[Solved]-Dependency injection with abstract class and object in Play Framework 2.5-scala

This is more related to how Guice handles inheritance and you have to do exactly what you would do if you were not using Guice, which is declaring the parameters to the superclass and calling the super constructor at your child classes. Guice even suggest it at its docs:

Wherever possible, use constructor injection to create immutable objects. Immutable objects are simple, shareable, and can be composed.

Constructor injection has some limitations:

• Subclasses must call super() with all dependencies. This makes constructor injection cumbersome, especially as the injected base class changes.

In pure Java, it will means doing something like this:

public abstract class Base {

  private final Dependency dep;

  public Base(Dependency dep) {
    this.dep = dep;
  }
}

public class Child extends Base {

  private final AnotherDependency anotherDep;

  public Child(Dependency dep, AnotherDependency anotherDep) {
    super(dep); // guaranteeing that fields at superclass will be properly configured
    this.anotherDep = anotherDep;
  }
}

Dependency injection won't change that and you will just have to add the annotations to indicate how to inject the dependencies. In this case, since Base class is abstract, and then no instances of Base can be created, we may skip it and just annotate Child class:

public abstract class Base {

  private final Dependency dep;

  public Base(Dependency dep) {
    this.dep = dep;
  }
}

public class Child extends Base {

  private final AnotherDependency anotherDep;

  @Inject
  public Child(Dependency dep, AnotherDependency anotherDep) {
    super(dep); // guaranteeing that fields at superclass will be properly configured
    this.anotherDep = anotherDep;
  }
}

Translating to Scala, we will have something like this:

abstract class Base(dep: Dependency) {
  // something else
}

class Child @Inject() (anotherDep: AnotherDependency, dep: Dependency) extends Base(dep) {
  // something else
}

Now, we can rewrite your code to use this knowledge and avoid deprecated APIs:

abstract class Microservice(serviceName: String, configuration: Configuration, ws: WSClient) {
    protected lazy val serviceURL: String = configuration.getString(s"microservice.$serviceName.url")
    // ...and functions using the injected WSClient...
}

// a class instead of an object
// annotated as a Singleton
@Singleton
class HelloWorldService(configuration: Configuration, ws: WSClient)
    extends Microservice("helloWorld", configuration, ws) {
    // ...
}

The last point is the implicit ExecutionContext and here we have two options:

1. Use the default execution context, which will be play.api.libs.concurrent.Execution.Implicits.defaultContext
2. Use other thread pools

This depends on you, but you can easily inject an ActorSystem to lookup the dispatcher. If you decide to go with a custom thread pool, you can do something like this:

abstract class Microservice(serviceName: String, configuration: Configuration, ws: WSClient, actorSystem: ActorSystem) {

    // this will be available here and at the subclass too
    implicit val executionContext = actorSystem.dispatchers.lookup("my-context")

    protected lazy val serviceURL: String = configuration.getString(s"microservice.$serviceName.url")
    // ...and functions using the injected WSClient...
}

// a class instead of an object
// annotated as a Singleton
@Singleton
class HelloWorldService(configuration: Configuration, ws: WSClient, actorSystem: ActorSystem)
    extends Microservice("helloWorld", configuration, ws, actorSystem) {
    // ...
}

How to use HelloWorldService?

Now, there are two things you need to understand in order to proper inject an instance of HelloWorldService where you need it.

From where HelloWorldService gets its dependencies?

Guice docs has a good explanation about it:

Dependency Injection

Like the factory, dependency injection is just a design pattern. The core principle is to separate behaviour from dependency resolution.

The dependency injection pattern leads to code that's modular and testable, and Guice makes it easy to write. To use Guice, we first need to tell it how to map our interfaces to their implementations. This configuration is done in a Guice module, which is any Java class that implements the Module interface.

And then, Playframework declare modules for WSClient and for Configuration. Both modules gives Guice enough information about how to build these dependencies, and there are modules to describe how to build the dependencies necessary for WSClient and Configuration. Again, Guice docs has a good explanation about it:

With dependency injection, objects accept dependencies in their constructors. To construct an object, you first build its dependencies. But to build each dependency, you need its dependencies, and so on. So when you build an object, you really need to build an object graph.

In our case, for HelloWorldService, we are using constructor injection to enable Guice to set/create our object graph.

How HelloWorldService is injected?

Just like WSClient has a module to describe how an implementation is binded to an interface/trait, we can do the same for HelloWorldService. Play docs has a clear explanation about how to create and configure modules, so I won't repeat it here.

But after creating an module, to inject a HelloWorldService to your controller, you just declare it as a dependency:

class MyController @Inject() (service: Microservice) extends Controller {

    def index = Action {
        // access "service" here and do whatever you want 
    }
}