java可以使用CountDownLatch來控制執行緒併發，使得一個或多個執行緒等待其他執行緒執行到某個操作後在執行。

如圖示：

1.在CountDownLatch例項化的時候定義需要等待的執行緒count數。

2.通過CountDownLatch的await方法，當count數為0時喚醒。

3.每次執行完需要先執行的執行緒時使用countDown方法，使得count數減1，就成功實現其他執行緒都執行完才執行await後操作

package study.joker.test;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.CountDownLatch;

public class test1 {
    static Logger logger = LoggerFactory.getLogger(test1.class);

    public static void main(String[] args) {
        try {
            final CountDownLatch latch = new CountDownLatch(3);

            new Thread(() -> {
                try {
                    logger.info("this is first Thread");
                    Thread.sleep(5000);
                    latch.countDown();
                    logger.info("this is first end ;latch.count={}", latch.getCount());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();

            new Thread(() -> {
                try {
                    logger.info("this is second Thread");
                    Thread.sleep(6000);
                    latch.countDown();
                    logger.info("this is second end;latch.count={}", latch.getCount());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();

            new Thread(() -> {
                try {
                    logger.info("this is third Thread begin");
                    Thread.sleep(7000);
                    latch.countDown();
                    logger.info("this is third Thread end；latch.count={}", latch.getCount());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();

            latch.await();//阻塞當前執行緒直到latch中數值為零才執行
//            Thread.sleep(10);
            logger.info("主執行緒執行！latch.count={}", latch.getCount());
            while (true) {
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}