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.

Play screenshot

I’ve uploaded the sample application here.

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