What happens to events when an actionlistener is hogging the event dispatch thread

[Alistair Parler]

Hi,

This is probably a really dumb question, but I'd appreciate any help: I'm just curious.

If I have an actionlistener which is executing its actionPerformed inside the AWT event dispatch thread (i.e. we haven't done the correct thing of either doing the work in a new thread so it doesn't hog the event dispatch thread, or doing the work in an invokeLater if it manipulates any Swing components), what happens to the peer events from the OS when this is running: do they get queued ready for dispatch when the actionPerformed returns?


Thanks,
Alistair

[Campbell Ritchie]

Welcome to JavaRanch

As far as I can remember, events (and anything else) queue until the thread is available, but I am not sure. You will have to try it out with two actionListeners; that might verify it. Does the threading policy link help?

[Alistair Parler]

Thanks Campbell!

I just wrote this program and tested it by clicking Btn1 and then Btn2 lots of times. After the sleep had completed, the remaining events were pumped, then the program terminated. So they are indeed queued.

package com.alistair.test;

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

import javax.swing.JButton;
import javax.swing.JFrame;

public class Main {
	public static void main(String[] args) {
		JFrame frame = new JFrame();
	
		JButton btn1 = new JButton("Btn1");
		btn1.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent e) {
				System.out.println("Btn 1 clicked");
				try {
					Thread.currentThread().sleep(5000);
				} catch (InterruptedException e1) {
					e1.printStackTrace();
				}
			}
		});
		
		JButton btn2 = new JButton("Btn2");
		btn2.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent e) {
				System.out.println("Btn 2 clicked");
			}
		});
				
		frame.setLayout(new FlowLayout());
		frame.add(btn1);
		frame.add(btn2);
		frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
		frame.pack();
		frame.setVisible(true);
	}
}

Thanks,
Alistair

[Campbell Ritchie]

Well done You have verified queuing for a simple example, so presumably queuing is normal behaviour.