Unexpected behavior of Java thread

Question

I came across this interesting situation while answering this question.

Try this piece of poorly designed code -

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

class abc extends JFrame implements ActionListener
{

boolean button_clicked = false;
JButton b1;

abc(){
    this.setSize (400, 400);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.createUI();
}

void createUI(){
    this.setLayout(null);
    b1 = new JButton("Click here");
    b1.setSize(110,30);
    b1.setLocation(10,210);
    this.add(b1);
    b1.addActionListener(this);
}

public boolean isButton_clicked()
{
    return button_clicked;
}

public void setButton_clicked(boolean button_clicked) {
    this.button_clicked = button_clicked;
}



public void actionPerformed(ActionEvent arg0) {
    button_clicked = true;
}

}

Here's the main method.

class tempMain extends JFrame
{

public static void main(String[] args) throws Exception
{
    abc temp = new abc();
    temp.setVisible(true);
    while(true)
    {
    //  Thread.sleep(200);
        if(temp.isButton_clicked())
        {
            JOptionPane.showMessageDialog(null, "Hello");
            temp.setButton_clicked(false);
        }
    }
}
}

When I ran this on my Windows 7 machine, nothing happened for at least about a minute (I didn't wait after that) after I clicked the button.

Now, just make one small change -

  Thread.sleep(200); // uncomment this from the main.

And surprisingly, it works and a JOptionPane message is displayed. Why is the message not displayed the first time?

Answer 1

And surprisingly, it works and a JOptionPane message is displayed. Why is the message not displayed the first time?

button_clicked is not marked as volatile and is being updated from a different thread than the main thread. Since the call-back is made from the event handler thread, the main thread will not see the update unless button_clicked is defined as a volatile boolean. Putting a sleep may allow a memory barrier to be crossed and button_clicked to be updated in the main thread luckily.

Here's some more reading about volatile and why it is important when we are dealing with multiple threads.

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Another issue is that you have an infinite loop which is spewing messages to System.out. This completely blocks after a while because the console can't display lines that fast which stops the checking for the click.