feat: allow `DefaultSqlSessionFactory` to provide a custom `SqlSession`

[epochcoder]

Hi Maintainers!

While extending SqlSessionFactoryBuilder allows users to fully customise their implementations of SqlSessionFactory and SqlSession, the drawback is that you cannot use the safe defaults MyBatis provides anymore, which leaves two options:

• Duplicate (copy/paste) DefaultSqlSession and DefaultSqlSessionFactory, the drawbacks here are that you do not get upstream updates anymore and have to compare and update these files every release.
• Use reflection, the drawbacks here are that this is quite dirty and suffers from the same problems mentioned above.

I added a minimal test case which shows a structure that would allow the use of the defaults while providing important extension points.

Below is parked for future discussion:

Additionally, allow subclasses of DefaultSqlSession to call selectList(...), which allows providing a system wide result handler if user code did not specify one.

I understand that this is definitely not a common use case, but believe the changes required are so minimal that it would provide a good tradeoff.

PS: note that the result handler is not the only reason here, we do quite some custom work (in our codebase) in the factory and session respectively.

[coveralls]

coverage: 87.17% (+0.001%) from 87.169% when pulling 67c6babee6d38522191b4b2bf6b051d4f150046c on epochcoder:feat/custom-sql-session-using-defaults into 043270b83a01162d50a05ae1656ee57f57336a88 on mybatis:master.

[epochcoder]

Hi @harawata, do you have some feedback on this one?

[harawata]

DefaultSqlSessionFactory looks OK.

I'm not sure about extending DefaultSqlSession, though. This particular change looks harmless and may be sufficient for your needs, but it could trigger future trickier requests.

If you revert the DefaultSqlSession, I will merge the DefaultSqlSessionFactory change. If you cannot give up the DefaultSqlSession change, I would like other devs' opinion.

[epochcoder]

@harawata I'm fine with reverting the change to DefaultSqlSession for now, we can discuss it at a later point. Ill update the PR with the changes

[epochcoder]

Please feel free to do it @harawata , if not, I'll do this by end of day GMT+2 tommorow 😊

[harawata]

Thank you, @epochcoder !