Java併發包之Latch

  Latch可以於當一個線程需要等候其它線程完成後再執行的情形java.util.concurrent包中CountDownLatch類實現了Latch功能，CountDownLatch（n）的構造函數中設置參與的線程數，await()用於等待其它線程，countDown()方法減少Latch的計數，如果計數達到零，則釋放所有等待的線程.

用例：

1. 要對某個方法做併發測試時（CountDownLatch參與者設爲1，其它線程run()方法裏調用await()，所有線程初始化完成後，主線程調用countDown()方法，讓所有線程同時執行），
2. 要從多個數據表中取得數據後，對這些數據進行加工，可以用多個線程去數據庫查詢數據，完成後設置countDown()，主線程用await()方法等候所有線程查詢完成後，分類彙總。

 

  用CountDownLatch實現一個旅遊的例子，一開始導遊在一個指定地方等所有遊客到來，所有遊客到齊後，行程開始。

  Tourist類繼承Thread類，代表一個遊客，讓Thread.sleep()生成一個隨機時間，代表遊客路上花費的時間，當到達後調用countDown方法，減少一個CountDownLatch的計數。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class Tourist extends Thread {
    private final CountDownLatch latch;
    private final String name;

    public Tourist(CountDownLatch latch, String name) {
        this.latch = latch;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            long duration = (long) (Math.random() * 10);
            TimeUnit.SECONDS.sleep(duration);
            System.out.printf("%s has arrived\n", name);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            latch.countDown();
        }
    }
}

 

Guide類代表一個導遊，當導遊到達集合地點，調 await()方法等Tourist到來，當所有人到達以後，則行程開始。

public class Guide  extends Thread {

    private CountDownLatch latch;

    public Guide(CountDownLatch latch) {

        this.latch=latch;

    }



    @Override

    public void run() {

        try {

            System.out.println("Guide  is waiting for Tourists ...");

            latch.await();

            System.out.println("Trip has started...");

        }catch (InterruptedException e) {

            e.printStackTrace();

        }

    }



    public static void main(String[] args) {

        CountDownLatch latch=new CountDownLatch(5);

        Guide guide =new Guide(latch);

        guide.start();

        for (int i = 1; i <= 5; i++) {

            Tourist tourist =new Tourist(latch, "Tourist" + i);

            tourist.start();

        }

    }

}

 

執行結果

Guide  is waiting for Tourists ...

Tourist2 has arrived

Tourist3 has arrived

Tourist4 has arrived

Tourist1 has arrived

Tourist5 has arrived

Trip has started...