java - 为什么 JMM 产生 (0, 0) 即使它被认为是禁止的结果

Question

我正在检查 JMM 的一些语句，并编写了一个 JCS 测试，如下所示:

@JCStressTest
@State
@Outcome(expect = ACCEPTABLE,  desc = "ACCEPTABLE")
public class ConcurrencyTest {
    private final int a = 1;
    private final int b = 2;

    public ConcurrencyTest instance;
    
    @Actor
    public void actor1() {
        instance = new ConcurrencyTest();
    }

    @Actor
    public void actor2(II_Result result) {
        ConcurrencyTest c = instance;
        if (c != null) {
            result.r1 = c.a;
            result.r2 = c.b;
        }
    }
}

运行此测试后，我看到以下结果:

(0, 0) (1, 2)

虽然 JMM 明确声明结果 (0, 0) 是禁止的，但为什么会发生这种情况？

Answer 1

让我们稍微修改一下代码:

@JCStressTest
@State
@Outcome(id = "0, 0", expect = Expect.FORBIDDEN)
@Outcome(id = "1, 2", expect = Expect.ACCEPTABLE)
@Outcome(id = "-1, -1", expect = Expect.ACCEPTABLE)
public class ConcurrencyTest {

    private final int a = 1;
    private final int b = 2;

    public ConcurrencyTest instance;

    @Actor
    public void actor1() {
        instance = new ConcurrencyTest();
    }

    @Actor
    public void actor2(II_Result result) {
        ConcurrencyTest c = instance;
        if (c != null) {
            result.r1 = c.a;
            result.r2 = c.b;
        } else {                   // <-- this is what you care about
            result.r1 = -1;
            result.r2 = -1;
        }
    }
}

您认为来自 @Outcome(id = "0, 0") 的值在哪里？来自？这些是您在 II_Result 中设置的持有两个 int s，默认值为 0 .
因此，当 c == null (意味着 actor1 没有运行)，即 if (c != null) { ... 不会被输入。所以，在你的代码中，你什么都不做:导致这些默认值 r1和 r2为零。您应该通过一个简单的 else 来处理这种默认情况。 ，就像我一样。