[interview: concurrent Article 27: multithreading: hesitation mode]

【 interview ： Concurrent articles 27： Multithreading ： Hesitation mode 】

00. Preface

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

01. Introduce

What is hesitation mode , It means that a thread finds another thread or thread has done something the same , Then this thread does not need to do any more , Straight back to .
Generally speaking, it's An object can only effectively call a method once , Then call directly to return to the end .

02. Two stage termination mode - Hesitation mode optimization

@Slf4j(topic = "c.TwoPhaseTermination")
public class Test13 {
    
    public static void main(String[] args) throws InterruptedException {
    
        TwoPhaseTermination tpt = new TwoPhaseTermination();
        tpt.start();
        tpt.start();
        tpt.start();

        Thread.sleep(3500);
        log.debug(" Stop monitoring ");
        tpt.stop();
    }
}

@Slf4j(topic = "c.TwoPhaseTermination")
class TwoPhaseTermination {
    
    //  Monitor threads 
    private Thread monitorThread;
    //  Stop mark 
    private volatile boolean stop = false;
    //  To judge whether or not  start  Method 
    private boolean starting = false;

    //  Start the monitoring thread 
    public void start() {
    
        synchronized (this) {
     // Hesitation mode main part code 
            if (starting) {
     // false
                return;
            }
            starting = true;
        }
        monitorThread = new Thread(() -> {
    
            while (true) {
    
                if (stop) {
    
                    log.debug(" Take care of future affairs ");
                    break;
                }
                try {
    
                    Thread.sleep(1000);
                    log.debug(" Execution monitoring record ");
                } catch (Exception e) {
    
                }
            }
        }, "monitor");
        monitorThread.start();
    }

    //  Stop monitoring thread 
    public void stop() {
    
        stop = true;
        monitorThread.interrupt();

    }
}

result

19:53:25.992 c.TwoPhaseTermination [monitor] - Execution monitoring record
19:53:27.004 c.TwoPhaseTermination [monitor] - Execution monitoring record
19:53:28.009 c.TwoPhaseTermination [monitor] - Execution monitoring record
19:53:28.487 c.TwoPhaseTermination [main] - Stop monitoring
19:53:28.487 c.TwoPhaseTermination [monitor] - Take care of future affairs

explain
We created one starting Variable The function is to judge whether this method has been called If not, execute the following code Direct return if any , Here we have implemented it three times start Method But it only ran once .
But one more thing to note Judge whether it is implemented if Statement needs to be locked , The purpose is to prevent multithreading Execute the same method repeatedly