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

September 24, 2013
Play! Play Guice Dependency Injection Modules

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

package services;

public interface GreetingService {
    String greeting();
}

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://localhost:9000.
```
$ play run
```
Your greeting should appear like the screenshot below.

I’ve upload the sample application here on https://github.com/codingricky/sample-play-with-guice.

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