java - Java不同步线程，这个答案怎么会出现，怎么解释

Question

哪两个是可能的结果？ （选择两个）

public class Cruiser {

    private int a = 0;

    public void foo() {
        Runnable r = new LittleCruiser();
        new Thread(r).start();
        new Thread(r).start();
    }

    public static void main(String arg[]) {
        Cruiser c = new Cruiser();
        c.foo();
    }

    public class LittleCruiser implements Runnable {
        public void run() {
            int current = 0;
            for (int i = 0; i < 4; i++) {
                current = a;
                System.out.print(current + ", ");
                a = current + 2;
            }
        }
    }
}

• A) 0, 2, 4, 0, 2, 4, 6, 6,
• B) 0, 2, 4, 6, 8, 10, 12, 14,
• C) 0, 2, 4, 6, 8, 10, 2, 4,
• D) 0, 0, 2, 2, 4, 4, 6, 6, 8, 8, 10, 10, 12, 12, 14, 14,
• E) 0, 2, 4, 6, 8, 10, 12, 14, 0, 2, 4, 6, 8, 10, 12, 14,

正确答案是 A 和 B，但是我真的不明白这段代码是如何在较低级别生成 A 的。

标签： javamultithreadingsynchronizationexplain

Answer 1

A) 0, 2, 4, 0, 2, 4, 6, 6,

Thread-1: read a = 0;
Thread-2: read a = 0;
Thread-1: output 0, 2, 4 
Thread-2: output 0
Thread-2: write 2 back to a
Thread-2: output 2, 4, 6
Thread-2: write 6 back to a;
Thread-1: read a = 6 and output 6