Daemon Threads Explanation
Some threads do background tasks, like sending keepalive packets, or performing periodic garbage collection, or whatever. These are only useful when the main program is running, and it's okay to kill them off once the other, non-daemon, threads have exited.
Without daemon threads, you'd have to keep track of them, and tell them to exit, before your program can completely quit. By setting them as daemon threads, you can let them run and forget about them, and when your program quits, any daemon threads are killed automatically.
Let's say you're making some kind of dashboard widget. As part of this, you want it to display the unread message count in your email box. So you make a little thread that will:
- Connect to the mail server and ask how many unread messages you have.
- Signal the GUI with the updated count.
- Sleep for a little while.
When your widget starts up, it would create this thread, designate it a daemon, and start it. Because it's a daemon, you don't have to think about it; when your widget exits, the thread will stop automatically.
A simpler way to think about it, perhaps: when main returns, your process will not exit if there are non-daemon threads still running.
A bit of advice: Clean shutdown is easy to get wrong when threads and synchronization are involved - if you can avoid it, do so. Use daemon threads whenever possible.