How is CountDownLatch used in Java Multithreading? -

can me understand java countdownlatch , when use it?

i don't have clear idea of how program works. understand 3 threads start @ once , each thread call countdownlatch after 3000ms. count down decrement 1 one. after latch becomes 0 program prints "completed". maybe way understood incorrect.

import java.util.concurrent.countdownlatch; import java.util.concurrent.executorservice; import java.util.concurrent.executors;  class processor implements runnable {     private countdownlatch latch;      public processor(countdownlatch latch) {         this.latch = latch;     }      public void run() {         system.out.println("started.");          try {             thread.sleep(3000);         } catch (interruptedexception e) {             e.printstacktrace();         }          latch.countdown();     } } 

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public class app {      public static void main(string[] args) {          countdownlatch latch = new countdownlatch(3); // coundown 3 0          executorservice executor = executors.newfixedthreadpool(3); // 3 threads in pool          for(int i=0; < 3; i++) {             executor.submit(new processor(latch)); // ref latch. each time call new processes latch count down 1         }          try {             latch.await();  // wait until latch counted down 0         } catch (interruptedexception e) {             e.printstacktrace();         }          system.out.println("completed.");     }  } 

yes, understood correctly. countdownlatch works in latch principle, main thread wait until gate open. 1 thread waits n number of threads specified while creating countdownlatch in java.

any thread, main thread of application, calls countdownlatch.await() wait until count reaches 0 or interrupted thread. other thread required count down calling countdownlatch.countdown() once completed or ready.

as count reaches zero, thread awaiting starts running. 1 of disadvantages/advantages of countdownlatch it's not reusable once count reaches 0 can not use countdownlatch more.

edit:

use countdownlatch when 1 thread main thread, requires wait 1 or more thread complete, before can start processing.

classical example of using countdownlatch in java server side core java application uses services architecture, multiple services provided multiple threads , application can not start processing until services have started successfully.

p.s. op's question has pretty straightforward example didn't include one.