13.3 The Main Thread
The runtime environment distinguishes between user threads and daemon threads. As long as a user thread is alive, the JVM does not terminate. A daemon thread is at the mercy of the runtime system: it is stopped if there are no more user threads running, thus terminating the program. Daemon threads exist only to serve user threads.
When a standalone application is run, a user thread is automatically created to execute the main() method of the application. This thread is called the main thread. If no other user threads are spawned, the program terminates when the main() method finishes executing. All other threads, called child threads, are spawned from the main thread, inheriting its user-thread status. The main() method can then finish, but the program will keep running until all user threads have completed. Calling the setDaemon(boolean) method in the Thread class marks the status of the thread as either daemon or user, but this must be done before the thread is started. Any attempt to change the status after the thread has been started, throws an IllegalThreadStateException. Marking all spawned threads as daemon threads ensures that the application terminates when the main thread dies.
When a GUI application is started, a special thread is automatically created to monitor the user–GUI interaction. This user thread keeps the program running, allowing interaction between the user and the GUI, even though the main thread might have completed after the main() method finished executing.