XDoclet, JBOSS-AOP Example3

[X_AWemner_X]

http://www.onjava.com/pub/a/onjava/2003/05/28/aop_jboss.html?page=2
http://www.jboss.org/index.html?module=html&op=userdisplay&id=developers/pro
jects/jboss/aop
http://xdoclet.sourceforge.net/

Ive just read first about XDoclet and jboss-AspectOrientedProgramming
articles. Below is a small example from jboss-aop standalone tutorial from
onjava site.

I did not download anything yet, but wanted to ask how it can work at first
place. See example code below and especially "Tracing tracing =
(Tracing)pojo;" cast. It should not work as POJO class does not implement
such tracing. Ok, aop does the magic and it does work. Then see example how
to compile and run it.

This is where Im lost in space: running "javac *.java" should give us
castexception at compile time. Clearly it does not do so.

What is that magic bullet to fool java compiler run successfully. Is it
that, they do not actually run a regular com.sun.compiler?
[compile and running instructions]
To compile and run this example:
$ cd oreilly-aop/example3
$ export CLASSPATH=.;jboss-common.jar;jboss-aop.jar;javassist.jar
$ javac *.java
$ java -Djava.system.class.loader=org.jboss.aop.standalone.SystemClassLoader
POJO

[example 3 code]
public class POJO {
public POJO() {}

public void helloWorld() { System.out.println("Hello World!"); }

/**
* @jboss-aop.metadata group="tracing"
* filter="true"
*/
public static void main(String[] args) {
POJO pojo = new POJO();
Tracing tracing = (Tracing)pojo;
pojo.helloWorld();
System.out.println("Turn off tracing");
tracing.disableTracing();
pojo.helloWorld();
System.out.println("Turn on tracing");
tracing.enableTracing();
pojo.helloWorld();
}
}