CountdownLatch(门闩)

àì夳堔傛蜴生んèń 2024-03-22 23:53 61阅读 0赞

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）。

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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。

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案例一：

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案例二：

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### 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][watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5ZaK5oiR5bCP546L5ZCn77yB_size_20_color_FFFFFF_t_70_g_se_x_16]

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][watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5ZaK5oiR5bCP546L5ZCn77yB_size_20_color_FFFFFF_t_70_g_se_x_16 1]

### 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 ---------- ");  
> \}

测试结果

![在这里插入图片描述][watermark_type_d3F5LXplbmhlaQ_shadow_50_text_Q1NETiBA5ZaK5oiR5bCP546L5ZCn77yB_size_20_color_FFFFFF_t_70_g_se_x_16 2]

### 最后 ###

CountDownLatch 可以用来计数，可以测试任务是否执行结束  
也可以用来停止一个线程，也可以用来线程运行结束完后通知事件，彼此工作的线程互相独立不关心。

[9af31ea88e934f5da7c175da166b3766.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/86dd805b825f4ccd85047dfba6ec8a09.png
[b5202f406fbe4385a63fb6564eae585e.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/2d87980e7a724030a11ed9e41bcba285.png
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[dc38aad132bd42fa8474c8f22cd25629.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/88209595ea4443d096ee16cddacb7f10.png
[51c08499f6044117af5ea796f0bf38a4.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/92f114c19da04853b7b75a49dafbd795.png
[d9518d3780f64ce19240745d1b188c13.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/fbd1e77c84d14da4bb9a189f0594bc45.png
[2ed3499b21894816ab60443393530f29.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/8a39c7a48ee94659accef974a29495ef.png
[2f1633f4407942a7ae3bbb5ddf9cbd32.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/c254d35e86154b2f895c827126b95fe3.png
[16575399bd0f41cfa297994af48c2cfe.png]: https://img-blog.csdnimg.cn/16575399bd0f41cfa297994af48c2cfe.png
[7f8444340c114ed4882df6ec1499bd2a.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/b99921d7e7d14f06a7369042ed5e0954.png
[2f053489561c4a66b11d40f1031e32d7.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/033f72ee7fc04725ab4484ebcb9b6ae3.png
[915b72e4ff7e43d2b0b2dd59d257cc88.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/4aee5c1701fd4b1b99d44faef4cb0347.png
[3d8fd383bc554c5799b627ff0b30c641.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/a8278f84c1e0402d836a7f3b0ef7c563.png
[907bd59e2b44447884fc1305c57c2826.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/7242e3cbf467497aa97bdb93baa9a3d4.png
[a8e3432a5d7a48d4af2ff16721751766.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/0da0f072038f4e94926d3c8b3f959212.png
[3e1e1b343a1443b8ba75e6c563506fcd.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/14/eff6dc7260604937a17cf18f8b00147e.png
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[CyclicBarrier]: https://so.csdn.net/so/search?q=CyclicBarrier&spm=1001.2101.3001.7020
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