java - cachedThreadPool 没有按我的预期工作

Question

我正在玩线程，但我不明白为什么这没有像我想象的那样工作。

我正在尝试使用线程计算总和，并期望线程池在打印结果时等待所有任务完成(由于 shutdown() 调用和 isTermination() 检查)。

我在这里缺少什么？

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

public class Test5 {

private Integer sum= new Integer(0);

public static void main(String[] args) {

    ExecutorService pool = Executors.newCachedThreadPool(); 
    Test5 obj = new Test5();

    for(int i=0; i<1000; i++){
            pool.execute(obj.new Adding());
    }

    pool.shutdown();

    while(!pool.isTerminated()) {
        //could be empty loop...
        System.out.println(" Is it done? : " + pool.isTerminated());
    }

    System.out.println(" Is it done? : " + pool.isTerminated());
    System.out.println("Sum is " + obj.sum);                
}

class Adding implements Runnable {

    public void run() {

        synchronized(this) {

            int tmp = sum;
            tmp+=1;
            sum=new Integer(tmp);           
        }                       
    }               
}
}

虽然我确实得到了很好的结果，但我也得到了如下输出:

Is it done? : true
Sum is 983

Answer 1

您需要在主对象实例上同步。我在下面使用 int，Integer 也可以工作(需要显式初始化为零)。

这是工作代码

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

public class AppThreadsSum {

    int sum;

    public static void main(String[] args) {

        ExecutorService pool = Executors.newCachedThreadPool();

        AppThreadsSum app = new AppThreadsSum();

        for (int i = 0; i < 1000; i++) {
            pool.execute(app.new Adding());
        }

        pool.shutdown();

        while (!pool.isTerminated()) {
            System.out.println(" Is it done? : " + pool.isTerminated());
        }

        System.out.println(" Is it done? : " + pool.isTerminated());
        System.out.println("Sum is " + app.sum);
    }

    class Adding implements Runnable {

        public void run() {

            synchronized (AppThreadsSum.this) {

                sum += 1;
            }
        }
    }
}

附:忙碌等待是一种需要避免的反模式(从邻居的答案中复制来完整并了解这一重要的事情，请参阅评论)