JVM什么时候退出

在Java的官方文档中：http://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.1
有如下的描述：
A program terminates all its activity and exits when one of two things happens:
All the threads that are not daemon threads terminate.
Some thread invokes the exit method of class Runtime or class System, and the exit operation is not forbidden by the security manager.

那么问题来了，什么是daemon thread？
请看如下:
Sometimes you want to create a thread that performs some helper function but you don’t want the existence of this thread to prevent the JVM from shutting down.This is what daemon threads are for.
Threads are divided into two types: normal threads and daemon threads. When the JVM starts up, all the threads it creates(such as garbage collector and other housekeeping threads) are daemon threads, except the main thread. When a new thread is created, it inherits the daemon status of the thread that created it, so by default any threads created by the main thread are also normal threads.
Normal threads and daemon threads differ only in what happens when they exit. When a thread exits, the JVM performs an inventory of running threads, and if the only threads that are left are daemon threads,it initiates an orderly shutdown. When the JVM halts, any remaining daemon threads are abandoned finally blocks are not executed, stacks are not unwound the JVM just exits.
Daemon threads should be used sparingly few processing activities can be safely abandoned at any time with no cleanup. In particular, it is dangerous to use daemon threads for tasks that might perform any sort of I/O. Daemon threads are best saved for “housekeeping” tasks, such as a background thread that periodically removes expired entries from an in memory cache.