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A More Complex Use Case

Without going into too much detail, we will have a look at a more complex use case for AOP. A good example is that provided by Java 2 Enterprise Edition. In order to deploy one 'Bean' (essentially a class which performs business logic) which is accessible by a J2EE server and clients, you require at least four files:

The bean class itself:
public function RecordEJB implements SessionBean { ... }
A 'remote' interface:
public interface Record extends EJBObject { ... }
A 'home' interface:
public interface RecordHome extends EJBHome { ... }
XML defining the bean name, type and other information

The contents of each of these files depend on what functions exist in the original Bean. As you can imagine, there is a lot of work to be done when a new function is added to the bean. In this case, a tool called xdoclet employs AOP to automatically generate a heap of scaffolding code for you at complile time. It does this by preprocessing the class and method comments looking for attributes of interest.

/**
 * @ejb-bean name="Record"
 */
public function RecordEJB implements SessionBean { ... }

Now instead of writing all of the afore mentioned files manually an automated build process can parse the @ejb-bean attribute, and automatically generate all of the required scaffolding. The tool used to do this supports multiple attributes, each contributin to different parts of the code generation. For example, just at the class level, the following attribute tags are available:

@ejb.bean, @ejb.dao, @ejb.data-object, @ejb.ejb-external-ref, @ejb.ejb-ref, 
@ejb.env-entry, @ejb.facade, @ejb.finder, @ejb.home, @ejb.interface, 
@ejb.persistence, @ejb.pk, @ejb.remote-facade, @ejb.resource-env-ref, 
@ejb.resource-ref, @ejb.security-identity, @ejb.security-role-ref, 
@ejb.security-roles, @ejb.util, @ejb.value-object

Each of these attributes mean something different to xdoclet, despite them just being plain text to the Java VM. You can see from the extent of the xdoclet tool that there is a multitude of uses that can be achieved. Incidentaly, the syntax used to annotate Java code with xdoclet attributes does not use the native Java Annotation sytax, presumably because that was a relatively new addition to the syntax, whereas xdoclet has been around for a while.

The only point about this example though, is that AOP is used as a preprocessing step during the build process. Because PHP is an interpreted language, this facility does not exist. The following example will show how we use AOP at runtime in a PHP application at IVT.