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<DAO - it's not about Layering, it's about Abstraction! ⁄ >

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Again there were some interesting debates in the near past, which once again discussed the role of the DAO within enterprise applications, especially based on JEE and EJB 3. One main argument for the dissapearance of DAOs is the very unlikely probability to exchange or replace your once chosen persistence technology, so that you may directly rely on EJBs Entity Manager within your Business Services (since it's in fact very unlikely that your underlying persistence technology will change). This argument is mostly accompanied with the concern, that the introduction of a DAO Layer might increase complexity and introduce some performance penalties, while direct usage of the Entity Manager let collapse two separate Layers into one.

All in all, those statements raise the impression, that DAOs form a kind of Layer in their own right, or represent at least the main part of your data access Layer.

Let me give you some counter arguments (using some input from 'J2EE Design and Development' as well as 'Domain Driven Design') which will show you that DAOs aren't so much about Layers but more importantly about Abstraction and domain specific Encapsulation (that said, with the disappearance of the DAO, collapsing two Layers into one as suggested, there is STILL a data access Layer, you only just jumbled business and data access logic).

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