multithreading - How does CountDownLatch works in Java? -

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• how countdownlatch used in java multithreading? 10 answers

i studying synchronization in java. not able understand exact mechanism of countdownlatch.

does countdownlatch 'counts down latch' (waits completion of number of threads) per number of threads given @ declaration?

here code tried understand:

public class latchexample implements runnable {     private countdownlatch latch;      private int id;      public latchexample(int id, countdownlatch latch){         this.id=id;         this.latch = latch;     }      public static void main(string[] args) {          countdownlatch latch = new countdownlatch(5);          executorservice executor = executors.newfixedthreadpool(3);          (int = 0; < 7; i++) {             executor.submit(new latchexample(i,latch));         }          try {             latch.await();             system.out.println("all process completed");         } catch (interruptedexception e) {             e.printstacktrace();         }           system.out.println();     }      @override     public void run() {          system.out.println("starting: "+id);         try {             thread.sleep(1000);         } catch (interruptedexception e) {             e.printstacktrace();         }          latch.countdown();     } } 

in example above:

7 threads spawned executorservice (from thread pool). understanding latch should wait completion of 6 threads (from 0 5), defined by:

countdownlatch latch = new countdownlatch(5); 

but output not constant every time. waits 6 threads complete , waits 7 e.g.:

starting: 1 starting: 0 starting: 2 starting: 3 starting: 5 starting: 4 starting: 6 process completed 

here output @ alternate times:

starting: 0 starting: 2 starting: 1 starting: 4 starting: 5 starting: 3 process completed  starting: 6 

edit : countdownlatch should ideally countdown until 5 tasks passed latch. here showing either 6 or 7.

what fix code, if want display 5 tasks before 'all process completed' ?

your latch need 5 countdown() calls reach 0 , let await return. however, way task executor written, more tasks start running needed release latch. helpful approach seeing code working through run step step, this:

1. tasks 0-2 start running on 3 threadpool threads;
2. you see starting: n printed 3 times, n ranges between 0 , 2, in arbitrary order;
3. a second passes;
4. the 3 tasks complete simultaneously, bringing latch's count down 2;
5. tasks 3-5 start running;
6. you see starting: n printed 3 times, n ranging between 3 , 5, in arbitrary order;
7. another second passes;
8. tasks 3-5 complete simultaneously, releasing latch. both main thread can continue , task 6 can start;
9. all processes completed prints @ same time starting: 6, in arbitrary order.

now, not quite clear expected code do, hope above way of reasoning bring state behavior aligns expectation.