As rule of thumb try to divide logical layers according to your context. Inspire you of the theory but use it with care. I give you my humble understanding of layer's interest with few examples. This vision is of course not complete but I hope it will help you to answer your questions.
- So is it overkill to use UIBean instead of Service DTO ? I would say it depends of your context.
Maybe there are user inputs data inside your UI beans ? You have to validate them with JSR 303 annotations for example. If those annotations have a meaning in this layer they are useless for underneath layers. That's why you will have a UIBean with JSR 303 annotations and a DTOBean without JSR 303 annotations.
But if they are exactly the same why duplicate ? Maybe at UIBean layer a date could be represented as a String type and you want to manipulate Date type instead of String at DTO layer. That's why you need to adapt your data between layers to work with objects that make sense to a particular layer. For example, you could add a BOAdapter (between UIView and Service) and DTOAdapter (between Service and DAO). Those adapters are usefull for transforming your data inside each POJO's format. For example, you could have in your BO(=UIBean) a date expressed inside three strings and you want a Date object for DTO so you transform it inside the BOAdapter:
public class BOAdapter(){
private BOAdapter(){}
public static DTO toDTO(BO objectBO){
DTO objectDTO = new DTO();
SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-aa");
objectDTO.setDate(df.parse(objectBO.getYear()+"-"+objectBO.getMonth()+"-"+objectBO.getDay());
[...]
}
}
- Why I need DTOAdapter ? Maybe you have a database that contains at least two tables Customers and Adresses with an integrity constraint between them. JPA will automatically generate the right code. But do you really need all this code up to UIView ? I mean if the functionnality you are coding needs only the name, surname and date of birth of your customer, their adress is useless. Again that's why you need to adapt your data between layers to work with objects that make sense to a particular layer. In this case you could create a DTO object only with name, surname and date of birth information and create a method inside your DTOadapter to transform your custom DTO into an heavy JPA object to work properly with database.
But I need the whole entity for coding my fonctionnality ? Maybe you need to add validation constraints inside this layer besides JSR 303. So it could be interesting to have DTO classes besides your entity for the same reason as BO objects.
But my entity is big how duplicate it easily ? Try to use a tool to map data (like dozer) automatically. If it is not too big do it manually.
