Demystifying AOP, Getting started with LTW (load time weaving)

Often, using aspectj AOP, especially with LTW (load time weaving) is shrouded in mystery.
Thought I would write up a little note about getting started with Aspectj AOP LTW.

here goes...all you would need is a simple eclipse java project looking like below.

We would need 

1. an aspect source file - MySimpleLoggerAspect.java
2. a sample service which will get AOP-ed - SampleService.java
3. a test class with main method - Tester.java
4. an Aspectj, LTW related config file - META-INF\aop.xml
5. aspectjrt-1.7.0.jar and aspectjweaver-1.7.0.jar in your project classpath

MySimpleLoggerAspect.java is the logging aspect. For further details about writing aspectj aspects please refer www.eclipse.org/aspectj/docs.php. Listing of the simple aspect in java is below.

----------------------------------------------------------------

package com.ghag.rnd.aspects.ltw;

import org.aspectj.lang.ProceedingJoinPoint;

import org.aspectj.lang.annotation.Around;

import org.aspectj.lang.annotation.Aspect;

import org.aspectj.lang.annotation.Pointcut;

@Aspect

public class MySimpleLoggerAspect {

@Pointcut("execution(* *(..))")

public void myTraceCall() {

}

@Around("com.ghag.rnd.aspects.ltw.MySimpleLoggerAspect.myTraceCall()")

public Object myTrace(ProceedingJoinPoint joinPoint) throws Throwable

{

System.out.println("myTrace:before call "

+joinPoint.getTarget().getClass().getName()

+"."+joinPoint.getSignature().getName());

Object retVal = null;

try

{

retVal = joinPoint.proceed();

}

finally

{

System.out.println("myTrace:after call "+

joinPoint.getTarget().getClass().getName()

+"."+joinPoint.getSignature().getName() + " retval=" +retVal);

}

return retVal;

}

}

SampleService.java is as easy as given below:

----------------------------------------------------------------

package com.ghag.rnd.aspects.sample;

public class SampleService {

public String doService(String in){

System.out.println("inside doService");

return in;

}

}

Tester.java listing is a few more lines of code:
----------------------------------------------------------------

package com.ghag.test;

import com.ghag.rnd.aspects.sample.SampleService;

public class Tester {

public static void main(String[] args) {

new SampleService().doService("Ganesh Ghag");

}

}

And finally the META-INF\aop.xml listing is simple a self explanatory, especially the package names ;-)
----------------------------------------------------------------

<aspectj>

<aspects>

<aspect name="com.ghag.rnd.aspects.ltw.MySimpleLoggerAspect" />

</aspects>

 

<!--  <weaver options="-verbose -debug -showWeaveInfo"> -->

<weaver>

<include within="com.ghag.rnd.aspects.sample.*" />

<include within="com.ghag.rnd.aspects.ltw.*" />

</weaver>

</aspectj>

Now when you run Tester.java, just ensure you have the following paramater supplied as JVM argument:

-javaagent:/your dev env local /path/to/aspectj\aspectjweaver-1.7.0.jar

Thats it folks, when you run, Tester.java, SampleService call will get AOP-ed and give following output:

----------------------------------------------------------------

myTrace:before call com.ghag.rnd.aspects.sample.SampleService.doService

inside doService

myTrace:after call com.ghag.rnd.aspects.sample.SampleService.doService retval=Ganesh Ghag


Getting started with AspectJ AOP with LTW is that easy, folks!