Embracing invokedynamic to tame class loaders in Java agents

Felix Barnsteiner and Rafael Winterhalter explore how the invokedynamic bytecode instruction can simplify Java agent development, particularly in overcoming complex class loader issues. Traditionally, Java agents face challenges with class loaders, visibility, and version conflicts, but the invokedynamic instruction helps tackle these by allowing for a straightforward mental model and enabling runtime updates without application restarts. This approach, adopted by the Elastic APM Java agent, mitigates issues associated with traditional advice dispatching methods by using a child class loader to load advice and helper classes, avoiding reliance on unsafe internal APIs and facilitating live updates. The invokedynamic-based method allows agents to isolate their classes from the application's class loader, thus preventing compatibility issues and simplifying debugging by eliminating class conflicts. Although it requires Java 7 or later, this method offers a more robust and flexible architecture for Java agents, especially when handling complex instrumentation tasks.