To apply an aspect, you define when to execute the aspect code. Those points in execution are called pointcuts. An analogy to a pointcut is a regular expression. Where a regular expression matches strings, a pointcut expression matches events/points within your application. For example, a valid pointcut definition would be "for all calls to the JDBC method executeQuery(), call the aspect that verifies SQL syntax."

An entry point could be a field access, or a method or constructor call. An event could be an exception being thrown. Some AOP implementations use languages akin to queries to specify pointcuts. Others use tags. JBoss AOP uses both. Listing Three shows how to define a pointcut for the metrics example.

Listing Three: Defining a pointcut in JBoss AOP

1. <bind pointcut="public void com.mc.BankAccountDAO->withdraw(double amount)">
2.       <interceptor class="com.mc.Metrics"/>
3. </bind >

4. <bind pointcut="* com.mc.billing.*->*(..)">
5.       <interceptor class="com.mc.Metrics"/>
6. </bind >

Lines 1-3 define a pointcut that applies the metrics aspect to the specific method BankAccountDAO.withdraw(). Lines 4-6 define a general pointcut that applies the metrics aspect to all methods in all classes in the com.mc.billing package.

There is also an optional annotation mapping if you do not like XML. See JBossAop Reference Guide for more information.

JBoss AOP has a rich set of pointcut expressions that you can use to define various points/events in your Java application so that you can apply your aspects. You can attach your aspects to a specific Java class in your application or you can use more complex compositional pointcuts to specify a wide range of classes within one expression.