Long process under Swing GUI : unexpected delay

Question

To explain my question here is an MCVE where clicking clicking a JButton on JDialog A opens JDialog B:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JDialog;

public class  DiagA extends JDialog  {

    private DiagB diag;

    public  DiagA() {

        super();
        setTitle("main diag");
        setSize(200, 150);
        setLocation(400,400);

        JButton btn = new JButton("Show DiagB");
        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {

                showDiag();
            }
        });
        add(btn, BorderLayout.NORTH);

        //make main frame visible
        setVisible(true);
    }

    void showDiag() {

        if(diag == null) {

            diag = new DiagB();

            //this prints out as expected
            System.out.println("set visible done");

            try {
                Thread.sleep(3000);
            } catch (InterruptedException ex) {}

            //only after the delay diag shows in full
        }
    }

    public static void main(String[] args) {
        new  DiagA();
    }
}

class DiagB extends JDialog  {

    public  DiagB() {

        super();
        setTitle("2nd diag");
        setSize(150, 100);
        setLocation(600,420);
        setLayout(new FlowLayout(FlowLayout.CENTER));
        getContentPane().setBackground(Color.YELLOW);
        setVisible(true);
    }
}

As you can see in the code I added a 3 sec delay after creating DiagB . Clicking the button DiagB shows like this:

Only after the 3 sec delay ends, DiagB shows in full:

My questions are:
a. Why doesn't DiagB show completely after it is constructed ? (It shows in full only when showDiag() returns).
b. The reason for my question is that DiagB needs to be updated, by long processes in DiagA .
What is the right way to update ? Does it require using a SwingWorker for every updating process ?

Answer 1

a. showDiag runs on the GUI thread. The GUI will be completely dead while you make the GUI thread sleep.

b. Yes, use a SwingWorker for long-running tasks and use SwingUtilities.invokeLater() to submit GUI-updating tasks back to the GUI thread. Alternatively, implement SwingWorker#done() which is a convenience method that runs on the GUI thread after the SwingWorker task completes.

Answer 2

Based on Marko Topolnik answer and Andrew Thompson comment, I warped the long process with a SwingWorker .
This works fine:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingWorker;

public class DiagA extends JDialog  {

    private FrameB frame;
    private JButton btn;

    public  DiagA() {

        super();
        setTitle("main frame");
        setSize(200, 150);
        setLocation(400,400);

        btn = new JButton("Show Frame B");
        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent arg0) {

                show2ndFrame();
            }
        });
        add(btn, BorderLayout.NORTH);
        setVisible(true);
    }

    void show2ndFrame() {

        if(frame == null) {

            frame = new FrameB();
            btn.setText("Exit");

        }else {

            System.exit(0);
        }

        doWork();
    }

    private void doWork() {

        SwingWorker<Void, Void> sw = new SwingWorker<Void, Void>() {

            @Override
            protected Void doInBackground() throws Exception {

                try {

                    for(int i = 1 ; i<=100 ; i++) {
                        //represents a long process
                        Thread.sleep(100);
                        frame.update(i);
                    }

                } catch (InterruptedException ex) {}
                return null;
            }
        };
        sw.execute();
    }

    public static void main(String[] args) {
        new  DiagA();
    }
}

class FrameB extends JFrame  {

   JLabel label;

    public  FrameB() {

        super();
        setTitle("2nd frame");
        setSize(150, 100);
        setLocation(600,420);
        setLayout(new FlowLayout(FlowLayout.CENTER));
        getContentPane().setBackground(Color.YELLOW);
        label = new JLabel("0");
        add(label);
        setVisible(true);
    }

    void update(int progress) {

        label.setText(String.valueOf(progress));
    }
}