为什么Java基本变量分配不是原子的

Question

all. I run following code to test whether Java basic variable assignment is atomic or not.

public class Test  {
static int x = 1 << 16 - 1, y = -1 << 16, i = x;
static volatile boolean flag = false;
public static void main(String... args) {
    ExecutorService executors = Executors.newFixedThreadPool(3);
    executors.execute(() -> {
        while (true) {
            if (flag) {
                return;
            }
            i = x;
        }
    });
    executors.execute(() -> {
        while (true) {
            if (flag) {
                return;
            }
            i = y;
        }
    });
    executors.execute(() -> {
        while (true) {
            if (i != x && i != y && (flag = true)) {
                System.out.println(i + "," + x + "," + y);
                throw new RuntimeException("Not Equal!!");
            }
        }
    });
}

It will throw a new exception(Follow text ), but i can not catch the actual i when condition (i != x && i != y) is true, because other thread will modify the variable i at same time.

Exception in thread "pool-1-thread-3" java.lang.RuntimeException: Not Equal!!
at common.Test.lambda$main$2(Test.java:31)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

32768,32768,-65536

Can someone give some useful advices to help me to find out the actual i which cause (if (i != x && i != y && (flag = true)) is true?

Answer 1

The problem you're seeing is not because int assignment is not atomic: it is!

Since you're modifying i from multiple threads all reads and writes to it should be synchronized otherwise threads might get stale values and that's what happens in your case, example:

1. assignment i = y
2. this condition is evaluated: if (i != x && i != y && (flag = true))
3. i != x is evaluated and returns true
4. assignment i = x
5. i != y is evaluated and returns true
6. we're inside the if