java concurrent programming notes eight

Park & Unpark

Basic use

They class method LockSupport

// 暂停当前线程
LockSupport.park();
// 恢复某个线程的运行
LockSupport.unpark(暂停线程对象)

Feature

Compared with the wait & notify Object

• wait, notify and notifyAll must be used in conjunction with the Object Monitor, and park, unpark do not have to
• park & ​​unpark thread is blocked in units of [] and [] wake up the thread, and notify only a random wake up waiting threads, notifyAll wakes up all waiting threads are, not so precise []
• park & ​​unpark can first unpark, and not to wait & notify notify

park unpark principle

Each thread has its own a Parker object consists of three parts counter, condand mutexdraw an analogy

• Thread like a traveler, Parker like his carry-on bag, backpack tent like condition variable ratio. _counter like backpack alternate dry food (0 depletion, sufficient 1)
• Call the park it is to see or need to stop and rest
  • If the spare rations depleted, then got into the tent to rest
  • If sufficient spare dry food, so do not need to stop and move on
• Call unpark, like so dry food sufficient
  • If you then still thread tent, so that he would wake up and move on
  • If the thread is still running at this time, then the next time he calls the park, only dry food is consumed spare, without staying to move on
    • Because of the limited bag space, multiple calls to unpark only would add a spare dry food
      

1. The current thread calls Unsafe.park () method
2. Check _counter, this situation is 0, this time, to obtain _mutex mutex
3. Thread into the blocked condition variable _cond
4. Provided _counter = 0
   
5. Call Unsafe.unpark (Thread_0) method, set _counter 1
6. Wake _cond condition variable Thread_0
7. Thread_0 resume operation
8. _Counter set to 0