CountDownLatch（应对并发问题的工具类）

CountDownLatch

CountDownLatch允许一个或多个线程等待其他线程完成操作以后，再执行当前线程；比如我们在主线程需要开启2个其他线程，当其他的线程执行完毕以后我们再去执行主线程，针对这

个需求我们就可以使用CountDownLatch来进行实现。CountDownLatch中count down是倒着数数的意思；CountDownLatch是通过一个计数器来实现的，每当一个线程完成了自己的

任务后，可以调用countDown()方法让计数器-1，当计数器到达0时，调用CountDownLatch的await()方法的线程阻塞状态解除，继续执行。

CountDownLatch的相关方法

public CountDownLatch(int count)						// 初始化一个指定计数器的CountDownLatch对象
public void await() throws InterruptedException			// 让当前线程等待
public void countDown()									// 计数器进行减1

案例演示：使用CountDownLatch完成上述需求(我们在主线程需要开启2个其他线程，当其他的线程执行完毕以后我们再去执行主线程)

实现思路：在main方法中创建一个CountDownLatch对象，把这个对象作为作为参数传递给其他的两个任务线程

线程任务类1

public class CountDownLatchThread01 implements Runnable {// CountDownLatch类型成员变量private CountDownLatch countDownLatch ;public CountDownLatchThread01(CountDownLatch countDownLatch) {      // 构造方法的作用：接收CountDownLatch对象this.countDownLatch = countDownLatch ;}@Overridepublic void run() {try {Thread.sleep(10000);System.out.println("10秒以后执行了CountDownLatchThread01......");} catch (InterruptedException e) {e.printStackTrace();}// 调用CountDownLatch对象的countDown方法对计数器进行-1操作countDownLatch.countDown();}}

线程任务类2

public class CountDownLatchThread02 implements Runnable {// CountDownLatch类型成员变量private CountDownLatch countDownLatch ;public CountDownLatchThread02(CountDownLatch countDownLatch) {      // 构造方法的作用：接收CountDownLatch对象this.countDownLatch = countDownLatch ;}@Overridepublic void run() {try {Thread.sleep(3000);System.out.println("3秒以后执行了CountDownLatchThread02......");} catch (InterruptedException e) {e.printStackTrace();}// 调用CountDownLatch对象的countDown方法对计数器进行-1操作countDownLatch.countDown();}}

测试类

public class CountDownLatchDemo01 {public static void main(String[] args) {//  1. 创建一个CountDownLatch对象CountDownLatch countDownLatch = new CountDownLatch(2) ;                 // CountDownLatch中的计数器的默认值就是2//  2. 创建线程任务类对象，并且把这个CountDownLatch对象作为构造方法的参数进行传递CountDownLatchThread01 countDownLatchThread01 = new CountDownLatchThread01(countDownLatch) ;//  3. 创建线程任务类对象，并且把这个CountDownLatch对象作为构造方法的参数进行传递CountDownLatchThread02 countDownLatchThread02 = new CountDownLatchThread02(countDownLatch) ;//  4. 创建线程对象，并启动线程Thread t1 = new Thread(countDownLatchThread01);Thread t2 = new Thread(countDownLatchThread02);t1.start();t2.start();//  5. 在主线程中调用 CountDownLatch中的await让主线程处于阻塞状态try {countDownLatch.await();} catch (InterruptedException e) {e.printStackTrace();}//  6. 程序结束的输出System.out.println("主线程执行了.... 程序结束了......");}}

控制台输出结果

3秒以后执行了CountDownLatchThread02......
10秒以后执行了CountDownLatchThread01......
主线程执行了.... 程序结束了......

CountDownLatchThread02线程先执行完毕，此时计数器-1；CountDownLatchThread01线程执行完毕，此时计数器-1；当计数器的值为0的时候，主线程阻塞状态接触，主线程向下执行。