262 lines
11 KiB
ReStructuredText
262 lines
11 KiB
ReStructuredText
.. _futures-java:
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Futures
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===============
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Introduction
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------------
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In the Scala Standard Library, a `Future <http://en.wikipedia.org/wiki/Futures_and_promises>`_ is a data structure
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used to retrieve the result of some concurrent operation. This result can be accessed synchronously (blocking)
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or asynchronously (non-blocking). To be able to use this from Java, Akka provides a java friendly interface
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in ``akka.dispatch.Futures``.
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See also :ref:`actor-java-lambda` for Java compatibility.
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Execution Contexts
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------------------
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In order to execute callbacks and operations, Futures need something called an ``ExecutionContext``,
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which is very similar to a ``java.util.concurrent.Executor``. if you have an ``ActorSystem`` in scope,
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it will use its default dispatcher as the ``ExecutionContext``, or you can use the factory methods provided
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by the ``ExecutionContexts`` class to wrap ``Executors`` and ``ExecutorServices``, or even create your own.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports1,imports7
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: diy-execution-context
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Use with Actors
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---------------
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There are generally two ways of getting a reply from an ``UntypedActor``: the first is by a sent message (``actorRef.tell(msg, sender)``),
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which only works if the original sender was an ``UntypedActor``) and the second is through a ``Future``.
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Using the ``ActorRef``\'s ``ask`` method to send a message will return a ``Future``.
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To wait for and retrieve the actual result the simplest method is:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports1
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: ask-blocking
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This will cause the current thread to block and wait for the ``UntypedActor`` to 'complete' the ``Future`` with it's reply.
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Blocking is discouraged though as it can cause performance problem.
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The blocking operations are located in ``Await.result`` and ``Await.ready`` to make it easy to spot where blocking occurs.
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Alternatives to blocking are discussed further within this documentation.
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Also note that the ``Future`` returned by an ``UntypedActor`` is a ``Future<Object>`` since an ``UntypedActor`` is dynamic.
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That is why the cast to ``String`` is used in the above sample.
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.. warning::
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``Await.result`` and ``Await.ready`` are provided for exceptional situations where you **must** block,
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a good rule of thumb is to only use them if you know why you **must** block. For all other cases, use
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asynchronous composition as described below.
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To send the result of a ``Future`` to an ``Actor``, you can use the ``pipe`` construct:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: pipe-to
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Use Directly
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------------
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A common use case within Akka is to have some computation performed concurrently without needing
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the extra utility of an ``UntypedActor``. If you find yourself creating a pool of ``UntypedActor``\s for the sole reason
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of performing a calculation in parallel, there is an easier (and faster) way:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports2
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: future-eval
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In the above code the block passed to ``future`` will be executed by the default ``Dispatcher``,
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with the return value of the block used to complete the ``Future`` (in this case, the result would be the string: "HelloWorld").
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Unlike a ``Future`` that is returned from an ``UntypedActor``, this ``Future`` is properly typed,
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and we also avoid the overhead of managing an ``UntypedActor``.
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You can also create already completed Futures using the ``Futures`` class, which can be either successes:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: successful
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Or failures:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: failed
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It is also possible to create an empty ``Promise``, to be filled later, and obtain the corresponding ``Future``:
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.. includecode:: code/docs/future/FutureDocTest.java#promise
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For these examples ``PrintResult`` is defined as follows:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: print-result
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Functional Futures
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------------------
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Scala's ``Future`` has several monadic methods that are very similar to the ones used by ``Scala``'s collections.
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These allow you to create 'pipelines' or 'streams' that the result will travel through.
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Future is a Monad
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^^^^^^^^^^^^^^^^^
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The first method for working with ``Future`` functionally is ``map``. This method takes a ``Mapper`` which performs
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some operation on the result of the ``Future``, and returning a new result.
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The return value of the ``map`` method is another ``Future`` that will contain the new result:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports2
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: map
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In this example we are joining two strings together within a ``Future``. Instead of waiting for f1 to complete,
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we apply our function that calculates the length of the string using the ``map`` method.
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Now we have a second ``Future``, f2, that will eventually contain an ``Integer``.
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When our original ``Future``, f1, completes, it will also apply our function and complete the second ``Future``
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with its result. When we finally ``get`` the result, it will contain the number 10.
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Our original ``Future`` still contains the string "HelloWorld" and is unaffected by the ``map``.
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Something to note when using these methods: passed work is always dispatched on the provided ``ExecutionContext``. Even if
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the ``Future`` has already been completed, when one of these methods is called.
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Composing Futures
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^^^^^^^^^^^^^^^^^
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It is very often desirable to be able to combine different Futures with each other,
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below are some examples on how that can be done in a non-blocking fashion.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports3
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: sequence
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To better explain what happened in the example, ``Future.sequence`` is taking the ``Iterable<Future<Integer>>``
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and turning it into a ``Future<Iterable<Integer>>``. We can then use ``map`` to work with the ``Iterable<Integer>`` directly,
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and we aggregate the sum of the ``Iterable``.
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The ``traverse`` method is similar to ``sequence``, but it takes a sequence of ``A`` and applies a function from ``A`` to ``Future<B>``
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and returns a ``Future<Iterable<B>>``, enabling parallel ``map`` over the sequence, if you use ``Futures.future`` to create the ``Future``.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports4
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: traverse
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It's as simple as that!
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Then there's a method that's called ``fold`` that takes a start-value,
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a sequence of ``Future``:s and a function from the type of the start-value, a timeout,
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and the type of the futures and returns something with the same type as the start-value,
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and then applies the function to all elements in the sequence of futures, non-blockingly,
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the execution will be started when the last of the Futures is completed.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports5
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: fold
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That's all it takes!
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If the sequence passed to ``fold`` is empty, it will return the start-value, in the case above, that will be empty String.
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In some cases you don't have a start-value and you're able to use the value of the first completing ``Future``
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in the sequence as the start-value, you can use ``reduce``, it works like this:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports6
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: reduce
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Same as with ``fold``, the execution will be started when the last of the Futures is completed, you can also parallelize
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it by chunking your futures into sub-sequences and reduce them, and then reduce the reduced results again.
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This is just a sample of what can be done.
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Callbacks
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---------
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Sometimes you just want to listen to a ``Future`` being completed, and react to that not by creating a new Future, but by side-effecting.
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For this Scala supports ``onComplete``, ``onSuccess`` and ``onFailure``, of which the last two are specializations of the first.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: onSuccess
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: onFailure
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: onComplete
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Ordering
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--------
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Since callbacks are executed in any order and potentially in parallel,
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it can be tricky at the times when you need sequential ordering of operations.
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But there's a solution! And it's name is ``andThen``, and it creates a new ``Future`` with
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the specified callback, a ``Future`` that will have the same result as the ``Future`` it's called on,
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which allows for ordering like in the following sample:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: and-then
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Auxiliary methods
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-----------------
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``Future`` ``fallbackTo`` combines 2 Futures into a new ``Future``, and will hold the successful value of the second ``Future``
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if the first ``Future`` fails.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: fallback-to
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You can also combine two Futures into a new ``Future`` that will hold a tuple of the two Futures successful results,
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using the ``zip`` operation.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: zip
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Exceptions
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----------
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Since the result of a ``Future`` is created concurrently to the rest of the program, exceptions must be handled differently.
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It doesn't matter if an ``UntypedActor`` or the dispatcher is completing the ``Future``, if an ``Exception`` is caught
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the ``Future`` will contain it instead of a valid result. If a ``Future`` does contain an ``Exception``,
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calling ``Await.result`` will cause it to be thrown again so it can be handled properly.
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It is also possible to handle an ``Exception`` by returning a different result.
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This is done with the ``recover`` method. For example:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: recover
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In this example, if the actor replied with a ``akka.actor.Status.Failure`` containing the ``ArithmeticException``,
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our ``Future`` would have a result of 0. The ``recover`` method works very similarly to the standard try/catch blocks,
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so multiple ``Exception``\s can be handled in this manner, and if an ``Exception`` is not handled this way
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it will behave as if we hadn't used the ``recover`` method.
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You can also use the ``recoverWith`` method, which has the same relationship to ``recover`` as ``flatMap`` has to ``map``,
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and is use like this:
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: try-recover
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After
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-----
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``akka.pattern.Patterns.after`` makes it easy to complete a ``Future`` with a value or exception after a timeout.
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: imports8
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.. includecode:: code/docs/future/FutureDocTest.java
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:include: after
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