I have recently become aware that @BeforeClass does not work as expected in Parametrized JUnit tests. Namely, it is not executed before any of the tests have been instantiated. Quick search on SO made me aware that others took notice of the issue, too:
What surprises me is that the solutions proposed on SO seem to miss the most obvious workaround - embedding the @BeforeClass in the @Parameters method. The latter is static and is executed only once - before any of the tests.
Here is an example.
I needed a JUnit test that validates all XML files in a particular directory against a schema stored in a particular XSD file. It would be best if the schema is instantiated once - and re-used for all of the individual tests. I tried to encapsulate the schema instantiation in the doSetup() method which I annotated as @BeforeClass. Unfortunately, I got NullPointerException in each of the tests as the @BeforeClass method was, apparently, not called and the schema was therefore not instantiated. Calling the doSetup() method with the @Parameters method data() did the job: