Due to restrictions imposed by JUnit 4, the SpringClassRule must be
declared as a public static field, which makes it impossible to be
declared directly within a nested (i.e., inner) test class.
This commit demonstrates a work-around that makes it possible to use
the SpringClassRule and SpringMethodRule in a nested (i.e., inner) test
class when using a custom JUnit 4 runner such as the
HierarchicalContextRunner from Stefan Bechtold.
The trick is to have inner test classes extend a class that properly
declares the SpringClassRule and SpringMethodRule. The
SpringRuleConfigurer in this commit serves as an example.
Note, however, that each such nested test class must declare its own
@ContextConfiguration. Furthermore, TestExecutionListeners in the
Spring TestContext Framework are not applied to the enclosing instance
of such an inner test class, meaning that @Autowired fields in the
enclosing instance will not be injected, etc.
Issue: SPR-14150
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