不知道JVM中是不是也有类似的动作呢！
求R大以及各位大神，好人的回复。  在此表示感谢了先！

只有Java线程才可以执行Java代码。
JNI的 AttachCurrentThread() 机制允许一个当前不是Java线程的线程归入JVM的控制，成为一个Java线程。
而 DetachCurrentThread() 允许一个当前是Java线程的线程脱离JVM的控制。

当一个线程要detach了，它就即将不能执行任何Java代码了。自然，它也就不能持有任何Java层面的锁（monitor），所以在detach时要把该线程持有的monitor都释放掉。就这么简单。
其它清理动作也是同理。

楼主可以先读读JNI Invocation API的说明：http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/invocation.html
然后再读Dalvik VM在代码里的注释：

/*
 * Detach the thread from the various data structures, notify other threads
 * that are waiting to "join" it, and free up all heap-allocated storage.
 *
 * Used for all threads.
 *
 * When we get here the interpreted stack should be empty.  The JNI 1.6 spec
 * requires us to enforce this for the DetachCurrentThread call, probably
 * because it also says that DetachCurrentThread causes all monitors
 * associated with the thread to be released.  (Because the stack is empty,
 * we only have to worry about explicit JNI calls to MonitorEnter.)
 *
 * THOUGHT:
 * We might want to avoid freeing our internal Thread structure until the
 * associated Thread/VMThread objects get GCed.  Our Thread is impossible to
 * get to once the thread shuts down, but there is a small possibility of
 * an operation starting in another thread before this thread halts, and
 * finishing much later (perhaps the thread got stalled by a weird OS bug).
 * We don't want something like Thread.isInterrupted() crawling through
 * freed storage.  Can do with a Thread finalizer, or by creating a
 * dedicated ThreadObject class for java/lang/Thread and moving all of our
 * state into that.
 */
void dvmDetachCurrentThread()

有点困惑于R大说的“JAVA 线程"  我一直都没有做这种区分呀  可能因为我最近才开始看，所以还不清楚～

还有就是注释中的栈 和java运行时栈指的是同一个意思吗？