Partially Mocking a Class in Java

This is an example of complete mocking, but not the kind of mocking this post discusses.

I love writing unit tests. i know this is an unpopular opinion but I just really like it. I love thinking of edge cases that could break code and then coming up with exactly how to create a test that will prevent that.

However, mocking everything stinks. Yes, we should be using in-memory fakes instead of mocks as much as possible, and I’ve been working toward this, but it is not realistic to make everything a fake.

My very smart coworker just alerted me to this method built into Mockito’s when method, which is used for controlling the behavior of mocks. If you want to stop reading now, just read the next two lines, and then you can go:

when(thing.method()).thenCallRealMethod()

This bypasses any mock behavior and goes back to the real class’s implementation. Handy!

For absolutely more details than you need, here’s an example:

class MyClass(){
 public int myMethod(){
  return 1;
 }
}

class SomeTestClass(){
 public MyClass myClass = mock(MyClass.class);

 int result = myClass.myMethod();

 assertEquals(1, result);
}

This contrived example will fail, because we haven’t explicitly declared the behavior of myMethod. (It will return null instead.)

So what I would have done yesterday would look like this:

class MyClass(){
 public int myMethod(){
  return 1;
 }
}

class SomeTestClass(){
 public MyClass myClass = mock(MyClass.class);
 when(myClass.myMethod()).thenReturn(1);
 int result = myClass.myMethod();

 assertEquals(1, result);
}

This will work (ignore the part where this test is a tautology!), but if your method under test has a lot of external calls that you’re mocking out, this can get messy. That’s where .thenCallRealMethod() comes in. If you had:

class MyClass(){
 public int myMethod(){
  return 1;
 }
 public int someOtherMethod(){
  return 3;
 }
}

class SomeTestClass(){
 public MyClass myClass = mock(MyClass.class);
 when(myClass.myMethod()).thenReturn(2);
 when(myClass.someOtherMethod()).thenCallRealMethod();
 int result = myClass.myMethod();
 int result2 = myClass.SomeOtherMethod();

 assertEquals(2, result); //true because we have mocked the behavior of this method
 assertEquals(3, result); //true because the real method returns 3
}

Like I said: Handy!