一文揭秘Java内存模型的隐匿陷阱与解决方案

public class VolatileExample {
    private volatile boolean flag = false;
    private int counter = 0;
 
    public void writer() {
        counter = 42;      // 非volatile写
        flag = true;       // volatile写
    }
 
    public void reader() {
        if (flag) {         // volatile读
            System.out.println(counter); // 输出什么？
        }
    }
}

private volatile int counter = 0;

public synchronized void writer() { ... }
public synchronized void reader() { ... }

private static final VarHandle COUNTER_HANDLE;
static {
    try {
        COUNTER_HANDLE = MethodHandles
            .lookup()
            .findVarHandle(VolatileExample.class, "counter", int.class);
    } catch (Exception e) { throw new Error(e); }
}
 
public void reader() {
    if (flag) {
        // 显式插入读屏障
        COUNTER_HANDLE.loadLoadFence(); 
        System.out.println(counter);
    }
}

private final AtomicInteger counter = new AtomicInteger(0);
 
public void writer() {
    counter.set(42);      // 内部包含volatile语义
    flag = true;
}
 
public void reader() {
    if (flag) {
        System.out.println(counter.get()); // 安全！
    }
}

@JCStressTest
@Outcome(id = "0", expect = Expect.ACCEPTABLE_INTERESTING, desc = "!!! 可见性失效 !!!")
@Outcome(id = "42", expect = Expect.ACCEPTABLE, desc = "正常可见")
public class VolatileTest {
    private boolean flag = false;
    private int counter = 0;
 
    @Actor
    public void writer() {
        counter = 42;
        flag = true;
    }
 
    @Actor
    public void reader(IntResult1 r) {
        if (flag) r.r1 = counter;
    }
}