Force initialization of Scala singleton object

Question

I\'m working on an automatic mapping framework built on top of Dozer. I won\'t go into specifics as it\'s not relevant to the question but in general it\'s supposed to allow

Answer 1

Short answer: You can't. Scala objects are lazy, and are not initialized until first reference. You could reference the object, but then you need a way of ensuring the executing code gets executed, reducing the problem back to the original problem.

Answer 2

In ended up doing this:

trait ProjektionAware with DelayedInit
{
  private val initCode = new ListBuffer[() => Unit]

  override def delayedInit(body: => Unit)
  {
    initCode += (() => body)
  }


  def registerProjektions()
  {
    for (proc <- initCode) proc()
  }
}

object A extends ProjektionAware {
  projekt[A].to[B]

}

Now I can use a classpath scanning library to initialize all instances of ProjektionAware on application bootstrap. Not ideal, but works for me.

Answer 3

As Dave Griffith and Don Roby already noted, it cannot be done at JVM startup in general. However maybe this initialization could wait until first use of your framework? If so, and if you don't mind resorting to fragile reflection tricks, in your --> method you could obtain reference to the companion object and get it initialize itself.

You can start at Getting object instance by string name in scala.

Answer 4

You can force the instantiation of A to involve the companion object by using an apply() method or some other sort of factory method defined in the object instead of directly using the new A() constructor.

This does not cause the object to be initialized when the JVM starts, which I think as noted in another answer can't generally be done.