淺析Java中CountDownLatch用法

CountDownLatch如其所寫，是一個倒計數的鎖存器，當計數減至0時觸發特定的事件。利用這種特性，可以讓主線程等待子線程的結束。下面以一個模擬運動員比賽的例子加以說明。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import android.util.Log;

public class CountDownLatchDemo {
	private static final int PLAYER_AMOUNT = 5;
	public CountDownLatchDemo() {
		// TODO Auto-generated constructor stub
	}
	/**
	* @param args
	*/
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		//對於每位運動員，CountDownLatch減1後即結束比賽
		CountDownLatch begin = new CountDownLatch(1);
		//對於整個比賽，所有運動員結束後纔算結束
		CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
		Player[] plays = new Player[PLAYER_AMOUNT];

		for(int i=0; i<PLAYER_AMOUNT; i++)
			plays[i] = new Player(i+1, begin, end);

		//設置特定的線程池，大小爲5
		ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
		for(Player p:plays)
			exe.execute(p);            //分配線程

		System.out.println("Race begins!");
		Log.i("countdownlatch", "Race begins!");
		begin.countDown();
		try {
			end.await();            //等待end狀態變爲0，即爲比賽結束
		} catch (InterruptedException e) {
			// TODO: handle exception
			e.printStackTrace();
		} finally {
			System.out.println("Race ends!");
			Log.i("countdownlatch", "Race ends!");
		}
		exe.shutdown();
	}
}
class Player implements Runnable {

	private int id;
	private CountDownLatch begin;
	private CountDownLatch end;
	public Player(int i, CountDownLatch begin, CountDownLatch end) {
		// TODO Auto-generated constructor stub
		super();
		this.id = i;
		this.begin = begin;
		this.end = end;
	}
	
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try {
			begin.await(); //等待begin的狀態爲0
			Thread.sleep((long)(Math.random()*100)); //隨機分配時間，即運動員完成時間
			System.out.println("Play"+id+" arrived.");
			Log.i("countdownlatch", "Play"+id+" arrived.");
		} catch (InterruptedException e) {
			// TODO: handle exception
			e.printStackTrace();
		} finally {
			end.countDown(); //使end狀態減1，最終減至0
		}
	}
}

原文地址