What is a daemon thread in Java?

Daemon threads are low-priority threads that provide services to user threads (e.g., Garbage Collector).

Key Characteristics:

• They do not prevent the JVM from exiting. When all user threads finish, the JVM terminates, even if daemon threads are still running.
• They must be set as daemon before starting the thread.

Code:

Thread t = new Thread(new MyRunnable());
t.setDaemon(true); // Must be called before start()
t.start();

If you call setDaemon(true) after start(), it throws an IllegalThreadStateException.