Re: null pointer exception in thread

"John B. Matthews" <nospam@nospam.invalid>
Sun, 02 Aug 2009 19:43:00 -0400
In article <op.ux1sbykm8jd0ej@macbook-pro.local>,
 "Peter Duniho" <> wrote:

On Sun, 02 Aug 2009 11:08:25 -0700, John B. Matthews
<nospam@nospam.invalid> wrote:


No Null Pointer Exception seen in my test. End of test.

** Just check what is happening on your **

Something on that line has an undefined reference, but I can not
say what it is was my editor formats your code differently and I
can not see what is on line 70.

I got similar results, but I think it's a fluke. As the GUI is not
built on the EDT but on the initial thread, the TextArea
(referenced at line 70) may not be visible to the AppServer thread
when it begins. Of course, building the GUI on the EDT in the usual
way blocks the EDT at accept(). One solution would be to delegate
the socket handling to a SwingWorker.

Or alternatively, use the usual convention of calling on the EDT a
Runnable that instantiates the frame (i.e. using invokeLater()).

Quite right; I just needed to move the socket/stream code to the
listener thread in order to avoid blocking the EDT.

I suspect the NPE is coming from a synchronization problem, which you
allude to when you write "may not be visible...". That is, the
variables "ta" and "input" are in fact both initialized in the
constructor, but without synchronization between the main thread and
the daemon thread, the values assigned in the main thread may not be
visible in the daemon thread by the time the daemon thread gets
around to looking at them.

That's what I was thinking; but as you say, it's hard to reproduce.

Making them "volatile" should be sufficient to correct that issue,
assuming that's the cause of the NPE in the first place (hard to say
for sure, when the problem isn't 100% reproducible).

OP: Can you try this example on your end? It uses JTextArea, which has
several thread-safe methods, including append().

package net;

import java.awt.*;
import java.awt.event.*;
import java.util.Scanner;
import javax.swing.*;

public class EchoServer implements ActionListener, Runnable {

    private static final int PORT = 12000;
    private final JTextField tf = new JTextField(20);;
    private final JTextArea ta = new JTextArea(15, 20);;
    private final JButton send = new JButton("Send");
    private volatile PrintWriter out;
    private Scanner in;

    public EchoServer() {
        JFrame f = new JFrame("Echo Server");
        f.add(tf, BorderLayout.NORTH);
        f.add(new JScrollPane(ta), BorderLayout.CENTER);
        f.add(send, BorderLayout.SOUTH);
        f.setLocation(300, 300);
        ta.append("Please telnet to port " + PORT + "\n");
        new Thread(this, "Listener").start();

    public void actionPerformed(ActionEvent ae) {
        String s = tf.getText();
        if (out != null) out.println(s);

    public void run() {
        try {
            ServerSocket ss = new ServerSocket(PORT);
            Socket socket = ss.accept();
            in = new Scanner(socket.getInputStream());
            out = new PrintWriter(socket.getOutputStream(), true);
            while (true) {
        } catch (Exception e) {

    private void display(String s) {
        ta.append(s + "\u23CE\n");

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                new EchoServer();

John B. Matthews
trashgod at gmail dot com

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