answer to stackoverflow question http://stackoverflow.com/questions/13835581/where-place-compiled-javascript-class-files-for-sbt-and-play-framework-2-1-to-fi#comment19130392_13835581

.gitignore

logs
project/project
project/target
target
tmp
.history
dist
/.idea
/*.iml
/out
/.idea_modules
/.classpath
/.project
/RUNNING_PID
/.settings

application.conf

# This is the main configuration file for the application.
# ~~~~~

# Secret key
# ~~~~~
# The secret key is used to secure cryptographics functions.
# If you deploy your application to several instances be sure to use the same key!
application.secret="[O62eawT;]t7lyQd9CsilTFQ2dJUo>iXlsbp``PAolHOIs6bopSPhGOvlqy:I@l?"

# The application languages
# ~~~~~
application.langs="en"

# Global object class
# ~~~~~
# Define the Global object class for this application.
# Default to Global in the root package.
# application.global=Global

# Router 
# ~~~~~
# Define the Router object to use for this application.
# This router will be looked up first when the application is starting up,
# so make sure this is the entry point. 
# Furthermore, it's assumed your route file is named properly. 
# So for an application router like `my.application.Router`,
# you may need to define a router file `conf/my.application.routes`.
# Default to Routes in the root package (and conf/routes)
# application.router=my.application.Routes

# Database configuration
# ~~~~~ 
# You can declare as many datasources as you want.
# By convention, the default datasource is named `default`
#
# db.default.driver=org.h2.Driver
# db.default.url="jdbc:h2:mem:play"
# db.default.user=sa
# db.default.password=""

# Evolutions
# ~~~~~
# You can disable evolutions if needed
# evolutionplugin=disabled

# Logger
# ~~~~~
# You can also configure logback (http://logback.qos.ch/), by providing a logger.xml file in the conf directory .

# Root logger:
logger.root=ERROR

# Logger used by the framework:
logger.play=INFO

# Logger provided to your application:
logger.application=DEBUG

Application.scala

package controllers

import play.api._
import play.api.mvc._

object Application extends Controller {
  
  def index = Action {


    Ok(so.Example.hello)
  }
  
}

build.properties

sbt.version=0.12.1

Build.scala

import org.apache.ivy.util.FileUtil
import sbt._
import sbt.Keys._
import play.Project._
import java.io.File

object ApplicationBuild extends Build {

  val appName         = "so13835581"
  val appVersion      = "1.0-SNAPSHOT"

  val appDependencies = Seq(
    // Add your project dependencies here,
    jdbc,
    anorm
  )

  val rhinoJarName = "rhino.jar"
  val folderForRhinoJar = unmanagedBase
  val rhinoClasspath = (baseDirectory) map { base => Attributed.blank(base / "rhino")}
  val rhinoClassesToJar = TaskKey[Unit]("rhino-classes-to-jar")
  val rhinoClassesToJarInitializer = (base: File, folderForJar: File) => {
    val rhinoFolder = new File(base, "rhino")
    val isClassFile = (file: File) => file.getName.endsWith("class")
    val classFiles =  rhinoFolder.***.filter(isClassFile).get
    val sources = classFiles map { file =>
      file -> file.getCanonicalPath.drop(rhinoFolder.getCanonicalPath.length + 1)
    }
    IO.jar(sources, folderForJar / rhinoJarName, new java.util.jar.Manifest())
  }

  val main = play.Project(appName, appVersion, appDependencies).settings(
    unmanagedClasspath in Compile <+= rhinoClasspath,
    unmanagedClasspath in Runtime <+= rhinoClasspath,
    rhinoClassesToJar <<= (baseDirectory, folderForRhinoJar) map rhinoClassesToJarInitializer,
    playStage <<= playStage.dependsOn(rhinoClassesToJar),
    playStage <<= (playStage, folderForRhinoJar) map {(unused, folder) =>
      IO.delete(folder / rhinoJarName)
    },
    cleanFiles <+= folderForRhinoJar { _ / rhinoJarName } //make sure jar will be deleted if play stage does not finish and cleanup jar
  )
}

Example.class

plugins.sbt

// Comment to get more information during initialization
logLevel := Level.Warn

// The Typesafe repository 
resolvers += "Typesafe repository" at "http://repo.typesafe.com/typesafe/releases/"

// Use the Play sbt plugin for Play projects
addSbtPlugin("play" % "sbt-plugin" % "2.1-RC1")

routes

# Routes
# This file defines all application routes (Higher priority routes first)
# ~~~~

# Home page
GET     /                           controllers.Application.index

# Map static resources from the /public folder to the /assets URL path
GET     /assets/*file               controllers.Assets.at(path="/public", file)