Question

Is there a way to call a method in a Grails service, from a Scala class that is running on the same JVM?
I have seen something similar done from Groovy/Griffon but cannot figure out how to accomplish that in Grails. (http://www.jroller.com/aalmiray/entry/griffon_groovy_scala_working_together)

Basically, one of my Grails controllers calls some Scala code, which should return some values asynchronously. So, I guess, the only way to return those values is by calling back a method in a Grails service.

Answer 1

I found a way of doing it, inspired by the link in the question above, and one of the FAQs in the Grails website.

On the Scala side:
Declare an object similar to the following:

package scalaCallback

object ScalaCallback{
    var cback: {def callback(example: String)} = null

    def setCallback(cb: {def callback(example: String)}){
        cback = cb
    }

    def invokeCallback(example: String){
        if(callback != null) cback.callback(example)
    }
}

On the Grails side:

Create a class in src/groovy similar to the following:

package groovyCallback
import org.codehaus.groovy.grails.commons.ApplicationHolder

class GroovyCallback{
    private GroovyCallback() {}
    private static final INSTANCE = new GroovyCallback()

    static getInstance(){ return INSTANCE }

    void callback(String example){
        ApplicationHolder.application.mainContext.yourService.yourMethod(example)   
    }
}

In your BootStrap.groovy init add the following:

scalaCallback.cback = groovyCallback.GroovyCallback.getInstance()

When you call invokeCallback(“example”) in Scala, it will call yourService.yourMethod(“example”)

Note: the jar file with your Scala class should be in the lib folder of you Grails application