Java CountDownLatch example 2017-07-16 03:48
In java, if you want do something after multiple threads finish CountDownLatch might help you. This class can help you count how many threads is finished. If there is no thread running, it will wake up the main thread. The code example is like following.
public class Worker implements Runnable {
private CountDownLatch latch;
public Worker(CountDownLatch countDownLatch) {
this.latch= countDownLatch;
}
@Override
public void run() {
try {
System.out.println("thread:" + Thread.currentThread().getName() + " is running");
Thread.sleep(3000);
System.out.println("thread:" + Thread.currentThread().getName() + " is finish");
latch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public class CountDownLatchClient {
public static void main(String[] args) {
final CountDownLatch latch = new CountDownLatch(2);
new Thread(new Worker(latch)).start();
new Thread(new Worker(latch)).start();
try {
System.out.println("waiting 2 threads..");
latch.await();
System.out.println("2 threads finish.");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
final CountDownLatch latch = new CountDownLatch(2) means this latch waiting for 2 threads finish. latch.await() in the client will block the main thread until both workers are finished. Both Worker accept the instance of CountDownLatch, when workers finish they will invoke the countDown method. This method will decrease the count in the latch. When the count of latch becomes 0 the main thread will be waked up.
The output is like following.
waiting 2 threads..
thread:Thread-1 is running
thread:Thread-0 is running
thread:Thread-1 is finish
thread:Thread-0 is finish
2 threads finish.
EOF