java - Mockito Spy'ing on the object being unit tested

Question

Is it a code smell to spy on an object that is being unit tested? For example say I have a LineCounter class whose job is to simply count the number of lines in a string. --

class LineCounter {
    public int getNumLines(String string) {
        String metadata = getStringMetadata(string);

        // count lines in file
        return numLines;
    }

    /** Expensive operation */
    protected String getStringMetadata(String string) {
        // do stuff with string
    }
}

Now I want to write a JUnit 4 test for this to test the getNumLines method while mocking out the expensive getStringMetadata call. I decide to use Mockito's spy mechanism to have getStringMetadata return a dummy value.

class LineCounterTests {
    @Test public void testGetNumLines() {
        LineCounter lineCounterSpy = Mockito.spy(new LineCounter());

        // Mock out expensive call to return dummy value.            
        Mockito.when(lineCounterSpy.getStringMetadata(Mockito.anyString()).thenReturn("foo");

        assertEquals(2, lineCounterSpy.getNumLines("hello
world");
    }
}

Is this a reasonable thing to do? I feel pretty weird testing a Spy object rather than the actual class, but I can't really think of a reason against it.

Answer 1

I will answer the question in two parts. First, yes it is code smell to mock or spy the class under test. That does not mean that it cannot be done correctly but that it is risk prone and should be avoided whenever possible.

WRT your specific example, I would see how the spy could be correctly used but that would be predicated on the assertion that you have elsewhere fully unit tested getStringMetadata. This then begs the question, if you have fully unit tested getStringMetadata elsewhere then you must know how to test it and therefore why not test getNumLines without the spy.

All this being said, millhouse makes a good point but either way you have to unit test the expensive code somewhere. His suggestion goes a long way to help isolate the expensive code and ensure that you only have to test / exercise it once.