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/*
* Copyright (C) 2006 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
* in compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License
* is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
* or implied. See the License for the specific language governing permissions and limitations under
* the License.
*/
package com.google.common.util.concurrent;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.util.concurrent.CollectionFuture.ListFuture;
import com.google.common.util.concurrent.ImmediateFuture.ImmediateCancelledFuture;
import com.google.common.util.concurrent.ImmediateFuture.ImmediateFailedCheckedFuture;
import com.google.common.util.concurrent.ImmediateFuture.ImmediateFailedFuture;
import com.google.common.util.concurrent.ImmediateFuture.ImmediateSuccessfulCheckedFuture;
import com.google.common.util.concurrent.ImmediateFuture.ImmediateSuccessfulFuture;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicInteger;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* Static utility methods pertaining to the {@link Future} interface.
*
* Many of these methods use the {@link ListenableFuture} API; consult the Guava User Guide
* article on {@code
* ListenableFuture}.
*
*
The main purpose of {@code ListenableFuture} is to help you chain together a graph of
* asynchronous operations. You can chain them together manually with calls to methods like {@link
* Futures#transform(ListenableFuture, Function, Executor) Futures.transform}, but you will often
* find it easier to use a framework. Frameworks automate the process, often adding features like
* monitoring, debugging, and cancellation. Examples of frameworks include:
*
*
* - Dagger Producers
*
*
* If you do chain your operations manually, you may want to use {@link FluentFuture}.
*
* @author Kevin Bourrillion
* @author Nishant Thakkar
* @author Sven Mawson
* @since 1.0
*/
@Beta
@GwtCompatible(emulated = true)
public final class Futures extends GwtFuturesCatchingSpecialization {
// A note on memory visibility.
// Many of the utilities in this class (transform, withFallback, withTimeout, asList, combine)
// have two requirements that significantly complicate their design.
// 1. Cancellation should propagate from the returned future to the input future(s).
// 2. The returned futures shouldn't unnecessarily 'pin' their inputs after completion.
//
// A consequence of these requirements is that the delegate futures cannot be stored in
// final fields.
//
// For simplicity the rest of this description will discuss Futures.catching since it is the
// simplest instance, though very similar descriptions apply to many other classes in this file.
//
// In the constructor of AbstractCatchingFuture, the delegate future is assigned to a field
// 'inputFuture'. That field is non-final and non-volatile. There are 2 places where the
// 'inputFuture' field is read and where we will have to consider visibility of the write
// operation in the constructor.
//
// 1. In the listener that performs the callback. In this case it is fine since inputFuture is
// assigned prior to calling addListener, and addListener happens-before any invocation of the
// listener. Notably, this means that 'volatile' is unnecessary to make 'inputFuture' visible
// to the listener.
//
// 2. In done() where we may propagate cancellation to the input. In this case it is _not_ fine.
// There is currently nothing that enforces that the write to inputFuture in the constructor is
// visible to done(). This is because there is no happens before edge between the write and a
// (hypothetical) unsafe read by our caller. Note: adding 'volatile' does not fix this issue,
// it would just add an edge such that if done() observed non-null, then it would also
// definitely observe all earlier writes, but we still have no guarantee that done() would see
// the inital write (just stronger guarantees if it does).
//
// See: http://cs.oswego.edu/pipermail/concurrency-interest/2015-January/013800.html
// For a (long) discussion about this specific issue and the general futility of life.
//
// For the time being we are OK with the problem discussed above since it requires a caller to
// introduce a very specific kind of data-race. And given the other operations performed by these
// methods that involve volatile read/write operations, in practice there is no issue. Also, the
// way in such a visibility issue would surface is most likely as a failure of cancel() to
// propagate to the input. Cancellation propagation is fundamentally racy so this is fine.
//
// Future versions of the JMM may revise safe construction semantics in such a way that we can
// safely publish these objects and we won't need this whole discussion.
// TODO(user,lukes): consider adding volatile to all these fields since in current known JVMs
// that should resolve the issue. This comes at the cost of adding more write barriers to the
// implementations.
private Futures() {}
/**
* Creates a {@link CheckedFuture} out of a normal {@link ListenableFuture} and a {@link Function}
* that maps from {@link Exception} instances into the appropriate checked type.
*
*
Warning: We recommend against using {@code CheckedFuture} in new projects. {@code
* CheckedFuture} is difficult to build libraries atop. {@code CheckedFuture} ports of methods
* like {@link Futures#transformAsync} have historically had bugs, and some of these bugs are
* necessary, unavoidable consequences of the {@code CheckedFuture} API. Additionally, {@code
* CheckedFuture} encourages users to take exceptions from one thread and rethrow them in another,
* producing confusing stack traces.
*
*
The given mapping function will be applied to an {@link InterruptedException}, a {@link
* CancellationException}, or an {@link ExecutionException}. See {@link Future#get()} for details
* on the exceptions thrown.
*
* @since 9.0 (source-compatible since 1.0)
* @deprecated {@link CheckedFuture} cannot properly support the chained operations that are the
* primary goal of {@link ListenableFuture}. {@code CheckedFuture} also encourages users to
* rethrow exceptions from one thread in another thread, producing misleading stack traces.
* Additionally, it has a surprising policy about which exceptions to map and which to leave
* untouched. Guava users who want a {@code CheckedFuture} can fork the classes for their own
* use, possibly specializing them to the particular exception type they use. We recommend
* that most people use {@code ListenableFuture} and perform any exception wrapping
* themselves. This method is scheduled for removal from Guava in January 2019.
*/
// TODO(b/72241575): Remove by 2019-01
@Deprecated
@GwtIncompatible // TODO
public static CheckedFuture makeChecked(
ListenableFuture future, Function mapper) {
return new MappingCheckedFuture<>(checkNotNull(future), mapper);
}
/**
* Creates a {@code ListenableFuture} which has its value set immediately upon construction. The
* getters just return the value. This {@code Future} can't be canceled or timed out and its
* {@code isDone()} method always returns {@code true}.
*/
public static ListenableFuture immediateFuture(@Nullable V value) {
if (value == null) {
// This cast is safe because null is assignable to V for all V (i.e. it is covariant)
@SuppressWarnings({"unchecked", "rawtypes"})
ListenableFuture typedNull = (ListenableFuture) ImmediateSuccessfulFuture.NULL;
return typedNull;
}
return new ImmediateSuccessfulFuture(value);
}
/**
* Returns a {@code CheckedFuture} which has its value set immediately upon construction.
*
* The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
* returns {@code true}. Calling {@code get()} or {@code checkedGet()} will immediately return the
* provided value.
*
* @deprecated {@link CheckedFuture} cannot properly support the chained operations that are the
* primary goal of {@link ListenableFuture}. {@code CheckedFuture} also encourages users to
* rethrow exceptions from one thread in another thread, producing misleading stack traces.
* Additionally, it has a surprising policy about which exceptions to map and which to leave
* untouched. Guava users who want a {@code CheckedFuture} can fork the classes for their own
* use, possibly specializing them to the particular exception type they use. We recommend
* that most people use {@code ListenableFuture} and perform any exception wrapping
* themselves. This method is scheduled for removal from Guava in January 2019.
*/
// TODO(b/72241893): Remove by 2019-01
@Deprecated
@GwtIncompatible // TODO
public static CheckedFuture immediateCheckedFuture(
@Nullable V value) {
return new ImmediateSuccessfulCheckedFuture<>(value);
}
/**
* Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
*
* The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
* returns {@code true}. Calling {@code get()} will immediately throw the provided {@code
* Throwable} wrapped in an {@code ExecutionException}.
*/
public static ListenableFuture immediateFailedFuture(Throwable throwable) {
checkNotNull(throwable);
return new ImmediateFailedFuture(throwable);
}
/**
* Creates a {@code ListenableFuture} which is cancelled immediately upon construction, so that
* {@code isCancelled()} always returns {@code true}.
*
* @since 14.0
*/
public static ListenableFuture immediateCancelledFuture() {
return new ImmediateCancelledFuture();
}
/**
* Returns a {@code CheckedFuture} which has an exception set immediately upon construction.
*
* The returned {@code Future} can't be cancelled, and its {@code isDone()} method always
* returns {@code true}. Calling {@code get()} will immediately throw the provided {@code
* Exception} wrapped in an {@code ExecutionException}, and calling {@code checkedGet()} will
* throw the provided exception itself.
*
* @deprecated {@link CheckedFuture} cannot properly support the chained operations that are the
* primary goal of {@link ListenableFuture}. {@code CheckedFuture} also encourages users to
* rethrow exceptions from one thread in another thread, producing misleading stack traces.
* Additionally, it has a surprising policy about which exceptions to map and which to leave
* untouched. Guava users who want a {@code CheckedFuture} can fork the classes for their own
* use, possibly specializing them to the particular exception type they use. We recommend
* that most people use {@code ListenableFuture} and perform any exception wrapping
* themselves. This method is scheduled for removal from Guava in January 2019.
*/
// TODO(b/72241500): Remove by 2019-01
@Deprecated
@GwtIncompatible // TODO
public static CheckedFuture immediateFailedCheckedFuture(
X exception) {
checkNotNull(exception);
return new ImmediateFailedCheckedFuture<>(exception);
}
/**
* Executes {@code callable} on the specified {@code executor}, returning a {@code Future}.
*
* @throws RejectedExecutionException if the task cannot be scheduled for execution
* @since 23.0
*/
public static ListenableFuture submitAsync(AsyncCallable callable, Executor executor) {
TrustedListenableFutureTask task = TrustedListenableFutureTask.create(callable);
executor.execute(task);
return task;
}
/**
* Schedules {@code callable} on the specified {@code executor}, returning a {@code Future}.
*
* @throws RejectedExecutionException if the task cannot be scheduled for execution
* @since 23.0
*/
@GwtIncompatible // java.util.concurrent.ScheduledExecutorService
@SuppressWarnings("GoodTime") // should accept a java.time.Duration
public static ListenableFuture scheduleAsync(
AsyncCallable callable,
long delay,
TimeUnit timeUnit,
ScheduledExecutorService executorService) {
TrustedListenableFutureTask task = TrustedListenableFutureTask.create(callable);
final Future scheduled = executorService.schedule(task, delay, timeUnit);
task.addListener(
new Runnable() {
@Override
public void run() {
// Don't want to interrupt twice
scheduled.cancel(false);
}
},
directExecutor());
return task;
}
/**
* Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
* primary input fails with the given {@code exceptionType}, from the result provided by the
* {@code fallback}. {@link Function#apply} is not invoked until the primary input has failed, so
* if the primary input succeeds, it is never invoked. If, during the invocation of {@code
* fallback}, an exception is thrown, this exception is used as the result of the output {@code
* Future}.
*
* Usage example:
*
*
{@code
* ListenableFuture fetchCounterFuture = ...;
*
* // Falling back to a zero counter in case an exception happens when
* // processing the RPC to fetch counters.
* ListenableFuture faultTolerantFuture = Futures.catching(
* fetchCounterFuture, FetchException.class, x -> 0, directExecutor());
* }
*
* When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
* functions passed to this method.
*
* @param input the primary input {@code Future}
* @param exceptionType the exception type that triggers use of {@code fallback}. The exception
* type is matched against the input's exception. "The input's exception" means the cause of
* the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
* different kind of exception, that exception itself. To avoid hiding bugs and other
* unrecoverable errors, callers should prefer more specific types, avoiding {@code
* Throwable.class} in particular.
* @param fallback the {@link Function} to be called if {@code input} fails with the expected
* exception type. The function's argument is the input's exception. "The input's exception"
* means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
* {@code get()} throws a different kind of exception, that exception itself.
* @param executor the executor that runs {@code fallback} if {@code input} fails
* @since 19.0
*/
@Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
public static ListenableFuture catching(
ListenableFuture input,
Class exceptionType,
Function fallback,
Executor executor) {
return AbstractCatchingFuture.create(input, exceptionType, fallback, executor);
}
/**
* Returns a {@code Future} whose result is taken from the given primary {@code input} or, if the
* primary input fails with the given {@code exceptionType}, from the result provided by the
* {@code fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has
* failed, so if the primary input succeeds, it is never invoked. If, during the invocation of
* {@code fallback}, an exception is thrown, this exception is used as the result of the output
* {@code Future}.
*
* Usage examples:
*
*
{@code
* ListenableFuture fetchCounterFuture = ...;
*
* // Falling back to a zero counter in case an exception happens when
* // processing the RPC to fetch counters.
* ListenableFuture faultTolerantFuture = Futures.catchingAsync(
* fetchCounterFuture, FetchException.class, x -> immediateFuture(0), directExecutor());
* }
*
* The fallback can also choose to propagate the original exception when desired:
*
*
{@code
* ListenableFuture fetchCounterFuture = ...;
*
* // Falling back to a zero counter only in case the exception was a
* // TimeoutException.
* ListenableFuture faultTolerantFuture = Futures.catchingAsync(
* fetchCounterFuture,
* FetchException.class,
* e -> {
* if (omitDataOnFetchFailure) {
* return immediateFuture(0);
* }
* throw e;
* },
* directExecutor());
* }
*
* When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
* functions passed to this method. (Specifically, {@code directExecutor} functions should avoid
* heavyweight operations inside {@code AsyncFunction.apply}. Any heavyweight operations should
* occur in other threads responsible for completing the returned {@code Future}.)
*
* @param input the primary input {@code Future}
* @param exceptionType the exception type that triggers use of {@code fallback}. The exception
* type is matched against the input's exception. "The input's exception" means the cause of
* the {@link ExecutionException} thrown by {@code input.get()} or, if {@code get()} throws a
* different kind of exception, that exception itself. To avoid hiding bugs and other
* unrecoverable errors, callers should prefer more specific types, avoiding {@code
* Throwable.class} in particular.
* @param fallback the {@link AsyncFunction} to be called if {@code input} fails with the expected
* exception type. The function's argument is the input's exception. "The input's exception"
* means the cause of the {@link ExecutionException} thrown by {@code input.get()} or, if
* {@code get()} throws a different kind of exception, that exception itself.
* @param executor the executor that runs {@code fallback} if {@code input} fails
* @since 19.0 (similar functionality in 14.0 as {@code withFallback})
*/
@CanIgnoreReturnValue // TODO(kak): @CheckReturnValue
@Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
public static ListenableFuture catchingAsync(
ListenableFuture input,
Class exceptionType,
AsyncFunction fallback,
Executor executor) {
return AbstractCatchingFuture.create(input, exceptionType, fallback, executor);
}
/**
* Returns a future that delegates to another but will finish early (via a {@link
* TimeoutException} wrapped in an {@link ExecutionException}) if the specified duration expires.
*
* The delegate future is interrupted and cancelled if it times out.
*
* @param delegate The future to delegate to.
* @param time when to timeout the future
* @param unit the time unit of the time parameter
* @param scheduledExecutor The executor service to enforce the timeout.
* @since 19.0
*/
@GwtIncompatible // java.util.concurrent.ScheduledExecutorService
@SuppressWarnings("GoodTime") // should accept a java.time.Duration
public static ListenableFuture withTimeout(
ListenableFuture delegate,
long time,
TimeUnit unit,
ScheduledExecutorService scheduledExecutor) {
if (delegate.isDone()) {
return delegate;
}
return TimeoutFuture.create(delegate, time, unit, scheduledExecutor);
}
/**
* Returns a new {@code Future} whose result is asynchronously derived from the result of the
* given {@code Future}. If the given {@code Future} fails, the returned {@code Future} fails with
* the same exception (and the function is not invoked).
*
* More precisely, the returned {@code Future} takes its result from a {@code Future} produced
* by applying the given {@code AsyncFunction} to the result of the original {@code Future}.
* Example usage:
*
*
{@code
* ListenableFuture rowKeyFuture = indexService.lookUp(query);
* ListenableFuture queryFuture =
* transformAsync(rowKeyFuture, dataService::readFuture, executor);
* }
*
* When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
* functions passed to this method. (Specifically, {@code directExecutor} functions should avoid
* heavyweight operations inside {@code AsyncFunction.apply}. Any heavyweight operations should
* occur in other threads responsible for completing the returned {@code Future}.)
*
*
The returned {@code Future} attempts to keep its cancellation state in sync with that of the
* input future and that of the future returned by the chain function. That is, if the returned
* {@code Future} is cancelled, it will attempt to cancel the other two, and if either of the
* other two is cancelled, the returned {@code Future} will receive a callback in which it will
* attempt to cancel itself.
*
* @param input The future to transform
* @param function A function to transform the result of the input future to the result of the
* output future
* @param executor Executor to run the function in.
* @return A future that holds result of the function (if the input succeeded) or the original
* input's failure (if not)
* @since 19.0 (in 11.0 as {@code transform})
*/
public static ListenableFuture transformAsync(
ListenableFuture input,
AsyncFunction function,
Executor executor) {
return AbstractTransformFuture.create(input, function, executor);
}
/**
* Returns a new {@code Future} whose result is derived from the result of the given {@code
* Future}. If {@code input} fails, the returned {@code Future} fails with the same exception (and
* the function is not invoked). Example usage:
*
* {@code
* ListenableFuture queryFuture = ...;
* ListenableFuture> rowsFuture =
* transform(queryFuture, QueryResult::getRows, executor);
* }
*
* When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
* functions passed to this method.
*
*
The returned {@code Future} attempts to keep its cancellation state in sync with that of the
* input future. That is, if the returned {@code Future} is cancelled, it will attempt to cancel
* the input, and if the input is cancelled, the returned {@code Future} will receive a callback
* in which it will attempt to cancel itself.
*
*
An example use of this method is to convert a serializable object returned from an RPC into
* a POJO.
*
* @param input The future to transform
* @param function A Function to transform the results of the provided future to the results of
* the returned future.
* @param executor Executor to run the function in.
* @return A future that holds result of the transformation.
* @since 9.0 (in 2.0 as {@code compose})
*/
public static ListenableFuture transform(
ListenableFuture input, Function function, Executor executor) {
return AbstractTransformFuture.create(input, function, executor);
}
/**
* Like {@link #transform(ListenableFuture, Function, Executor)} except that the transformation
* {@code function} is invoked on each call to {@link Future#get() get()} on the returned future.
*
* The returned {@code Future} reflects the input's cancellation state directly, and any
* attempt to cancel the returned Future is likewise passed through to the input Future.
*
*
Note that calls to {@linkplain Future#get(long, TimeUnit) timed get} only apply the timeout
* to the execution of the underlying {@code Future}, not to the execution of the
* transformation function.
*
*
The primary audience of this method is callers of {@code transform} who don't have a {@code
* ListenableFuture} available and do not mind repeated, lazy function evaluation.
*
* @param input The future to transform
* @param function A Function to transform the results of the provided future to the results of
* the returned future.
* @return A future that returns the result of the transformation.
* @since 10.0
*/
@GwtIncompatible // TODO
public static Future lazyTransform(
final Future input, final Function function) {
checkNotNull(input);
checkNotNull(function);
return new Future() {
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
return input.cancel(mayInterruptIfRunning);
}
@Override
public boolean isCancelled() {
return input.isCancelled();
}
@Override
public boolean isDone() {
return input.isDone();
}
@Override
public O get() throws InterruptedException, ExecutionException {
return applyTransformation(input.get());
}
@Override
public O get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
return applyTransformation(input.get(timeout, unit));
}
private O applyTransformation(I input) throws ExecutionException {
try {
return function.apply(input);
} catch (Throwable t) {
throw new ExecutionException(t);
}
}
};
}
/**
* Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
* input futures, if all succeed.
*
* The list of results is in the same order as the input list.
*
*
Canceling this future will attempt to cancel all the component futures, and if any of the
* provided futures fails or is canceled, this one is, too.
*
* @param futures futures to combine
* @return a future that provides a list of the results of the component futures
* @since 10.0
*/
@Beta
@SafeVarargs
public static ListenableFuture> allAsList(ListenableFuture... futures) {
return new ListFuture(ImmutableList.copyOf(futures), true);
}
/**
* Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
* input futures, if all succeed.
*
* The list of results is in the same order as the input list.
*
*
Canceling this future will attempt to cancel all the component futures, and if any of the
* provided futures fails or is canceled, this one is, too.
*
* @param futures futures to combine
* @return a future that provides a list of the results of the component futures
* @since 10.0
*/
@Beta
public static ListenableFuture> allAsList(
Iterable> futures) {
return new ListFuture(ImmutableList.copyOf(futures), true);
}
/**
* Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
* successful.
*
* @since 20.0
*/
@SafeVarargs
public static FutureCombiner whenAllComplete(ListenableFuture... futures) {
return new FutureCombiner(false, ImmutableList.copyOf(futures));
}
/**
* Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
* successful.
*
* @since 20.0
*/
public static FutureCombiner whenAllComplete(
Iterable> futures) {
return new FutureCombiner(false, ImmutableList.copyOf(futures));
}
/**
* Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
*
* If any input fails, the returned future fails immediately.
*
* @since 20.0
*/
@SafeVarargs
public static FutureCombiner whenAllSucceed(ListenableFuture... futures) {
return new FutureCombiner(true, ImmutableList.copyOf(futures));
}
/**
* Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
*
* If any input fails, the returned future fails immediately.
*
* @since 20.0
*/
public static FutureCombiner whenAllSucceed(
Iterable> futures) {
return new FutureCombiner(true, ImmutableList.copyOf(futures));
}
/**
* A helper to create a new {@code ListenableFuture} whose result is generated from a combination
* of input futures.
*
* See {@link #whenAllComplete} and {@link #whenAllSucceed} for how to instantiate this class.
*
*
Example:
*
*
{@code
* final ListenableFuture loginDateFuture =
* loginService.findLastLoginDate(username);
* final ListenableFuture> recentCommandsFuture =
* recentCommandsService.findRecentCommands(username);
* ListenableFuture usageFuture =
* Futures.whenAllSucceed(loginDateFuture, recentCommandsFuture)
* .call(
* () ->
* new UsageHistory(
* username,
* Futures.getDone(loginDateFuture),
* Futures.getDone(recentCommandsFuture)),
* executor);
* }
*
* @since 20.0
*/
@Beta
@CanIgnoreReturnValue // TODO(cpovirk): Consider removing, especially if we provide run(Runnable)
@GwtCompatible
public static final class FutureCombiner {
private final boolean allMustSucceed;
private final ImmutableList> futures;
private FutureCombiner(
boolean allMustSucceed, ImmutableList> futures) {
this.allMustSucceed = allMustSucceed;
this.futures = futures;
}
/**
* Creates the {@link ListenableFuture} which will return the result of calling {@link
* AsyncCallable#call} in {@code combiner} when all futures complete, using the specified {@code
* executor}.
*
* If the combiner throws a {@code CancellationException}, the returned future will be
* cancelled.
*
*
If the combiner throws an {@code ExecutionException}, the cause of the thrown {@code
* ExecutionException} will be extracted and returned as the cause of the new {@code
* ExecutionException} that gets thrown by the returned combined future.
*
*
Canceling this future will attempt to cancel all the component futures.
*/
public ListenableFuture callAsync(AsyncCallable combiner, Executor executor) {
return new CombinedFuture(futures, allMustSucceed, executor, combiner);
}
/**
* Creates the {@link ListenableFuture} which will return the result of calling {@link
* Callable#call} in {@code combiner} when all futures complete, using the specified {@code
* executor}.
*
* If the combiner throws a {@code CancellationException}, the returned future will be
* cancelled.
*
*
If the combiner throws an {@code ExecutionException}, the cause of the thrown {@code
* ExecutionException} will be extracted and returned as the cause of the new {@code
* ExecutionException} that gets thrown by the returned combined future.
*
*
Canceling this future will attempt to cancel all the component futures.
*/
@CanIgnoreReturnValue // TODO(cpovirk): Remove this
public ListenableFuture call(Callable combiner, Executor executor) {
return new CombinedFuture(futures, allMustSucceed, executor, combiner);
}
/**
* Creates the {@link ListenableFuture} which will return the result of running {@code combiner}
* when all Futures complete. {@code combiner} will run using {@code executor}.
*
* If the combiner throws a {@code CancellationException}, the returned future will be
* cancelled.
*
*
Canceling this Future will attempt to cancel all the component futures.
*
* @since 23.6
*/
public ListenableFuture run(final Runnable combiner, Executor executor) {
return call(
new Callable() {
@Override
public Void call() throws Exception {
combiner.run();
return null;
}
},
executor);
}
}
/**
* Returns a {@code ListenableFuture} whose result is set from the supplied future when it
* completes. Cancelling the supplied future will also cancel the returned future, but cancelling
* the returned future will have no effect on the supplied future.
*
* @since 15.0
*/
public static ListenableFuture nonCancellationPropagating(ListenableFuture future) {
if (future.isDone()) {
return future;
}
NonCancellationPropagatingFuture output = new NonCancellationPropagatingFuture<>(future);
future.addListener(output, directExecutor());
return output;
}
/** A wrapped future that does not propagate cancellation to its delegate. */
private static final class NonCancellationPropagatingFuture
extends AbstractFuture.TrustedFuture implements Runnable {
private ListenableFuture delegate;
NonCancellationPropagatingFuture(final ListenableFuture delegate) {
this.delegate = delegate;
}
@Override
public void run() {
// This prevents cancellation from propagating because we don't call setFuture(delegate) until
// delegate is already done, so calling cancel() on this future won't affect it.
ListenableFuture localDelegate = delegate;
if (localDelegate != null) {
setFuture(localDelegate);
}
}
@Override
protected String pendingToString() {
ListenableFuture localDelegate = delegate;
if (localDelegate != null) {
return "delegate=[" + localDelegate + "]";
}
return null;
}
@Override
protected void afterDone() {
delegate = null;
}
}
/**
* Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
* successful input futures. The list of results is in the same order as the input list, and if
* any of the provided futures fails or is canceled, its corresponding position will contain
* {@code null} (which is indistinguishable from the future having a successful value of {@code
* null}).
*
* Canceling this future will attempt to cancel all the component futures.
*
* @param futures futures to combine
* @return a future that provides a list of the results of the component futures
* @since 10.0
*/
@Beta
@SafeVarargs
public static ListenableFuture> successfulAsList(
ListenableFuture... futures) {
return new ListFuture(ImmutableList.copyOf(futures), false);
}
/**
* Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
* successful input futures. The list of results is in the same order as the input list, and if
* any of the provided futures fails or is canceled, its corresponding position will contain
* {@code null} (which is indistinguishable from the future having a successful value of {@code
* null}).
*
* Canceling this future will attempt to cancel all the component futures.
*
* @param futures futures to combine
* @return a future that provides a list of the results of the component futures
* @since 10.0
*/
@Beta
public static ListenableFuture> successfulAsList(
Iterable> futures) {
return new ListFuture(ImmutableList.copyOf(futures), false);
}
/**
* Returns a list of delegate futures that correspond to the futures received in the order that
* they complete. Delegate futures return the same value or throw the same exception as the
* corresponding input future returns/throws.
*
* "In the order that they complete" means, for practical purposes, about what you would
* expect, but there are some subtleties. First, we do guarantee that, if the output future at
* index n is done, the output future at index n-1 is also done. (But as usual with futures, some
* listeners for future n may complete before some for future n-1.) However, it is possible, if
* one input completes with result X and another later with result Y, for Y to come before X in
* the output future list. (Such races are impossible to solve without global synchronization of
* all future completions. And they should have little practical impact.)
*
*
Cancelling a delegate future propagates to input futures once all the delegates complete,
* either from cancellation or because an input future has completed. If N futures are passed in,
* and M delegates are cancelled, the remaining M input futures will be cancelled once N - M of
* the input futures complete. If all the delegates are cancelled, all the input futures will be
* too.
*
* @since 17.0
*/
@Beta
public static ImmutableList> inCompletionOrder(
Iterable> futures) {
// Can't use Iterables.toArray because it's not gwt compatible
final Collection> collection;
if (futures instanceof Collection) {
collection = (Collection>) futures;
} else {
collection = ImmutableList.copyOf(futures);
}
@SuppressWarnings("unchecked")
ListenableFuture[] copy =
(ListenableFuture[])
collection.toArray(new ListenableFuture[collection.size()]);
final InCompletionOrderState state = new InCompletionOrderState<>(copy);
ImmutableList.Builder> delegatesBuilder = ImmutableList.builder();
for (int i = 0; i < copy.length; i++) {
delegatesBuilder.add(new InCompletionOrderFuture(state));
}
final ImmutableList> delegates = delegatesBuilder.build();
for (int i = 0; i < copy.length; i++) {
final int localI = i;
copy[i].addListener(
new Runnable() {
@Override
public void run() {
state.recordInputCompletion(delegates, localI);
}
},
directExecutor());
}
@SuppressWarnings("unchecked")
ImmutableList> delegatesCast = (ImmutableList) delegates;
return delegatesCast;
}
// This can't be a TrustedFuture, because TrustedFuture has clever optimizations that
// mean cancel won't be called if this Future is passed into setFuture, and then
// cancelled.
private static final class InCompletionOrderFuture extends AbstractFuture {
private InCompletionOrderState state;
private InCompletionOrderFuture(InCompletionOrderState state) {
this.state = state;
}
@Override
public boolean cancel(boolean interruptIfRunning) {
InCompletionOrderState localState = state;
if (super.cancel(interruptIfRunning)) {
localState.recordOutputCancellation(interruptIfRunning);
return true;
}
return false;
}
@Override
protected void afterDone() {
state = null;
}
@Override
protected String pendingToString() {
InCompletionOrderState localState = state;
if (localState != null) {
// Don't print the actual array! We don't want inCompletionOrder(list).toString() to have
// quadratic output.
return "inputCount=["
+ localState.inputFutures.length
+ "], remaining=["
+ localState.incompleteOutputCount.get()
+ "]";
}
return null;
}
}
private static final class InCompletionOrderState {
// A happens-before edge between the writes of these fields and their reads exists, because
// in order to read these fields, the corresponding write to incompleteOutputCount must have
// been read.
private boolean wasCancelled = false;
private boolean shouldInterrupt = true;
private final AtomicInteger incompleteOutputCount;
private final ListenableFuture[] inputFutures;
private volatile int delegateIndex = 0;
private InCompletionOrderState(ListenableFuture[] inputFutures) {
this.inputFutures = inputFutures;
incompleteOutputCount = new AtomicInteger(inputFutures.length);
}
private void recordOutputCancellation(boolean interruptIfRunning) {
wasCancelled = true;
// If all the futures were cancelled with interruption, cancel the input futures
// with interruption; otherwise cancel without
if (!interruptIfRunning) {
shouldInterrupt = false;
}
recordCompletion();
}
private void recordInputCompletion(
ImmutableList> delegates, int inputFutureIndex) {
ListenableFuture inputFuture = inputFutures[inputFutureIndex];
// Null out our reference to this future, so it can be GCed
inputFutures[inputFutureIndex] = null;
for (int i = delegateIndex; i < delegates.size(); i++) {
if (delegates.get(i).setFuture(inputFuture)) {
recordCompletion();
// this is technically unnecessary, but should speed up later accesses
delegateIndex = i + 1;
return;
}
}
// If all the delegates were complete, no reason for the next listener to have to
// go through the whole list. Avoids O(n^2) behavior when the entire output list is
// cancelled.
delegateIndex = delegates.size();
}
private void recordCompletion() {
if (incompleteOutputCount.decrementAndGet() == 0 && wasCancelled) {
for (ListenableFuture toCancel : inputFutures) {
if (toCancel != null) {
toCancel.cancel(shouldInterrupt);
}
}
}
}
}
/**
* Registers separate success and failure callbacks to be run when the {@code Future}'s
* computation is {@linkplain java.util.concurrent.Future#isDone() complete} or, if the
* computation is already complete, immediately.
*
* The callback is run on {@code executor}. There is no guaranteed ordering of execution of
* callbacks, but any callback added through this method is guaranteed to be called once the
* computation is complete.
*
*
Example:
*
*
{@code
* ListenableFuture future = ...;
* Executor e = ...
* addCallback(future,
* new FutureCallback() {
* public void onSuccess(QueryResult result) {
* storeInCache(result);
* }
* public void onFailure(Throwable t) {
* reportError(t);
* }
* }, e);
* }
*
* When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. All its warnings about heavyweight listeners are also applicable to heavyweight
* callbacks passed to this method.
*
*
For a more general interface to attach a completion listener to a {@code Future}, see {@link
* ListenableFuture#addListener addListener}.
*
* @param future The future attach the callback to.
* @param callback The callback to invoke when {@code future} is completed.
* @param executor The executor to run {@code callback} when the future completes.
* @since 10.0
*/
public static void addCallback(
final ListenableFuture future,
final FutureCallback callback,
Executor executor) {
Preconditions.checkNotNull(callback);
future.addListener(new CallbackListener(future, callback), executor);
}
/** See {@link #addCallback(ListenableFuture, FutureCallback, Executor)} for behavioral notes. */
private static final class CallbackListener implements Runnable {
final Future future;
final FutureCallback callback;
CallbackListener(Future future, FutureCallback callback) {
this.future = future;
this.callback = callback;
}
@Override
public void run() {
final V value;
try {
value = getDone(future);
} catch (ExecutionException e) {
callback.onFailure(e.getCause());
return;
} catch (RuntimeException | Error e) {
callback.onFailure(e);
return;
}
callback.onSuccess(value);
}
@Override
public String toString() {
return MoreObjects.toStringHelper(this).addValue(callback).toString();
}
}
/**
* Returns the result of the input {@code Future}, which must have already completed.
*
* The benefits of this method are twofold. First, the name "getDone" suggests to readers that
* the {@code Future} is already done. Second, if buggy code calls {@code getDone} on a {@code
* Future} that is still pending, the program will throw instead of block. This can be important
* for APIs like {@link #whenAllComplete whenAllComplete(...)}{@code .}{@link
* FutureCombiner#call(Callable, Executor) call(...)}, where it is easy to use a new input from
* the {@code call} implementation but forget to add it to the arguments of {@code
* whenAllComplete}.
*
*
If you are looking for a method to determine whether a given {@code Future} is done, use the
* instance method {@link Future#isDone()}.
*
* @throws ExecutionException if the {@code Future} failed with an exception
* @throws CancellationException if the {@code Future} was cancelled
* @throws IllegalStateException if the {@code Future} is not done
* @since 20.0
*/
@CanIgnoreReturnValue
// TODO(cpovirk): Consider calling getDone() in our own code.
public static V getDone(Future future) throws ExecutionException {
/*
* We throw IllegalStateException, since the call could succeed later. Perhaps we "should" throw
* IllegalArgumentException, since the call could succeed with a different argument. Those
* exceptions' docs suggest that either is acceptable. Google's Java Practices page recommends
* IllegalArgumentException here, in part to keep its recommendation simple: Static methods
* should throw IllegalStateException only when they use static state.
*
*
* Why do we deviate here? The answer: We want for fluentFuture.getDone() to throw the same
* exception as Futures.getDone(fluentFuture).
*/
checkState(future.isDone(), "Future was expected to be done: %s", future);
return getUninterruptibly(future);
}
/**
* Returns the result of {@link Future#get()}, converting most exceptions to a new instance of the
* given checked exception type. This reduces boilerplate for a common use of {@code Future} in
* which it is unnecessary to programmatically distinguish between exception types or to extract
* other information from the exception instance.
*
* Exceptions from {@code Future.get} are treated as follows:
*
*
* - Any {@link ExecutionException} has its cause wrapped in an {@code X} if the cause
* is a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
* RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
*
- Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
* interrupt).
*
- Any {@link CancellationException} is propagated untouched, as is any other {@link
* RuntimeException} (though {@code get} implementations are discouraged from throwing such
* exceptions).
*
*
* The overall principle is to continue to treat every checked exception as a checked
* exception, every unchecked exception as an unchecked exception, and every error as an error. In
* addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
* new stack trace matches that of the current thread.
*
*
Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
* that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
* (preferring constructors with at least one {@code String}) and calling the constructor via
* reflection. If the exception did not already have a cause, one is set by calling {@link
* Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
* IllegalArgumentException} is thrown.
*
* @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
* whose cause is not itself a checked exception
* @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
* {@code RuntimeException} as its cause
* @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
* Error} as its cause
* @throws CancellationException if {@code get} throws a {@code CancellationException}
* @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
* does not have a suitable constructor
* @since 19.0 (in 10.0 as {@code get})
*/
@CanIgnoreReturnValue
@GwtIncompatible // reflection
public static V getChecked(Future future, Class exceptionClass)
throws X {
return FuturesGetChecked.getChecked(future, exceptionClass);
}
/**
* Returns the result of {@link Future#get(long, TimeUnit)}, converting most exceptions to a new
* instance of the given checked exception type. This reduces boilerplate for a common use of
* {@code Future} in which it is unnecessary to programmatically distinguish between exception
* types or to extract other information from the exception instance.
*
* Exceptions from {@code Future.get} are treated as follows:
*
*
* - Any {@link ExecutionException} has its cause wrapped in an {@code X} if the cause
* is a checked exception, an {@link UncheckedExecutionException} if the cause is a {@code
* RuntimeException}, or an {@link ExecutionError} if the cause is an {@code Error}.
*
- Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
* interrupt).
*
- Any {@link TimeoutException} is wrapped in an {@code X}.
*
- Any {@link CancellationException} is propagated untouched, as is any other {@link
* RuntimeException} (though {@code get} implementations are discouraged from throwing such
* exceptions).
*
*
* The overall principle is to continue to treat every checked exception as a checked
* exception, every unchecked exception as an unchecked exception, and every error as an error. In
* addition, the cause of any {@code ExecutionException} is wrapped in order to ensure that the
* new stack trace matches that of the current thread.
*
*
Instances of {@code exceptionClass} are created by choosing an arbitrary public constructor
* that accepts zero or more arguments, all of type {@code String} or {@code Throwable}
* (preferring constructors with at least one {@code String}) and calling the constructor via
* reflection. If the exception did not already have a cause, one is set by calling {@link
* Throwable#initCause(Throwable)} on it. If no such constructor exists, an {@code
* IllegalArgumentException} is thrown.
*
* @throws X if {@code get} throws any checked exception except for an {@code ExecutionException}
* whose cause is not itself a checked exception
* @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with a
* {@code RuntimeException} as its cause
* @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
* Error} as its cause
* @throws CancellationException if {@code get} throws a {@code CancellationException}
* @throws IllegalArgumentException if {@code exceptionClass} extends {@code RuntimeException} or
* does not have a suitable constructor
* @since 19.0 (in 10.0 as {@code get} and with different parameter order)
*/
@CanIgnoreReturnValue
@GwtIncompatible // reflection
@SuppressWarnings("GoodTime") // should accept a java.time.Duration
public static V getChecked(
Future future, Class exceptionClass, long timeout, TimeUnit unit) throws X {
return FuturesGetChecked.getChecked(future, exceptionClass, timeout, unit);
}
/**
* Returns the result of calling {@link Future#get()} uninterruptibly on a task known not to throw
* a checked exception. This makes {@code Future} more suitable for lightweight, fast-running
* tasks that, barring bugs in the code, will not fail. This gives it exception-handling behavior
* similar to that of {@code ForkJoinTask.join}.
*
* Exceptions from {@code Future.get} are treated as follows:
*
*
* - Any {@link ExecutionException} has its cause wrapped in an {@link
* UncheckedExecutionException} (if the cause is an {@code Exception}) or {@link
* ExecutionError} (if the cause is an {@code Error}).
*
- Any {@link InterruptedException} causes a retry of the {@code get} call. The interrupt is
* restored before {@code getUnchecked} returns.
*
- Any {@link CancellationException} is propagated untouched. So is any other {@link
* RuntimeException} ({@code get} implementations are discouraged from throwing such
* exceptions).
*
*
* The overall principle is to eliminate all checked exceptions: to loop to avoid {@code
* InterruptedException}, to pass through {@code CancellationException}, and to wrap any exception
* from the underlying computation in an {@code UncheckedExecutionException} or {@code
* ExecutionError}.
*
*
For an uninterruptible {@code get} that preserves other exceptions, see {@link
* Uninterruptibles#getUninterruptibly(Future)}.
*
* @throws UncheckedExecutionException if {@code get} throws an {@code ExecutionException} with an
* {@code Exception} as its cause
* @throws ExecutionError if {@code get} throws an {@code ExecutionException} with an {@code
* Error} as its cause
* @throws CancellationException if {@code get} throws a {@code CancellationException}
* @since 10.0
*/
@CanIgnoreReturnValue
public static V getUnchecked(Future future) {
checkNotNull(future);
try {
return getUninterruptibly(future);
} catch (ExecutionException e) {
wrapAndThrowUnchecked(e.getCause());
throw new AssertionError();
}
}
private static void wrapAndThrowUnchecked(Throwable cause) {
if (cause instanceof Error) {
throw new ExecutionError((Error) cause);
}
/*
* It's an Exception. (Or it's a non-Error, non-Exception Throwable. From my survey of such
* classes, I believe that most users intended to extend Exception, so we'll treat it like an
* Exception.)
*/
throw new UncheckedExecutionException(cause);
}
/*
* Arguably we don't need a timed getUnchecked because any operation slow enough to require a
* timeout is heavyweight enough to throw a checked exception and therefore be inappropriate to
* use with getUnchecked. Further, it's not clear that converting the checked TimeoutException to
* a RuntimeException -- especially to an UncheckedExecutionException, since it wasn't thrown by
* the computation -- makes sense, and if we don't convert it, the user still has to write a
* try-catch block.
*
* If you think you would use this method, let us know. You might also also look into the
* Fork-Join framework: http://docs.oracle.com/javase/tutorial/essential/concurrency/forkjoin.html
*/
/**
* A checked future that uses a function to map from exceptions to the appropriate checked type.
*/
@GwtIncompatible // TODO
private static class MappingCheckedFuture
extends AbstractCheckedFuture {
final Function mapper;
MappingCheckedFuture(ListenableFuture delegate, Function mapper) {
super(delegate);
this.mapper = checkNotNull(mapper);
}
@Override
protected X mapException(Exception e) {
return mapper.apply(e);
}
}
}