Concurrent Park and unpark description

1 park And unpark Use

park and unpark yes LockSupport Methods in class

//  Pause current thread 
LockSupport.park(); 
//  Resume the running of a thread 
LockSupport.unpark( Pause thread object )

Case study 1

First park after unpark

Thread t1 = new Thread(() -> {
    
 log.debug("start...");
 sleep(1);
 log.debug("park...");
 LockSupport.park();
 log.debug("resume...");
},"t1");
t1.start();
sleep(2);
log.debug("unpark...");
LockSupport.unpark(t1);

/*  Running results : 18:42:52.585 c.TestParkUnpark [t1] - start... 18:42:53.589 c.TestParkUnpark [t1] - park... 18:42:54.583 c.TestParkUnpark [main] - unpark... 18:42:54.583 c.TestParkUnpark [t1] - resume... */

According to the results :

t1 Thread sleep 1s After execution park Method , main Threads sleep 2s After execution unpark Method

Case study 2

First unpark after park

Thread t1 = new Thread(() -> {
    
 log.debug("start...");
 sleep(2);
 log.debug("park...");
 LockSupport.park();
 log.debug("resume...");
}, "t1");
t1.start();
sleep(1);
log.debug("unpark...");
LockSupport.unpark(t1);
/*  Running results : 18:43:50.765 c.TestParkUnpark [t1] - start... 18:43:51.764 c.TestParkUnpark [main] - unpark... 18:43:52.769 c.TestParkUnpark [t1] - park... 18:43:52.769 c.TestParkUnpark [t1] - resume... */

According to the results :

t1 Thread sleep 2s After execution park Method , And no sleep , Direct output statement , main Threads sleep 1s After execution unpark Method .

characteristic

And Object Medium wait and notify Methods to compare :

• wait,notify and notifyAll Must cooperate Object Monitor Use it together , and park,unpark No need

• park & unpark It's based on threads 【 Blocking 】 and 【 Wake up the 】 Threads , and notify Only one waiting thread can be wakened randomly ,notifyAll Is to wake up all waiting threads , It's not that 【 accurate 】

• park & unpark You can start with unpark, and wait & notify Not first notify

2 park And unpark Principle

Each thread has its own Parker object , It's made up of three parts _counter , _cond and _mutex.

Take travelers for example :

• Thread is like a traveler ,Parker Like his backpack , Conditional variables are like tents in backpacks ._counter It's like in a backpack Spare dry food （0 To exhaust ,1 For the sake of sufficiency ）
• call park It depends on whether you need to stop and rest
  • If the spare dry food runs out , So get into the tent and rest
  • If there's enough dry food in reserve , Then there is no need to stay , To move forward
• call unpark, It's like having enough dry food
  • If the thread is still in the tent , Just wake him up and let him move on
  • If the thread is still running , So next time he calls park when , Just consume spare dry food , No need to stop and move on
    • Because the backpack space is limited , Multiple calls unpark Only one spare dry food will be added \

situation 1

1 Current thread call Unsafe.park() Method

2 Check _counter , This situation is 0, At this time , get _mutex The mutex

3 Thread entry _cond Conditional variables block

4 Set up _counter = 0

situation 2

1 call Unsafe.unpark(Thread_0) Method , Set up _counter by 1

2 Wake up the _cond In a conditional variable Thread_0

3 Thread_0 Resume operation

4 Set up _counter by 0

situation 3

1 call Unsafe.unpark(Thread_0) Method , Set up _counter by 1

2 Current thread call Unsafe.park() Method

3 Check _counter , This situation is 1, At this time, the thread does not need to block , Continue operation

4 Set up _counter by 0