651f699893
- akka.Main will start an ActorSystem and one actor; when that actor terminates the system is shut down - HelloWorld sample with two actors
44 lines
2.3 KiB
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44 lines
2.3 KiB
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##########################
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The Obligatory Hello World
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##########################
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Since every programming paradigm needs to solve the tough problem of printing a
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well-known greeting to the console we’ll introduce you to the actor-based
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version.
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.. includecode:: ../scala/code/docs/actor/IntroDocSpec.scala#hello-world
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The ``HelloWorld`` actor is the application’s “main” class; when it terminates
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the application will shut down—more on that later. The main business logic
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happens in the :meth:`preStart` method, where a ``Greeter`` actor is created
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and instructed to issue that greeting we crave for. When the greeter is done it
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will tell us so by sending back a message, and when that message has been
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received it will be passed into the behavior described by the :meth:`receive`
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method where we can conclude the demonstration by stopping the ``HelloWorld``
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actor. You will be very curious to see how the ``Greeter`` actor performs the
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actual task:
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.. includecode:: ../scala/code/docs/actor/IntroDocSpec.scala#greeter
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This is extremely simple now: after its creation this actor will not do
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anything until someone sends it a message, and if that happens to be an
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invitation to greet the world then the ``Greeter`` complies and informs the
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requester that the deed has been done.
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As a Scala developer you will probably want to tell us that there is no
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``main(Array[String])`` method anywhere in these classes, so how do we run this
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program? The answer is that the appropriate :meth:`main` method is implemented
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in the generic launcher class :class:`akka.Main` which expects only one command
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line argument: the class name of the application’s main actor. This main method
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will then create the infrastructure needed for running the actors, start the
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given main actor and arrange for the whole application to shut down once the
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main actor terminates. Thus you will be able to run the above code with a
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command similar to the following::
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java -classpath <all those JARs> akka.Main com.example.HelloWorld
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This conveniently assumes placement of the above class definitions in package
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``com.example`` and it further assumes that you have the required JAR files for
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``scala-library`` and ``akka-actor`` available. The easiest would be to manage
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these dependencies with a build tool, see :ref:`build-tool`.
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