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When using SSH(Struts+Spring+Hibernate), data transform and transfer is painful, usually, from ActionForm(Web) --> VO(Service) --> PO(DAO) .... --> PO --> VO --> ActionForm. As for EJB2, it is very similar that you have to convert ActionForm(or whatever other objects depending on what web layer framework you use) into DTO(Data Transfer Object) and then into Entity Bean.

However, that is the cost of layering your application, which makes your system more flexible and loosely coupled.

The problem is when using the above architectures, there is too much duplication of classes and its “getter and setter” and too much simple “copy” of code. For system maintenance and extension, it’s a nightmare.   

So I would say your problem is not only about passing data from service layer into DAO layer, but also about transferring data between different layers in system.

The solution probably is using POJO and new web frameworks and new Persistence APIs.  

If you want to continue to use Struts and Spring, using Struts2+spring+JPA is your choice. If you are going to adopt EJB, using JSF+EJB3 is your choice. The key of both architectures is their using POJO(+Annotation) as the vehicle of data transfer and manipulation. 

For example, JSF components can be directly bound to POJO’s field; JPA EntityManager uses POJO+Annotion as Entity Bean to manipulate data in database. Struts2 also discarded the ActionForm and uses POJO as HTML input or output value binding.

So put it simple, forget VO,DTO,PO,ActionForm, only use POJO(+Annotation),which is also called “domain objects”, you will find that the system becomes so clean and no any duplication for data transfer at all.

Here is a very good example for Strut2+Spring+JPA:http://cwiki.apache.org/S2WIKI/struts-2-spring-2-jpa-ajax.html. 

Anyway, you may still worry about using one single POJO in every layers; it probably makes the system tightly coupled. But in fact it does not. You have to change the way you regard the “domain objects”(POJO+Annotation), that move between different layers of the system,carring data, being presented, changed, saved into database, etc. These “domain objects” are the blood of the system, and can be used by Web Framework, Business Logic code, Persistence API. These “domain objects” almost only contain data and its getters and setters, or some small logic for conversion of data. They are the only artifacts created, used, changed, transferred by the components in every layers in the system.

As for JPA, don’t worry about it at all, it just absorbs all the essence of Hibernate and uses Annotation instead of hbm.xml. If you are familiar with Hibernate, it will not take you long time to master it. 

Another issue is DAO layer seems to disappear. Yes, if you use JPA, write another layer of DAO seems to be meaningless. In my point of view, DAO is only useful when you use JDBC or a very big system with multi-layer even in business logic code. If you has already use ORM tool such as Hibernate, JPA, it really doesn’t give much benefit for the system, unless it’s a very big system. 

I hope this will give you little help.