Java SwingUtilities.invokeLater update TextArea

Question

I have the following classes:

MainServer
TCPServer
UDPServer

I create new instances of TCPServer and UDPServer classes (start) from MainServer class, where my GUI is initialized. In this GUI I have a textArea, which either TCP or UDP classes needs to update to display log information (errors, status, etc). I did some search and I know i may need to use EDT in MainServer, but dont know how to access this object in MainServer class from TCPServer or UDPServer. Right now I can only print to console which is undesirable. How to access MainServer.printlog from TCPServer? or Mainserver.textArea object? If I create a new MainServer instance from TCPServer or UDPServer this seems not to be working.

This is my function in MainServer class:

public void printLog (final String log, final int level) {

    SwingUtilities.invokeLater(
    new Runnable() 
    {
        public void run() 
        {
        if (level == 1) 
            textArea.append("INFO\t" + log);
        if (level == 2)
            textArea.append("WARN\t" + log);
        if (level == 3)
            textArea.append("ERROR\t" + log);
        }
    }
    );
}

Edit: I tried to create a new instance of MainServer and access printLog but I get:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at MultithreadedBarcodeReader.MultithreadedBarcodeReaderServer$2.run(
        MultithreadedBarcodeReaderServer.java:68)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:682)
    at java.awt.EventQueue.access$000(EventQueue.java:85)
    at java.awt.EventQueue$1.run(EventQueue.java:643)
    at java.awt.EventQueue$1.run(EventQueue.java:641)

This part looks OK; for reference, there's a working example [here](http://stackoverflow.com/a/3245805/230513). — trashgod, Sep 02 '12 at 21:36
As you state that “this seems not to be working”, please be more precise as to *what* the actual *problem* is you're observing. — MvG, Sep 02 '12 at 22:00
Hi MvG, I have 6 different class files: *MainServerInit, * MainServer, *TCPServer *UPDServer *Decode *Result. In Decode and Result I process anything I obtain from my clients. But the MainServerInit class initialized MainServer which starts the GUI and contains the printLog method. At this stage as I initialized a MainServer instance which also starts TCP and UDP servers, I'm not sure which methods or the sintaxis to access MainServer server = new MainServer; server.printLog("TCP Server() Error",2); from TCP, UDP, Decode, etc. — gogasca, Sep 03 '12 at 00:04
Please don't put code or exception output into code comments. Edit it into the question (as already done by @trashgod). — Andrew Thompson, Sep 03 '12 at 01:15

score 2 · Answer 1 · answered Sep 03 '12 at 01:07

The NullPointerException occurs on line 68 in an anonymous class nested in MultithreadedBarcodeReaderServer, presumably the Runnable cited. One may guess that textArea is null, as log is not dereferenced and level is primitive. You'll need to break near that line in the debugger to be certain. Also verify that your GUI components are constructed and manipulated only on the event dispatch thread.

score 1 · Accepted Answer · answered Sep 03 '12 at 07:19

Consider using a separate logging frame-work, or at least a separate logging class with static methods (or a logging factory with static methods). For your application, it should't matter whether they log to the console, to a JTextArea, ... and you certainly do not want to pass the MainServer instance around only for logging purposes.

Then your application can simply 'log' those messages, and if you want to display those on a certain JTextArea, you can simply add a handler which does exactly this. This handler can then be created in the same location as where you create the actual JTextArea, and of course can append the log messages on the Event Dispatch Thread.

Other advantages of using a decent logging framework are that you can activate the logging on a per-class basis, configure the logging level on a per-class basis, ... and all that without making any changes to your code. So this allows to gather "debug-information" from a deployed application (perhaps even information you do not want to appear in the JTextArea but you want to know to diagnostic a problem).

Java SwingUtilities.invokeLater update TextArea

2 Answers2