Field interception

Overview

JBoss AOP allows you to intercept a field access and insert behavior their. If you look at Driver.java and POJO.java you will see that they are accessing a number of fields.

How do I apply an Interceptor to a field execution?

To bind an interceptor to the access of a field, you must create an XML file. Open up jboss-aop.xml and take a look. Let's take a look at the first bindings.

<aop>
...
   <bind pointcut="get(private java.lang.String POJO->var1)">
       <interceptor class="GetInterceptor"/>
   </bind>
...
</aop>

This bindings states that when POJO.var1 is accessed for a read, invoke the GetInterceptor.

The next one is:

...
   <bind pointcut="set(private java.lang.String POJO->var2)">
       <interceptor class="SetInterceptor"/>
   </bind>
...

Whenever the field POJO.var2 is written to, invoke the SetInterceptor.

The next one is:

...
   <bind pointcut="field(public static * POJO->*)">
       <interceptor class="FieldInterceptor"/>
   </bind>
...

Whenever any public static field in POJO is accessed for a read or a write, call the FieldInterceptor. The field expression matches both get/set of a field.

Running

Let's try these babies out.... To compile and run:

  $ ant

It will javac the files and then run the AOPC precompiler to manipulate the bytecode, then finally run the example. The output should read as follows:

run:
     [java] --- pojo.getVar1(); ---
     [java] <<< Entering GetInterceptor for: var1
     [java] >>> Leaving GetInterceptor
     [java] hello
     [java] --- pojo.setVar2(); ---
     [java] <<< Entering SetInterceptor for: var2
     [java] >>> Leaving SetInterceptor
     [java] --- pojo.getVar2(); ---
     [java] world
     [java] --- POJO.var3++; ---
     [java] <<< Entering FieldInterceptor for: var3 type: org.jboss.aop.joinpoint.FieldReadInvocation
     [java] >>> Leaving FieldInterceptor
     [java] <<< Entering FieldInterceptor for: var3 type: org.jboss.aop.joinpoint.FieldWriteInvocation
     [java] >>> Leaving FieldInterceptor