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CountDownLatch literally means countdown latches. The two typical uses of CountDownLatch are either one-to-one or one-to-one. The two uses are demonstrated here, and it is important to note that CountDownLatch cannot be reused

Usage 1 (first class more)

A thread waits for multiple threads to complete before resuming its work

package org.example.concurrent; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; /** * this class is used to demonstrate a use of CountDownLatch ** a thread waits for all other threads to complete ** this class assumes that the main thread waits for all five child threads to complete before moving on to the next stage ** @author Catch */ public class CountDownLatchUsage1 { public static void main(String[] args) throws InterruptedException { CountDownLatch latch=new CountDownLatch(5); ExecutorService executorService = Executors.newFixedThreadPool(5); for (int i = 0; i < 5; i++) { int no=i+1; Runnable r=()->{ try { Thread.sleep((long) (Math.random()*10000)); System.out.println("No."+ No +" finished checking "); } catch (InterruptedException e) { e.printStackTrace(); }finally{ latch.countDown(); }}; executorService.submit(r); } system.out. println(" Wait for 5 child threads to complete the check...") ); latch.await(); System.out.println(" All child threads have finished checking and are starting the next phase."); }}Copy the code

Usage 2 (more than one)

Below is a scenario where multiple threads wait for one thread to finish work, and then multiple threads start work at the same time

package org.example.concurrent; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; /** * This class is used to demonstrate the use of CountDownLatch. * Multiple threads wait for one thread to complete work, and then multiple threads start working at the same time. * * @author Catch */ public class CountDownLatchUsage2 {public static void CountDownLatchUsage2 {public static void CountDownLatchUsage2 main(String[] args) throws InterruptedException { CountDownLatch begin=new CountDownLatch(1); CountDownLatch end = new CountDownLatch(1000); ExecutorService executorService = Executors.newFixedThreadPool(1000); for (int i = 0; i < 1000; I++) {Runnable r=()->{try {// some preparation or initialization can be done before begin.await(); begin.await(); Thread.sleep((long) (math.random () * 5000)); System.out.println(thread.currentThread ().getName())+ ); } catch (InterruptedException e) { e.printStackTrace(); }finally{ end.countDown(); }}; executorService.submit(r); } Thread.sleep(3000); System.out.println(" All threads ready, pressure test started!") ); begin.countDown(); end.await(); System.out.println(" All threads completed, pressure finished!") ); }}Copy the code