My http request becomes null inside an Akka future

My server application uses Scalatra, with json4s, and Akka.

Most of the requests it receives are POSTs, and they return immediately to the client with a fixed response. The actual responses are sent asynchronously to a server socket at the client. To do this, I need to getRemoteAddr from the http request. I am trying with the following code:

case class MyJsonParams(foo:String, bar:Int)

class MyServices extends ScalatraServlet {
  implicit val formats = DefaultFormats

  post("/test") {
    withJsonFuture[MyJsonParams]{ params =>
      // code that calls request.getRemoteAddr goes here
      // sometimes request is null and I get an exception
      println(request)
    }
  }

  def withJsonFuture[A](closure: A => Unit)(implicit mf: Manifest[A]) = {
    contentType = "text/json"
    val params:A = parse(request.body).extract[A]
    future{
      closure(params)
    }      
    Ok("""{"result":"OK"}""")
  }    
}

The intention of the withJsonFuture function is to move some boilerplate out of my route processing.

This sometimes works (prints a non-null value for request) and sometimes request is null, which I find quite puzzling. I suspect that I must be "closing over" the request in my future. However, the error also happens with controlled test scenarios when there are no other requests going on. I would imagine request to be immutable (maybe I'm wrong?)

In an attempt to solve the issue, I have changed my code to the following:

case class MyJsonParams(foo:String, bar:Int)

class MyServices extends ScalatraServlet {
  implicit val formats = DefaultFormats

  post("/test") {
    withJsonFuture[MyJsonParams]{ (addr, params) =>
      println(addr)
    }
  }

  def withJsonFuture[A](closure: (String, A) => Unit)(implicit mf: Manifest[A]) = {
    contentType = "text/json"
    val addr = request.getRemoteAddr()
    val params:A = parse(request.body).extract[A]
    future{
      closure(addr, params)
    }      
    Ok("""{"result":"OK"}""")
  }    
}

This seems to work. However, I really don't know if it is still includes any bad concurrency-related programming practice that could cause an error in the future ("future" meant in its most common sense = what lies ahead :).

I don't know Scalatra, but it's fishy that you are accessing a value called request that you do not define yourself. My guess is that it is coming as part of extending ScalatraServlet. If that's the case, then it's probably mutable state that it being set (by Scalatra) at the start of the request and then nullified at the end. If that's happening, then your workaround is okay as would be assigning request to another val like val myRequest = request before the future block and then accessing it as myRequest inside of the future and closure.