CountDownLatch倒计数器
目录
CountDownLatch场景
想要做当前事情之前,必须具备前期的若干事件都准备完毕,当前事件才能进行。用程序来说,就是:
当前线程处于阻塞状态,必须等到其它的若干线程都运行完毕之后,当前线程才被唤醒得以执行。
CountDownLatch原理
CountDownLatch类的源码:内部私有类Sync继承AQS,最主要的两个方法await()、countDown()。CountDownLatch类是共享模式的同步器。至于AQS,concurrent包的基石,建议参考https://www.cnblogs.com/chengxiao/p/7141160.html
CountDownLatch类其实是实现了线程之间的通讯,其背后实现的本质是利用了Object对象的wait()、notify()、notifyAll()方法来达到线程之间的通讯。
await()方法,功能类似Object的wait()方法,进行当前线程的阻塞。
countDown()方法,其内部逻辑是计数递减1,然后判断计数是否为零。直到计数当前为零,才唤醒阻塞的线程。功能类似Object的notify()、notifyAll()方法
CountDownLatch示例
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
class PrepareEvent implements Runnable
{
private CountDownLatch c ;
public PrepareEvent(CountDownLatch c){
this.c = c ;
}
public void run(){
try{
long time = (long)(Math.random() * 5000); //模拟前期准备事件耗时
Thread.sleep(time);
String tName = Thread.currentThread().getName();
System.out.println(tName+"' working time is: "+time);
}catch(InterruptedException e){}
c.countDown();
}
}
public class CountDownLatchTest
{
public static void main(String[] args)
{
int count = 5 ;//计数5
CountDownLatch c = new CountDownLatch(count);
Thread[] ts = new Thread[count];
System.out.println("CountDownLatch init, now count is: "+c.getCount()+"\n\r");
System.out.println("because Of await(), main blocking...\n\r");
for(int i=count-1; i>=0; i--){
ts[i] = new Thread(new PrepareEvent(c),"T"+i);
ts[i].start();
}
try{
c.await();//这里阻塞,直至计数降为零
}catch(InterruptedException e){}
System.out.println(" \n\rafter await(), main continue...\n\r");
System.out.println("CountDownLatch used, now count is: "+c.getCount()+"\n\r");
}
}
运行结果如下:
CountDownLatch init, now count is: 5
because Of await(), main blocking...
T3' working time is: 1099
T2' working time is: 3320
T0' working time is: 3448
T4' working time is: 3647
T1' working time is: 3825
after await(), main continue...
CountDownLatch used, now count is: 0