CountDownLatch用法

CountDownLatch可以理解为是一个计数器。
关于CountDownLatch类方法如下

//构造实现类，count代表计数器大小
public CountDownLatch(int count);
//线程挂起等待计数器为0
public void await();
//线程在有限时间内挂起等待计数器为0
public boolean await(long timeout, TimeUnit unit);
//计数器减一
public void countDown();

CountDownLatch底层也是通过AQS实现，我们可以把我们定义的计数器大小理解为我们同时加了几把锁，countDown每次释放掉一把锁。

例1

public class CountDownLatchDemo {

    public static CountDownLatch countDownLatch = new CountDownLatch(1);

    static class ThreadDemo implements Runnable{
        public String name;

        public ThreadDemo(String name){
            this.name = name;
        }


        @Override
        public void run() {
            System.out.println(name+"开始"+getDate());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            countDownLatch.countDown();
            System.out.println(name+"结束"+getDate());
        }
    }

    static class ThreadDemo1 implements Runnable{
        public String name;

        public ThreadDemo1(String name){
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(name+"开始"+getDate());
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(name+"结束"+getDate());
        }
    }

    public static String getDate(){
        Date date = new Date();
        SimpleDateFormat dateFormat= new SimpleDateFormat("yyyy-MM-dd :hh:mm:ss");
        return  dateFormat.format(date);
    }
    public static void main(String []args) {
        Thread thread1 = new Thread(new ThreadDemo("t1"));
        Thread thread2 = new Thread(new ThreadDemo1("t2"));
        thread1.start();
        thread2.start();
    }
}

结果如下，因为t2执行了await，所以它会等待t1执行完countDown再往下执行。

t2开始2019-11-17 :11:30:26
t1开始2019-11-17 :11:30:26
t1结束2019-11-17 :11:30:28
t2结束2019-11-17 :11:30:28

例2

public class CountDownLatchDemo2 {

    public static CountDownLatch countDownLatch = new CountDownLatch(1);

    static class ThreadDemo implements Runnable{
        public String name;

        public ThreadDemo(String name){
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(name+"开始"+getDate());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            countDownLatch.countDown();
            System.out.println(name+"结束"+getDate());
        }
    }

    static class ThreadDemo1 implements Runnable{
        public String name;

        public ThreadDemo1(String name){
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(name+"开始"+getDate());
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(name+"结束"+getDate());
        }
    }

    public static String getDate(){
        Date date = new Date();
        SimpleDateFormat dateFormat= new SimpleDateFormat("yyyy-MM-dd :hh:mm:ss");
        return  dateFormat.format(date);
    }
    public static void main(String []args) {
        Thread thread1 = new Thread(new ThreadDemo("t1"));
        Thread thread2 = new Thread(new ThreadDemo1("t2"));
        Thread thread3 = new Thread(new ThreadDemo1("t3"));
        thread1.start();
        thread2.start();
        thread3.start();
    }
}

结果如下,可以看到我们启用2个线程await，也是会等待countDown完毕才会执行。

t2开始2019-11-17 :11:43:58
t1开始2019-11-17 :11:43:58
t3开始2019-11-17 :11:43:58
t1结束2019-11-17 :11:44:00
t3结束2019-11-17 :11:44:00
t2结束2019-11-17 :11:44:00

例3

public class CountDownLatchDemo3 {

    public static CountDownLatch countDownLatch = new CountDownLatch(1);

    static class ThreadDemo implements Runnable{
        public String name;

        public ThreadDemo(String name){
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(name+"开始"+getDate());
            countDownLatch.countDown();
            countDownLatch.countDown();
            System.out.println(name+"结束"+getDate());
        }
    }

    public static String getDate(){
        Date date = new Date();
        SimpleDateFormat dateFormat= new SimpleDateFormat("yyyy-MM-dd :hh:mm:ss");
        return  dateFormat.format(date);
    }
    public static void main(String []args) {
        Thread thread1 = new Thread(new ThreadDemo("t1"));
        thread1.start();
    }
}

可以看到可以正常执行的

t1开始2019-11-17 :11:49:03
t1结束2019-11-17 :11:49:04

例4

public class CountDownLatchDemo4 {

    public static CountDownLatch countDownLatch = new CountDownLatch(1);

    static class ThreadDemo implements Runnable{
        public String name;

        public ThreadDemo(String name){
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(name+"开始"+getDate());
            countDownLatch.countDown();
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            countDownLatch.countDown();
            System.out.println(name+"结束"+getDate());
        }
    }

    public static String getDate(){
        Date date = new Date();
        SimpleDateFormat dateFormat= new SimpleDateFormat("yyyy-MM-dd :hh:mm:ss");
        return  dateFormat.format(date);
    }
    public static void main(String []args) {
        Thread thread1 = new Thread(new ThreadDemo("t1"));
        thread1.start();
    }
}

可以看到例4也可以正常执行的

t1开始2019-11-17 :11:51:07
t1结束2019-11-17 :11:51:07