Mockito acts strangely when I assign multiple custom matchers to a single method

Question

I'm wanting to use two custom matchers for a single method. Basically, if I pass the method VALUE_A, I want it to return RESULT_A, and if I pass it VALUE_B, I want it to return RESULT_B. So here's a code excerpt :

class IsNonEmpty extends ArgumentMatcher<Get> {
    public boolean matches(Object get) {
        //For some reason, this method is called when I assign the IsEmpty matcher to MockHtable.get()
        //When this happens, the value of the get argument is null, so this method throws an NPE

        return Arrays.equals(((Get) get).getRow(), SERIALIZATION_HELPER.getValidBytes(key)); 
    }
}

class IsEmpty extends ArgumentMatcher<Get> {
    public boolean matches(Object get) {
        return !(Arrays.equals(((Get) get).getRow(), SERIALIZATION_HELPER.getValidBytes(key))); 
    }
}      

[...]

//This line executes just fine
Mockito.when(mockHTable.get(Mockito.argThat(new IsNonEmpty()))).thenReturn(dbResult);

[...]

//This line calls IsNonEmpty.matches() for some reason.  IsNonEmpty.matches() throws an NPE
Mockito.when(mockHTable.get(Mockito.argThat(new IsEmpty()))).thenReturn(emptyResult);

When I assign the IsEmpty custom matcher to mockHTable.get() method, it calls the IsNonEmpty.matches() function. No idea why it's doing this. So I change the IsNonEmpty class to this :

class IsNonEmpty extends ArgumentMatcher<Get> {
    public boolean matches(Object get) {
        //For some reason, this method is called when I assign the IsEmpty matcher.  Weird, no?
        if(get == null) {
            return false;
        }

        return Arrays.equals(((Get) get).getRow(), SERIALIZATION_HELPER.getValidBytes(key)); 
    }
}

and then everything works just fine! IsNonEmpty.matches() is still called when I assign the IsEmpty matcher to the mockHTable.get() function, but my matchers work exactly how they should.

So what's the deal? Why does this happen? Is my work-around an adequate way to compensate for this quirky behavior, or am I Doing It Wrong?

Answer 1

The reason why IsNonEmpty.matches() gets called on the second line of stubbing is that the Mockito.argThat(new IsEmpty()) returns null, which is then passed to mockHTable.get(). This call has to be checked against the earlier stubbing, to see whether it's a match; and that means calling IsNonEmpty.matches().

I'm not sure why this makes your test fail - it's hard to tell without seeing all of the code.

But, I would seriously recommend using doReturn...when instead of when...thenReturn whenever you have to stub the same mock more than once. You won't encounter issues like this if you do. In fact, I prefer to use doReturn...when in preference to when...thenReturn always (and similarly doThrow and doAnswer), although most people prefer when...thenReturn.

Re-writing one of your stubbing lines with the doReturn...when syntax looks like the following. The other is similar.

Mockito.doReturn(dbResult).when(mockHTable).get(Mockito.argThat(new IsNonEmpty()));  

Lastly, a plea, on behalf of the Mockito development team (of which I am a member). If you think there is a bug in Mockito here - and from your description, I think there may well be - please EITHER

• send a message to the Mockito mailing group ([email protected]) OR
• raise an issue on the Mockito issues list (http://code.google.com/p/mockito/issues/list).

It's useful to the Mockito team if you can actually post a complete example, rather than just what you think the key lines are - sometimes the cause of a Mockito problem is in quite an unexpected place.