java - 为什么我无法证明我的代码中需要 AtomicReference？

Question

我有以下代码。显然，Reference 类不是线程安全的，因为它不保护其“引用”属性。我如何证明我需要通过例如原子引用来保护它？

当我运行以下 JUnit 测试时，它在 Windows:Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz 和 Linux:Intel(R) Xeon(R) CPU X5670 @ 2.93GHz(使用 JRE 1.7)上成功.0_15.

import java.util.concurrent.CountDownLatch;
import org.junit.Test;
import static org.junit.Assert.assertTrue;

public class AssignReferenceTest {
    private static class Reference {
        private Object reference = null;

        private void setReference(Object reference) {
            this.reference = reference;
        }

        boolean hasReference() {
            return reference != null;
        }
    }

    @Test
    public void runManyTimes() throws Exception {
        for (int i = 0; i < 100000; i++) {
            testReferenceVisibilityProblem();
        }
    }

    public void testReferenceVisibilityProblem() throws Exception {
        final Reference reference = new Reference();
        final CountDownLatch latch = new CountDownLatch(1);

        Thread writeThread = new Thread(new Runnable() {
            public void run() {
                reference.setReference(new Object());
                latch.countDown();
            }
        });
        Thread readThread = new Thread(new Runnable() {
            public void run() {
                try {
                    latch.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                assertTrue("Should have the reference", reference.hasReference());
            }
        });

        writeThread.start();
        readThread.start();
        writeThread.join();
        readThread.join();
    }
}

Answer 1

您的代码是线程安全的，因为 CountDownLatch 保证在 await() 返回之前完成的每个更改都发生在之后完成的所有操作之前。

参见http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/package-summary.html :

Actions prior to "releasing" synchronizer methods such as Lock.unlock, Semaphore.release, and CountDownLatch.countDown happen-before actions subsequent to a successful "acquiring" method such as Lock.lock, Semaphore.acquire, Condition.await, and CountDownLatch.await on the same synchronizer object in another thread.