[interview: concurrent Article 23: multithreading: Join] re understanding of join

【 interview ： Concurrent articles 23： Multithreading ：join】join Understand again

00. Preface

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

01. Introduce

Yesterday I suddenly thought of a question join The underlying implementation of the method is wait, perform wait Thread waiting for , So how does it wake up ？ With this problem in mind, I looked for a blog and finally found this article https://blog.csdn.net/x541211190/article/details/109322537, It introduces notifyAll Unlocking time of , I found such a passage stay java in ,Thread Class thread finished executing run() After the method , It will be automatically executed notifyAll() Method This sentence is like an epiphany , I immediately understood join In the method wait How to wake up .
Next, let me simulate join The process of .

02. simulation

join Source code

We focus on two places , The first is the while The loop condition isAlive() Its function is Judgment call join Is the thread alive or not If alive, execute wait() here wait The role of the thread is to execute join The thread of , Note that there The difference between call and execution . For example ： We are t2 Executed in thread t1.join(), that isAlive() The judgment is t1 Is the thread alive ,wait It's Jean t2 Thread caught waiting

join Example

@Slf4j(topic = "c.TestInterJoin")
public class TestInterJoin {
    
    public static void main(String[] args) {
    
        Thread t1 = new Thread(()->{
    
            for (int i=0;i<5;i++){
    
                log.debug("t1");
            }
        },"t1");

        Thread t2 = new Thread(()->{
    
            try {
    
                t1.join();
            } catch (InterruptedException e) {
    
                e.printStackTrace();
            }
            for (int i=0;i<5;i++){
    
                log.debug("t2");
            }
        },"t2");
        t2.start();
        t1.start();

    }
}

result

15:08:50.125 c.TestInterJoin [t1] - t1
15:08:50.127 c.TestInterJoin [t1] - t1
15:08:50.127 c.TestInterJoin [t1] - t1
15:08:50.127 c.TestInterJoin [t1] - t1
15:08:50.127 c.TestInterJoin [t1] - t1
15:08:50.127 c.TestInterJoin [t2] - t2
15:08:50.128 c.TestInterJoin [t2] - t2
15:08:50.128 c.TestInterJoin [t2] - t2
15:08:50.128 c.TestInterJoin [t2] - t2
15:08:50.128 c.TestInterJoin [t2] - t2

explain
We created two threads t1 t2, We are t2 Execute in thread t1.join bring t1 The thread is synchronized to t2 Threads , The result can also prove this conclusion

use wait simulation join Example

@Slf4j(topic = "c.TestInterJoinMN")
public class TestInterJoinMN {
    
    public static void main(String[] args) {
    
        Thread t1 = new Thread(()->{
    
            Sleeper.sleep(2);
            for (int i=0;i<5;i++){
    
                log.debug("t1");
            }
        },"t1");

        Thread t2 = new Thread(()->{
    
            synchronized (t1){
    
                try {
    
                    t1.wait();
                } catch (InterruptedException e) {
    
                    e.pri(ntStackTrace();
                }
            }
            for (int i=0;i<5;i++){
    
                log.debug("t2");
            }
        },"t2");
        t2.start();
        t1.start();
    }
}

result

15:12:16.361 c.TestInterJoinMN [t1] - t1
15:12:16.363 c.TestInterJoinMN [t1] - t1
15:12:16.363 c.TestInterJoinMN [t1] - t1
15:12:16.363 c.TestInterJoinMN [t1] - t1
15:12:16.363 c.TestInterJoinMN [t1] - t1
15:12:16.363 c.TestInterJoinMN [t2] - t2
15:12:16.363 c.TestInterJoinMN [t2] - t2
15:12:16.363 c.TestInterJoinMN [t2] - t2
15:12:16.363 c.TestInterJoinMN [t2] - t2
15:12:16.363 c.TestInterJoinMN [t2] - t2

explain
We found that we were t2 The thread creates a with t1 The thread object is the lock of the monitor , And perform the wait() Methods lead to t2 Thread caught waiting , At this time, if we don't notify or notifyAll Method words Can't wake up t2 Thread , But something magical happened here We found that eventually t2 The thread still executes . The reason is still this sentence stay java in ,Thread Class thread finished executing run() After the method , It will be automatically executed notifyAll() Method , Because we put t1 Threads are treated as lock objects , But as the t1 The running of the thread ends namely run End of method run t1 The thread executes t1.notifyAll(), So I woke up at this time All with t1 Thread for lock , therefore t2 The thread is awakened . This is it. join Implementation principle of .