Client Reuse

Client.scala

class Client(connection: String) {
  println("New Client: " + connection)
}

object Client {
  private val clients = new mutable.HashMap[String, Client]()

  def apply(connection: String): Client = {
    clients.getOrElse(connection, createClient(connection))
  }

  private def createClient(connection: String): Client = {
    val client = new Client(connection)
    clients.put(connection, client)
    client
  }
}

Ok so lazy val cant take param so your approach is fine. Another approach would be as follow. The only advantage of this approach is that Scala is taking care of synchronization. Your approach has a small threading issue and apply should really be synchronized

object env{
 object dev {
  lazy val client = new Client("dev connection string")
}
object prod{
 lazy val client = new Client("prod connection string")
}
}