DAL 简介

The Data Access Layer (DAL) is an object-oriented service foraccessing and updating data in distributed databases and other persistent dataresources.

Applications, SOA, etc....     ------------->         DAL    ---------------->          DB, etc

All create, read, update, and delete (CRUD) operations on persistentdata resources are performed through DAL's abstraction of the data. 
The Data Access Layer:

· Decouplesthe persistence layer from the application layer
· Performs object/relationalmapping (ORM) — mapping data between the persistence layer's native form(e.g. a relational database schema) and Java objects
· Encapsulatesmechanisms for locating distributed data
· Encapsulatesfailover mechanisms
· Encapsulatescaching mechanisms

DAL provides a consistent, transparent, objectoriented API for accessing and updating data in distributed persistent dataresources.