CountDownLatch用法详解

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10sexecutorService.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

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1countDownLatch.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3countDownLatch.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3thread.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;}@Overridepublic 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

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

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。

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CountDownLatch用法详解

CountDownLatch用法详解

CountDownLatch使用场景

自定义计数器

最后

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