Java Agent

Starting with JDK 1.5, the javaagent feature has been around which provides for a hook to get access to the classes as they are loaded and consequently do instrumentation of the class to serve your purpose. This along with Javassist will allow you to achieve some useful tasks like enabling specific debugging etc. Below are some useful links for the same

http://www.csg.ci.i.u-tokyo.ac.jp/~chiba/javassist/tutorial/tutorial.html
http://blog.javabenchmark.org/2013/05/java-instrumentation-tutorial.html
https://today.java.net/article/2008/04/22/add-logging-class-load-time-java-instrumentation

I used to face issues related to the agent class not been able to find javassist related classes. To tackle this you can add an index file to the agent jar like below.

jar -i agent.jar lib\javassist.jar

and ensure that during deployment the javassist.jar file is in the lib folder which itselfs exists in the same directory as the agent jar.

The insertBefore and insertAfter methods of javaassist can be very useful when instrumenting methods for tasks like profiling the timings, checking the input parameters and checking return values.

slf4j-ext has a good helper class (JavassistHelper) which provides for string to be used for logging the signature (input params) and the return value.

http://www.slf4j.org/extensions.html

In order to use the insertAt method of javassist, you can use the ExprEditor like below

method.instrument(new ExprEditor() {
 public void edit(MethodCall m) throws CannotCompileException {
 }
}

This will iterate through the method body. The required instrumentation can be done within this.