Java: Testing if an exception is caught within a larger system

Question

I know there are a number of questions on this topic, but all of them seem to assume one of two things:

1. You just want to test if an exception was thrown and not caught,
2. You should test the function that is inside of the try block directly

I'm not sure how I can apply those options to this case. I have a small try/catch block, like so:

try {
    o.getDataContainer().build(...);
    o2.setDataContainer(o.getDataContainer());
} catch (final Exception e) {
    LOGGER.error("Data set failed", e);
}

As you can see, if o.getDataContainer() returns null, an exception would be triggered. However, that exception is then caught, and the test tool considers it a successful test. Is it possible to test that the exception occurred without changing the code?

I ask because our logging system triggers a trouble ticket if it picks up an exception that is caught and logged. Since it is common human error to forget to guard something like this, I would like to write UTs that can test if an exception was triggered and caught. I can't remove the whole-program protection provided by the catch block, but the error would also cause a degradation of the user experience, since the data isn't being passed along. (I work in a place where minutes of site downtime equal millions of dollars lost.)

In other words: The exception is an error, and we want to log it and investigate it, but in case this manages to trigger on prod, we don't want to risk the whole site going down.

Note: This try/catch sits inside a much larger function with many such try/catch blocks. One could easily argue bad overall design, but fixing it is not an option (without a huge amount of free dev time, at least).

Update: As the task at hand does not allow me to spend a great deal of time on this, I went with a very simple generic test that would fail if the guard and catch were both removed so that I could move on. But, I'm leaving the question unanswered for now in hopes of continuing conversation. I would love to be able to write a simple UT for each new feature that fails if any exceptions are triggered and caught.

Answer 1

Ignoring the issues with this code (Sometimes you've gotta put lipstick on a pig, I guess), this is how I might handle the situation.

I'd use Mockito and mock o2, then use an Answer to ensure the method is invoked.

A test might look like this:

@RunWith(MockitoJUnitRunner.class)
public class TestClass{
    @Mock
    O2 o2;
    @Mock
    O1 o1;

    boolean exceptionThrown = false;

    @Test
    public void test(){
        Mockito.doAnswer(new Answer<Void>(){     
           public Void answer(InvocationOnMock invocation) throws Throwable {
              exceptionThrown = true; 
           throw new RuntimeException("some message");
        }            
      }).when(o2).setDataContainer(DataContainer.class);
    }
}

Essentially, you can Mock out o2 in your example, and force the exception.

If this doesn't quite do what you want, you may need to mock LOGGER and verify that it's invoked with LOGGER.error("some message");. Unfortunately, mocking statics is not at all elegant, but it can be done with PowerMock.

Answer 2

You could add a custom handler to LOGGER that just throws when an error is logged. For java.util.logging you could do something like:

LOGGER.addHandler(new Handler() {
    public void publish(LogRecord record) {
        if ("Data set failed".equals(record.getMessage())) {
            throw new RuntimeException(record.getThrown());
        }
    }
    public void flush() {}
    public void close() throws SecurityException {}
});

I think log4j calls it "Appender," but the same principle should work. See How to create a own Appender in log4j? or How to Create a Custom Appender in log4j2?