java concurrent 之 CountDownLatch

java concurrent 之 CountDownLatch

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
A CountDownLatch is initialized with a given count. The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.

A CountDownLatch is a versatile synchronization tool and can be used for a number of purposes. A CountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all threads invoking await wait at the gate until it is opened by a thread invoking countDown(). A CountDownLatch initialized to N can be used to make one thread wait until N threads have completed some action, or some action has been completed N times.

A useful property of a CountDownLatch is that it doesn't require that threads calling countDown wait for the count to reach zero before proceeding, it simply prevents any thread from proceeding past an await until all threads could pass.

demo

package com.viashare;

import java.util.concurrent.CountDownLatch;

/**
 * Created by Jeffy on 16/01/11.
 */
public class CountDownLatchMain {

    private static final int COUNT_NUM = 10;

    public static void main(String[] args) throws InterruptedException {

        CountDownLatch countDownLatch = new CountDownLatch(COUNT_NUM);

        for(int i = 0;i<COUNT_NUM;i++){
            new Thread(new ThreadTask(countDownLatch,i)).start();

        }
        Thread.sleep(5000);//主要是模拟等待
        for(int i = 0;i<COUNT_NUM;i++){
            countDownLatch.countDown();
        }

    }

    static class ThreadTask implements Runnable{
        CountDownLatch countDownLatch = null;

        private int no;

        public ThreadTask(CountDownLatch countDownLatch, int no) {
            this.countDownLatch = countDownLatch;
            this.no = no;
        }

        @Override
        public void run() {
            try {


                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.err.println(no);
        }
    }
}