Guicing up the Play Framework: Dependency Injection with Guice and Play

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:

	name := “sample-play-with-guice”
	version := “1.0-SNAPSHOT”
	libraryDependencies ++= Seq(
	javaJdbc,
	javaEbean,
	cache,
	“com.google.inject” % “guice” % “4.0-beta”
	)
	play.Project.playJavaSettings

• 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

	package services;
	public class RealGreetingService implements GreetingService {
	@Override
	public String greeting() {
	return “bonjour”;
	}
	}

• 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.

	package controllers;
	import com.google.inject.Inject;
	import play.mvc.Controller;
	import play.mvc.Result;
	import services.GreetingService;
	import views.html.index.*;
	public class Application extends Controller {
	@Inject
	private GreetingService greetingService;
	public Result index() {
	return ok(index.render(greetingService.greeting()));
	}
	}

• Go to your routes file and put an @ in front of the index route to indicate that it is no longer static.

	# Routes
	# This file defines all application routes (Higher priority routes first)
	# ~~~~
	# Home page
	GET / @controllers.Application.index()
	# Map static resources from the /public folder to the /assets URL path
	GET /assets/*file controllers.Assets.at(path=”/public”, file)

• 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.

	import com.google.inject.AbstractModule;
	import com.google.inject.Guice;
	import com.google.inject.Injector;
	import play.Application;
	import play.GlobalSettings;
	import services.GreetingService;
	import services.RealGreetingService;
	public class Global extends GlobalSettings {
	private Injector injector;
	@Override
	public void onStart(Application application) {
	injector = Guice.createInjector(new AbstractModule() {
	@Override
	protected void configure() {
	bind(GreetingService.class).to(RealGreetingService.class);
	}
	});
	}
	@Override
	public <T> T getControllerInstance(Class<T> aClass) throws Exception {
	return injector.getInstance(aClass);
	}
	}

• If this has all worked, you should be able to run the application and see the message by visiting http://localhos:9000.

$ play run

Your greeting should appear like the screenshot below.

I’ve uploaded the sample application here.