/*
* 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.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Queues;
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.List;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ConcurrentLinkedQueue;
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 javax.annotation.Nullable;
/**
* Static utility methods pertaining to the {@link Future} interface.
*
* <p>Many of these methods use the {@link ListenableFuture} API; consult the Guava User Guide
* article on <a href="https://github.com/google/guava/wiki/ListenableFutureExplained">
* {@code ListenableFuture}</a>.
*
* @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.
*
* <p><b>Warning:</b> 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.
*
* <p>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)
*/
@GwtIncompatible // TODO
public static <V, X extends Exception> CheckedFuture<V, X> makeChecked(ListenableFuture<V> future, Function<? super Exception, X> mapper) {
return new MappingCheckedFuture<V, X>(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 <V> ListenableFuture<V> 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<V> typedNull = (ListenableFuture) ImmediateSuccessfulFuture.NULL;
return typedNull;
}
return new ImmediateSuccessfulFuture<V>(value);
}
/**
* Returns a {@code CheckedFuture} which has its value set immediately upon construction.
*
* <p>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.
*/
@GwtIncompatible // TODO
public static <V, X extends Exception> CheckedFuture<V, X> immediateCheckedFuture(@Nullable V value) {
return new ImmediateSuccessfulCheckedFuture<V, X>(value);
}
/**
* Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
*
* <p>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 <V> ListenableFuture<V> immediateFailedFuture(Throwable throwable) {
checkNotNull(throwable);
return new ImmediateFailedFuture<V>(throwable);
}
/**
* Creates a {@code ListenableFuture} which is cancelled immediately upon construction, so that
* {@code isCancelled()} always returns {@code true}.
*
* @since 14.0
*/
public static <V> ListenableFuture<V> immediateCancelledFuture() {
return new ImmediateCancelledFuture<V>();
}
/**
* Returns a {@code CheckedFuture} which has an exception set immediately upon construction.
*
* <p>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.
*/
@GwtIncompatible // TODO
public static <V, X extends Exception> CheckedFuture<V, X> immediateFailedCheckedFuture(X exception) {
checkNotNull(exception);
return new ImmediateFailedCheckedFuture<V, X>(exception);
}
/**
* 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}.
*
* <p>Usage example:
*
* <pre> {@code
* ListenableFuture<Integer> fetchCounterFuture = ...;
*
* // Falling back to a zero counter in case an exception happens when
* // processing the RPC to fetch counters.
* ListenableFuture<Integer> faultTolerantFuture = Futures.catching(
* fetchCounterFuture, FetchException.class,
* new Function<FetchException, Integer>() {
* public Integer apply(FetchException e) {
* return 0;
* }
* });}</pre>
*
* <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
* choice in some cases. See the discussion in the {@link ListenableFuture#addListener
* ListenableFuture.addListener} documentation. The documentation's warnings about "lightweight
* listeners" refer here to the work done during {@code Function.apply}.
*
* @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.
* @since 19.0
*/
@Partially.GwtIncompatible("AVAILABLE but requires exceptionType to be Throwable.class")
public static <V, X extends Throwable> ListenableFuture<V> catching(
ListenableFuture<? extends V> input,
Class<X> exceptionType,
Function<? super X, ? extends V> fallback) {
return AbstractCatchingFuture.create(input, exceptionType, fallback);
}
/**
* 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}.
*
* <p>Usage example:
*
* <pre> {@code
* ListenableFuture<Integer> fetchCounterFuture = ...;
*
* // Falling back to a zero counter in case an exception happens when
* // processing the RPC to fetch counters.
* ListenableFuture<Integer> faultTolerantFuture = Futures.catching(
* fetchCounterFuture, FetchException.class,
* new Function<FetchException, Integer>() {
* public Integer apply(FetchException e) {
* return 0;
* }
* }, directExecutor());}</pre>
*
* <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. The documentation's warnings about "lightweight listeners" refer here to the
* work done during {@code Function.apply}.
*
* @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 <V, X extends Throwable> ListenableFuture<V> catching(ListenableFuture<? extends V> input, Class<X> exceptionType, Function<? super X, ? extends V> 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}.
*
* <p>Usage examples:
*
* <pre> {@code
* ListenableFuture<Integer> fetchCounterFuture = ...;
*
* // Falling back to a zero counter in case an exception happens when
* // processing the RPC to fetch counters.
* ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
* fetchCounterFuture, FetchException.class,
* new AsyncFunction<FetchException, Integer>() {
* public ListenableFuture<Integer> apply(FetchException e) {
* return immediateFuture(0);
* }
* });}</pre>
*
* <p>The fallback can also choose to propagate the original exception when desired:
*
* <pre> {@code
* ListenableFuture<Integer> fetchCounterFuture = ...;
*
* // Falling back to a zero counter only in case the exception was a
* // TimeoutException.
* ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
* fetchCounterFuture, FetchException.class,
* new AsyncFunction<FetchException, Integer>() {
* public ListenableFuture<Integer> apply(FetchException e)
* throws FetchException {
* if (omitDataOnFetchFailure) {
* return immediateFuture(0);
* }
* throw e;
* }
* });}</pre>
*
* <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
* choice in some cases. See the discussion in the {@link ListenableFuture#addListener
* ListenableFuture.addListener} documentation. The documentation's warnings about "lightweight
* listeners" refer here to the work done during {@code AsyncFunction.apply}, not to any work done
* to complete 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.
* @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 <V, X extends Throwable> ListenableFuture<V> catchingAsync(
ListenableFuture<? extends V> input,
Class<X> exceptionType,
AsyncFunction<? super X, ? extends V> fallback) {
return AbstractCatchingFuture.create(input, exceptionType, fallback);
}
/**
* 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}.
*
* <p>Usage examples:
*
* <pre> {@code
* ListenableFuture<Integer> fetchCounterFuture = ...;
*
* // Falling back to a zero counter in case an exception happens when
* // processing the RPC to fetch counters.
* ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
* fetchCounterFuture, FetchException.class,
* new AsyncFunction<FetchException, Integer>() {
* public ListenableFuture<Integer> apply(FetchException e) {
* return immediateFuture(0);
* }
* }, directExecutor());}</pre>
*
* <p>The fallback can also choose to propagate the original exception when desired:
*
* <pre> {@code
* ListenableFuture<Integer> fetchCounterFuture = ...;
*
* // Falling back to a zero counter only in case the exception was a
* // TimeoutException.
* ListenableFuture<Integer> faultTolerantFuture = Futures.catchingAsync(
* fetchCounterFuture, FetchException.class,
* new AsyncFunction<FetchException, Integer>() {
* public ListenableFuture<Integer> apply(FetchException e)
* throws FetchException {
* if (omitDataOnFetchFailure) {
* return immediateFuture(0);
* }
* throw e;
* }
* }, directExecutor());}</pre>
*
* <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. The documentation's warnings about "lightweight listeners" refer here to the
* work done during {@code AsyncFunction.apply}, not to any work done to complete 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 <V, X extends Throwable> ListenableFuture<V> catchingAsync(ListenableFuture<? extends V> input, Class<X> exceptionType, AsyncFunction<? super X, ? extends V> 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.
*
* <p>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
public static <V> ListenableFuture<V> withTimeout(
ListenableFuture<V> delegate,
long time,
TimeUnit unit,
ScheduledExecutorService scheduledExecutor) {
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).
*
* <p>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:
*
* <pre> {@code
* ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
* AsyncFunction<RowKey, QueryResult> queryFunction =
* new AsyncFunction<RowKey, QueryResult>() {
* public ListenableFuture<QueryResult> apply(RowKey rowKey) {
* return dataService.read(rowKey);
* }
* };
* ListenableFuture<QueryResult> queryFuture =
* transformAsync(rowKeyFuture, queryFunction);}</pre>
*
* <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
* choice in some cases. See the discussion in the {@link ListenableFuture#addListener
* ListenableFuture.addListener} documentation. The documentation's warnings about "lightweight
* listeners" refer here to the work done during {@code AsyncFunction.apply}, not to any work done
* to complete the returned {@code Future}.
*
* <p>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 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
* @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 <I, O> ListenableFuture<O> transformAsync(ListenableFuture<I> input, AsyncFunction<? super I, ? extends O> function) {
return AbstractTransformFuture.create(input, function);
}
/**
* 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).
*
* <p>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:
*
* <pre> {@code
* ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
* AsyncFunction<RowKey, QueryResult> queryFunction =
* new AsyncFunction<RowKey, QueryResult>() {
* public ListenableFuture<QueryResult> apply(RowKey rowKey) {
* return dataService.read(rowKey);
* }
* };
* ListenableFuture<QueryResult> queryFuture =
* transformAsync(rowKeyFuture, queryFunction, executor);}</pre>
*
* <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. The documentation's warnings about "lightweight listeners" refer here to the
* work done during {@code AsyncFunction.apply}, not to any work done to complete the returned
* {@code Future}.
*
* <p>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 <I, O> ListenableFuture<O> transformAsync(
ListenableFuture<I> input,
AsyncFunction<? super I, ? extends O> 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:
*
* <pre> {@code
* ListenableFuture<QueryResult> queryFuture = ...;
* Function<QueryResult, List<Row>> rowsFunction =
* new Function<QueryResult, List<Row>>() {
* public List<Row> apply(QueryResult queryResult) {
* return queryResult.getRows();
* }
* };
* ListenableFuture<List<Row>> rowsFuture =
* transform(queryFuture, rowsFunction);}</pre>
*
* <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
* choice in some cases. See the discussion in the {@link ListenableFuture#addListener
* ListenableFuture.addListener} documentation. The documentation's warnings about "lightweight
* listeners" refer here to the work done during {@code Function.apply}.
*
* <p>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.
*
* <p>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. This will be run in the thread that notifies input it is complete.
* @return A future that holds result of the transformation.
* @since 9.0 (in 1.0 as {@code compose})
*/
public static <I, O> ListenableFuture<O> transform(ListenableFuture<I> input, Function<? super I, ? extends O> function) {
return AbstractTransformFuture.create(input, function);
}
/**
* 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:
*
* <pre> {@code
* ListenableFuture<QueryResult> queryFuture = ...;
* Function<QueryResult, List<Row>> rowsFunction =
* new Function<QueryResult, List<Row>>() {
* public List<Row> apply(QueryResult queryResult) {
* return queryResult.getRows();
* }
* };
* ListenableFuture<List<Row>> rowsFuture =
* transform(queryFuture, rowsFunction, executor);}</pre>
*
* <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation. The documentation's warnings about "lightweight listeners" refer here to the
* work done during {@code Function.apply}.
*
* <p>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.
*
* <p>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 <I, O> ListenableFuture<O> transform(ListenableFuture<I> input, Function<? super I, ? extends O> function, Executor executor) {
return AbstractTransformFuture.create(input, function, executor);
}
/**
* Like {@link #transform(ListenableFuture, Function)} except that the transformation {@code
* function} is invoked on each call to {@link Future#get() get()} on the returned future.
*
* <p>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.
*
* <p>Note that calls to {@linkplain Future#get(long, TimeUnit) timed get} only apply the timeout
* to the execution of the underlying {@code Future}, <em>not</em> to the execution of the
* transformation function.
*
* <p>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 <I, O> Future<O> lazyTransform(
final Future<I> input, final Function<? super I, ? extends O> function) {
checkNotNull(input);
checkNotNull(function);
return new Future<O>() {
@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);
}
}
};
}
/**
* Returns a new {@code ListenableFuture} whose result is the product of calling {@code get()} on
* the {@code Future} nested within the given {@code Future}, effectively chaining the futures one
* after the other. Example:
*
* <pre> {@code
* SettableFuture<ListenableFuture<String>> nested = SettableFuture.create();
* ListenableFuture<String> dereferenced = dereference(nested);}</pre>
*
* <p>Most users will not need this method. To create a {@code Future} that completes with the
* result of another {@code Future}, create a {@link SettableFuture}, and call {@link
* SettableFuture#setFuture setFuture(otherFuture)} on it.
*
* <p>{@code dereference} has the same cancellation and execution semantics as {@link
* #transformAsync(ListenableFuture, AsyncFunction)}, in that the returned {@code Future}
* attempts to keep its cancellation state in sync with both the input {@code Future} and the
* nested {@code Future}. The transformation is very lightweight and therefore takes place in
* the same thread (either the thread that called {@code dereference}, or the thread in which
* the dereferenced future completes).
*
* @param nested The nested future to transform.
* @return A future that holds result of the inner future.
* @since 13.0
*/
@SuppressWarnings({"rawtypes", "unchecked"})
public static <V> ListenableFuture<V> dereference(ListenableFuture<? extends ListenableFuture<? extends V>> nested) {
return transformAsync((ListenableFuture) nested, (AsyncFunction) DEREFERENCER);
}
/**
* Helper {@code Function} for {@link #dereference}.
*/
private static final AsyncFunction<ListenableFuture<Object>, Object> DEREFERENCER = new AsyncFunction<ListenableFuture<Object>, Object>() {
@Override
public ListenableFuture<Object> apply(ListenableFuture<Object> input) {
return input;
}
};
/**
* Creates a new {@code ListenableFuture} whose value is a list containing the values of all its
* input futures, if all succeed. If any input fails, the returned future fails immediately.
*
* <p>The list of results is in the same order as the input list.
*
* <p>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 <V> ListenableFuture<List<V>> allAsList(ListenableFuture<? extends V>... futures) {
return new ListFuture<V>(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. If any input fails, the returned future fails immediately.
*
* <p>The list of results is in the same order as the input list.
*
* <p>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 <V> ListenableFuture<List<V>> allAsList(Iterable<? extends ListenableFuture<? extends V>> futures) {
return new ListFuture<V>(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 <V> FutureCombiner<V> whenAllComplete(ListenableFuture<? extends V>... futures) {
return new FutureCombiner<V>(false, ImmutableList.copyOf(futures));
}
/**
* Creates a {@link FutureCombiner} that processes the completed futures whether or not they're
* successful.
*
* @since 20.0
*/
public static <V> FutureCombiner<V> whenAllComplete(Iterable<? extends ListenableFuture<? extends V>> futures) {
return new FutureCombiner<V>(false, ImmutableList.copyOf(futures));
}
/**
* Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
*
* <p>If any input fails, the returned future fails immediately.
*
* @since 20.0
*/
@SafeVarargs
public static <V> FutureCombiner<V> whenAllSucceed(ListenableFuture<? extends V>... futures) {
return new FutureCombiner<V>(true, ImmutableList.copyOf(futures));
}
/**
* Creates a {@link FutureCombiner} requiring that all passed in futures are successful.
*
* <p>If any input fails, the returned future fails immediately.
*
* @since 20.0
*/
public static <V> FutureCombiner<V> whenAllSucceed(Iterable<? extends ListenableFuture<? extends V>> futures) {
return new FutureCombiner<V>(true, ImmutableList.copyOf(futures));
}
/**
* A helper to create a new {@code ListenableFuture} whose result is generated from a combination
* of input futures.
*
* <p>See {@link #whenAllComplete} and {@link #whenAllSucceed} for how to instantiate this class.
*
* <p>Example:
*
* <pre> {@code
* final ListenableFuture<Instant> loginDateFuture =
* loginService.findLastLoginDate(username);
* final ListenableFuture<List<String>> recentCommandsFuture =
* recentCommandsService.findRecentCommands(username);
* Callable<UsageHistory> usageComputation =
* new Callable<UsageHistory>() {
* public UsageHistory call() throws Exception {
* return new UsageHistory(
* username, loginDateFuture.get(), recentCommandsFuture.get());
* }
* };
* ListenableFuture<UsageHistory> usageFuture =
* Futures.whenAllSucceed(loginDateFuture, recentCommandsFuture)
* .call(usageComputation, executor);}</pre>
*
* @since 20.0
*/
@Beta
@CanIgnoreReturnValue // TODO(cpovirk): Consider removing, especially if we provide run(Runnable)
@GwtCompatible
public static final class FutureCombiner<V> {
private final boolean allMustSucceed;
private final ImmutableList<ListenableFuture<? extends V>> futures;
private FutureCombiner(
boolean allMustSucceed, ImmutableList<ListenableFuture<? extends V>> 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}.
*
* <p>If the combiner throws a {@code CancellationException}, the returned future will be
* cancelled.
*
* <p>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.
*
* <p>Canceling this future will attempt to cancel all the component futures.
*/
public <C> ListenableFuture<C> callAsync(AsyncCallable<C> combiner, Executor executor) {
return new CombinedFuture<C>(futures, allMustSucceed, executor, combiner);
}
/**
* Like {@link #callAsync(AsyncCallable, Executor)} but using {@linkplain
* MoreExecutors#directExecutor direct executor}.
*/
public <C> ListenableFuture<C> callAsync(AsyncCallable<C> combiner) {
return callAsync(combiner, directExecutor());
}
/**
* 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}.
*
* <p>If the combiner throws a {@code CancellationException}, the returned future will be
* cancelled.
*
* <p>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.
*
* <p>Canceling this future will attempt to cancel all the component futures.
*/
@CanIgnoreReturnValue
public <C> ListenableFuture<C> call(Callable<C> combiner, Executor executor) {
return new CombinedFuture<C>(futures, allMustSucceed, executor, combiner);
}
/**
* Like {@link #call(Callable, Executor)} but using {@linkplain MoreExecutors#directExecutor
* direct executor}.
*/
@CanIgnoreReturnValue
public <C> ListenableFuture<C> call(Callable<C> combiner) {
return call(combiner, directExecutor());
}
/*
* TODO(cpovirk): Evaluate demand for a run(Runnable) version. Would it allow us to remove
* @CanIgnoreReturnValue from the call() methods above?
* https://github.com/google/guava/issues/2371
*/
}
/**
* Creates a new {@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
*/
@GwtIncompatible // TODO
public static <V> ListenableFuture<V> nonCancellationPropagating(ListenableFuture<V> future) {
return new NonCancellationPropagatingFuture<V>(future);
}
/**
* A wrapped future that does not propagate cancellation to its delegate.
*/
@GwtIncompatible // TODO
private static final class NonCancellationPropagatingFuture<V> extends AbstractFuture.TrustedFuture<V> {
NonCancellationPropagatingFuture(final ListenableFuture<V> delegate) {
delegate.addListener(new Runnable() {
@Override
public void run() {
// This prevents cancellation from propagating because we don't assign delegate until
// delegate is already done, so calling cancel() on it is a no-op.
setFuture(delegate);
}
}, directExecutor());
}
}
/**
* 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}).
*
* <p>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 <V> ListenableFuture<List<V>> successfulAsList(ListenableFuture<? extends V>... futures) {
return new ListFuture<V>(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}).
*
* <p>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 <V> ListenableFuture<List<V>> successfulAsList(Iterable<? extends ListenableFuture<? extends V>> futures) {
return new ListFuture<V>(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.
*
* <p>Cancelling a delegate future has no effect on any input future, since the delegate future
* does not correspond to a specific input future until the appropriate number of input futures
* have completed. At that point, it is too late to cancel the input future. The input future's
* result, which cannot be stored into the cancelled delegate future, is ignored.
*
* @since 17.0
*/
@Beta
@GwtIncompatible // TODO
public static <T> ImmutableList<ListenableFuture<T>> inCompletionOrder(Iterable<? extends ListenableFuture<? extends T>> futures) {
// A CLQ may be overkill here. We could save some pointers/memory by synchronizing on an
// ArrayDeque
final ConcurrentLinkedQueue<SettableFuture<T>> delegates = Queues.newConcurrentLinkedQueue();
ImmutableList.Builder<ListenableFuture<T>> listBuilder = ImmutableList.builder();
// Using SerializingExecutor here will ensure that each CompletionOrderListener executes
// atomically and therefore that each returned future is guaranteed to be in completion order.
// N.B. there are some cases where the use of this executor could have possibly surprising
// effects when input futures finish at approximately the same time _and_ the output futures
// have directExecutor listeners. In this situation, the listeners may end up running on a
// different thread than if they were attached to the corresponding input future. We believe
// this to be a negligible cost since:
// 1. Using the directExecutor implies that your callback is safe to run on any thread.
// 2. This would likely only be noticeable if you were doing something expensive or blocking on
// a directExecutor listener on one of the output futures which is an antipattern anyway.
SerializingExecutor executor = new SerializingExecutor(directExecutor());
for (final ListenableFuture<? extends T> future : futures) {
SettableFuture<T> delegate = SettableFuture.create();
// Must make sure to add the delegate to the queue first in case the future is already done
delegates.add(delegate);
future.addListener(
new Runnable() {
@Override
public void run() {
delegates.remove().setFuture(future);
}
},
executor);
listBuilder.add(delegate);
}
return listBuilder.build();
}
/**
* 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.
*
* <p>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: <pre> {@code
* ListenableFuture<QueryResult> future = ...;
* addCallback(future,
* new FutureCallback<QueryResult>() {
* public void onSuccess(QueryResult result) {
* storeInCache(result);
* }
* public void onFailure(Throwable t) {
* reportError(t);
* }
* });}</pre>
*
* <p>This overload, which does not accept an executor, uses {@code directExecutor}, a dangerous
* choice in some cases. See the discussion in the {@link ListenableFuture#addListener
* ListenableFuture.addListener} documentation.
*
* <p>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.
* @since 10.0
*/
public static <V> void addCallback(ListenableFuture<V> future, FutureCallback<? super V> callback) {
addCallback(future, callback, directExecutor());
}
/**
* 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.
*
* <p>The callback is run in {@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: <pre> {@code
* ListenableFuture<QueryResult> future = ...;
* Executor e = ...
* addCallback(future,
* new FutureCallback<QueryResult>() {
* public void onSuccess(QueryResult result) {
* storeInCache(result);
* }
* public void onFailure(Throwable t) {
* reportError(t);
* }
* }, e);}</pre>
*
* <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
* the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
* documentation.
*
* <p>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 <V> void addCallback(
final ListenableFuture<V> future,
final FutureCallback<? super V> callback,
Executor executor) {
Preconditions.checkNotNull(callback);
Runnable callbackListener = new Runnable() {
@Override
public void run() {
final V value;
try {
value = getDone(future);
} catch (ExecutionException e) {
callback.onFailure(e.getCause());
return;
} catch (RuntimeException e) {
callback.onFailure(e);
return;
} catch (Error e) {
callback.onFailure(e);
return;
}
callback.onSuccess(value);
}
};
future.addListener(callbackListener, executor);
}
/**
* Returns the result of the input {@code Future}, which must have already completed.
*
* <p>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) 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}.
*
* <p>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> V getDone(Future<V> 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.
*
* <p>Exceptions from {@code Future.get} are treated as follows:
* <ul>
* <li>Any {@link ExecutionException} has its <i>cause</i> 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}.
* <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
* interrupt).
* <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
* RuntimeException} (though {@code get} implementations are discouraged from throwing such
* exceptions).
* </ul>
*
* <p>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.
*
* <p>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, X extends Exception> V getChecked(Future<V> future, Class<X> 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.
*
* <p>Exceptions from {@code Future.get} are treated as follows:
* <ul>
* <li>Any {@link ExecutionException} has its <i>cause</i> 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}.
* <li>Any {@link InterruptedException} is wrapped in an {@code X} (after restoring the
* interrupt).
* <li>Any {@link TimeoutException} is wrapped in an {@code X}.
* <li>Any {@link CancellationException} is propagated untouched, as is any other {@link
* RuntimeException} (though {@code get} implementations are discouraged from throwing such
* exceptions).
* </ul>
*
* <p>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.
*
* <p>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
public static <V, X extends Exception> V getChecked(Future<V> future, Class<X> 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}.
*
* <p>Exceptions from {@code Future.get} are treated as follows:
* <ul>
* <li>Any {@link ExecutionException} has its <i>cause</i> wrapped in an {@link
* UncheckedExecutionException} (if the cause is an {@code Exception}) or {@link
* ExecutionError} (if the cause is an {@code Error}).
* <li>Any {@link InterruptedException} causes a retry of the {@code get} call. The interrupt is
* restored before {@code getUnchecked} returns.
* <li>Any {@link CancellationException} is propagated untouched. So is any other {@link
* RuntimeException} ({@code get} implementations are discouraged from throwing such
* exceptions).
* </ul>
*
* <p>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}.
*
* <p>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
@GwtIncompatible // TODO
public static <V> V getUnchecked(Future<V> future) {
checkNotNull(future);
try {
return getUninterruptibly(future);
} catch (ExecutionException e) {
wrapAndThrowUnchecked(e.getCause());
throw new AssertionError();
}
}
@GwtIncompatible // TODO
private static void wrapAndThrowUnchecked(Throwable cause) {
if (cause instanceof Error) {
throw new ExecutionError((Error) cause);
}
/*
* 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<V, X extends Exception> extends AbstractCheckedFuture<V, X> {
final Function<? super Exception, X> mapper;
MappingCheckedFuture(ListenableFuture<V> delegate, Function<? super Exception, X> mapper) {
super(delegate);
this.mapper = checkNotNull(mapper);
}
@Override
protected X mapException(Exception e) {
return mapper.apply(e);
}
}
}
