JProgressBar update and thread scheduling

Question

Hello stack exchangers,

I have a problem with progress bars in java Swing. I think my confusions arise from my poor understanding of threads and the Swing event queue (I don't know much about java Threads, and exactly what is happening on the AWTEventQueue, although I generally understand what multithreading is about).

The context is that a JButton is pressed to start a long calculation. Before the calculation starts, I make a progress bar in a JFrame, which I thought would be painted, but it isn't. The frame appears, but it is just grey. The button, in this example has "clickMe" written on it.

In the "clickMe" action listener, I first make and display a JFrame in a subtask which is "run" (I'm not clear on when this is scheduled TBH). Then, I call doTask() which is running in the same thread as the action listener (which I think is the AWTEventThread??). The doTask() runs, printing out numbers to the Console. Intermixed with the doTask() output are iteration counts of the progressbar (from when the action listener started makeProgressBar()).

So, from the output, it looks like both the progress bar is running and the AWTEventThread, but the value set in the JProgressBar GUI is never updated.

How can I change my code so that the GUI gets updated? I've tried understanding the JProgressBar tutorial and hunted around the web, but I think my problem is more a conceptual understanding of Java Tasks.

This is my code:

package problemclass;

import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JProgressBar;
import javax.swing.SwingUtilities;

public class ProblemClass 
{

    void progressBarButtonClick()
    {
        JFrame buttonInAFrame = new JFrame();
        JPanel buttonInAFramePanel = new JPanel();
        JButton clickMe = new JButton("Click me!");

        buttonInAFramePanel.add(clickMe);       


        clickMe.addActionListener(new ActionListener() {
            @Override
                    public void actionPerformed(ActionEvent e) 
                {
                JFrame progBarFrame = makeProgressBar();                
                doTask();
                progBarFrame.dispose();
                    }
        });


        buttonInAFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        buttonInAFrame.add(buttonInAFramePanel);
        buttonInAFrame.pack();
        buttonInAFrame.setVisible(true);
    }

    private void doTask()
    {

        for(int i = 0; i < 20000; i++)
        {
            if (i % 100 == 0) 
            {
                System.out.println("TASK iteration " + i);

                try 
                {
                    Thread.sleep(1000);
                } 
                catch (InterruptedException e) {}
            }
        }
    }

    private JFrame makeProgressBar()
    {
        JFrame progBarFrame = new JFrame();
        JPanel progBarPanel = new JPanel();

        JProgressBar progressBar = new JProgressBar();
        progBarPanel.add(progressBar);      
        progressBar.setValue(0);
            progressBar.setStringPainted(true);
            progressBar.setIndeterminate(true);


        new Thread(new Runnable() 
        {
            public void run() 
            {
                for (int i = 0; i <= 100; i++) 
                {

                    final int j = i; 
                    System.out.println("Progress Iteration " + j);

                    SwingUtilities.invokeLater(new Runnable() 
                    {
                        public void run() 
                        {
                           progressBar.setValue(j);
                        }
                    });

                    try 
                    {
                        java.lang.Thread.sleep(100);
                    }
                    catch(Exception e) { }
                }
            }
        }).start();

        progBarFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        progBarFrame.add(progBarPanel);
        progBarFrame.pack();
        progBarFrame.setVisible(true);

        return progBarFrame;
    }
    public static void main(String[] args)
    {
        EventQueue.invokeLater(() ->
        {       
            new ProblemClass().progressBarButtonClick();
        });
    }
}

Answer 1

JFrame progBarFrame = makeProgressBar();                
doTask();

Not sure exactly what you are trying to do.

The above code has two loops:

1. In the makePrgressBar() method you start a Thread and invoke SwingUtilities.invokeLater(…), to update the progress bar, which is correct.

2. but then in doTack() you start another loop. This time you don't start a Thread so the code is invoked on the EDT and since you use Thread.sleep, the EDT will sleep and the GUI will not repaint itself until the entire loop is finished.

I would suggest you get rid of the doTask() method since I don't know why you need two blocks of code that loop. Or if you really need it, then you also need to use a Thread and invokeLater(…).

Answer 2

Just like you, I recently did some work on progress bars and threading and went nuts until I realized that it is just so simple.In a nutshell this is the code I have when my button is clicked:

 // Create 2 threads. One handles your GUI. Other does the task
        Thread t1 = new Thread(new Runnable() {
          @Override
          public void run() {
          // code goes here.
          //In here I choose to hide the button, display the progress bar
                      }
          });  
        t1.start();

       Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            // code goes here.
            //In here I get the task done, then hide the progress bar

             }
           });  
        t2.start();

Works like a charm every time. Hope it helps!