How do I call a Scala Object method using reflection?

reflection scala object

19,870

Solution 1

def companion[T](name : String)(implicit man: Manifest[T]) : T = 
    Class.forName(name + "$").getField("MODULE$").get(man.erasure).asInstanceOf[T]

val result = companion[SomeTrait]("SomeObject").someMethod

Solution 2

Since scala 2.10, we can use the reflection of Module:

import scala.reflect.runtime.universe

val runtimeMirror = universe.runtimeMirror(getClass.getClassLoader)
val module = runtimeMirror.staticModule("SomeObject")
val obj = runtimeMirror.reflectModule(module)
val someTrait:SomeTrait = obj.instance.asInstanceOf[SomeTrait]  
someTrait.someMethod

Solution 3

For classes, this can be done pretty easily using the standard java reflection classOf method. For Scala objects, it is a bit more work, but it still can be done:


trait SomeTrait { def someMethod: String}
object SomeObject extends SomeTrait { def someMethod = "something"}

class SomeClass extends SomeTrait { def someMethod = "something"}

object Main {
 def main(args:Array[String]) = {
    val someClassTrait:SomeTrait = Class.forName("SomeClass").newInstance().asInstanceOf[SomeTrait]
    println("calling someClassTrait: " + someClassTrait.someMethod)
    val objectName = "SomeObject$"
    val cons = Class.forName(objectName).getDeclaredConstructors(); 
    cons(0).setAccessible(true);
    val someObjectTrait:SomeTrait = cons(0).newInstance().asInstanceOf[SomeTrait]
    println("calling someObjectTrait: " + someObjectTrait.someMethod)
  }
}

//prints:
calling someClassTrait: something
calling someObjectTrait: something

19,870

Author by

sanjib

Updated on June 07, 2022

Comments

sanjib almost 2 years

say, I have the following:

trait SomeTrait {
  def someMethod: String;
}

object SomeObject extends SomeTrait {
  def someMethod = "something";
}

I would like to call "someMethod" using reflection as I have the object name as a String. Something like:

val objectName = "SomeObject"  
val someTrait:SomeTrait = ???.asInstanceOf[SomeTrait]  
someTrait.someMethod

or something similar.

Thanks

Thomas Jung almost 14 years

I think it does break the expectation of clients of a companion (singleton) object if you create new instances of it.
sanjib almost 14 years

Arjan - thanks. That's exactly what I was looking for. Thomas - That's true. But you could always wrap the code in an object factory and provide the singleton behavior yourself.
sanjib almost 14 years

This is even better. Thanks Thomas.
0__ almost 11 years

Do you need the manifest? I found that Class.forName(name + "$").getField("MODULE$").get(null) is sufficient. This is allowed for static fields.
bhericher almost 11 years

How does it translate in 2.10 with reflection?
Kevin Meredith about 10 years

Using Scala 2.10.2, my code threw an exception on Class.forName(name + "$").getField("MODULE$")
BdEngineer over 5 years

@Thomas Jung , what is the "$" here , what it refer too , sorry me new to scala.