Package com.google.common.util.concurrent

Class Futures

  • @GwtCompatible(emulated=true)
public final class Futures
extends Object
    Static utility methods pertaining to the Future interface.

    Many of these methods use the ListenableFuture API; consult the Guava User Guide article on ListenableFuture.

    The main purpose of ListenableFuture is to help you chain together a graph of asynchronous operations. You can chain them together manually with calls to methods like 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:

    If you do chain your operations manually, you may want to use FluentFuture.

    Since:
    1.0
    Author:
    Kevin Bourrillion, Nishant Thakkar, Sven Mawson

    • Method Detail

      • immediateFuture

        public static <V> ListenableFuture<V> immediateFuture​(@NullableDecl
                                                      V value)
        Creates a ListenableFuture which has its value set immediately upon construction. The getters just return the value. This Future can't be canceled or timed out and its isDone() method always returns true.

      • immediateVoidFuture

        public static ListenableFuture<Void> immediateVoidFuture()
        Returns a successful ListenableFuture<Void>. This method is equivalent to immediateFuture(null) except that it is restricted to produce futures of type Void.
        Since:
        29.0

      • immediateFailedFuture

        public static <V> ListenableFuture<V> immediateFailedFuture​(Throwable throwable)
        Returns a ListenableFuture which has an exception set immediately upon construction.

        The returned Future can't be cancelled, and its isDone() method always returns true. Calling get() will immediately throw the provided Throwable wrapped in an ExecutionException.

      • immediateCancelledFuture

        public static <V> ListenableFuture<V> immediateCancelledFuture()
        Creates a ListenableFuture which is cancelled immediately upon construction, so that isCancelled() always returns true.
        Since:
        14.0

      • catching

        @Beta
@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)
        Returns a Future whose result is taken from the given primary input or, if the primary input fails with the given exceptionType, from the result provided by the fallback. Function.apply(F) is not invoked until the primary input has failed, so if the primary input succeeds, it is never invoked. If, during the invocation of fallback, an exception is thrown, this exception is used as the result of the output Future.

        Usage example: 

        
 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, x -> 0, directExecutor());

        When selecting an executor, note that directExecutor is dangerous in some cases. See the warnings the MoreExecutors.directExecutor() documentation.

        Parameters:
        input - the primary input Future
        exceptionType - the exception type that triggers use of fallback. The exception type is matched against the input's exception. "The input's exception" means the cause of the ExecutionException thrown by input.get() or, if 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 Throwable.class in particular.
        fallback - the Function to be called if 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 ExecutionException thrown by input.get() or, if get() throws a different kind of exception, that exception itself.
        executor - the executor that runs fallback if input fails
        Since:
        19.0

      • catchingAsync

        @Beta
@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)
        Returns a Future whose result is taken from the given primary input or, if the primary input fails with the given exceptionType, from the result provided by the fallback. AsyncFunction.apply(I) is not invoked until the primary input has failed, so if the primary input succeeds, it is never invoked. If, during the invocation of fallback, an exception is thrown, this exception is used as the result of the output Future.

        Usage examples: 

        
 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, x -> immediateFuture(0), directExecutor());

        The fallback can also choose to propagate the original exception when desired: 

        
 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,
     e -> {
       if (omitDataOnFetchFailure) {
         return immediateFuture(0);
       }
       throw e;
     },
     directExecutor());

        When selecting an executor, note that directExecutor is dangerous in some cases. See the warnings the MoreExecutors.directExecutor() documentation.

        Parameters:
        input - the primary input Future
        exceptionType - the exception type that triggers use of fallback. The exception type is matched against the input's exception. "The input's exception" means the cause of the ExecutionException thrown by input.get() or, if 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 Throwable.class in particular.
        fallback - the AsyncFunction to be called if 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 ExecutionException thrown by input.get() or, if get() throws a different kind of exception, that exception itself.
        executor - the executor that runs fallback if input fails
        Since:
        19.0 (similar functionality in 14.0 as withFallback)

      • transformAsync

        @Beta
public static <I,​O> ListenableFuture<O> transformAsync​(ListenableFuture<I> input,
                                                             AsyncFunction<? super I,​? extends O> function,
                                                             Executor executor)
        Returns a new Future whose result is asynchronously derived from the result of the given Future. If the given Future fails, the returned Future fails with the same exception (and the function is not invoked).

        More precisely, the returned Future takes its result from a Future produced by applying the given AsyncFunction to the result of the original Future. Example usage: 

        
 ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
 ListenableFuture<QueryResult> queryFuture =
     transformAsync(rowKeyFuture, dataService::readFuture, executor);

        When selecting an executor, note that directExecutor is dangerous in some cases. See the warnings the MoreExecutors.directExecutor() documentation.

        The returned 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 Future is cancelled, it will attempt to cancel the other two, and if either of the other two is cancelled, the returned Future will receive a callback in which it will attempt to cancel itself.

        Parameters:
        input - The future to transform
        function - A function to transform the result of the input future to the result of the output future
        executor - Executor to run the function in.
        Returns:
        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 transform)

      • transform

        @Beta
public static <I,​O> ListenableFuture<O> transform​(ListenableFuture<I> input,
                                                        Function<? super I,​? extends O> function,
                                                        Executor executor)
        Returns a new Future whose result is derived from the result of the given Future. If input fails, the returned Future fails with the same exception (and the function is not invoked). Example usage: 
        
 ListenableFuture<QueryResult> queryFuture = ...;
 ListenableFuture<List<Row>> rowsFuture =
     transform(queryFuture, QueryResult::getRows, executor);

        When selecting an executor, note that directExecutor is dangerous in some cases. See the warnings the MoreExecutors.directExecutor() documentation.

        The returned Future attempts to keep its cancellation state in sync with that of the input future. That is, if the returned Future is cancelled, it will attempt to cancel the input, and if the input is cancelled, the returned 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.

        Parameters:
        input - The future to transform
        function - A Function to transform the results of the provided future to the results of the returned future.
        executor - Executor to run the function in.
        Returns:
        A future that holds result of the transformation.
        Since:
        9.0 (in 2.0 as compose)

      • lazyTransform

        @Beta
@GwtIncompatible
public static <I,​O> Future<O> lazyTransform​(Future<I> input,
                                                  Function<? super I,​? extends O> function)
        Like transform(ListenableFuture, Function, Executor) except that the transformation function is invoked on each call to get() on the returned future.

        The returned 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 timed get only apply the timeout to the execution of the underlying Future, not to the execution of the transformation function.

        The primary audience of this method is callers of transform who don't have a ListenableFuture available and do not mind repeated, lazy function evaluation.

        Parameters:
        input - The future to transform
        function - A Function to transform the results of the provided future to the results of the returned future.
        Returns:
        A future that returns the result of the transformation.
        Since:
        10.0

      • allAsList

        @Beta
@SafeVarargs
public static <V> ListenableFuture<List<V>> allAsList​(ListenableFuture<? extends V>... futures)
        Creates a new 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.

        This differs from successfulAsList(ListenableFuture[]) in that it will return a failed future if any of the items fails.

        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.

        Parameters:
        futures - futures to combine
        Returns:
        a future that provides a list of the results of the component futures
        Since:
        10.0

      • allAsList

        @Beta
public static <V> ListenableFuture<List<V>> allAsList​(Iterable<? extends ListenableFuture<? extends V>> futures)
        Creates a new 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.

        This differs from successfulAsList(Iterable) in that it will return a failed future if any of the items fails.

        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.

        Parameters:
        futures - futures to combine
        Returns:
        a future that provides a list of the results of the component futures
        Since:
        10.0

      • nonCancellationPropagating

        @Beta
public static <V> ListenableFuture<V> nonCancellationPropagating​(ListenableFuture<V> future)
        Returns a 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

      • successfulAsList

        @Beta
@SafeVarargs
public static <V> ListenableFuture<List<V>> successfulAsList​(ListenableFuture<? extends V>... futures)
        Creates a new 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 null (which is indistinguishable from the future having a successful value of null).

        The list of results is in the same order as the input list.

        This differs from allAsList(ListenableFuture[]) in that it's tolerant of failed futures for any of the items, representing them as null in the result list.

        Canceling this future will attempt to cancel all the component futures.

        Parameters:
        futures - futures to combine
        Returns:
        a future that provides a list of the results of the component futures
        Since:
        10.0

      • successfulAsList

        @Beta
public static <V> ListenableFuture<List<V>> successfulAsList​(Iterable<? extends ListenableFuture<? extends V>> futures)
        Creates a new 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 null (which is indistinguishable from the future having a successful value of null).

        The list of results is in the same order as the input list.

        This differs from allAsList(Iterable) in that it's tolerant of failed futures for any of the items, representing them as null in the result list.

        Canceling this future will attempt to cancel all the component futures.

        Parameters:
        futures - futures to combine
        Returns:
        a future that provides a list of the results of the component futures
        Since:
        10.0

      • inCompletionOrder

        @Beta
public static <T> ImmutableList<ListenableFuture<T>> inCompletionOrder​(Iterable<? extends ListenableFuture<? extends T>> futures)
        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

      • addCallback

        public static <V> void addCallback​(ListenableFuture<V> future,
                                   FutureCallback<? super V> callback,
                                   Executor executor)
        Registers separate success and failure callbacks to be run when the Future's computation is complete or, if the computation is already complete, immediately.

        The callback is run on 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.

        Exceptions thrown by a callback will be propagated up to the executor. Any exception thrown during Executor.execute (e.g., a RejectedExecutionException or an exception thrown by direct execution) will be caught and logged.

        Example: 

        
 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);

        When selecting an executor, note that directExecutor is dangerous in some cases. See the warnings the MoreExecutors.directExecutor() documentation.

        For a more general interface to attach a completion listener to a Future, see addListener.

        Parameters:
        future - The future attach the callback to.
        callback - The callback to invoke when future is completed.
        executor - The executor to run callback when the future completes.
        Since:
        10.0

      • getDone

        @CanIgnoreReturnValue
public static <V> V getDone​(Future<V> future)
                     throws ExecutionException
        Returns the result of the input Future, which must have already completed.

        The benefits of this method are twofold. First, the name "getDone" suggests to readers that the Future is already done. Second, if buggy code calls getDone on a Future that is still pending, the program will throw instead of block. This can be important for APIs like whenAllComplete(...).call(...), where it is easy to use a new input from the call implementation but forget to add it to the arguments of whenAllComplete.

        If you are looking for a method to determine whether a given Future is done, use the instance method Future.isDone().

        Throws:
        ExecutionException - if the Future failed with an exception
        CancellationException - if the Future was cancelled
        IllegalStateException - if the Future is not done
        Since:
        20.0

      • getChecked

        @Beta
@CanIgnoreReturnValue
@GwtIncompatible
public static <V,​X extends Exception> V getChecked​(Future<V> future,
                                                         Class<X> exceptionClass)
                                                  throws X extends Exception
        Returns the result of Future.get(), converting most exceptions to a new instance of the given checked exception type. This reduces boilerplate for a common use of Future in which it is unnecessary to programmatically distinguish between exception types or to extract other information from the exception instance.

        Exceptions from Future.get are treated as follows:

        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 ExecutionException is wrapped in order to ensure that the new stack trace matches that of the current thread.

        Instances of exceptionClass are created by choosing an arbitrary public constructor that accepts zero or more arguments, all of type String or Throwable (preferring constructors with at least one String) and calling the constructor via reflection. If the exception did not already have a cause, one is set by calling Throwable.initCause(Throwable) on it. If no such constructor exists, an IllegalArgumentException is thrown.

        Throws:
        X - if get throws any checked exception except for an ExecutionException whose cause is not itself a checked exception
        UncheckedExecutionException - if get throws an ExecutionException with a RuntimeException as its cause
        ExecutionError - if get throws an ExecutionException with an Error as its cause
        CancellationException - if get throws a CancellationException
        IllegalArgumentException - if exceptionClass extends RuntimeException or does not have a suitable constructor
        X extends Exception
        Since:
        19.0 (in 10.0 as get)

      • getChecked

        @Beta
@CanIgnoreReturnValue
@GwtIncompatible
public static <V,​X extends Exception> V getChecked​(Future<V> future,
                                                         Class<X> exceptionClass,
                                                         long timeout,
                                                         TimeUnit unit)
                                                  throws X extends Exception
        Returns the result of 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 Future in which it is unnecessary to programmatically distinguish between exception types or to extract other information from the exception instance.

        Exceptions from Future.get are treated as follows:

        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 ExecutionException is wrapped in order to ensure that the new stack trace matches that of the current thread.

        Instances of exceptionClass are created by choosing an arbitrary public constructor that accepts zero or more arguments, all of type String or Throwable (preferring constructors with at least one String) and calling the constructor via reflection. If the exception did not already have a cause, one is set by calling Throwable.initCause(Throwable) on it. If no such constructor exists, an IllegalArgumentException is thrown.

        Throws:
        X - if get throws any checked exception except for an ExecutionException whose cause is not itself a checked exception
        UncheckedExecutionException - if get throws an ExecutionException with a RuntimeException as its cause
        ExecutionError - if get throws an ExecutionException with an Error as its cause
        CancellationException - if get throws a CancellationException
        IllegalArgumentException - if exceptionClass extends RuntimeException or does not have a suitable constructor
        X extends Exception
        Since:
        19.0 (in 10.0 as get and with different parameter order)

      • getUnchecked

        @CanIgnoreReturnValue
public static <V> V getUnchecked​(Future<V> future)
        Returns the result of calling Future.get() uninterruptibly on a task known not to throw a checked exception. This makes 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 ForkJoinTask.join.

        Exceptions from Future.get are treated as follows:

        The overall principle is to eliminate all checked exceptions: to loop to avoid InterruptedException, to pass through CancellationException, and to wrap any exception from the underlying computation in an UncheckedExecutionException or ExecutionError.

        For an uninterruptible get that preserves other exceptions, see Uninterruptibles.getUninterruptibly(Future).

        Throws:
        UncheckedExecutionException - if get throws an ExecutionException with an Exception as its cause
        ExecutionError - if get throws an ExecutionException with an Error as its cause
        CancellationException - if get throws a CancellationException
        Since:
        10.0