Why put a DAO layer over a persistence layer (like JDO or Hibernate)

Question

Data Access Objects (DAOs) are a common design pattern, and recommended by Sun. But the earliest examples of Java DAOs interacted directly with relational databases -- they were

Answer 1

It depends what your layer's goals are. You put an abstraction in to supply a different set of semantics over another set. Generally further layers are there to simplify somethings such as development of future maintennance. But they could have other uses.

For example a DAO (or persistence handling) layer over an ORM code supply specialised recovery and error handling functionality that you didn't want polluting the business logic.