A class inherited from Thread makes CPU run 100%

Question

I have such code in my application which I want to run in background:

import java.awt.*;
import java.awt.datatransfer.*;

class Main extends Thread implements ClipboardOwner {

    private Clipboard sysClip = Toolkit.getDefaultToolkit().getSystemClipboard();

    public void run() {
      Transferable trans = sysClip.getContents(this);
      regainOwnership(trans);
      System.out.println("Listening to board...");
      while (true) { }
    }

    public static void main(String[] args) {
      Main b = new Main();
      b.start();
    }
  //............

It listens for Ctrl+C and prints the content of clipboard once this key combination is pressed. However, it makes CPU run 99% all the time. If I remove while (true) { } then the application will just stop in a second.

I tried to launch it as java -jar myapp.jar & but the result was the same. And it would be the same on Windows I guess, although I care about Linux for now.

What do I do about that?

@Alex http://stackoverflow.com/questions/541487/implements-runnable-vs-extends-thread — Philipp Sander, Nov 13 '13 at 12:59

score 3 · Accepted Answer · answered Nov 13 '13 at 12:34

3

You have a thread running an infinite loop. That's exactly the sort of thing that will use 100% of your CPU for nothing. If nothing else, putting a couple second Thread.sleep() in the loop will bring the CPU usage down.

answered Nov 13 '13 at 12:34

Kayaman

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the maximum amount of time it can sleep is 0.5 sec. Does it make sense to use Thread.sleep? – Incerteza Nov 13 '13 at 12:35
@Alex yes. Half a second is a huge time for a cpu. – Joeri Hendrickx Nov 13 '13 at 12:37
@Alex You should be able to pass any arbitrary long argument to [`Thread.sleep`](http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#sleep(long)). – Nov 13 '13 at 12:37
Is it? I thought only starting from a few seconds would be reasonable enough. – Incerteza Nov 13 '13 at 12:43
Put Thread.sleep(1) in the loop and notice how it lowers the CPU usage. – Kayaman Nov 13 '13 at 12:44
Yes, you're right. However, shouldn't I use runnable instead? – Incerteza Nov 13 '13 at 12:49
Well, using Runnable won't change the general execution of the code. It's just a cleaner approach. – Kayaman Nov 13 '13 at 13:18

score 0 · Answer 2 · edited May 23 '17 at 10:25

0

You should avoid using eternal loops to check if a variable has changed, but rather use a listener. To be more precise, a FlavorListener will allow you to execute code when the clipboard has changed.

It would be something like this:

    Toolkit.getDefaultToolkit().getSystemClipboard().addFlavorListener(
        new FlavorListener() { 
        @Override 
        public void flavorsChanged(FlavorEvent e) { 
            System.out.println("Clipboard contents: " + e.toString()); 
        } 
    });

edited May 23 '17 at 10:25

Community

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answered Nov 13 '13 at 12:43

Luke

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I saw this code already and it didn't work. Could you give a full working example? – Incerteza Nov 13 '13 at 12:44
I don't, but these guys have some pointers: http://www.coderanch.com/t/377833/java/java/listen-clipboard – Luke Nov 13 '13 at 12:56
and where is FlavorListener there? – Incerteza Nov 13 '13 at 13:38

A class inherited from Thread makes CPU run 100%

2 Answers2