Aspect-Oriented Programming and JBoss
Pages: 1, 2

Example 2. Using Metadata

The TracingInterceptor does not trace field access because it is a bit too verbose. It is a common practice for developers to implement get() and set() methods to encapsulate field access. It would be nice if the TracingInterceptor could filter out and not trace these methods. This example shows you how to use JBoss AOP metadata to implement this filtering on a per-method basis. Usually, metadata is used for more complex things like defining transaction attributes, per-method security roles, or persistence mappings, but this example should be good enough to illustrate how metadata could be used in an AOP-enabled application.

Defining Class Metadata

To add this filtering functionality, we will provide a flag that you can use to turn tracing off. We will go back to our AOP XML file to define the tags that will remove tracing for get() and set() methods. Actually, it's kind of pointless to be tracing the main() function, so let's filter that out, too.

<?xml version="1.0" encoding="UTF-8">
<aop>
   <class-metadata group="tracing" class="POJO">
      <method name="(get.*)|(set.*)">
        <filter>true</filter>
      </method>
      <method name="main">
        <filter>true</filter>
      </method>
   </class-metadata>
</aop>

The above XML defines a grouping of attributes called tracing. The filter attribute will be attached to every method starting with get or set. The regular expression format uses JDK-1.4-defined expressions. This metadata can be accessed within the TracingInterceptor through the Invocation object.

Accessing Metadata

For this metadata to be useful, it must be accessible at runtime. Class metadata is accessible through the Invocation object. To make use of it in our example, the TracingInterceptor must be modified a tiny bit.

public class TracingInterceptor implements Interceptor
{
   public String getName() { return TracingInterceptor; }
   public InvocationResponse invoke(Invocation invocation) 
       throws Throwable
   {
      String filter = (String)invocation.getMetaData(tracing, filter);
      if (filter != null && filter.equals(true))
	  	return invocation.invokeNext();

      String message = null;

      if (invocation.getType() == InvocationType.METHOD)
      {
         Method method = MethodInvocation.getMethod(invocation);
         message       = method:  + method.getName();
      }
      else if (invocation.getType() == InvocationType.CONSTRUCTOR)
      {
         Constructor c = ConstructorInvocation.getConstructor(invocation);
         message       = constructor:  + c.toString();
      }
      else
      {
         // Do nothing for fields.  Just too verbose.
         return invocation.invokeNext();
      }

      System.out.println(Entering  + message);

      // Continue on.  Invoke the real method or constructor.
      InvocationResponse rsp = invocation.invokeNext();
      System.out.println(Leaving  + message);
      return rsp;
   }
}

Running Example 2

The POJO class has been expanded a bit to add get() and set() methods.

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

   private int counter = 0;

   public int getCounter() { return counter; }
   public void setCounter(int val) { counter = val; }
   public static void main(String[] args)
   {
      POJO pojo = new POJO();
      pojo.helloWorld();
      pojo.setCounter(32);
      System.out.println(counter is:  + pojo.getCounter());
   }
}

The TracingInterceptor will intercept calls to main(), POJO(), and helloWorld(). The output should look like this:

Entering constructor: public POJO()
Leaving constructor: public POJO()
Entering method: helloWorld
Hello World!
Leaving method: helloWorld

You can download JBoss AOP and sample code here. To compile and execute:

$ cd oreilly-aop/example2
$ 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. Using Introductions

It would be cool if we could turn tracing off and on for specific instances. JBoss AOP has an API to attach metadata to an object instance, but let's pretend an actual tracing API is a better solution. In this example, we'll change the definition of the POJO class itself through the use of an introduction. We will force the POJO class to implement a tracing interface and provide a mix-in class that handles the new tracing API. This will be the tracing interface:

public interface Tracing
{
   public void enableTracing();
   public void disableTracing();
}

Defining a Mix-in Class

The Tracing interface will be implemented in a mix-in class. An instance of this mix-in class will be attached to the POJO class when a POJO is instantiated. Here's the implementation:

import org.jboss.aop.Advised;

public class TracingMixin implements Tracing
{
   Advised advised;

   Public TracingMixin(Object obj)
   {
      this.advised = (Advised)obj;
   }

   public void enableTracing()
   {
      advised._getInstanceAdvisor().getMetaData().addMetaData(
	  	"tracing", "filter", true);
   }

   public void disableTracing()
   {
      advised._getInstanceAdvisor().getMetaData().addMetaData(
	  	"tracing", "filter", false);
   }
}

The enableTracing() method attaches the filter attribute to the object instance. The disableTracing() method does the same, but turns the filter attribute to false. These two methods are examples of how metadata can be applied to more than the class level. That metadata can applied at the instance level, as well.

Attaching an Introduction

Okay, so we've defined the tracing interface and implemented the mix-in class. The next step is to apply the introduction to the POJO class. As for interceptors, we must define another pointcut in XML. Let's see what this looks like.

<?xml version="1.0" encoding="UTF-8">
<aop>
   <introduction-pointcut class="POJO">
      <mixin>
         <interfaces>Tracing</interfaces>
         <class>TracingMixin</class>
         <construction>new TracingMixin(this)</construction>
      </mixin>
   </introduction-pointcut>
</aop>

The above pointcut will force the POJO class to implement the Tracing interface. Now, when an instance of POJO is instantiated, an instance of TracingMixin will also be instantiated. The way TracingMixin is instantiated is defined in the <construction> tag. You can put any one-line Java code you want in the <construction> tag.

Running Example 3

The POJO class has been expanded a bit to show how the Tracing API can now be accessed. The TracingInterceptor hasn't changed from Example 2.

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

   public static void main(String[] args)
   {
      POJO pojo     = new POJO();
      Tracing trace = (Tracing)this;
      pojo.helloWorld();

      System.out.println("Turn off tracing.");

      trace.disableTracing();
      pojo.helloWorld();

      System.out.println("Turn on tracing.");

      trace.enableTracing();
      pojo.helloWorld();
   }
}

Notice that we can typecast POJO to the Tracing interface. The output should look like this:

Entering constructor: POJO()
Leaving constructor: POJO()
Entering method: helloWorld
Hello World!
Leaving method: helloWorld
Turn off tracing.
Entering method: disableTracing
Leaving method: disableTracing
Hello World!
Turn on tracing.
Entering method: helloWorld
Hello World!
Leaving method: helloWorld

Notice that the interceptor-pointcut that added the TracingInterceptor also applies to the methods introduced by the Tracing introduction.

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

Conclusions

Aspect-oriented programming is a powerful new tool for software development. With JBoss 4.0, you can implement your own interceptors, metadata, and introductions, to make your software development process more dynamic and fluid. Come visit us at www.jboss.org for more detailed documentation. There are a few surprises waiting for you, as we've implemented a suite of services on top of our new framework. Have a look and happy coding.

Bill Burke is a Fellow at the JBoss division of REd Hat Inc. A long time JBoss contributor and architect, his current project is RESTEasy, RESTful Web Services for Java.

Adrian Brock is Director of Support for JBossGroup, LLC.

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