J2ME thread programming

Question

I am creating a j2me application which is interacting with a database on a server. Therefore I launch a thread to connect to the server. I handle all my command actions in a separate class which implements CommandListener.

When I try to get the response the server returned I get an empty String. I have tried waiting for the thread that connects to the server to return but this makes the application unresponsive.

Below is the code I am using.

//my network worker class.
public class NetworkConnector implements Runnable {
    //constructor
    public NetworkConnector(String url){
       //url =>server url to connect to.
       Thread thread = new Thread(this);
       thread.start();
    }

    private String serverReply="";
    private String url="

    //method to connect to the server and return the 
    //response from the server.
    public String sendData(String serverUrl) {

      //open streams to connect to the Server.
      httpConn = (HttpConnection)  
      Connector.open(serverUrl,Connector.READ_WRITE, true);
      httpConn.setRequestMethod(HttpConnection.GET);

      inStream = httpConn.openInputStream();
      int read;
      while ((read = inStream.read()) != -1) {
        sb.append((char) read);
      }

      return sb.toString();
   }


   public String getServerReply() {
     //serverReply is a class variable.
     return serverReply;
   }

   public void run(){
      //call the send method that connects to the server.
       serverResponse = sendData(Url);
   }

} //end of connector class.

// this class is where all my Command Actions are
//implemented. 
public class CommandActionController implements
CommandListener, DataReceiver {

public void commandAction(Command cmd, Displayable d) {
   //networkMgr => is the worker class that makes
   //connection to the server.
   networkMgr = new NetworkConnector("http://localhost
   /JsonPhp/login.php?uname=" + loginUserInfo.userName +
   "&passwd=" + loginUserInfo.password);

   //here am getting the response from the server.
   String serverResponse = networkMgr.getServerReply();

   //at this point the ServerReponse String is Empty =>
   //yet the server is supposed to return some String.
   System.out.println("xxxxxxxxxxxxxxxxxx
   ServerResponse =" + serverResponse);
   }

   }//end of CommandListener class.

Answer 1

Your expectations about serverResponse value are wrong; it can be empty at the moment when you attempt to obtain it in commandAction with the code snippet in the question.

In order to get non-empty value, you need to redesign method NetworkConnector.getServerReply() to make it properly wait until thread finishes and server response is indeed obtained.

If you do that, however, you will also have to redesign CommandActionController to keep user interface responsive - your testing with "waiting for the thread that connects to the server" has shown just that.

Redesigned code should just launch the server connect thread and exit the commandAction as soon as possible (possibly changing to some "wait screen" to let user see what happens). Next screen update should be triggered only after server response is obtained.

Consider studying a tutorial specifically targeted on explaining how to properly design this kind applications - Networking, User Experience, and Threads:

This article explains how your MIDlet can make network connections without compromising your user interface. It includes six iterative examples that illustrate multithreaded networking and the use of a wait screen...