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reflection - Getting object instance by string name in scala - Stack Overflow

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Clipped on: 2012-08-09

I need the object (or "singleton object" or "companion object"... anything but the class) defined by a string name. In other words, if I have:

package myPackage
object myObject

...then is there anything like this:

GetSingletonObjectByName("myPackage.myObject") match {
 
case instance: myPackage.myObject => "instance is what I wanted"
}
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asked Dec 16 '09 at 8:12
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3 Answers

up vote 6 down vote accepted

Scala is still missing a reflection API. You can get the an instance of the companion object by loading the companion object class:

import scala.reflect._
def companion[T](implicit man: Manifest[T]) : T =
  man
.erasure.getField("MODULE$").get(man.erasure).asInstanceOf[T]


scala
> companion[List$].make(3, "s")
res0
: List[Any] = List(s, s, s)

To get the untyped companion object you can use the class directly:

import scala.reflect.Manifest
def companionObj[T](implicit man: Manifest[T]) = {
 
val c = Class.forName(man.erasure.getName + "$")
  c
.getField("MODULE$").get(c)
}


scala
> companionObj[List[Int]].asInstanceOf[List$].make(3, "s")
res0
: List[Any] = List(s, s, s)

This depends on the way scala is mapped to java classes.

answered Dec 16 '09 at 8:18
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Holy cow. Do you know if this syntax is a fixed part of the Scala spec (as fixed as anything else in the language, anyway)? It seems like a bad idea to rely on this. And since my goal was to make the code clearer... Thank you! – Dave Dec 17 '09 at 11:24
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As he mentioned, there is no reflection API in Scala yet, so whether this is covered by the Scala spec or not, this is the only way for you to do it. I noticed this question / answer is over a year old, are there any news here? – pdinklag Feb 17 '11 at 5:37
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Barring reflection tricks, you can't. Note, for instance, how the method companion is defined on Scala 2.8 collections -- it is there so an instance of a class can get the companion object, which is otherwise not possible.

answered Dec 16 '09 at 10:49
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Could you add a link to the source, please? – Thomas Jung Dec 16 '09 at 12:11
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lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src/library/…, though, hopefully, 2.8's Scaladoc 2 will soon contain links to the source, as already happens with 2.7. – Daniel C. Sobral Dec 17 '09 at 11:24
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Adjustment to Thomas Jung's answer above: you would do better to say companion[List.type] because a) this should be a stable way to refer to it, not dependant on the name mangling scheme and b) you get the unerased types.

def singleton[T](implicit man: reflect.Manifest[T]) = {
 
val name = man.erasure.getName()
  assert
(name endsWith "$", "Not an object: " + name)
 
val clazz = java.lang.Class.forName(name)

  clazz
.getField("MODULE$").get(clazz).asInstanceOf[T]
}  

scala
> singleton[List.type].make(3, "a")                      
res0
: List[java.lang.String] = List(a, a, a)
answered Jan 11 '10 at 19:50
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Very nice! Still a little wonky for my tastes but I may use this... – Dave Jan 12 '10 at 1:18
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How is this better than just List.make(3, "a")? The OP wanted to get the object by string name – IttayD Nov 7 '10 at 12:51
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Thanks! This is exactly what I was looking for. It's certainly a hack, but less dirty than what I managed to come up with. – Tomer Gabel Jun 27 '11 at 14:19
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