为什么给一个线程私有的Random设置了种子后，在多线程环境下，输出new Random(47).nextInt(3)的值是随机的呢？

Question

下面是我的测试用例，请指教！

public class Test {
    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool();
        for(int i = 0; i < 10; i++) {
           exec.execute(new LocalThread());
        }
    }
}

class LocalThread implements Runnable{
    private final Random rand = new Random(47);

    @Override
    public void run() {
        try {
            while(!Thread.interrupted()) {
                System.out.println(Thread.currentThread().getId() + " - " + rand.nextInt(3));
                TimeUnit.MILLISECONDS.sleep(100);
            }
        }catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

输出:

0 - 2
3 - 2
2 - 2
1 - 2
4 - 2
2 - 2
4 - 2
0 - 2
1 - 2
3 - 2
3 - 1
2 - 1
0 - 1
4 - 1
1 - 1

把暂停去掉

class LocalThread implements Runnable{
    private final Random rand = new Random(47);
    private final int id;
    public LocalThread(int id) {
        this.id = id;
    }

    @Override
    public void run() {
        while(!Thread.interrupted()) {
            System.out.println(id + " - " + rand.nextInt(3));
        }
    }
}

输出

1 - 1
1 - 1
1 - 0
1 - 2
1 - 0
1 - 0
1 - 2
1 - 0
1 - 1
1 - 2
0 - 0
0 - 1
3 - 0
...

Answer 1

public class TestRandom {
    public static void main(String[] args) {
        ExecutorService exec = newCachedThreadPool();
        for (int i = 0; i < 4; i++) {
            exec.execute(new LocalThread());
        }
    }
}

class LocalThread implements Runnable {
    private final Random rand = new Random(47);

    @Override
    public void run() {
        int total = 1;
        try {
            while (total <= 5) {
                System.out.println(Thread.currentThread().getId() + " - " + total + "-" + rand.nextInt(3));
                total++;
                TimeUnit.MILLISECONDS.sleep(3000);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

这是我改了时间和增加打印运行次数的例子 结果就是下面

11 - 1-2
13 - 1-2
12 - 1-2
14 - 1-2
13 - 2-2
11 - 2-2
12 - 2-2
14 - 2-2
11 - 3-1
13 - 3-1
12 - 3-1
14 - 3-1
11 - 4-2
12 - 4-2
13 - 4-2
14 - 4-2
12 - 5-1
11 - 5-1
13 - 5-1
14 - 5-1

每个线程的序列都是 2 2 1 2 1 这个就是这个种子伪随机创建出来的随机序列