Answer by Daniel Alexiuc for Spring beans redefinition in unit test environment

There are some very complicated and powerful solutions listed here.

But there is a FAR, FAR simpler way to accomplish what Stas has asked, which doesn't involve modifying anything other than one line of code in the test method. It works for unit tests and Spring integration tests alike, for autowired dependencies, private and protected fields.

Here it is:

junitx.util.PrivateAccessor.setField(testSubject, "fieldName", mockObject);