Question

Hi I’m new to Java programming. I am trying to execute an external command from Java and then show the command prompt output in JTextArea in real time. That external program will generate 1 line of output every second and then exit after 10 seconds.

The following are my Java codes:

original codes have been deleted to save space after reading Kumra's answer

When I run program.exe manually in command prompt window, the output is updated in real time like this:

<some warning message of the .exe program
    which should not affect the output of the program> // shown at t=0
output line 1   //shown at t=1
output line 2   //shown at t=2
output line 3   //shown at t=3
output line 4   //shown at t=4
output line 5   //shown at t=5
output line 6   //shown at t=6
output line 7   //shown at t=7
output line 8   //shown at t=8
output line 9   //shown at t=9
output line 10  //shown at t=10

Done. //shown at t=10.

When I run my Java program at above, I thought JTextArea will be updated in real time to show the command prompt output. Unfortunately, it is not working. The actual output is like this:

<some warning message of the .exe program
    which should not affect the output of the program> // shown at t=0

From t=0 until t=10, the JTextArea get stuck at the above output. At t=11, it suddenly show the complete output:

<some warning message of the .exe program
    which should not affect the output of the program> // shown at t=0
output line 1   //shown at t=11
output line 2   //shown at t=11
output line 3   //shown at t=11
output line 4   //shown at t=11
output line 5   //shown at t=11
output line 6   //shown at t=11
output line 7   //shown at t=11
output line 8   //shown at t=11
output line 9   //shown at t=11
output line 10  //shown at t=11

Done. //shown at t=11

May I know what’s wrong with my codes? Can anyone teach me how to display command prompt output in JTextArea in real time? Thanks.

EDIT 1:
I have edited the codes based on Kumar’s answer but it is still not working. Below are the latest codes.

MyUI.java

public class MyUI extends JFrame
implements ActionListener, KeyListener, ChangeListener, WindowListener
{
    ...
    private JTextArea output;

    public void showMessage(String message)
    {
        output.append(message + "\n");
        output.setCaretPosition(output.getDocument().getLength());
    }
    ...

    public void actionPerformed(final ActionEvent e)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                if (xxxxx)
                {
                    myThreadInstance = new MyThread(xx,xxx,xx,xx,xx);
                    myThreadInstance.start();
                }
            }
        }
    }

}

MyThread.java

public class MyThread extends Thread
{
    ...
    public MyUI myFrame;
    ...

    public void run
    {
        try
        {
            String command = "program.exe arg1 arg2 arg3 arg4";

            List<String> items = Arrays.asList(command.split("\\s+"));

            builder = new ProcessBuilder(items);
            builder.redirectErrorStream(true);

            process = builder.start();

            input = new BufferedReader(new InputStreamReader(process.getInputStream()));

            String inputline = null;
            while ((inputline = input.readLine()) != null)
            {
                myFrame.showMessage(inputline);
            }
        }
        catch(){}
        finally{}
    }
}

Answer 1

Try this…

1. Always keep the UI work on the UI thread and Non-UI work on the Non-UI thread.
2. Event Dispatcher Thread is the UI thread, which is responsible for the Gui.
3. You are doing the process of counting from t=0 to t=10 on the UI thread, that’s the reason
   you don’t see any output till t=11.
4. Create a separate thread to do the counting on the cmd, or use SwingWorker provided in
   Swing to synchronize the UI and Non-UI thread.