Juc-7.0-thread collaboration countdownlatch

Statement

The running and timing of threads are beyond our control , In some cases, we may need multiple threads to complete some tasks , Like threads A etc. B The thread runs after execution

CountDownLatch

CountDownLatch Is a tool class for thread collaboration , It's a counter , The following methods are mainly used :

• Construction method , Parameter is count , Is the value that needs to be counted backwards
• await(): call await() Method will be suspended , wait until count by 0 Will continue to execute when
• countDown(): Each call will count Value reduction 1, Until 0 The waiting thread will be awakened

Common use

Use a

CountDownLatch The first use of is that a thread waits for multiple threads to complete execution , Continue your work , The code example is as follows

@Slf4j
public class CountDownLatchTest {

    public static void main(String[] args) throws InterruptedException {
        ExecutorService threadPool = Executors.newFixedThreadPool(5);
        CountDownLatch countDownLatch = new CountDownLatch(5);

        for (int i = 0; i < 5; i++) {
            threadPool.submit(()->{
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }finally {
                    log.info(" Completed ...");
                    countDownLatch.countDown();
                }
            });
        }

        log.info(" Waiting for other threads ....");
        countDownLatch.await();
        log.info(" All threads are finished ...");

    }

}

The operation output is as follows :

10:25:42.359 [main] INFO com.learning.java.cooperation.CountDownLatchTest -  Waiting for other threads ....
10:25:43.352 [pool-1-thread-2] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:25:43.352 [pool-1-thread-3] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:25:43.352 [pool-1-thread-1] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:25:43.353 [pool-1-thread-5] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:25:43.353 [pool-1-thread-4] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:25:43.353 [main] INFO com.learning.java.cooperation.CountDownLatchTest -  All threads are finished ...

First, create a new one count by 5 Of CountDownLatch Counter , Then build 5 Threads , Then the main thread calls await() wait for , wait for 5 All threads have called countDown() after , count by 0, The main thread continues to run

Use two

The second usage is that multiple threads wait for the signal of a thread , Then start executing at the same time , The code example is as follows :

@Slf4j
public class CountDownLatchTest {

    public static void main(String[] args) throws InterruptedException {
        ExecutorService threadPool = Executors.newFixedThreadPool(5);
        CountDownLatch countDownLatch = new CountDownLatch(1);

        for (int i = 0; i < 5; i++) {
            threadPool.submit(()->{
                try {
                    log.info(" be ready , Wait for the main thread signal ");
                    //  Hung thread , Wait for the main thread signal 
                    countDownLatch.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }finally {
                    log.info(" Completed ...");
                }
            });
        }

        Thread.sleep(2000);
        log.info(" The main thread is ready ..");
        countDownLatch.countDown();
    }
}

The console output is as follows

10:41:52.079 [pool-1-thread-1] INFO com.learning.java.cooperation.CountDownLatchTest -  be ready , Wait for the main thread signal 
10:41:52.079 [pool-1-thread-5] INFO com.learning.java.cooperation.CountDownLatchTest -  be ready , Wait for the main thread signal 
10:41:52.080 [pool-1-thread-2] INFO com.learning.java.cooperation.CountDownLatchTest -  be ready , Wait for the main thread signal 
10:41:52.088 [pool-1-thread-3] INFO com.learning.java.cooperation.CountDownLatchTest -  be ready , Wait for the main thread signal 
10:41:52.092 [pool-1-thread-4] INFO com.learning.java.cooperation.CountDownLatchTest -  be ready , Wait for the main thread signal 
10:41:54.072 [main] INFO com.learning.java.cooperation.CountDownLatchTest -  The main thread is ready ..
10:41:54.072 [pool-1-thread-1] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:41:54.072 [pool-1-thread-5] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:41:54.072 [pool-1-thread-2] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:41:54.072 [pool-1-thread-3] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...
10:41:54.072 [pool-1-thread-4] INFO com.learning.java.cooperation.CountDownLatchTest -  Completed ...

This scenario is generally used for pressure measurement , When multiple threads are ready , Send the request to the server together

summary

• understand CountDownLatch Two common usage scenarios : Wait more and wait more
• CountDownLatch When instantiating, you need to pass in the count down , Wait until the countdown is 0 When , All called await() The thread of the method will be awakened
• CountDownLatch Cannot roll back reset , That is, it cannot be reused