Did you know the Java Instrumentation API exists already since Java 5? It is actually a way you can spy legally in the JVM and can help you uncover all the details inside the JVM.
The ByteBuddy library uses that power of the JVM and makes it easy to do bytecode manipulation, but how to use it?
There is a practical way to use it, OpenTelemetry Java agent is both using the Instrumentation API and ByteBuddy to instrument your application. But did you know you can easily create your own extension on top of that?

10 years ago, most Java applications weren't built with observability in mind. Barely any logs, no metrics, no traces — just darkness.
What if you could add complete distributed tracing to your (legacy) Java application or vendor application without modifying its source code, rebuilding it, or even having access to the original codebase?

In this talk, we'll explore how the Java Instrumentation API is working, you will learn more about ByteBuddy, OpenTelemetry Java agent and custom extensions, all to get valuable insights about the running application.
I will demonstrate how to intercept method calls and even change the outcome of a method if you like.
You will also learn how to create distributed traces and custom metrics by instrumenting code via the Java agent. This approach delivers the deep observability needed to finally understand your system's actual behavior.

You'll leave with working code examples and practical patterns you can apply immediately — perfect for anyone who needs to understand what's really happening inside that "legacy system no one wants to touch."