2013-11-15 11:53:21 +01:00
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<head>
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<title>The Obligatory Hello World</title>
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</head>
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<body>
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<div>
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<h2>The Obligatory Hello World</h2>
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<p>
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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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</p>
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<p>
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Open <a href="#code/src/main/scala/sample/hello/HelloWorld.scala" class="shortcut">HelloWorld.scala</a>
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</p>
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<p>
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The <code>HelloWorld</code> 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 <code>preStart</code> method, where a <code>Greeter</code> 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 <code>receive</code>
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method where we can conclude the demonstration by stopping the <code>HelloWorld</code>
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actor.
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</p>
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</div>
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<div>
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<h2>The Greeter</h2>
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<p>
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You will be very curious to see how the <code>Greeter</code> actor performs the
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actual task. Open <a href="#code/src/main/scala/sample/hello/Greeter.scala" class="shortcut">Greeter.scala</a>.
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</p>
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<p>
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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 <code>Greeter</code> complies and informs the
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requester that the deed has been done.
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</p>
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</div>
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<div>
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<h2>Main class</h2>
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<p>
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Go to the <a href="#run" class="shortcut">Run</a> tab, and start the application main class
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<code>sample.hello.Main</code>. In the log output you can see the "Hello World!" greeting.
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</p>
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<p>
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<a href="#code/src/main/scala/sample/hello/Main.scala" class="shortcut">Main.scala</a>
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is actually just a small wrapper around the generic launcher class <code>akka.Main</code>,
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which expects only one argument: the class name of the application’s main actor. This main
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method 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 application with a
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command similar to the following:
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</p>
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<pre><code>
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java -classpath <all those JARs> akka.Main sample.hello.HelloWorld
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</code></pre>
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<p>
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This conveniently assumes placement of the above class definitions in package
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<code>sample.hello</code> and it further assumes that you have the required JAR files for
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<code>scala-library</code>, <code>typesafe-config</code> and <code>akka-actor</code> available.
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The easiest would be to manage these dependencies with a
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2014-03-07 13:43:05 +01:00
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<a href="http://doc.akka.io/docs/akka/2.4-SNAPSHOT/intro/getting-started.html#Using_a_build_tool" target="_blank">build tool</a>.
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2013-11-15 11:53:21 +01:00
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</p>
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<p>
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If you need more control of the startup code than what is provided by <code>akka.Main</code>
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you can easily write your own main class such as
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<a href="#code/src/main/scala/sample/hello/Main2.scala" class="shortcut">Main2.scala</a>
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</p>
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<p>
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Try to run the <code>sample.hello.Main2</code> class
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by selecting it in the 'Main class' menu in the <a href="#run" class="shortcut">Run</a> tab.
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</p>
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</div>
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</body>
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</html>
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