1.背景:
- countDownLatch是在java1.5被引入,跟它一起被引入的工具类还有CyclicBarrier、Semaphore、concurrentHashMap和BlockingQueue。
- 存在于java.util.cucurrent包下
2.概念
- countDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。
- 是通过一个计数器来实现的,计数器的初始值是线程的数量。每当一个线程执行完毕后,计数器的值就-1,当计数器的值为0时,表示所有线程都执行完毕,然后在闭锁上等待的线程就可以恢复工作了。
3.源码
- countDownLatch类中只提供了一个构造器:
//参数count为计数值 public CountDownLatch(int count) { };
- 类中有三个方法是最重要的:
//调用await()方法的线程会被挂起,它会等待直到count值为0才继续执行 public void await() throws InterruptedException { }; //和await()类似,只不过等待一定的时间后count值还没变为0的话就会继续执行 public boolean await(long timeout, TimeUnit unit) throws InterruptedException { }; //将count值减1 public void countDown() { };
4.示例
普通示例:public class CountDownLatchTest { public static void main(String[] args) { final CountDownLatch latch = new CountDownLatch(2); System.out.println("主线程开始执行…… ……"); //第一个子线程执行 ExecutorService es1 = Executors.newSingleThreadExecutor(); es1.execute(new Runnable() { @Override public void run() { try { Thread.sleep(3000); System.out.println("子线程:"+Thread.currentThread().getName()+"执行"); } catch (InterruptedException e) { e.printStackTrace(); } latch.countDown(); } }); es1.shutdown(); //第二个子线程执行 ExecutorService es2 = Executors.newSingleThreadExecutor(); es2.execute(new Runnable() { @Override public void run() { try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("子线程:"+Thread.currentThread().getName()+"执行"); latch.countDown(); } }); es2.shutdown(); System.out.println("等待两个线程执行完毕…… ……"); try { latch.await(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("两个子线程都执行完毕,继续执行主线程"); } }
结果集:
主线程开始执行…… …… 等待两个线程执行完毕…… …… 子线程:pool-1-thread-1执行 子线程:pool-2-thread-1执行 两个子线程都执行完毕,继续执行主线程
模拟并发示例:
public class Parallellimit { public static void main(String[] args) { ExecutorService pool = Executors.newCachedThreadPool(); CountDownLatch cdl = new CountDownLatch(100); for (int i = 0; i < 100; i++) { CountRunnable runnable = new CountRunnable(cdl); pool.execute(runnable); } } } class CountRunnable implements Runnable { private CountDownLatch countDownLatch; public CountRunnable(CountDownLatch countDownLatch) { this.countDownLatch = countDownLatch; } @Override public void run() { try { synchronized (countDownLatch) { /*** 每次减少一个容量*/ countDownLatch.countDown(); System.out.println("thread name:"+Thread.currentThread().getName()+",thread counts = " + (countDownLatch.getCount())); } countDownLatch.await(); System.out.println("thread name:"+Thread.currentThread().getName()+",concurrency counts = " + (100 - countDownLatch.getCount())); } catch (InterruptedException e) { e.printStackTrace(); } } }
结果集:
thread name:pool-1-thread-1,thread counts = 99 thread name:pool-1-thread-5,thread counts = 98 thread name:pool-1-thread-4,thread counts = 97 thread name:pool-1-thread-6,thread counts = 96 thread name:pool-1-thread-3,thread counts = 95 thread name:pool-1-thread-7,thread counts = 94 thread name:pool-1-thread-2,thread counts = 93 thread name:pool-1-thread-8,thread counts = 92 thread name:pool-1-thread-9,thread counts = 91 thread name:pool-1-thread-10,thread counts = 90 thread name:pool-1-thread-11,thread counts = 89 thread name:pool-1-thread-12,thread counts = 88 thread name:pool-1-thread-13,thread counts = 87 ... thread name:pool-1-thread-98,thread counts = 2 thread name:pool-1-thread-99,thread counts = 1 thread name:pool-1-thread-100,thread counts = 0 thread name:pool-1-thread-100,concurrency counts = 100 thread name:pool-1-thread-1,concurrency counts = 100 thread name:pool-1-thread-4,concurrency counts = 100 thread name:pool-1-thread-5,concurrency counts = 100 ...
CountDownLatch和CyclicBarrier区别:
1.countDownLatch是一个计数器,线程完成一个记录一个,计数器递减,只能只用一次 2.CyclicBarrier的计数器更像一个阀门,需要所有线程都到达,然后继续执行,计数器递增,提供reset功能,可以多次使用
https://www.jianshu.com/p/e233bb37d2e6