测试Java方法同步的一个好方法是什么?

如果你只是想检查一个方法是否有 synchronized 修饰符,除了显而易见的(查看源代码/javadoc)之外,您还可以使用反射。

Modifier.isSynchronized(method.getModifiers())

如果一个方法在所有并发场景中都能保证正确的同步,那么测试这个更一般的问题可能是一个无法确定的问题。

这些都是可怕的想法,但你可以这么做…

一

    // Substitute this LOCK with your monitor (could be you object you are
    // testing etc.)
    final Object LOCK = new Object();
    Thread locker = new Thread() {
        @Override
        public void run() {
            synchronized (LOCK) {
                try {
                    Thread.sleep(Long.MAX_VALUE);
                } catch (InterruptedException e) {
                    System.out.println("Interrupted.");
                    return;
                }
            }
        }
    };

    locker.start();

    Thread attempt = new Thread() {
        @Override
        public void run() {
            // Do your test.
        }
    };

    attempt.start();
    try {
        long longEnough = 3000 * 1000;// It's in nano seconds

        long before = System.nanoTime();
        attempt.join(longEnough);
        long after = System.nanoTime();

        if (after - before < longEnough) {
            throw new AssertionError("FAIL");
        } else {
            System.out.println("PASS");
        }
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
        return;
    }
    locker.interrupt();

二

如果知道参数上的方法总是在任何实现中调用,则可以传递伪装为参数的模拟对象并调用holdslock()。

如此:

class Mock implements Argument {
    private final Object LOCK;
    private final Argument real;
    public Mock(Object obj, Argument real){
       this.LOCK=obj;
       this.real = real;
    }

    @Overrides
    public void something(){
        System.out.println("held:"+Thread.holdsLock(LOCK));
        this.real.something();
    }

然后等待类对参数调用something()。

非常感谢zwei steinen写下了我使用的方法。在我处理过的示例代码中有一些问题,所以我认为在这里发布我的发现是值得的。

• 调用join()需要毫秒,而不是纳秒。
• 两个线程必须协调,否则尝试线程可以在锁定线程获取锁之前启动并完成所有操作。
• 在记录开始时间之前,不应启动尝试线程。否则,该线程获得足够的启动时间,记录的时间可能会略小于超时时间,从而导致虚假的失败。

下面是作为scala特性的同步测试代码:

trait SynchronizedTestTrait
{
    val classUnderTest: AnyRef

    class Gate
    {
        val latch = new java.util.concurrent.CountDownLatch(1)

        def open()
        {
            this.latch.countDown
        }

        def await()
        {
            this.latch.await
        }
    }

    def nanoTime(code: => Unit) =
    {
        val before = System.nanoTime
        code
        val after = System.nanoTime
        after - before
    }

    def assertSynchronized(code: => Unit)
    {
        this.assertThreadSafety(threadSafe = true, millisTimeout = 10L)(code)
    }

    def assertNotSynchronized(code: => Unit)
    {
        this.assertThreadSafety(threadSafe = false, millisTimeout = 60L * 1000L)(code)
    }

    def assertThreadSafety(threadSafe: Boolean, millisTimeout: Long)(code: => Unit)
    {
        def spawn(code: => Unit) =
        {
            val result = new Thread
            {
                override def run = code
            }
            result.start()
            result
        }

        val gate = new Gate

        val lockHolderThread = spawn
        {
            this.classUnderTest.synchronized
            {
                // Don't let the other thread start until we've got the lock
                gate.open()

                // Hold the lock until interruption
                try
                {
                    Thread.sleep(java.lang.Long.MAX_VALUE)
                }
                catch
                {
                    case ignore: InterruptedException => return;
                }
            }
        }

        val measuredNanoTime = nanoTime
        {
            // Don't start until the other thread is synchronized on classUnderTest
            gate.await()
            spawn(code).join(millisTimeout, 0)
        }

        val nanoTimeout = millisTimeout * 1000L * 1000L

        Assert.assertEquals(
            "Measured " + measuredNanoTime + " ns but timeout was " + nanoTimeout + " ns.",
            threadSafe,
            measuredNanoTime > nanoTimeout)

        lockHolderThread.interrupt
        lockHolderThread.join
    }
}

现在假设我们要测试一个简单的类:

class MySynchronized
{
    def synch = this.synchronized{}
    def unsynch = {}
}

测试结果如下:

class MySynchronizedTest extends SynchronizedTestTrait
{
    val classUnderTest = new MySynchronized


    @Test
    def synch_is_synchronized
    {
        this.assertSynchronized
        {
            this.classUnderTest.synch
        }
    }

    @Test
    def unsynch_not_synchronized
    {
        this.assertNotSynchronized
        {
            this.classUnderTest.unsynch
        }
    }
}

使用反射,获取方法的方法对象,并对其调用toString()。“synchronized”关键字应该出现在tostring()的输出中。