Java-CountDownLatch

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CountDownLatch是什么？

CountDownLatch 位于java.util.concurrent包下，是JDK5.0的新特性，根据包名便可知，它是用于处理多线程并发的工具类。

CountDownLatch能干什么？

CountDownLatch 能够使一个线程处于等待状态，直到它期望的其他线程达到预期，再恢复运行状态。

CountDownLatch的运行原理

CountDownLatch 是通过一个计数器来实现的，计数器的初始值为线程数。
每当一个线程完成任务后，计数器的值就会减1。当计数器的值为0时，它表示所线程已经完成任务，然后在闭锁上等待线程就恢复运行状态。

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1. Main thread start
2. Create CountDownLatch for N threads
3. Create and start N threads
4. Main thread wait on latch
5. N threads completes there tasks are returns
6. Main thread resume execution

CountDownLatch原理

CountDownLatch主要API

  1. 构造函数

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    //Constructs a CountDownLatch initialized with the given count.
    public void CountDownLatch(int count) {…}

  2. 让线程处于等待状态的函数

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    CountDownLatch.await()

  3. 让线程恢复执行状态的函数（cnt=0时，线程恢复运行状态）

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    CountDownLatch.countDown()

示例

主线程启动三个子线程，完成各自的验证工作后，主线程再继续运行，使用CountDownLatch进行实现。

主程序入口

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package com.atopom.jdk5.concurrent.countdownlatch;
public class _Main {
	public static void main(String[] args) {
		boolean result = false;
		try {
			result = ApplicationStartupUtil.checkExternalServices();
		} catch (Exception e) {
			e.printStackTrace();
		}
		System.out.println("External services validation completed !! Result was :: " + result);
	}
}

运行结果

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Checking Cache Service
Checking Database Service
Checking Network Service
Database Service is UP
Cache Service is UP
Network Service is UP
External services validation completed !! Result was :: true

代码地址

https://github.com/atopom/java_familiar_strange/tree/master/Code/CountDownLatchDemo