gistfile1.txt

class User(val firstName: String, val lastName: String)

trait UserDao {
  def findByFirstName(firstName: String): User
}

object defaultUserDao extends UserDao {
  override def findByFirstName(firstName: String) = new User(firstName, "Lewis")
}

object AppConfig {
  var userDao: () => UserDao = () => defaultUserDao
}

trait UserServiceConfig {
  val userDao: () => UserDao = () => AppConfig.userDao()
}

class UserService extends UserServiceConfig {
  def findUser(firstName: String) = {
    userDao().findByFirstName(firstName)
  }
}


var us = new UserService
println(us.findUser("Chris").firstName) // "Chris"

AppConfig.userDao = () => new UserDao() {
  override def findByFirstName(firstName: String) = new User("Mocked", "User")
}

println(us.findUser("Chris").firstName) // "Mocked"