Elastic's contribution: Invokedynamic in the OpenTelemetry Java agent

To overcome the above-mentioned limitations in developing and maintaining auto-instrumentation modules in the OpenTelemetry Java agent, Elastic started contributing its invokedynamic-based instrumentation approach to the OpenTelemetry Java agent in July 2023.  

To explain the improvement, you should know that in Java, a common approach to do auto-instrumentation of applications is through utilizing Java agents that do bytecode instrumentation at runtime. Byte Buddy is a popular and widespread utility that helps with bytecode instrumentation without the need to deal with Java’s bytecode directly. Instrumentation logic that collects observability data from the target application’s code lives in so-called advice methods. Byte Buddy provides different ways of hooking these advice methods into the target application’s methods:

• Advice inlining: The advice method’s code is being copied into the instrumented target method.
• Static advice dispatching: The instrumented target method invokes static advice methods that need to be visible by the instrumented code.
• Advice dispatching with invokedynamic: The instrumented target method uses the JVM’s invokedynamic bytecode instruction to call advice methods that are isolated from the instrumented code.

These different approaches are described in great detail in our related blog post on Elastic’s Java APM agent using invokedynamic. In a nutshell, both approaches, advice inlining and dispatching to static advice methods come with some limitations with respect to writing and maintaining the advice code. So far, the OpenTelemetry Java agent has used advice inlining for its bytecode instrumentation. The resulting limitations on developing instrumentations are documented in corresponding developer guidelines. Among other things, the limitation of not being able to debug advice code is a painful restriction when developing and maintaining instrumentation code.

Elastic’s APM Java agent has been using the invokedynamic approach with its benefits for years — field-proven by thousands of customers. To help improve the OpenTelemetry Java agent, Elastic started contributing the invokedynamic approach with the goal to simplify and improve the development and maintainability of auto-instrumentation modules. The contribution proposal and the implementation outline is documented in more detail in this GitHub issue. 

With the new approach in place, Elastic will help migrate existing instrumentations so the OTel Java community can benefit from the invokedynamic-based instrumentation approach.