutor);
407  }
408
409  /**
410   * Returns a new {@code Future} whose result is asynchronously derived from the result of the
411   * given {@code Future}. If the given {@code Future} fails, the returned {@code Future} fails with
412   * the same exception (and the function is not invoked).
413   *
414   * <p>More precisely, the returned {@code Future} takes its result from a {@code Future} produced
415   * by applying the given {@code AsyncFunction} to the result of the original {@code Future}.
416   * Example usage:
417   *
418   * <pre>{@code
419   * ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
420   * ListenableFuture<QueryResult> queryFuture =
421   *     transformAsync(rowKeyFuture, dataService::readFuture, executor);
422   * }</pre>
423   *
424   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
425   * the warnings the {@link MoreExecutors#directExecutor} documentation.
426   *
427   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
428   * input future and that of the future returned by the chain function. That is, if the returned
429   * {@code Future} is cancelled, it will attempt to cancel the other two, and if either of the
430   * other two is cancelled, the returned {@code Future} will receive a callback in which it will
431   * attempt to cancel itself.
432   *
433   * @param input The future to transform
434   * @param function A function to transform the result of the input future to the result of the
435   *     output future
436   * @param executor Executor to run the function in.
437   * @return A future that holds result of the function (if the input succeeded) or the original
438   *     input's failure (if not)
439   * @since 19.0 (in 11.0 as {@code transform})
440   */
441  @Beta
442  public static <I, O> ListenableFuture<O> transformAsync(
443      ListenableFuture<I> input,
444      AsyncFunction<? super I, ? extends O> function,
445      Executor executor) {
446    return AbstractTransformFuture.create(input, function, executor);
447  }
448
449  /**
450   * Returns a new {@code Future} whose result is derived from the result of the given {@code
451   * Future}. If {@code input} fails, the returned {@code Future} fails with the same exception (and
452   * the function is not invoked). Example usage:
453   *
454   * <pre>{@code
455   * ListenableFuture<QueryResult> queryFuture = ...;
456   * ListenableFuture<List<Row>> rowsFuture =
457   *     transform(queryFuture, QueryResult::getRows, executor);
458   * }</pre>
459   *
460   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
461   * the warnings the {@link MoreExecutors#directExecutor} documentation.
462   *
463   * <p>The returned {@code Future} attempts to keep its cancellation state in sync with that of the
464   * input future. That is, if the returned {@code Future} is cancelled, it will attempt to cancel
465   * the input, and if the input is cancelled, the returned {@code Future} will receive a callback
466   * in which it will attempt to cancel itself.
467   *
468   * <p>An example use of this method is to convert a serializable object returned from an RPC into
469   * a POJO.
470   *
471   * @param input The future to transform
472   * @param function A Function to transform the results of the provided future to the results of
473   *     the returned future.
474   * @param executor Executor to run the function in.
475   * @return A future that holds result of the transformation.
476   * @since 9.0 (in 2.0 as {@code compose})
477   */
478  @Beta
479  public static <I, O> ListenableFuture<O> transform(
480      ListenableFuture<I> input, Function<? super I, ? extends O> function, Executor executor) {
481    return AbstractTransformFuture.create(input, function, executor);
482  }
483
484  /**
485   * Like {@link #transform(ListenableFuture, Function, Executor)} except that the transformation
486   * {@code function} is invoked on each call to {@link Future#get() get()} on the returned future.
487   *
488   * <p>The returned {@code Future} reflects the input's cancellation state directly, and any
489   * attempt to cancel the returned Future is likewise passed through to the input Future.
490   *
491   * <p>Note that calls to {@linkplain Future#get(long, TimeUnit) timed get} only apply the timeout
492   * to the execution of the underlying {@code Future}, <em>not</em> to the execution of the
493   * transformation function.
494   *
495   * <p>The primary audience of this method is callers of {@code transform} who don't have a {@code
496   * ListenableFuture} available and do not mind repeated, lazy function evaluation.
497   *
498   * @param input The future to transform
499   * @param function A Function to transform the results of the provided future to the results of
500   *     the returned future.
501   * @return A future that returns the result of the transformation.
502   * @since 10.0
503   */
504  @Beta
505  @GwtIncompatible // TODO
506  public static <I, O> Future<O> lazyTransform(
507      final Future<I> input, final Function<? super I, ? extends O> function) {
508    checkNotNull(input);
509    checkNotNull(function);
510    return new Future<O>() {
511
512      @Override
513      public boolean cancel(boolean mayInterruptIfRunning) {
514        return input.cancel(mayInterruptIfRunning);
515      }
516
517      @Override
518      public boolean isCancelled() {
519        return input.isCancelled();
520      }
521
522      @Override
523      public boolean isDone() {
524        return input.isDone();
525      }
526
527      @Override
528      public O get() throws InterruptedException, ExecutionException {
529        return applyTransformation(input.get());
530      }
531
532      @Override
533      public O get(long timeout, TimeUnit unit)
534          throws InterruptedException, ExecutionException, TimeoutException {
535        return applyTransformation(input.get(timeout, unit));
536      }
537
538      private O applyTransformation(I input) throws ExecutionException {
539        try {
540          return function.apply(input);
541        } catch (Throwable t) {
542          throw new ExecutionException(t);
543        }
544      }
545    };
546  }
547
548  /**
549   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
550   * input futures, if all succeed.
551   *
552   * <p>The list of results is in the same order as the input list.
553   *
554   * <p>This differs from {@link #successfulAsList(ListenableFuture[])} in that it will return a
555   * failed future if any of the items fails.
556   *
557   * <p>Canceling this future will attempt to cancel all the component futures, and if any of the
558   * provided futures fails or is canceled, this one is, too.
559   *
560   * @param futures futures to combine
561   * @return a future that provides a list of the results of the component futures
562   * @since 10.0
563   */
564  @Beta
565  @SafeVarargs
566  public static <V> ListenableFuture<List<V>> allAsList(ListenableFuture<? extends V>... futures) {
567    return new ListFuture<V>(ImmutableList.copyOf(futures), true);
568  }
569
570  /**
571   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
572   * input futures, if all succeed.
573   *
574   * <p>The list of results is in the same order as the input list.
575   *
576   * <p>This differs from {@link #successfulAsList(Iterable)} in that it will return a failed future
577   * if any of the items fails.
578   *
579   * <p>Canceling this future will attempt to cancel all the component futures, and if any of the
580   * provided futures fails or is canceled, this one is, too.
581   *
582   * @param futures futures to combine
583   * @return a future that provides a list of the results of the component futures
584   * @since 10.0
585   */
586  @Beta
587  public static <V> ListenableFuture<List<V>> allAsList(
588      Iterable<? extends ListenableFuture<? extends V>> futures) {
589    return new ListFuture<V>(ImmutableList.copyOf(futures), true);
590  }
591
592  /**
593   * Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
594   * successful.
595   *
596   * <p>Any failures from the input futures will not be propagated to the returned future.
597   *
598   * @since 20.0
599   */
600  @Beta
601  @SafeVarargs
602  public static <V> FutureCombiner<V> whenAllComplete(ListenableFuture<? extends V>... futures) {
603    return new FutureCombiner<V>(false, ImmutableList.copyOf(futures));
604  }
605
606  /**
607   * Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
608   * successful.
609   *
610   * <p>Any failures from the input futures will not be propagated to the returned future.
611   *
612   * @since 20.0
613   */
614  @Beta
615  public static <V> FutureCombiner<V> whenAllComplete(
616      Iterable<? extends ListenableFuture<? extends V>> futures) {
617    return new FutureCombiner<V>(false, ImmutableList.copyOf(futures));
618  }
619
620  /**
621   * Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
622   *
623   * <p>If any input fails, the returned future fails immediately.
624   *
625   * @since 20.0
626   */
627  @Beta
628  @SafeVarargs
629  public static <V> FutureCombiner<V> whenAllSucceed(ListenableFuture<? extends V>... futures) {
630    return new FutureCombiner<V>(true, ImmutableList.copyOf(futures));
631  }
632
633  /**
634   * Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
635   *
636   * <p>If any input fails, the returned future fails immediately.
637   *
638   * @since 20.0
639   */
640  @Beta
641  public static <V> FutureCombiner<V> whenAllSucceed(
642      Iterable<? extends ListenableFuture<? extends V>> futures) {
643    return new FutureCombiner<V>(true, ImmutableList.copyOf(futures));
644  }
645
646  /**
647   * A helper to create a new {@code ListenableFuture} whose result is generated from a combination
648   * of input futures.
649   *
650   * <p>See {@link #whenAllComplete} and {@link #whenAllSucceed} for how to instantiate this class.
651   *
652   * <p>Example:
653   *
654   * <pre>{@code
655   * final ListenableFuture<Instant> loginDateFuture =
656   *     loginService.findLastLoginDate(username);
657   * final ListenableFuture<List<String>> recentCommandsFuture =
658   *     recentCommandsService.findRecentCommands(username);
659   * ListenableFuture<UsageHistory> usageFuture =
660   *     Futures.whenAllSucceed(loginDateFuture, recentCommandsFuture)
661   *         .call(
662   *             () ->
663   *                 new UsageHistory(
664   *                     username,
665   *                     Futures.getDone(loginDateFuture),
666   *                     Futures.getDone(recentCommandsFuture)),
667   *             executor);
668   * }</pre>
669   *
670   * @since 20.0
671   */
672  @Beta
673  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing, especially if we provide run(Runnable)
674  @GwtCompatible
675  public static final class FutureCombiner<V> {
676    private final boolean allMustSucceed;
677    private final ImmutableList<ListenableFuture<? extends V>> futures;
678
679    private FutureCombiner(
680        boolean allMustSucceed, ImmutableList<ListenableFuture<? extends V>> futures) {
681      this.allMustSucceed = allMustSucceed;
682      this.futures = futures;
683    }
684
685    /**
686     * Creates the {@link ListenableFuture} which will return the result of calling {@link
687     * AsyncCallable#call} in {@code combiner} when all futures complete, using the specified {@code
688     * executor}.
689     *
690     * <p>If the combiner throws a {@code CancellationException}, the returned future will be
691     * cancelled.
692     *
693     * <p>If the combiner throws an {@code ExecutionException}, the cause of the thrown {@code
694     * ExecutionException} will be extracted and returned as the cause of the new {@code
695     * ExecutionException} that gets thrown by the returned combined future.
696     *
697     * <p>Canceling this future will attempt to cancel all the component futures.
698     */
699    public <C> ListenableFuture<C> callAsync(AsyncCallable<C> combiner, Executor executor) {
700      return new CombinedFuture<C>(futures, allMustSucceed, executor, combiner);
701    }
702
703    /**
704     * Creates the {@link ListenableFuture} which will return the result of calling {@link
705     * Callable#call} in {@code combiner} when all futures complete, using the specified {@code
706     * executor}.
707     *
708     * <p>If the combiner throws a {@code CancellationException}, the returned future will be
709     * cancelled.
710     *
711     * <p>If the combiner throws an {@code ExecutionException}, the cause of the thrown {@code
712     * ExecutionException} will be extracted and returned as the cause of the new {@code
713     * ExecutionException} that gets thrown by the returned combined future.
714     *
715     * <p>Canceling this future will attempt to cancel all the component futures.
716     */
717    @CanIgnoreReturnValue // TODO(cpovirk): Remove this
718    public <C> ListenableFuture<C> call(Callable<C> combiner, Executor executor) {
719      return new CombinedFuture<C>(futures, allMustSucceed, executor, combiner);
720    }
721
722    /**
723     * Creates the {@link ListenableFuture} which will return the result of running {@code combiner}
724     * when all Futures complete. {@code combiner} will run using {@code executor}.
725     *
726     * <p>If the combiner throws a {@code CancellationException}, the returned future will be
727     * cancelled.
728     *
729     * <p>Canceling this Future will attempt to cancel all the component futures.
730     *
731     * @since 23.6
732     */
733    public ListenableFuture<?> run(final Runnable combiner, Executor executor) {
734      return call(
735          new Callable<Void>() {
736            @Override
737            public Void call() throws Exception {
738              combiner.run();
739              return null;
740            }
741          },
742          executor);
743    }
744  }
745
746  /**
747   * Returns a {@code ListenableFuture} whose result is set from the supplied future when it
748   * completes. Cancelling the supplied future will also cancel the returned future, but cancelling
749   * the returned future will have no effect on the supplied future.
750   *
751   * @since 15.0
752   */
753  @Beta
754  public static <V> ListenableFuture<V> nonCancellationPropagating(ListenableFuture<V> future) {
755    if (future.isDone()) {
756      return future;
757    }
758    NonCancellationPropagatingFuture<V> output = new NonCancellationPropagatingFuture<>(future);
759    future.addListener(output, directExecutor());
760    return output;
761  }
762
763  /** A wrapped future that does not propagate cancellation to its delegate. */
764  private static final class NonCancellationPropagatingFuture<V>
765      extends AbstractFuture.TrustedFuture<V> implements Runnable {
766    private ListenableFuture<V> delegate;
767
768    NonCancellationPropagatingFuture(final ListenableFuture<V> delegate) {
769      this.delegate = delegate;
770    }
771
772    @Override
773    public void run() {
774      // This prevents cancellation from propagating because we don't call setFuture(delegate) until
775      // delegate is already done, so calling cancel() on this future won't affect it.
776      ListenableFuture<V> localDelegate = delegate;
777      if (localDelegate != null) {
778        setFuture(localDelegate);
779      }
780    }
781
782    @Override
783    protected String pendingToString() {
784      ListenableFuture<V> localDelegate = delegate;
785      if (localDelegate != null) {
786        return "delegate=[" + localDelegate + "]";
787      }
788      return null;
789    }
790
791    @Override
792    protected void afterDone() {
793      delegate = null;
794    }
795  }
796
797  /**
798   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
799   * successful input futures. The list of results is in the same order as the input list, and if
800   * any of the provided futures fails or is canceled, its corresponding position will contain
801   * {@code null} (which is indistinguishable from the future having a successful value of {@code
802   * null}).
803   *
804   * <p>The list of results is in the same order as the input list.
805   *
806   * <p>This differs from {@link #allAsList(ListenableFuture[])} in that it's tolerant of failed
807   * futures for any of the items, representing them as {@code null} in the result list.
808   *
809   * <p>Canceling this future will attempt to cancel all the component futures.
810   *
811   * @param futures futures to combine
812   * @return a future that provides a list of the results of the component futures
813   * @since 10.0
814   */
815  @Beta
816  @SafeVarargs
817  public static <V> ListenableFuture<List<V>> successfulAsList(
818      ListenableFuture<? extends V>... futures) {
819    return new ListFuture<V>(ImmutableList.copyOf(futures), false);
820  }
821
822  /**
823   * Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
824   * successful input futures. The list of results is in the same order as the input list, and if
825   * any of the provided futures fails or is canceled, its corresponding position will contain
826   * {@code null} (which is indistinguishable from the future having a successful value of {@code
827   * null}).
828   *
829   * <p>The list of results is in the same order as the input list.
830   *
831   * <p>This differs from {@link #allAsList(Iterable)} in that it's tolerant of failed futures for
832   * any of the items, representing them as {@code null} in the result list.
833   *
834   * <p>Canceling this future will attempt to cancel all the component futures.
835   *
836   * @param futures futures to combine
837   * @return a future that provides a list of the results of the component futures
838   * @since 10.0
839   */
840  @Beta
841  public static <V> ListenableFuture<List<V>> successfulAsList(
842      Iterable<? extends ListenableFuture<? extends V>> futures) {
843    return new ListFuture<V>(ImmutableList.copyOf(futures), false);
844  }
845
846  /**
847   * Returns a list of delegate futures that correspond to the futures received in the order that
848   * they complete. Delegate futures return the same value or throw the same exception as the
849   * corresponding input future returns/throws.
850   *
851   * <p>"In the order that they complete" means, for practical purposes, about what you would
852   * expect, but there are some subtleties. First, we do guarantee that, if the output future at
853   * index n is done, the output future at index n-1 is also done. (But as usual with futures, some
854   * listeners for future n may complete before some for future n-1.) However, it is possible, if
855   * one input completes with result X and another later with result Y, for Y to come before X in
856   * the output future list. (Such races are impossible to solve without global synchronization of
857   * all future completions. And they should have little practical impact.)
858   *
859   * <p>Cancelling a delegate future propagates to input futures once all the delegates complete,
860   * either from cancellation or because an input future has completed. If N futures are passed in,
861   * and M delegates are cancelled, the remaining M input futures will be cancelled once N - M of
862   * the input futures complete. If all the delegates are cancelled, all the input futures will be
863   * too.
864   *
865   * @since 17.0
866   */
867  @Beta
868  public static <T> ImmutableList<ListenableFuture<T>> inCompletionOrder(
869      Iterable<? extends ListenableFuture<? extends T>> futures) {
870    // Can't use Iterables.toArray because it's not gwt compatible
871    final Collection<ListenableFuture<? extends T>> collection;
872    if (futures instanceof Collection) {
873      collection = (Collection<ListenableFuture<? extends T>>) futures;
874    } else {
875      collection = ImmutableList.copyOf(futures);
876    }
877    @SuppressWarnings("unchecked")
878    ListenableFuture<? extends T>[] copy =
879        (ListenableFuture<? extends T>[])
880            collection.toArray(new ListenableFuture[collection.size()]);
881    final InCompletionOrderState<T> state = new InCompletionOrderState<>(copy);
882    ImmutableList.Builder<AbstractFuture<T>> delegatesBuilder = ImmutableList.builder();
883    for (int i = 0; i < copy.length; i++) {
884      delegatesBuilder.add(new InCompletionOrderFuture<T>(state));
885    }
886
887    final ImmutableList<AbstractFuture<T>> delegates = delegatesBuilder.build();
888    for (int i = 0; i < copy.length; i++) {
889      final int localI = i;
890      copy[i].addListener(
891          new Runnable() {
892            @Override
893            public void run() {
894              state.recordInputCompletion(delegates, localI);
895            }
896          },
897          directExecutor());
898    }
899
900    @SuppressWarnings("unchecked")
901    ImmutableList<ListenableFuture<T>> delegatesCast = (ImmutableList) delegates;
902    return delegatesCast;
903  }
904
905  // This can't be a TrustedFuture, because TrustedFuture has clever optimizations that
906  // mean cancel won't be called if this Future is passed into setFuture, and then
907  // cancelled.
908  private static final class InCompletionOrderFuture<T> extends AbstractFuture<T> {
909    private InCompletionOrderState<T> state;
910
911    private InCompletionOrderFuture(InCompletionOrderState<T> state) {
912      this.state = state;
913    }
914
915    @Override
916    public boolean cancel(boolean interruptIfRunning) {
917      InCompletionOrderState<T> localState = state;
918      if (super.cancel(interruptIfRunning)) {
919        localState.recordOutputCancellation(interruptIfRunning);
920        return true;
921      }
922      return false;
923    }
924
925    @Override
926    protected void afterDone() {
927      state = null;
928    }
929
930    @Override
931    protected String pendingToString() {
932      InCompletionOrderState<T> localState = state;
933      if (localState != null) {
934        // Don't print the actual array! We don't want inCompletionOrder(list).toString() to have
935        // quadratic output.
936        return "inputCount=["
937            + localState.inputFutures.length
938            + "], remaining=["
939            + localState.incompleteOutputCount.get()
940            + "]";
941      }
942      return null;
943    }
944  }
945
946  private static final class InCompletionOrderState<T> {
947    // A happens-before edge between the writes of these fields and their reads exists, because
948    // in order to read these fields, the corresponding write to incompleteOutputCount must have
949    // been read.
950    private boolean wasCancelled = false;
951    private boolean shouldInterrupt = true;
952    private final AtomicInteger incompleteOutputCount;
953    private final ListenableFuture<? extends T>[] inputFutures;
954    private volatile int delegateIndex = 0;
955
956    private InCompletionOrderState(ListenableFuture<? extends T>[] inputFutures) {
957      this.inputFutures = inputFutures;
958      incompleteOutputCount = new AtomicInteger(inputFutures.length);
959    }
960
961    private void recordOutputCancellation(boolean interruptIfRunning) {
962      wasCancelled = true;
963      // If all the futures were cancelled with interruption, cancel the input futures
964      // with interruption; otherwise cancel without
965      if (!interruptIfRunning) {
966        shouldInterrupt = false;
967      }
968      recordCompletion();
969    }
970
971    private void recordInputCompletion(
972        ImmutableList<AbstractFuture<T>> delegates, int inputFutureIndex) {
973      ListenableFuture<? extends T> inputFuture = inputFutures[inputFutureIndex];
974      // Null out our reference to this future, so it can be GCed
975      inputFutures[inputFutureIndex] = null;
976      for (int i = delegateIndex; i < delegates.size(); i++) {
977        if (delegates.get(i).setFuture(inputFuture)) {
978          recordCompletion();
979          // this is technically unnecessary, but should speed up later accesses
980          delegateIndex = i + 1;
981          return;
982        }
983      }
984      // If all the delegates were complete, no reason for the next listener to have to
985      // go through the whole list. Avoids O(n^2) behavior when the entire output list is
986      // cancelled.
987      delegateIndex = delegates.size();
988    }
989
990    private void recordCompletion() {
991      if (incompleteOutputCount.decrementAndGet() == 0 && wasCancelled) {
992        for (ListenableFuture<?> toCancel : inputFutures) {
993          if (toCancel != null) {
994            toCancel.cancel(shouldInterrupt);
995          }
996        }
997      }
998    }
999  }
1000
1001  /**
1002   * Registers separate success and failure callbacks to be run when the {@code Future}'s
1003   * computation is {@linkplain java.util.concurrent.Future#isDone() complete} or, if the
1004   * computation is already complete, immediately.
1005   *
1006   * <p>The callback is run on {@code executor}. There is no guaranteed ordering of execution of
1007   * callbacks, but any callback added through this method is guaranteed to be called once the
1008   * computation is complete.
1009   *
1010   * <p>Exceptions thrown by a {@code callback} will be propagated up to the executor. Any exception
1011   * thrown during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an
1012   * exception thrown by {@linkplain MoreExecutors#directExecutor direct execution}) will be caught
1013   * and logged.
1014   *
1015   * <p>Example:
1016   *
1017   * <pre>{@code
1018   * ListenableFuture<QueryResult> future = ...;
1019   * Executor e = ...
1020   * addCallback(future,
1021   *     new FutureCallback<QueryResult>() {
1022   *       public void onSuccess(QueryResult result) {
1023   *         storeInCache(result);
1024   *       }
1025   *       public void onFailure(Throwable t) {
1026   *         reportError(t);
1027   *       }
1028   *     }, e);
1029   * }</pre>
1030   *
1031   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
1032   * the warnings the {@link MoreExecutors#directExecutor} documentation.
1033   *
1034   * <p>For a more general interface to attach a completion listener to a {@code Future}, see {@link
1035   * ListenableFuture#addListener addListener}.
1036   *
1037   * @param future The future attach the callback to.
1038   * @param callback The callback to invoke when {@code future} is completed.
1039   * @param executor The executor to run {@code callback} when the future completes.
1040   * @since 10.0
1041   */
1042  public static <V> void addCallback(
1043      final ListenableFuture<V> future,
1044      final FutureCallback<? super V> callback,
1045      Executor executor) {
1046    Preconditions.checkNotNull(callback);
1047    future.addListener(new CallbackListener<V>(future, callback), executor);
1048  }
1049
1050  /** See {@link #addCallback(ListenableFuture, FutureCallback, Executor)} for behavioral notes. */
1051  private static final class CallbackListener<V> implements Runnable {
1052    final Future<V> future;
1053    final FutureCallback<? super V> callback;
1054
1055    CallbackListener(Future<V> future, FutureCallback<? super V> callback) {
1056      this.future = future;
1057      this.callback = callback;
1058    }
1059
1060    @Override
1061    public void run() {
1062      if (future instanceof InternalFutureFailureAccess) {
1063        Throwable failure =
1064            InternalFutures.tryInternalFastPathGetFailure((InternalFutureFailureAccess) future);
1065        if (failure != null) {
1066          callback.onFailure(failure);
1067          return;
1068        }
1069      }
1070      final V value;
1071      try {
1072        value = getDone(future);
1073      } catch (ExecutionException e) {
1074        callback.onFailure(e.getCause());
1075        return;
1076      } catch (RuntimeException | Error e) {
1077        callback.onFailure(e);
1078        return;
1079      }
1080      callback.onSuccess(value);
1081    }
1082
1083    @Override
1084    public String toString() {
1085      return MoreObjects.toStringHelper(this).addValue(callback).toString();
1086    }
1087  }
1088
1089  /**
1090   * Returns the result of the input {@code Future}, which must have already completed.
1091   *
1092   * <p>The benefits of this method are twofold. First, the name "getDone" suggests to readers that
1093   * the {@code Future} is already done. Second, if buggy code calls {@code getDone} on a {@code
1094   * Future} that is still pending, the program will throw instead of block. This can be important
1095   * for APIs like {@link #whenAllComplete whenAllComplete(...)}{@code .}{@link
1096   * FutureCombiner#call(Callable, Executor) call(...)}, where it is easy to use a new input from
1097   * the {@code call} implementation but forget to add it to the arguments of {@code
1098   * whenAllComplete}.
1099   *
1100   * <p>If you are looking for a method to determine whether a given {@code Future} is done, use the
1101   * instance method {@link Future#isDone()}.
1102   *
1103   * @throws ExecutionException if the {@code Future} failed with an exception
1104   * @throws CancellationException if the {@code Future} was cancelled
1105   * @throws IllegalStateException if the {@code Future} is not done
1106   * @since 20.0
1107   */
1108  @CanIgnoreReturnValue
1109  // TODO(cpovirk): Consider calling getDone() in our own code.
1110  public static <V> V getDone(Future<V> future) throws ExecutionException {
1111    /*
1112     * We throw IllegalStateException, since the call could succeed later. Perhaps we "should" throw
1113     * IllegalArgumentException, since the call could succeed with a different argument. Those
1114     * exceptions' docs suggest that either is acceptable. Google's Java Practices page recommends
1115     * IllegalArgumentException here, in part to keep its recommendation simple: Static methods
1116     * should throw IllegalStateException only when they use static state.
1117     *
1118     * Why do we deviate here? The answer: We want for fluentFuture.getDone() to throw the same
1119     * exception as Futures.getDone(fluentFuture).
1120     */
1121    checkState(future.isDone(), "Future was expected to be done: %s", future);
1122    return getUninterruptibly(future);
1123  }
1124
1125  /**
1126   * Returns the result of {@link Future#get()}, converting most exceptions to a new instance of the
1127   * given checked exception type. This reduces boilerplate for a common use of {@code Future} in
1128   * which it is unnecessary to programmatically distinguish between exception types or to extract
1129   * other information from the exception instance.
1130   *
1131   * <p>Exceptions from {@code Future.get} are treated as follows:
1132   *
1133   * <ul>
1134   *   <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@code X} if the cause
1135   *       is a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
1136   *       RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
1137   *   <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
1138   *       interrupt).
1139   *   <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
1140   *       RuntimeException} (though {@code get} implementations are discouraged from throwing such
1141   *       exceptions).
1142   * </ul>
1143   *
1144   * <p>The overall principle is to continue to treat every checked exception as a checked
1145   * exception, every unchecked exception as an unchecked exception, and every error as an error. In
1146   * addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
1147   * new stack trace matches that of the current thread.
1148   *
1149   * <p>Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
1150   * that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
1151   * (preferring constructors with at least one {@code String}) and calling the constructor via
1152   * reflection. If the exception did not already have a cause, one is set by calling {@link
1153   * Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
1154   * IllegalArgumentException} is thrown.
1155   *
1156   * @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
1157   *     whose cause is not itself a checked exception
1158   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
1159   *     {@code RuntimeException} as its cause
1160   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1161   *     Error} as its cause
1162   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1163   * @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
1164   *     does not have a suitable constructor
1165   * @since 19.0 (in 10.0 as {@code get})
1166   */
1167  @Beta
1168  @CanIgnoreReturnValue
1169  @GwtIncompatible // reflection
1170  public static <V, X extends Exception> V getChecked(Future<V> future, Class<X> exceptionClass)
1171      throws X {
1172    return FuturesGetChecked.getChecked(future, exceptionClass);
1173  }
1174
1175  /**
1176   * Returns the result of {@link Future#get(long, TimeUnit)}, converting most exceptions to a new
1177   * instance of the given checked exception type. This reduces boilerplate for a common use of
1178   * {@code Future} in which it is unnecessary to programmatically distinguish between exception
1179   * types or to extract other information from the exception instance.
1180   *
1181   * <p>Exceptions from {@code Future.get} are treated as follows:
1182   *
1183   * <ul>
1184   *   <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@code X} if the cause
1185   *       is a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
1186   *       RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
1187   *   <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
1188   *       interrupt).
1189   *   <li>Any {@link TimeoutException} is wrapped in an {@code X}.
1190   *   <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
1191   *       RuntimeException} (though {@code get} implementations are discouraged from throwing such
1192   *       exceptions).
1193   * </ul>
1194   *
1195   * <p>The overall principle is to continue to treat every checked exception as a checked
1196   * exception, every unchecked exception as an unchecked exception, and every error as an error. In
1197   * addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
1198   * new stack trace matches that of the current thread.
1199   *
1200   * <p>Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
1201   * that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
1202   * (preferring constructors with at least one {@code String}) and calling the constructor via
1203   * reflection. If the exception did not already have a cause, one is set by calling {@link
1204   * Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
1205   * IllegalArgumentException} is thrown.
1206   *
1207   * @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
1208   *     whose cause is not itself a checked exception
1209   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
1210   *     {@code RuntimeException} as its cause
1211   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1212   *     Error} as its cause
1213   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1214   * @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
1215   *     does not have a suitable constructor
1216   * @since 28.0
1217   */
1218  @Beta
1219  @CanIgnoreReturnValue
1220  @GwtIncompatible // reflection
1221  public static <V, X extends Exception> V getChecked(
1222      Future<V> future, Class<X> exceptionClass, Duration timeout) throws X {
1223    return getChecked(future, exceptionClass, toNanosSaturated(timeout), TimeUnit.NANOSECONDS);
1224  }
1225
1226  /**
1227   * Returns the result of {@link Future#get(long, TimeUnit)}, converting most exceptions to a new
1228   * instance of the given checked exception type. This reduces boilerplate for a common use of
1229   * {@code Future} in which it is unnecessary to programmatically distinguish between exception
1230   * types or to extract other information from the exception instance.
1231   *
1232   * <p>Exceptions from {@code Future.get} are treated as follows:
1233   *
1234   * <ul>
1235   *   <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@code X} if the cause
1236   *       is a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
1237   *       RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
1238   *   <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
1239   *       interrupt).
1240   *   <li>Any {@link TimeoutException} is wrapped in an {@code X}.
1241   *   <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
1242   *       RuntimeException} (though {@code get} implementations are discouraged from throwing such
1243   *       exceptions).
1244   * </ul>
1245   *
1246   * <p>The overall principle is to continue to treat every checked exception as a checked
1247   * exception, every unchecked exception as an unchecked exception, and every error as an error. In
1248   * addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
1249   * new stack trace matches that of the current thread.
1250   *
1251   * <p>Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
1252   * that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
1253   * (preferring constructors with at least one {@code String}) and calling the constructor via
1254   * reflection. If the exception did not already have a cause, one is set by calling {@link
1255   * Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
1256   * IllegalArgumentException} is thrown.
1257   *
1258   * @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
1259   *     whose cause is not itself a checked exception
1260   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
1261   *     {@code RuntimeException} as its cause
1262   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1263   *     Error} as its cause
1264   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1265   * @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
1266   *     does not have a suitable constructor
1267   * @since 19.0 (in 10.0 as {@code get} and with different parameter order)
1268   */
1269  @Beta
1270  @CanIgnoreReturnValue
1271  @GwtIncompatible // reflection
1272  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
1273  public static <V, X extends Exception> V getChecked(
1274      Future<V> future, Class<X> exceptionClass, long timeout, TimeUnit unit) throws X {
1275    return FuturesGetChecked.getChecked(future, exceptionClass, timeout, unit);
1276  }
1277
1278  /**
1279   * Returns the result of calling {@link Future#get()} uninterruptibly on a task known not to throw
1280   * a checked exception. This makes {@code Future} more suitable for lightweight, fast-running
1281   * tasks that, barring bugs in the code, will not fail. This gives it exception-handling behavior
1282   * similar to that of {@code ForkJoinTask.join}.
1283   *
1284   * <p>Exceptions from {@code Future.get} are treated as follows:
1285   *
1286   * <ul>
1287   *   <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@link
1288   *       UncheckedExecutionException} (if the cause is an {@code Exception}) or {@link
1289   *       ExecutionError} (if the cause is an {@code Error}).
1290   *   <li>Any {@link InterruptedException} causes a retry of the {@code get} call. The interrupt is
1291   *       restored before {@code getUnchecked} returns.
1292   *   <li>Any {@link CancellationException} is propagated untouched. So is any other {@link
1293   *       RuntimeException} ({@code get} implementations are discouraged from throwing such
1294   *       exceptions).
1295   * </ul>
1296   *
1297   * <p>The overall principle is to eliminate all checked exceptions: to loop to avoid {@code
1298   * InterruptedException}, to pass through {@code CancellationException}, and to wrap any exception
1299   * from the underlying computation in an {@code UncheckedExecutionException} or {@code
1300   * ExecutionError}.
1301   *
1302   * <p>For an uninterruptible {@code get} that preserves other exceptions, see {@link
1303   * Uninterruptibles#getUninterruptibly(Future)}.
1304   *
1305   * @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with an
1306   *     {@code Exception} as its cause
1307   * @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
1308   *     Error} as its cause
1309   * @throws CancellationException if {@code get} throws a {@code CancellationException}
1310   * @since 10.0
1311   */
1312  @CanIgnoreReturnValue
1313  public static <V> V getUnchecked(Future<V> future) {
1314    checkNotNull(future);
1315    try {
1316      return getUninterruptibly(future);
1317    } catch (ExecutionException e) {
1318      wrapAndThrowUnchecked(e.getCause());
1319      throw new AssertionError();
1320    }
1321  }
1322
1323  private static void wrapAndThrowUnchecked(Throwable cause) {
1324    if (cause instanceof Error) {
1325      throw new ExecutionError((Error) cause);
1326    }
1327    /*
1328     * It's an Exception. (Or it's a non-Error, non-Exception Throwable. From my survey of such
1329     * classes, I believe that most users intended to extend Exception, so we'll treat it like an
1330     * Exception.)
1331     */
1332    throw new UncheckedExecutionException(cause);
1333  }
1334
1335  /*
1336   * Arguably we don't need a timed getUnchecked because any operation slow enough to require a
1337   * timeout is heavyweight enough to throw a checked exception and therefore be inappropriate to
1338   * use with getUnchecked. Further, it's not clear that converting the checked TimeoutException to
1339   * a RuntimeException -- especially to an UncheckedExecutionException, since it wasn't thrown by
1340   * the computation -- makes sense, and if we don't convert it, the user still has to write a
1341   * try-catch block.
1342   *
1343   * If you think you would use this method, let us know. You might also also look into the
1344   * Fork-Join framework: http://docs.oracle.com/javase/tutorial/essential/concurrency/forkjoin.html
1345   */
1346}