[Junit-devel] ClassRules

[Alistair I. <ai...@gm...>]

Hi, list.

Hope I'm not overstepping by just barging in like this. Anyway, I'm
one of those early adopters of JUnit 4.7's new @Rules.

As I use them more and more I've also been seeing the usefulness of
class-level rules more and more.

A concrete use case of mine is initializing a JPA (Hibernate)
persistence context backed by an embedded Derby database, then using
DbUnit to load fixtures and populate the database.

Using JUnit 4.7, I'm able to accomplish the above using a single
MethodRule that does everything for every test method. The performance
drawback should be obvious—we need to initialize Derby DB, set up
Hibernate JPA, scan for annotated classes and create the tables to
match the entities for _every test method_.

I've taken the liberty of forking, branching, and committing some
changes to JUnit that allow for class-level rules. It's up on GitHub,
and the meat of the changes are here:
  http://github.com/AlistairIsrael/junit/commit/79ef5a7e1d7fa144cc81c9414f4791aaea8b3d75

The gist of it:
  * Created the ClassRule interface that declares the apply(Statement,
TestClass) method.
  * Modified ParentRunner to scan for fields that are a) annotated
with @Rule, b) static, and c) implement ClassRule.
  * If any are found, then apply them in the order found, allowing
them to append to the head of the Statement chain.
  * Modified BlockJUnit4ClassRunner so validation treats ClassRules
appropriately, and only apply MethodRules in methodBlock()

I've also added a ClassRulesTest to AllTests to verify the above.

Using a local-build, I successfully refactored my specific use-case
and split the compound rule into a separate ClassRule (set up Derby
and Hibernate) and a MethodRule (load DbUnit fixtures). Not only are
the resulting tests more efficient, this strategy lets each Rule to be
used independently, even allows for mixing and matching of different
database, JPA or fixtures providers.

I have a few more ideas on where this can go, but I'd like to get some
feedback first if this is something that's welcome or if you guys have
another direction you think we should take.

Thanks,
- alistair
-- 
http://alistairisrael.wordpress.com