CountDownLatch倒計數器

目錄

CountDownLatch倒計數器

CountDownLatch場景

CountDownLatch原理

CountDownLatch示例 

CountDownLatch場景

 想要做當前事情之前，必須具備前期的若干事件都準備完畢，當前事件才能進行。用程式來說，就是：

當前執行緒處於阻塞狀態，必須等到其它的若干執行緒都執行完畢之後，當前執行緒才被喚醒得以執行。

CountDownLatch原理

CountDownLatch類的原始碼：內部私有類Sync繼承AQS，最主要的兩個方法await()、countDown()。CountDownLatch類是共享模式的同步器。至於AQS，concurrent包的基石，建議參考

CountDownLatch類其實是實現了執行緒之間的通訊，其背後實現的本質是利用了Object物件的wait()、notify()、notifyAll()方法來達到執行緒之間的通訊。

await()方法，功能類似Object的wait()方法，進行當前執行緒的阻塞。

countDown()方法，其內部邏輯是計數遞減1，然後判斷計數是否為零。直到計數當前為零，才喚醒阻塞的執行緒。功能類似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); //模擬前期準備事件耗時
			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 ;//計數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();//這裡阻塞,直至計數降為零
		}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");

	}
}
 

執行結果如下：

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