FIELD-level replication only replicates modified data fields inside objects stored in the session. It could potentially drastically reduce the data traffic between clustered nodes, and hence improve the performance of the whole cluster. To use FIELD-level replication, you have to first prepare your Java class to indicate which fields are to be replicated. This is done via JDK 1.4 style annotations embedded in JavaDocs:

To annotate your POJO, we provide two annotations: @@org.jboss.web.tomcat.tc5.session.AopMarker and @@org.jboss.web.tomcat.tc5.session.InstanceAopMarker. When you annotate your class with AopMarker, you indicate that instances of this class will be used in FIELD-level replication. For exmaple,

/*
 * My usual comments here first.
 * @@org.jboss.web.tomcat.tc5.session.AopMarker
 */
public class Address 
{
...
}

If you annotate it with InstanceAopMarker instead, then all of its sub-class will be automatically annotated as well. For example,

/*
 *
 * @@org.jboss.web.tomcat.tc5.session.InstanceOfAopMarker
 */
public class Person 
{
...
}

then when you have a sub-class like

public class Student extends Person
{
...
}

there will be no need to annotate Student. It will be annotated automatically because it is a sub-class of Person.

However, since we only support JDK 1.4 style annotation (provided by JBoss Aop) now, you will need to perform a pre-processing step. You need to use the JBoss AOP pre-compiler annotationc and post-compiler aopc to process the above source code before and after they are compiled by the Java compiler. Here is an example on how to invoke those commands from command line.

$ annotationc [classpath] [source files or directories]
$ javac -cp [classpath] [source files or directories]
$ aopc [classpath] [class files or directories]            
            

Please see the JBoss AOP documentation for the usage of the pre- and post-compiler. The JBoss AOP project also provides easy to use ANT tasks to help integrate those steps into your application build process. In the next AS release, JDK 5.0 annotation support will be provided for greater transparency. But for now, it is important that you perform the pre- and post-compilation steps for your source code.

When you deploy the web application into JBoss AS, make sure that the following configurations are correct:

Finally, let's see an example on how to use FIELD-level replication on those data classes. Notice that there is no need to call session.setAttribute() after you make changes to the data object, and all changes to the fields are automatically replicated across the cluster.

// Do this only once. So this can be in init(), e.g.
if(firstTime)
{
  Person joe = new Person("Joe", 40);
  Person mary = new Person("Mary", 30);
  Address addr = new Address();
  addr.setZip(94086);

  joe.setAddress(addr);
  mary.setAddress(addr); // joe and mary share the same address!

  session.setAttribute("joe", joe); // that's it.
  session.setAttribute("mary", mary); // that's it.
}

Person mary = (Person)session.getAttribute("mary");
mary.getAddress().setZip(95123); // this will update and replicate the zip code.            
            

Besides plain objects, you can also use regular Java collections of those objects as session attributes. JBoss cache automatically figures out how to handle those collections and replicate field changes in their member objects.