Reflect Scala object from Java

Question

Is it possible to get a reference to a Scala Object class from Java, using reflection?

To illustrate what I mean, this is the Scala code for doing this (assume MyClass is a Scala Object):

  val runtimeMirror = universe.runtimeMirror(getClass.getClassLoader)
  val module = runtimeMirror.staticModule("MyClass")
  val obj = runtimeMirror.reflectModule(module)
  val realObject: MyInterface = obj.instance.asInstanceOf[MyInterface]

In Java, I successfully loaded the Class, but I can't seem to be able to get an actual reference to the object; calling newInstance() on the class obviously fails since it doesn't have a public constructor.

I've managed to workaround it by getting its constructor by reflection and setting it to be accessible, but that seems like a hack to me.. is there a BKM for doing this sort of thing?

lots of reflection code does that... setAcessible(true) that is. You are already using reflection, which is already sort of a hack — Eugene
– Eugene, Commented Apr 4, 2017 at 14:08
Scala's object compiles to 2 .class files, one containing only the static forwarder to the implementation (singleton), called $MODULE and other one, inner class with $ appended. See this answer stackoverflow.com/a/12785219/4496364. I guess you want the MODULE$ instance... — insan-e
– insan-e, Commented Apr 4, 2017 at 14:41
@Eugene However, this particular use of setAccessible will give wrong results. — Alexey Romanov
– Alexey Romanov, Commented Apr 4, 2017 at 17:02

Dan Markhasin · Accepted Answer · 2017-04-04 18:33:42Z

1

The comment from insan-e mentioning MODULE$ did the trick.

The code below now works as expected (loading the class from an external jar file):

String className = "MyClass";
File f = new File("C:\\TEMP\\source.jar");
URLClassLoader urlCl = new URLClassLoader(new URL[]{f.toURI().toURL()}, getClass().getClassLoader());
Class c = urlCl.loadClass(className);
Field module = c.getField("MODULE$");
if (module.get(this) instanceof MyInterface) 
{
   MyInterface obj = (MyInterface) module.get(this);
}

answered Apr 4, 2017 at 18:33

Dan Markhasin

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Community · Accepted Answer · 2017-05-23 12:25:49Z

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See the answers to this question about the Java-equivalent implementation of Scala objects to understand how Scala objects translate to Java concepts

What is the Java equivalent of a Scala object?

edited May 23, 2017 at 12:25

CommunityBot

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answered Apr 4, 2017 at 14:36

radumanolescu

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