Migrating away from JPA back to simple SQL
jdbc, jpa
Solution
I hope you guys have some good unit and integration test. If not, then this is where to start.
After that, in a first step you could create an `abstract factory` through which you provide access to all your DAOs and entities and change all access from clients to go over that factory. In this step you'll have to provide an implementation for that factory, create a concrete factory `JpaAccessFactory` and let its methods return DAOs and entities filled using JPA.
When you are sure nothing went bad with the first step. then proceed with the second step.
Create a factory implementation `SqlAccessFactory` witch fills the DAOs and entities (objects of entity classes are here actually just DAOs) using SQL. Write some unit tests witch uses both factories and make the asserts using `expected` provided by `JpaAccessFactory` and `actual` provided by `SqlAccessFactory`.
When you are done with implementing the factory method for a table and all its dependencies in `SqlAccessFactory`, let the client / page that uses the DAO or entity provided by this method, use the new factory for retrieving it. You could make this configurable so you wont have to change the code to change witch factory is beeing used. This also would have the benefit to easily switch back if you find out that something is not as it should be.
Factory methods witch are not yet implemented in `SqlAccessFactory` could just throw an Exception
throw new UnsupportedOperationException
("Not yet implemented, please configure your client / page to use JPA.");
I hope this gives some hints one where and how to start.
Problem
After years of development with TopLink/EclipseLink in a production application with around 100 tables we have decided that enough is enough and that JPA is not worth the added complexity and uncertainty of its actual operations, and that SQL (with some wrapper like DBUtil or something like that) can do the job just right for us. Can you suggest how one would go about migrating a pretty large JPA application to JDBC/SQL in a way that would leave JPA still running (i.e. in the webapps with GUI) but that would allow us to still start with the "downgrade" to JDBC? We have entities and DAOs, but my real worry is the JPA entitycache (primary one) - is it possible to disable it completely so that JPA acts as a simple connection.begin(); entries... connection.commit(); in the interim, until we get rid of it for good?