Being pretty unfamiliar with design patterns and architecture, I'm having trouble explaining to others exactly how my latest application is designed. I've switched between thinking it's a pure n-tier, pure MVC and n-tier with MVC in the presentation layer. Currently I think the latter is correct, but I want thoughts from more experienced developers.
How it works:
- Browser sends HTTP request to Tomcat. Maps the request via web.xml to a servlet (which I call controller)
- The controller instantiates one or more business object and calls methods on these, i.e.
customerBO.getById(12)which again will perform business logic/validation before calling one or more DAO methods, i.e.customerDAO.getById(12). The BO returns a list of CustomerVO's to the controller - The controller prepares attributes for the view (JSP) (
request.setAttribute("customers", customers);) and chooses a .jsp file to use which in turn will iterate the list and render XHTML back to the browser.
Structure (my proposal/understanding)
Presentation tier: currently using what I think is a MVC web-implementation: servlets (controllers), jsp (views) and my own implementation of OO XHTML forms (ie. CustomerForm) lies here. It should be possible to use a Swing/JavaFX/Flex GUI by switching out this presentation layer and without the need to change anything on the layers below.
Logic tier: Divided into two layers, with Business Objects (BO) on top. Responsible for business logic, but I haven't found much to put in here besides input validation since the application mostly consists of simple CRUD actions... In many cases the methods just call a method with the same name on the DAO layer.
DAO classes with CRUD methods, which again contacts the data tier below. Also has a convertToVO(ResultSet res) methods which perform ORM from the database and to (lists of) value objects. All methods take value objects as input, i.e. customerDAO->save(voter) and return the updated voter on success and null on failure.
Data tier: At the bottom data is stored in a database or as XML files. I have not "coded" anything here, except some MySQL stored procedures and triggers.
Questions (besides the one in the title):
- The M in MVC. I'm not sure if I can call this n-tier MVC when the models are lists/VO's returned from business objects in the logic tier? Are the models required to reside within the presentation layer when the controller/view is here? And can the form templates in the presentation layer be called models? If so; are both the forms and lists from BO to be considered as the M in MVC?
- From my understanding, in MVC the view is supposed to observe the model and update on change, but this isn't possible in a web-application where the view is a rendered XHTML page? This in turn leads me to the question: is MVC implemented differently for web-applications vs. regular desktop applications?
- I'm not using a Front Controller pattern when all HTTP requests are explicitly mapped in web.xml right? To use Front Controller I need to forward all requests to a standard servlet/controller that in turn evalutes the request and calls another controller?
- The Business Layer felt a little "useless" in my application. What do you normally put in this layer/objects? Should one always have a business layer? I know it should contain "business logic", but what is this exactly? I just perform input validation and instantiate one or more DAOs and calls the appropriate methods on them...
I realize there is MVC frameworks such as Struts for Java, but since this my first Java web-application I tried to get a deeper understanding of how things work. Looking in retrospect I hope you can answer some of the questions I stumbled upon.