This is for testing only.
Context contain the Factory implementation specific
non-default values and mechanism for overriding the default
instance type returned by the Factory.
Factory implementation can extend the Context interface
to capture its specific override values.
If, for example, a Factory implementation uses a ThreadLocal
to override the default instance type for unit tests, then the
Context
will hold the current value of the ThreadLocal.
Getting the Context, clears the ThreadLocal value.
This allows the tester who wishes to create code that will provide
a wrapper around the default instance type to register their
wrapper class name with the ThreadLocal, and, within
the wrapper constructor, get the Context, get
a default instance, and then restore the Context.