Thread 的join的实现中有点迷惑的地方
结果：
Connected to the target VM, address: '127.0.0.1:1759', transport: 'socket'
begin >>> main isAlive: true
joinExtend_currentThread---> main
this ---> Thread-0 alive---> true currentThread---> main
==============================================================
Thread-0 isAlive: true
java.lang.ThreadGroup[name=main,maxpri=10]
ThreadGroup_child: main state: WAITING
ThreadGroup_child: Thread-0 state: RUNNABLE
==============================================================
Thread-0 isAlive: true
java.lang.ThreadGroup[name=main,maxpri=10]
ThreadGroup_child: main state: WAITING
ThreadGroup_child: Thread-0 state: RUNNABLE
==============================================================
Thread-0 isAlive: true
java.lang.ThreadGroup[name=main,maxpri=10]
ThreadGroup_child: main state: WAITING
ThreadGroup_child: Thread-0 state: RUNNABLE
==============================================================
Thread-0 isAlive: true
java.lang.ThreadGroup[name=main,maxpri=10]
ThreadGroup_child: main state: WAITING
ThreadGroup_child: Thread-0 state: RUNNABLE
end >>> main isAlive: true
Disconnected from the target VM, address: '127.0.0.1:1759', transport: 'socket'
Process finished with exit code 0
