The JVM is Not Observable Enough (and What To Do About It)

Bytecode instrumentation is a preferred technique for build-ing profiling, debugging and monitoring tools targeting the Java Virtual Machine (JVM), yet is fundamentally danger-ous. We illustrate its dangers with several examples gath-ered while building the DiSL instrumentation framework. We argue that no Java platform mechanism provides simul-taneously adequate performance, reliability and expressive-ness, but that this weakness is fixable. To elaborate, we con-trast internal with external observation, and sketch some ap-proaches and requirements for a hybrid mechanism