Refactor classes to be domain clean

The beauty of object oriented programming is that you can have a program that resembles the real world objects that it is to represent. Domain driven design is the notion that you create models based on a "domain". That is a subject area. This can be fuzzy, or it can be industry agreed upon. The catch is that you are writing an application, and not a domain model. What does that mean? It means that you will be tempted to add fields to your classes that meet your application's needs, but are not really part of your domain.
Transcendental Beans is here to save you from polluting your domain model with application schemata. You have the power of AspectJ introduction to separate application schema from domain schema.
Try aspect refactoring with AspectJ on your persistent classes with Transcendental Beans

Introduce JAXB to your POJOs

Suppose you want to keep your POJO really a POJO? If you use JPA annotations on a class, is it still really a POJO? A POJO should be usable as a domain object independent of whether it came from a database, or from a remote service. So If I see @Entity on your POJO, I know it is really a database object, and I start thinking all about it's dependencies, do I need a transaction, do I need a JDBC connection, what JAR has the JPA annotation, maybe it isn't in my JRE, no more a "plain" Java object.

Ok, but I want to store my POJO in a database, what do I do? Ok, that will be my next article... Today, what if I want to use JAXB to serialize the POJO as XML? Then I need to use @javax.xml.bind.annotation.XmlRootElement to annotate the class. That just ruined my POJO. AspectJ can add fields, methods, annotations, and parents to classes. Why not just introduce the JAXB annotations necessary to marshal the object at runtime just for the situation that I need?

Here is my POJO:

public class Order {
   private String number;
   private String product;
   private String customerEmail;
   private String customerName;
}

Now, here is the POJO with the JAXB annotation:

import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement
public class Order {
   private String number;
   private String product;
   private String customerEmail;
   private String customerName;
}

Isn't marshaling an object to XML a totally different concern from if an Order needs the customer's email address? So, let's separate those two concerns:

import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement
public aspect Jaxbify {
   declare @type : Order : @XmlRootElement;
}

This aspect introduces the XmlRootElement annotation to the Order class, keeping Orders, Orders.
Next time I'll show how to run this example.