CountdownLatch(门闩)

news2024/9/22 9:34:41

CountDownLatch是一个同步工具类，用来协调多个线程之间的同步，或者说起到线程之间的通信（而不是用作互斥的作用）。

CountDownLatch能够使一个线程在等待另外一些线程完成各自工作之后，再继续执行。使用一个计数器进行实现。计数器初始值为线程的数量。
当每一个线程完成自己任务后，计数器的值就会减一。当计数器的值为0时，表示所有的线程都已经完成一些任务，然后在CountDownLatch上等待的线程就可以恢复执行接下来的任务。

案例一：

案例二：

CountDownLatch使用场景

线程计数器用于线程执行任务,计数等待线程结束

用法一: 等待所有的事情都做完

//程序计数器
CountDownLatch countDownLatch = new CountDownLatch(10000);

//2个线程
ExecutorService executorService = Executors.newFixedThreadPool(2);

AtomicInteger count = new AtomicInteger(0);
for (int i = 0; i < 10000; i++) {

executorService.submit(() -> {
count.getAndIncrement();//自增
System.out.println(Thread.currentThread().getName() + " : " + count.get());
countDownLatch.countDown();
});
}

//线程池等待10s
executorService.awaitTermination(10, TimeUnit.SECONDS);

//关闭线程其实是将线程状态设置为中断标志必须等待所有线程处理完任务,才能完全关闭
executorService.shutdown();

//必须等待两个线程执行完会一直等待下去，当然也可以设置指定时间等待超时 await(timeout);
countDownLatch.await();

}

始终是2个线程在做事情，等2个线程做完事情才会停止下来。

用法二：假设2个线程做事情,刚开始并行做事情,等一个执行完成之后,另一个才能执行(实际还是计数)

//程序计数器
CountDownLatch countDownLatch = new CountDownLatch(1);

Thread thread1 =new Thread(()->{

System.out.println(" ---------------- 1 准备 ---------------- ");
try {
countDownLatch.await();
System.out.println(" ---------------- 1 finsh ---------------- ");
} catch (InterruptedException e) {
e.printStackTrace();
}
});
thread1.start();

Thread thread2 = new Thread(() -> {

System.out.println(" ---------------- 2 准备 ---------------- ");
try {
Thread.sleep(1_000);
System.out.println(" ---------------- 异步做事情 ---------------- ");
} catch (InterruptedException e) {
e.printStackTrace();
}finally {
countDownLatch.countDown();
}
});
thread2.start();

//main 在等main 结束死循环
Thread.currentThread().join();

刚开始一起在准备状态，然后分开做事情

在这里插入图片描述

用法三：退出条件

中断一个线程 count 到0

//程序计数器
CountDownLatch countDownLatch = new CountDownLatch(1);
Thread thread = Thread.currentThread();
Thread thread1 = new Thread(() -> {
try {
Thread.sleep(10_000);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 1 中断条件1
countDownLatch.countDown();

});

thread1.start();

countDownLatch.await();

System.out.println(" ----------------- ");

}

等待时间中断

//程序计数器
CountDownLatch countDownLatch = new CountDownLatch(1);
Thread thread = Thread.currentThread();
Thread thread1 = new Thread(() -> {
try {
Thread.sleep(10_000);
} catch (InterruptedException e) {
e.printStackTrace();
}
});

thread1.start();

//2 中断条件3
countDownLatch.await(5, TimeUnit.SECONDS);

System.out.println(" ----------------- ");

}

就中断条件而言: 当前还可以父线程中断

//程序计数器

Thread thread1 = new Thread(() -> {
try {
Thread.sleep(10_000);
} catch (InterruptedException e) {
e.printStackTrace();
}

// 中断条件3
thread.interrupt();
});

thread1.start();

System.out.println(" ----------------- ");

用法四：封装结束完后通知事件

封装结束完后通知事件参考 CyclicBarrier 通知

public class CountDownLatchTest4 extends CountDownLatch {

private Runnable runnable;

public CountDownLatchTest4(int count, Runnable runnable) {
super(count);
this.runnable = runnable;
}

@Override
public void countDown() {
super.countDown();
if (super.getCount()==0){
runnable.run();
}
}

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

//程序计数器
CountDownLatchTest4 countDownLatch = new CountDownLatchTest4(1,()->{

System.out.println(" 计数结束 .... ");
});

Thread thread1 = new Thread(() -> {
try {
Thread.sleep(2_000);
countDownLatch.countDown();
System.out.println(" thread 1 do something ");
} catch (InterruptedException e) {
e.printStackTrace();
}
});

Thread thread2 = new Thread(() -> {
try {
Thread.sleep(1_000);
System.out.println(" thread 2 do something ");
countDownLatch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
});

thread1.start();
thread2.start();

countDownLatch.await();

System.out.println(" ----------------- main 结束 ----------------- ");

}
}

可以看到运行结束，通知事件

在这里插入图片描述

自定义计数器

当然我们也可以实现自己的计数器

/**
* 自定义 CountDown 计数器
*/
public class CountDown {

//计数器
private int count = 0;
private final int total;

public CountDown(int total) {
this.total = total;
}

public void countDown() {
synchronized (this) {
this.count++;
//锁住 ++ 通知其他线程
this.notifyAll();
}
}

public void aWait() throws InterruptedException {
synchronized (this) {
while (total != count) {
//不等于则继续等待
this.wait();
}
}
}
}

测试

CountDown countDown = new CountDown( 5);

System.out.println(" 准备多线程处理任务 ");

IntStream.rangeClosed(1, 5).forEach(x -> {
new Thread(() -> {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(" 线程开始 ----- " + Thread.currentThread().getName());
countDown.countDown();
}, x + "").start();

});

try {
countDown.aWait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(" 准备多线程处理任务结束 ");
System.out.println(" ---------------------- ");
System.out.println(" 结束 mian ---------- ");
}