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Here are just some of the many sources out there on Data Access Objects and patterns.

Data Access Object (DAO) J2EE Pattern
http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html

Transfer Object (DTO) J2EE Pattern
http://java.sun.com/blueprints/corej2eepatterns/Patterns/TransferObject.html

Pattern Problem
http://forum.java.sun.com/thread.jspa?threadID=569418&tstart=225

General Design with BO's, DTO's and DAO's
http://forum.java.sun.com/thread.jspa?threadID=582832&tstart=134

Using BeanUtils to avoid duplication between DTOs and BOs
http://www.javaranch.com/newsletter/July2003/TouringTheCommonsPart1.html

DAO Generator
http://titaniclinux.net/daogen/

DAO Examples
http://daoexamples.sourceforge.net/

One big Service class or several small classes?
http://www.theserverside.com/news/thread.tss?thread_id=23705

Custom-Grained Data Transfer Objects
http://www.practicalsoftwarearchitect.com/articles/customgrained/customgrained.html

Dynamically generate DTOs with DynaBeans
http://www.javaranch.com/newsletter/200404/Commons_Part3.html