Client Server Thread Problems

Question

When I invoke the method getNextJob() in my Client.
This sends a this message (CLNT::GETNEXTJOB) to the server which then triggers the Server to invoke the corresponding method and send back another message. (Which as far as I know it does).
My problem is this, when I run the getNextJob() method a thread is supposed to be created and ran in parallel which listens for the Servers reply and unfortunately this thread keeps telling me that its get a NullPointerException (specifically at my Socket connection clientSocket). If I uncomment out this.openConnection(); within my run() method it works BUT I get no response from my server even though it is telling me it has sent a message.
I hope you can help.
OUTPUTTED TEXT
CLIENT:  Building GUI
CLIENT:  Get Next Job Invoked
CLIENT:  Opening Connection
CLIENT:  Socket is now OPEN
CLIENT:  Listerner Loaded
Exception in thread "Thread-2" java.lang.NullPointerException
CLIENT ->CLNT::GETNEXTJOB JAVA CODE
package Client;
import java.net.*;
import java.io.*;
public class Kernel implements Runnable {
    //Socket Variables & Streams
    Socket clientSocket;
    //variables
    private boolean msgReceived = true;
    private String faultNumber;
    //listening thread
    public void run()  {
            String serverMessage = "ERROR: run";
            System.out.println("CLIENT:  Listerner Loaded");
            //builds listener
            try {
                 //this.openConnection();
                //opens socket connection
                 BufferedReader in = new BufferedReader(
                                                new InputStreamReader(
                                                this.clientSocket.getInputStream()));
                 BufferedReader stdIn = new BufferedReader(
                                                new InputStreamReader(
                                                System.in));
                while((stdIn.readLine()) != null) {
                    System.out.println("CLIENT:  Listener Received Line = " + stdIn.readLine());
                    serverMessage = stdIn.readLine();
               System.out.println(in.readLine());
                msgReceived = false;
                in.close();
                stdIn.close();
            } catch(IOException e) {System.err.println("Error Details:" + e);}
            //sets current Tail to new value returned
            this.setCurrentTail(serverMessage);
   private void openConnection() throws IOException{
        try {
            System.out.println("CLIENT:  Opening Connection");
            this.clientSocket = new Socket("localhost", 4444);
        } catch (UnknownHostException e) {
            System.err.println("CLIENT:  Host not found");
            System.exit(1);
        } catch (IOException e) {
            System.err.println("CLIENT:  Couldn't get I/O for the connection to: Brenda.");
            System.exit(1);
    public String getNextJob()
        System.out.println("CLIENT:  Get Next Job Invoked");
        try{
            //opens socket connection
            this.openConnection();
            //waits for the socket to open
            System.out.println("CLIENT:  Socket is now OPEN");
            //invokes Listener
            (new Thread(new Kernel())).start();
            //sends a message
            this.sendMessage("CLNT::GETNEXTJOB");
            //waits for a reply from the server
            //while(msgReceived){}
            //closes socket connection
            //this.closeConnection();
        }catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection to: taranis.");
            System.exit(1);
        return this.getCurrentTail();
    private void setCurrentTail(String tail) {
        this.faultNumber = tail;
    private String getCurrentTail() {
        return this.faultNumber;
    private void closeConnection()
        try{
            clientSocket.close();
            System.out.println("CLIENT:  Connection Closed");
        }catch (IOException e) {e.printStackTrace();}
    private void sendMessage(String strMessage){
            try{
                PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
                out.println(strMessage);
                System.out.println("CLIENT ->" + strMessage);
                //Closes output stream.
                out.close();
            }catch(IOException e){System.err.println(e);}
}Edited by: DaveyB on Sep 2, 2009 3:19 PM

Answer 1

I have made some modifications based on the above information but I have hit another brick wall.
The below code returns this to the output pane:
run:
CLIENT:  Building GUI
CLIENT:  Get Next Job Invoked
Never Reached Statement Within getNext StartValue
CLIENT:  Listerner Loaded
CLIENT:  Opening Connection
CLIENT ->CLNT::GETNEXTJOB
STATEMENT NEVER REACHED
null
ErrorWhat you can see is the CLIENT sets up the connection and invokes the Listener thread (run). When this is ran it hits the while loop and even when a non null value is returned via the .ReadLine() as shown via the second value in the output pane {'null', 'Error'} it doesn't exit the loop.
Any ideas why this would be.
package webtoolclient;
import java.net.*;
import java.io.*;
public class Kernel implements Runnable {
    //Socket Variables & Streams
    Socket clientSocket = null;
    BufferedReader in = null;
    PrintWriter out = null;
    //variables
    private String faultNumber = "StartValue";
    private boolean isListenerLoaded = false;
    //listening thread
    public void run()  {
            //String serverMessage = "ERROR: run";
            System.err.println("CLIENT:  Listerner Loaded");
            //builds listener
            try {
                //opens socket connection
                this.openConnection();
                //sends a message
                this.sendMessage("CLNT::GETNEXTJOB");
                //creates inputstreams
                 in = new BufferedReader(
                                                new InputStreamReader(
                                                this.clientSocket.getInputStream()));
                String line = null; // in.readLine();
                while((line = in.readLine()) != null){
                    System.out.println(line);
                System.out.println("STATEMENT NEVER REACHED WITHIN THREAD line = " + line);
                this.setCurrentTail(line);
                this.isListenerLoaded = true;
                //in.close();
                //stdIn.close();
                //this.closeConnection();
            } catch(IOException e) {System.err.println("Error In run: " + e);}
            //sets current Tail to new value returned
            //this.setCurrentTail(line);
   private void openConnection() throws IOException{
        try {
            System.err.println("CLIENT:  Opening Connection");
            this.clientSocket = new Socket("localhost", 4444);
        } catch (UnknownHostException e) {
            System.err.println("CLIENT:  Host not found");
            System.exit(1);
        } catch (IOException e) {
            System.err.println("CLIENT:  Couldn't get I/O for the connection to: Brenda.");
            System.exit(1);
    public String getNextJob()
        System.err.println("CLIENT:  Get Next Job Invoked");
        //try{
            //opens socket connection
            //this.openConnection();
            //waits for the socket to open
           //System.err.println("CLIENT:  Socket is now OPEN");
            //invokes Listener
            new Thread(this).start();
            //ensures Listerner is open before message is sent.
            while(this.isListenerLoaded){}
            //waits for a reply from the server
            //while(this.getCurrentTail().equals("StartValue")){}
            System.err.println("Never Reached Statement Within getNext " + this.getCurrentTail());
            //closes socket connection
            //this.closeConnection();
            return this.getCurrentTail();
        /*}catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection to: taranis.");
            System.exit(1);
       //return this.getCurrentTail();
    private void setCurrentTail(String tail) {
        this.faultNumber = tail;
    private String getCurrentTail() {
        return this.faultNumber;
    private void closeConnection()
        try{
            clientSocket.close();
            System.err.println("CLIENT:  Connection Closed");
        }catch (IOException e) {e.printStackTrace();}
    private void sendMessage(String strMessage){
           try{
                out = new PrintWriter(clientSocket.getOutputStream(), true);
                out.println(strMessage);
                System.err.println("CLIENT ->" + strMessage);
                //Closes output stream.
                //out.close();
            }catch(IOException e){System.err.println(e);}
}