When are daemon threads useful?

I know that Deamon threads background threads. We can create our own daemon thread by calling setDaemon(true).

My question is: why and when do we need to create our thread as daemon thread?

The JVM exits when all the running threads are daemon threads. So imagine you're writing a simple game where your main method loops until you decide to quit. And imagine that at the start of the game, you start a thread that will endlessly poll some website to trigger alerts. You would like the JVM to exit when you decide to end the game. You don't want the endless polling to prevent the game from ending. So you make this polling thread a daemon thread.

A Deamon thread is automatically terminated by the JVM when all "normal" threads are terminated. Normal threads are never automatically terminated.

Services that you wish to offer to your consumers without any user-interation by way of essentially user-threads form the primary use-case for setting a user thread as a daemon.

As a consequence, until user-threads exist JVM gurantees that daemon threads run continously. You can find examples like GC, UI Thread etc.. those are daemons.

Hope it helps.