Use of @Singleton in Scala play framework

Question

When defining Scala controller it to mark a class as singleton use the @Singleton annotation :

@Singleton
class Application

https://docs.oracle.com/javaee/7/api/javax/inject/Singleton.html defines singleton as 'Identifies a type that the injector only instantiates once. Not inherited.' so is Scala play dependency injection framework relying on Java dependency injection ?

From https://www.playframework.com/documentation/2.5.x/ScalaDependencyInjection 'Play supports both runtime dependency injection based on JSR 330 (described in this page) and compile time dependency injection in Scala.' Is using @Singleton utilizing 'dependency injection based on JSR 330' so to use 'compile time dependency injection in Scala' what is required ?

suish suish · Accepted Answer · 2016-05-23T01:34:59

is Scala play dependency injection framework relying on Java dependency injection ?

Yes,So You need to write import javax.inject._ everyfile you use DI.

What you need to do basically is

・ Define interface as trait

trait FooService {
  def getBar(baz: String):Future[Bar]
}

・ Implement the interface

class FooServiceImpl extends FooService {
  def getBar(baz: String) = ???
}

・ Bind them via Module.scala(guice style)

class Module extends AbstractModule {
  override def configure() = {
    bind(classOf[FooService]).to(classOf[FooServiceImpl])
  }
}

・Use it

class FooController @Inject()(fooService: FooService)(implicit exec: ExecutionContext) extends Controller {
  def index = Action.async = {
    fooService.getBar("fooBar").map{_.doWhatEverYouWant}
    .....
  }
}

As You can see, You need to define class parameters when you use this way of DI.This is the reason why You cannot use Scala object and use @Singleton instead.

Use of @Singleton in Scala play framework

2 Answers