One of the features lacking out of the box for Play is Dependency Injection. Coming from a background of JEE, this seemed to be quite a significant omission.
Fortunately, it is not very difficult to set it up. This article will take you through the steps of adding Guice to your Play application.
The first step is to add the Guice dependency to your build.sbt file or Build.scala (if you’re using Play 2.1.X or lower). Your build.sbt should look like this:
Now let’s create a simple service that we will be injecting into our controller. First create an interface at the path app/services/GreetingService.java.
Followed up by its implementation here: app/services/RealGreetingService.java
Now let’s go to the Application controller and inject the GreetingService into it. The key things here are the instance variable with the @Inject annotation and the controller index method not being static anymore, as it needs access to the greetingService instance variable.
Go to your routes file and put an @ in front of the index route to indicate that it is no longer static.
Lastly, create a Global.java class at app/Global.java. Create an injector and override the getControllerInstance method to return instances from the injector. When a route has a prefix of @, Play will call this method.
If this has all worked, you should be able to run the application and see the message by visiting http://localhos:9000.
Your greeting should appear like the screenshot below.
I’ve uploaded the sample application here.