14.4. Daemon Threads

In Java, every Thread object has a boolean property named daemon that impacts when a program can exit – its value can be accessed using isDaemon(). Under normal circumstances, a Java program will only exit under two conditions:

• explicit exit: the exit(int) method in the Runtime class is called (e.g., as is done by System.exit(int)); or

• implicit exit: all the threads currently running are daemon threads (i.e., they have their daemon property set to true).

Thread Type	isDaemon()	Prevent implicit exit?
non-daemon	false	true
daemon	true	false

A thread’s daemon property cannot be changed after the thread has started executing (i.e., after it’s start() method has been called). When a new thread is created, its daemon property is set to the same value as the thread that created it.

The main thread is a non-daemon thread, so any new threads that are created in the main thread will also be non-daemon threads, i.e., unless their daemon value is changed to true using using setDaemon(boolean) before they are started.