Mockit | A simple mocking framework for Swift | Mock library

Earlier versions of JMockit allowed mocking private methods, and honestly, I thought it was a brilliant differentiator with other mocking-frameworks. Sadly, more recent versions have eliminated the ability to mock privates - became a warning in 1.45 and an exception in 1.47.

There is no real official explanation, although supposition is that private methods should be so simple they do not need testing/mocking. By extension, if you are trying to access it for purposes of testing, then it should not be private. People (other than you) would likely want to also alter the behavior, and that your need to access it for test purposes is strongly suggesting the method ought to be accessible. Make it protected or package-private. FWIW, there are annotations like "@VisibleForTesting" that can be used to help indicate the intent.

So you know, 1.47 also removed the "Deencapsulation" mechanism which was one of my favorite tools for inspecting/setting private data. Painful at the time I had to convert, because it littered my test code, but in hind sight, @Tested/@Injectable (the replacement) is way cleaner. As the maintainer indicates, JMockit is not intended as a way to get at privates, there are other frameworks that do that and no sense in doing the job that they do better. I switched over to Apache's commons-lang3 (FieldUtils/MethodUtils/etc), but other frameworks exist

Return function is used to return values on call of the mocked function. You can't change the signature of the mocked function.

Here you're returning func(*types.Query) bool (function that returns boolean) instead of bool, as the error message say.

You can have the function to make some computation and then return the value. You just have to add () (or whatever arguments your function accepts) after function definition to invoke it and you'll be good to go.

This works:

If this is for the purpose of learning or analysis, why don't you just use a debugger? I am going to show you an example in IntelliJ IDEA:

Given this code somewhere in your main method or test:

This is something you can configure in the "Run Configuration". Right after failing to run the test class, bring up the "Run Configurations" dialog (multiple ways to get there), the one you just ran should be selected. Click on the "Arguments" tab, put the required "javaagent" command-line arguments into that field and store them.

If you don't like having to edit this for every test class, you might consider changing your mocking technology. Mockito has no need for something like this.