A best practice is to use a layered architecture. Arne illustrated one
with three basic layers: front end, middleware logic and back end data
system.
Refinements of the scheme might split middleware into control logic
("Controller"), presentation logic ("View") and business logic
("Model"), for example. The business logic might communicate to the
data system via a connection layer, such as a Java Persistence API (JPA)
framework (TopLink, Hibernate, OpenJPA).
How many layers you architect, and how thick or thin each one is,
depends on the needs of the particular project. Three layers seems the
minimum, more than seven would be suspect.
The separation of model, view and controller (the
"Model-View-Controller" or "MVC" architecture) has many advantages for
robustness, correctness, stability, maintainability and expandability.
This is a case of the principle of separation of concerns - each layer
has a narrow focus of responsibility and clean, simple ways of
communication with its nearest neighbor layers (and no others).
A Java Enterprise architecture would use JSPs or XHTML pages for the
presentation artifacts, quite likely script enhanced. A framework like
Java Server Faces (JSF) provides the interface components, also
controller and presentation logic components. A custom or
framework-provided servlet (or small set of servlets) provides the glue
that ties those front-end components to the business logic components.
There might be specialized communication libraries to carry messages
between layers, such as web-services frameworks or message queues. The
business logic comprises Java components variously realized as POJO
(likely JavaBean-ish) classes or Enterprise JavaBeans (EJBs), which are
not the same as non-Enterprise JavaBeans. They handle things like
coordination of authentication, stitching together requested data into
meaningful structures and so on. Business logic can represent entities,
the nouns of the application domain, or processes, the verbs of the domain.
Business logic connects to the data layer, data layer talks to the
datastore, oh, oh, dem bones.
I just made it sound complex, and it can be if the requirements are big
enough. It doesn't need to be. Simple web apps use
XHTML/JSPs/JSF/scripting for the front end, a little bit of servlet
controlling things, Java objects handling business logic, JPA framework,
PostgreSQL or Derby (JavaDB) on the back end and you're good to go.
