Service layer/DTO question

[yarco]

1. Does the @Service layer know about DTO's (can on of its methods return it or take it as an argument)?If not, that which layer know about them?
2. Is the form backing object considered to be a DTO?
3. Can @Controller layer create Domain objects (for example crate a new Domain object from the data provided in the form backing object)?

Please do help me understand this, as I asked many people, and nobody seems to have a clear answer.

[Marten Deinum]

1) Why not... It is just an java class return/create whatever you like
2) It depends on your project it can be but it doesn't have to be.
3) Again why not, although why would your formBackingObject be something different then your actual domain object?

[yarco]

The reason I asked this question is that a while ago I read this post :
http://forum.springsource.org/showthread.php?t=64862
and specifically this

Never pass Command objects to a business service!

This is why I would like to know if backing objects(presentation-tier-specific) can be considered to be DTO's.
Looking at your previous post I understand that It is perfectly valid for my @Service layer to have a method that can convert my domain object to a DTO (i.e. Article to ArticleDTO or visa versa....that can be used by....umm.....lets say DWR for example), and looking back at constv's post he says that it isn't valid for my @Service method to know about commandObject (which can be a DTO too).

To sum up:
a)DTO's can be part of my domain model,@Service layer knows about them
b)Command objects are presentation-tier-specific, and @Service layer doesn't know about them.
So my question is : how is the command object any different than a regular DTO , they both are used to transfer data ( command object transfers it to the jsp(view) and back from the form);

[Marten Deinum]

The command object doesn't have to be presentation-tier-specific object that is where your logic is flawed. You can perfectly use your domain objects as command objects. You don't have to have a seperate object for that...

Also you don't have to have DTO's (however it depends on your architecture) if you already use DTO's then simply use ther DTO's as command objects why create another layer which you need to maintain?????

[yarco]

thanks for the reply,
yes we try to use domain objects as command objects too. This is certainly easier!
Anyway now I understood that my @Service layer knows about DTO's, and can have methods to convert to/from regular domain object.
In spring mvc, when would it be appropriate to use DTO's instead of domain objects. can anybody give a small example please. thank you so much

[Marten Deinum]

I tend to use DTO or specific command classes when there is a mismatch in what you try to do in yuor view and with your domain model. (Just like you have a relation database and OO design mismatch!). Then sometimes it can be easier to create dedicated objects. But I always would strive for as much reuse as possible.

[João Vieira da Luz]

If your architecture is runnable on a single vm (you don't for example EJB) I think that you would benefit a lot removing the DTO stuff as Marten tell you.

But, if you need the DTO and that copy to/from trick you should take a look to dozer.sf.net.