Question

I have a quite large codebase. In many places I have a piece of code like this:

for (MyObjectType myobj : myList) {
   //...do something with myobj
}

MyObjectType is the basic object in my application, and I iterate over many of them very often. I have some log4j set up so that I log info about the MyObjectType instance I am dealing with:

for (MyObjectType myobj : myList) {
   MDC.put("myobj", myobj.identify());
   ...
   logger.error("this message contains info about myobj")
}

This is really helpful. Unfortunately there are tons of for looks like this where I forgot to use the mdc.put(). So I was thinking…would be some way to instrument the code so that:

1. I detect there is a for looping over a collection of MyObjectType
2. I insert this as first instruction inside the loop: MDC.put("myobj",
myobj.identify());

If there is a way (using aop, instrumentation, some java agent?), how difficult would it be (maybe it’s not worth the effort).

I have never used any java bytecode library, just spring aop lightly.

Answer 1

You can change/add classes at runtime using javassist.
It’s practically black magic.

I have also used janino with great success.