Java annotation, scala object and javassist don't get along well

[Update: in the next paragraph, "showstopper" is clearly too strong - put it in my bad english level. The title is perhaps better, I wanted to say that things didn't came along as well as I hoped initially. In no case I wanted to sound over-pessimistic. Actually, the workaround is rather simple : I can use a direct call to the look-up service of T5 IOC each time I need a named service in place of the direct injection of that service.]

In my quest of using Tapestry 5 with a Scala back-end, I believe I fall on a showstopper. I seems that Javassist doesn't see annotation on method parameter in Scala object, although the annotation (being marked with Runtime retention) is seen with the Java reflexion API.

I tested with both Scala 2.7.3 and 2.8.0-nigtly, and both Javassist 3.9.0.GA and 3.10 - the behaviour is consistent in all combinations.

In fact, what seems to happens is that when one defines a Scala object "Foo.scala", scalac generates two classes : "Foo.class" that is a kind of singleton facade and initializer, and "Foo$.class" that is the actual class.

When the java reflexion API is used, I think that scala delegates the call in the rigth way. The problem seems to be that Javassist directly read the bytecode of "Foo.class", and doesn't see the annotation - it seems to be only on the method in "Foo$.class".

You can you see this behaviour like that:

Make a simple Java annotation, and compile it, something like @InjectService in Tapestry5-annotation package:

@Target({PARAMETER, FIELD})
@Retention(RUNTIME)
@Documented
public @interface InjectService
{
    String value();
}

Create a scala class that make use of that annotation and test it visibility thanks to Java reflexion API and Javassist :

import org.apache.tapestry5.ioc.annotations.InjectService
import javassist._
import javassist.runtime._


object ObjectTestAnnotation {
  def methodWithParamAnnot(@InjectService("service id") s:String) = s
}

class ClassTestAnnotation {
  def methodWithParamAnnot(@InjectService("service id") s:String) = s  
}

object Main {

  def main(args: Array[String]) { 
    /*
     * Using Java reflexion API
     */
    val methodWithParamAnnot = ObjectTestAnnotation.
      getClass.getMethod("methodWithParamAnnot", classOf[String])
    val annotations = methodWithParamAnnot.getParameterAnnotations
    println("* with java reflexion API, on ObjectTestAnnotation")
    annotations.foreach(_.foreach( x => println(x)))
    // print:
    // * with java reflexion API, on ObjectTestAnnotation
    // @org.apache.tapestry5.ioc.annotations.InjectService(value=service id)

    val methodWithParamAnnot1 = classOf[ClassTestAnnotation].
      getMethod("methodWithParamAnnot", classOf[String])
    val annotations1 = methodWithParamAnnot1.getParameterAnnotations
    println("* with java reflexion API, on ClassTestAnnotation")
    annotations1.foreach(_.foreach( x => println(x)))
    // print:
    // * with java reflexion API, on ClassTestAnnotation
    // @org.apache.tapestry5.ioc.annotations.InjectService(value=service id)

    /*
     * Using Javassist
     */
    val cp = ClassPool.getDefault()
    val ctString = cp.get("java.lang.String")
    val methodWithParamAnnot2  = cp.get("test.ObjectTestAnnotation")
      .getDeclaredMethod("methodWithParamAnnot",Array(ctString))
    val annotations2 =  methodWithParamAnnot2.getParameterAnnotations
    println("* with javassist, on ObjectTestAnnotation")
    annotations2.foreach(_.foreach( x => println(x)))

    // print (AND THAT'S BAD, the annotation is not seen !)
    
    // * with javassist, on ObjectTestAnnotation
      
    val methodWithParamAnnot2_bis  = cp.get("test.ObjectTestAnnotation$")
      .getDeclaredMethod("methodWithParamAnnot",Array(ctString))
    val annotations2_bis =  methodWithParamAnnot2_bis.getParameterAnnotations
    println("* with javassist, on ObjectTestAnnotation$")
    annotations2_bis.foreach(_.foreach( x => println(x)))
    // print:
    // * with javassist, on ObjectTestAnnotation$
    // @org.apache.tapestry5.ioc.annotations.InjectService(value="service id")
    
  
    val methodWithParamAnnot3  = cp.get("test.ClassTestAnnotation")
      .getDeclaredMethod("methodWithParamAnnot",Array(ctString))
    val annotations3 =  methodWithParamAnnot3.getParameterAnnotations
    println("* with javassist, on ClassTestAnnotation")
    annotations3.foreach(_.foreach( x => println(x)))
    // print:
    // * with javassist, on ClassTestAnnotation
    // @org.apache.tapestry5.ioc.annotations.InjectService(value="service id")
    
    println("done")
    
    
  } 
  
}

Cry ! Because, of course, Tapestry 5 uses these annotations in such a way that a bug here make the whole thing unusable - the core of T5-IOC container relies on them.
I would have to see if I can use a class in place of the object...