【 interview ： Concurrent articles 19： Multithreading ：Park&Unpark】

00. Preface

If you have any questions, please point out , thank .

01. Introduce

Basic use

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

When used in a thread LockSupport.park(); This thread becomes WATING state

02. Example

@Slf4j(topic = "c.TestParkUnpark")
public class TestParkUnpark {
    
    public static void main(String[] args) {
    
        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);
    }
}

result

17:22:16.403 c.TestParkUnpark [t1] - start…
17:22:17.412 c.TestParkUnpark [t1] - park…
17:22:18.402 c.TestParkUnpark [main] - unpark…
17:22:18.402 c.TestParkUnpark [t1] - resume…

explain
You can see that we are 1s when LockSupport.park(); Thread paused t1 stay 2s when LockSupport.unpark(t1); hold t1 Thread recovery

Be careful

If we were in 1s When it's time to recover t1 Threads ,2s Then pause t1 What happens to threads ？

@Slf4j(topic = "c.TestParkUnpark")
public class TestParkUnpark {
    
    public static void main(String[] args) {
    
        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);
    }
}

result

17:27:11.949 c.TestParkUnpark [t1] - start…
17:27:12.959 c.TestParkUnpark [main] - unpark…
17:27:13.958 c.TestParkUnpark [t1] - park…
17:27:13.958 c.TestParkUnpark [t1] - resume…

explain
It can be seen that we first unpark Again park,t1 The thread is still running normally , With us wait/notify The understanding is different

03. characteristic

LockSupport Of park/unpark And Object Of wait/notify comparison ：

1.wait notify and notifyAll Must cooperate Object Monitor Use together, that is, it can only be used under the condition of heavyweight lock , and park unpark Unwanted
2.park unpark Is to block and wake up threads on a thread by thread basis The tag can accurately wake up the specified waiting thread , and notify Only one waiting thread can be wakened randomly notifyAll Is to wake up all waiting threads , Imprecise
3.park unpark You can start with unpark, however wait notify Not first notify

04. principle

Each thread has its own Parker object , It's made up of three parts _counter, _ cond and _mtuex, Make a comparison
Thread is like a traveller ,Parker It's the backpack it carries , Conditional variables are like tents in backpacks ,_ counter It's like spare dry food in a backpack (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 , Then go to the tent to rest
If there's enough dry food in reserve , Then there's no need to stay To move forward
call unpark, It's like having enough dry food
If the thread is still in the tent Wake him up for so long 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

Premise ： Not yet park, To explore the park Changes in the process

1. Current thread call park Method
2. Check _counter This situation is 0 here get _mutex The mutex
3. Thread entry _cond Conditional variables block
4. Set up _counter=0

Premise ： At this point has been park After that , To explore the unpark Changes in the process

1. call 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

Premise ： Explore first unpark after park The situation of

1. call unpark(Thread_0) Method , Set up _counter by 1
2. Current thread call park Method
3. Check _counter This situation is 1 At this point, the thread does not need to block Continue operation
4. Set up _counter by 0