Around ten months ago I wrote a blog post together with Mikaël Francoeur on how to instrument instrumenters:

Have you ever wondered how libraries like Spring and Mockito modify your code at run-time to implement all their advanced features? Wouldn’t it be cool to get a peek behind the curtains? This is the premise of my meta-agent, a Java agent to instrument instrumenters, to get these insights and what this blog post is about.

Who instruments the instrumenters?

To use the meta-agent you had to attach it manually as an agent:

java -javaagent:target/meta-agent.jar=server -jar your-program.jar

This launched a website under localhost:7071 where you could view the actions of every instrument and transformer. The only problem? It’s cumbersome to use, especially programmatically. Join me in this short blog post to learn about the newest edition of meta-agent and what it can offer.

Instrumentation Handler

An idea that came up at the recent ConFoo conference in discussion with Mikaël and Jonatan Ivanov was to add a new handler mechanism to call code every time a new transformer is added or a class is instrumented. So I got to work.