如何测试AtomicBoolean原子性？

Question

我正在为AtomicInteger和AtomicBoolean编写单元测试.它们将用作参考测试,用于在objective-c中测试这些类的仿真,以用于翻译项目.

我认为AtomicInteger测试很好,主要是通过在大量for循环中执行可预测数量的递增,递减,加法和减法操作,每个循环都在自己的线程中运行(每个操作类型有很多线程).实际操作使用CountDownLatch同时启动.

当所有线程完成后,我通过比较原子整数和预期的整数值来断言,基于线程数,每个线程的迭代次数和每次迭代的预期增加/减少.这个测试通过.

但是如何测试AtomicBoolean？基本操作是get和set,因此在许多线程中多次调用并期望最终结果为true或false似乎没有意义.我正在考虑的方向是使用两个应始终具有相反值的AtomicBooleans.像这样:

@Test
public void testAtomicity() throws Exception {

    // ====  SETUP  ====
    final AtomicBoolean booleanA = new AtomicBoolean(true);
    final AtomicBoolean booleanB = new AtomicBoolean(false);

    final int threadCount = 50;

    final int iterationsPerThread = 5000;

    final CountDownLatch startSignalLatch = new CountDownLatch(1);
    final CountDownLatch threadsFinishedLatch = new CountDownLatch(threadCount);

    final AtomicBoolean assertFailed = new AtomicBoolean(false);

    // ====  EXECUTE: start all threads ====
    for (int i = 0; i < threadCount; i++) {

        // ====  Create the thread  =====
        AtomicOperationsThread thread;
        thread = new AtomicOperationsThread("Thread #" + i, booleanA, booleanB, startSignalLatch, threadsFinishedLatch, iterationsPerThread, assertFailed);
        System.out.println("Creating Thread #" + i);

        // ====  Start the thread (each thread will wait until the startSignalLatch is triggered)  =====
        thread.start();
    }

    startSignalLatch.countDown();

    // ====  VERIFY: that the AtomicInteger has the expected value after all threads have finished  ====
    final boolean allThreadsFinished;
    allThreadsFinished = threadsFinishedLatch.await(60, TimeUnit.SECONDS);

    assertTrue("Not all threads have finished before reaching the timeout", allThreadsFinished);
    assertFalse(assertFailed.get());

}

private static class AtomicOperationsThread extends Thread {

    // #####  Instance variables  #####

    private final CountDownLatch startSignalLatch;
    private final CountDownLatch threadsFinishedLatch;

    private final int iterations;

    private final AtomicBoolean booleanA, booleanB;

    private final AtomicBoolean assertFailed;

    // #####  Constructor  #####

    private AtomicOperationsThread(final String name, final AtomicBoolean booleanA, final AtomicBoolean booleanB, final CountDownLatch startSignalLatch, final CountDownLatch threadsFinishedLatch, final int iterations, final AtomicBoolean assertFailed) {

        super(name);
        this.booleanA = booleanA;
        this.booleanB = booleanB;
        this.startSignalLatch = startSignalLatch;
        this.threadsFinishedLatch = threadsFinishedLatch;
        this.iterations = iterations;
        this.assertFailed = assertFailed;
    }

    // #####  Thread implementation  #####

    @Override
    public void run() {

        super.run();

        // ====  Wait for the signal to start (so all threads are executed simultaneously)  =====
        try {
            System.out.println(this.getName() + " has started. Awaiting startSignal.");
            startSignalLatch.await();  /* Awaiting start signal */
        } catch (InterruptedException e) {
            throw new RuntimeException("The startSignalLatch got interrupted.", e);
        }

        // ====  Perform the atomic operations  =====
        for (int i = 0; i < iterations; i++) {

            final boolean booleanAChanged;
            booleanAChanged = booleanA.compareAndSet(!booleanB.get(), booleanB.getAndSet(booleanA.get()));  /* Set A to the current value of B if A is currently the opposite of B, then set B to the current value of A */

            if (!booleanAChanged){
                assertFailed.set(true);
                System.out.println("Assert failed in thread: " + this.getName());
            }
        }

        // ====  Mark this thread as finished  =====
        threadsFinishedLatch.countDown();
    }
}

这适用于一个线程,但失败了多个.我想这是因为booleanAChanged = booleanA.compareAndSet(!booleanB.get(), booleanB.getAndSet(booleanA.get()));不是一个原子操作.

有什么建议？

Answer 1

我会集中精力，这才是普通人和普通人compareAndSet的真正区别。AtomicBooleanboolean

例如，用于compareAndSet(false, true)控制关键区域。循环执行直到返回false，然后进入临界区。在临界区中，做一些如果两个或多个线程同时运行就很可能失败的事情。例如，在读取旧值和写入新值之间通过短暂的睡眠来增加计数器。在关键区域的末尾，将 设为AtomicBooleanfalse。

在启动线程之前将 初始化AtomicBoolean为 false，并初始化为零。globalCounter

for(int i=0; i<iterations; i++) {
  while (!AtomicBooleanTest.atomic.compareAndSet(false, true));
  int oldValue = AtomicBooleanTest.globalCounter;
  Thread.sleep(1);
  AtomicBooleanTest.globalCounter = oldValue + 1;
  AtomicBooleanTest.atomic.set(false);
}

最后，该globalCounter值应该是线程数t*iterations。t

线程的数量应该与硬件可以同时运行的数量相似——这在多处理器上比在单处理器上更有可能失败。失败的最高风险发生在 AtomicBoolean 变为 false 后。所有可用的处理器应该同时尝试获得对它的独占访问，将其视为 false，并自动将其更改为 true。