如何在junit中使用未捕获的异常处理程序进行多线程测试？

Question

我有以下代码，我希望这些代码能够成功运行到完成，但代码在"fail("this not be reached");“行失败。谁能解释一下为什么不调用默认的未捕获异常处理程序：

public class UncaughtExceptionTest extends TestCase
    implements UncaughtExceptionHandler {

    private final List<Throwable> uncaughtExceptions =
        new CopyOnWriteArrayList<Throwable>();

    class UncaughtExceptionTestInnerClass implements Runnable {
        private final ScheduledThreadPoolExecutor executor =
            new ScheduledThreadPoolExecutor(1);
        private final CountDownLatch latch;

        UncaughtExceptionTestInnerClass(CountDownLatch latch) {
            this.latch = latch;
            executor.schedule(this, 50, TimeUnit.MILLISECONDS);
        }

        @Override
        public void run() {
            System.out.println("This is printed");
            fail("this should fail");
            latch.countDown();
        }
    }

    @Test
    public void testUncaughtExceptions() {
        Thread.setDefaultUncaughtExceptionHandler(this);
        CountDownLatch latch = new CountDownLatch(1);
        UncaughtExceptionTestInnerClass testTheInnerClass =
                new UncaughtExceptionTestInnerClass(latch);
        try {
            if (!latch.await(1, TimeUnit.SECONDS)) {
                if (uncaughtExceptions.size() > 0) {
                    Throwable exception = uncaughtExceptions.get(0);
                    System.out.println("First uncaught exception: " + 
                                    exception.getMessage());
                }
                else {
                    fail("this should not be reached");
                }
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void uncaughtException(Thread t, Throwable e) {
        uncaughtExceptions.add(e);
    }
}

Answer 1

这与您使用Executor来运行任务的事实有关。只有当线程由于未捕获的异常而将要终止时，才调用未捕获的异常处理程序。如果您将实现更改为使用普通线程，则线程将以异常终止，您将看到预期的行为。

根据您提交任务的方式，executor线程可能会捕获所有Throwable并处理它们。因此，线程不会因为这些异常而终止，因此不涉及未捕获的异常处理程序。例如，ThreadPoolExecutor.execute(Runnable)将触发未捕获的异常处理程序。但是，ThreadPoolExecutor.submit(Callable)不需要。此外，ScheduledThreadPoolExecutor.schedule()也不需要(这与他们使用FutureTask实现有关)。

使用executor服务访问意外异常的更好方法是通过Future。

Answer 2

ScheduledThreadPoolExecutor.schedule()接受Runnable/Callable参数，而不是Thread。前者没有运行时异常处理程序。在您的run或call方法中为RuntimeException设置一个try/catch块。