To be honest I’ve not really seen much written detailing good (or even standard) ways of unit testing private methods without making their scope less restrictive than private. Thus I’ll detail the way a co-worker a technique a co-worker and I came up with that seems like it will work pretty well. If you see any potential problems with this approach please just let me know in the comments.

The approach is fairly simple. We create a helper class that extends and “wraps” our class to be tested. This helper class then has simple methods in it that expose the private methods via a wrapper. So let’s say we had a class, MoneyLender, with the private method of computeInterest(). Let’s ignore whether that method should be private or not for arguments sake. We would create a MoneyLenderTestHelper class with a method in it called computerInterestWrapper() which actually just returns the private computeInterest method.

Right off the bat I have one potential concern and that is will the other private things (methods, member variables, etc) still be exposed to the private method that is being returned? I haven’t tested this out yet so I don’t know.

Anyway, my test would then instantiate the MoneyLenderTestHelper class, and call the computeInterestWrapper() method to get a reference to the private method to be tested. Then I could execute the method and perform the test. If the test needed to analyze the state of any private member variables I could create a helper method in MoneyLenderTestHelper that would inspect the private variable.

The main reason for returning a reference to the method in the helper class, instead of calling the helper method there, was so that I don’t have to maintain the method signature in both the class being tested and in the helper class. My test obviously has to know the signature but overall this seems more maintainable.

What do you think? I’ll post more as I muck around with the approach and let you know how well it all works. We’ve already implemented the idea on a few test cases but the private methods don’t actually touch any private member variables; however, in the cases we have tried it with the technique has worked out well.


Marc Esher

the “makePublic” stuff in mxunit basically just does some dynamic cfc generation and some method-name swapping. it ain’t purty, but then most class/method monkeying isn’t.

when I added it to mxunit, I added it for pretty much exactly the reasons Bill described in his last comment. I use testing to help me code better and faster, bottom line. And when I’m writing private functions, sometimes they make themselves test-worthy. Sometimes that’s an immediate code smell to me and I catch myself. But sometimes, it’s legitimate. Sometimes the private function is doing something complicated that I just can’t get my mind around and I need to see what’s going on, and it’s easiest for me to whip up a test and execute that private method directly. Often, I will take out the private method tests from my test cases because they’ve served their purpose, which is: to help me run my code in isolation so that I can achieve the behavior I want to achieve and move on to the next thing.

In my own experience, I haven’t seen that the existence of makePublic leads me to a bias towards testing private methods. I have found that I’m glad it’s there when I need it.

thanks for the post Bill!


I don’t create unit tests for all of my private methods but occasionally one pops up that needs specific fine grain testing.

The private methods are a foundation upon which my public methods are built upon, if the private method ends up with a bug then my public method will fail; sometimes it is more efficient to make sure the private method works exactly as expected (via unit testing).

I could just write a unit test against the public methods that call the private methods but that is neither efficient (in regards to testing time) nor is it always clear what functionality exactly is being tested.

In the end I suppose it might be a personal preference; but some methods are critical enough that I think they warrant their own set of unit tests even if the method is private. Likewise, if an error is discovered through usage of the system and, via, debugging, I find out the error is specifically related to a private method I like to write my unit test to duplicate the problem such that I am testing the failing private method directly with data that I know caused the problem. This way if the public method has logic that transforms the data I don’t have to worry about that transformation logic changing and thus invalidating my very specific unit test.

Tom Chiverton

Why would you want to unit test private methods ? Those are almost by definition part of the internal workings of the class, whereas you should just be making sure the exposed interface is correct, no ?


You can read more at, but basically it seems like the original method is renamed (at least for the instance of the object at runtime for your unit test), and a new public method with the same name and functionality is injected.


well that’s an interesting and handy feature. What does it do, make a copy of the function and use method injection?


If you are using MXUnit, you can just call the makePublic() method, and then test your private method like it were public.