How to bind Class with Google Guice using a FQDN String - scala

Question

I'm using Play 2.5 with the Guice dependency injection.
Normal bind works like:
bind(classOf[SomeClass]).to(classOf[DefaultClass])
I now only need to be able to bind a class where the className has been specified in the Configuration.
I tried something like:
val className = config.getString("someClass.className")
val x: Class[_] = Class.forName(className)
bind(classOf[SomeClass]).to(classOf[x])
But then the types are wrong.
It needs to be done via Guice, since the SomeClass has an argument that needs to be injected, otherwise I would have used something like
val className = config.getString("someClass.className")
val x = Class.forName(className).newInstance().asInstanceOf[SomeClass]
bind(classOf[SomeClass]).toInstance(x)
Does anybody have an idea on how to bind it via guice?

Answer 1

I think you are looking for something like this...
lazy val injector = (new GuiceApplicationBuilder).injector()
def inject[T : ClassTag]: T = {
injector.instanceOf[T]
}
This is the simplest version, but doesn't handle arguments. You would create a class and then call this as inject[SomeDep].
I've not found a good way of injecting on-the-fly, the only way of really doing it via #inject at the top most invoked class. We only use inject for unit tests at present.

Answer 2

Thanks for the reply, but I finally got it to work using Scala's Reflection.
object Reflection {
import scala.reflect.api
import reflect.runtime.universe._
import reflect.ClassTag
def classTagToClass[T: reflect.ClassTag]: Class[T] = {
def ctag = implicitly[reflect.ClassTag[T]]
ctag.runtimeClass.asInstanceOf[Class[T]]
}
def typeToClassTag[T: TypeTag]: ClassTag[T] = {
ClassTag[T]( typeTag[T].mirror.runtimeClass( typeTag[T].tpe ) )
}
def stringToTypeTag[A](name: String): TypeTag[A] = {
val c = Class.forName(name) // obtain java.lang.Class object from a string
val mirror = runtimeMirror(c.getClassLoader) // obtain runtime mirror
val sym = mirror.staticClass(name) // obtain class symbol for `c`
val tpe = sym.selfType // obtain type object for `c`
// create a type tag which contains above type object
TypeTag(mirror, new api.TypeCreator {
def apply[U <: api.Universe with Singleton](m: api.Mirror[U]): U#Type =
if (m eq mirror) {
tpe.asInstanceOf[U#Type]
}
else {
throw new IllegalArgumentException(s"Type tag defined in $mirror cannot be migrated to other mirrors.")
}
})
}
}
using the above object you can bind a class using it FQDN in the following way:
configuration.getString("config.className")
.map(className =>
bind(classOf[AbstractClass]).to(classTagToClass(typeToClassTag(stringToTypeTag[AbstractClass](className))))
).getOrElse(bind(classOf[AbstractClass]).to(classOf[AbstractClassImpl]))