Why adding element to JList makes it empty in this particular case?

Question

As the title says, when i try to dynamically add an element to a JList, which is inside a JScrollPane, it becomes empty. I think it has something to do with multithreading.

The GUI is a simple monitoring interface for a server I am working on. I'll post parts of the code I think is responsable for this:

But first, this is the frame:

public class EventMonitor extends JFrame{

private JList eventList;
private DefaultListModel<String> dlm ;
JScrollPane jPane;

public EventMonitor()
{
    super("Server Monitor");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLayout(new FlowLayout());

    dlm = new DefaultListModel<String> ();

    eventList = new JList();
    jPane = new JScrollPane(eventList);
    jPane.setPreferredSize(new Dimension(950, 600));
    add(jPane);

}

public void addEvent(String event)
 {
    dlm.addElement(event);
    eventList.setModel(dlm);
    jPane.revalidate();
    jPane.repaint();

 }
}

In main() from Server class:

     //some code
    eventMonitor = new EventMonitor();
    eventMonitor.setBounds(250, 250, 1000, 650);
    eventMonitor.setVisible(true);
     //some other code
    eventMonitor.addEvent("Server is waiting for new connections..."); // this is shown in the list
    while(true)
        {
            ClientHandler clientHandler = new ClientHandler(listener.accept());// listener is ServerSocket
            clientList.add(clientHandler);
            clientHandler.start();
        }

This is what makes the list go empty: inClientHandler constructor:

public ClientHandler(Socket socket){
//some code
Server.addEvent("The client is: " + user.GetName() + ", with the ip:" + user.GetIPv4() + ", and using the port: "  + user.GetPORT());
//some more code 
} // where Server.addEvent(String event) just calls eventMonitor.addEvent(event)

After this, 'run' from 'ClientHandler' listens for messages and they are posted via Server.addEvent(), and it works without a problem, but when another client connects it goes empty again. The first event shown after the one in ClientHandler constructor makes the list whole (showing the event that made it empty too)

1- Swing is not thread safe and the call to `dlm.addElement(event);` should be done from within the context of the Event Dispatching Thread, wrapping in a `SwingUtilities.invokeLater` should help. 2- There should be no need for `eventList.setModel(dlm);`, `jPane.revalidate();`, `jPane.repaint();` within the `addEvent` method — MadProgrammer, Dec 03 '14 at 07:15
For further reading I recommend: http://docs.oracle.com/javase/tutorial/uiswing/concurrency/ — keuleJ, Dec 03 '14 at 07:16

score 2 · Accepted Answer · answered Dec 03 '14 at 07:16

2

It looks like you're blocking the UI thread in the loop - when you call accept(), that's going to wait for a new connection, which means the UI thread can't repaint the UI. You should do that in a separate thread.

All UI interactions should be performed in the UI thread though, e.g. using SwingUtilities.invokeLater.

answered Dec 03 '14 at 07:16

Jon Skeet

1,421,763
867
9,128
9,194

I will try that and I'll post an update after. Thank you for answering. – laurGanta Dec 03 '14 at 07:18
@laurGanta: A related example is seen [here](http://stackoverflow.com/a/3245805/230513). – trashgod Dec 03 '14 at 12:19

Why adding element to JList makes it empty in this particular case?

1 Answers1