OpenJIT is an open-ended, reflective JIT compiler framework for Java being researched and developed in a joint project by Tokyo Inst. Tech. and Fujitsu Ltd. Although in general self-descriptive systems have been studied in various contexts such as reflection and interpreter/compiler bootstrapping, OpenJIT is a first system we know to date that offers a stable, full-fledged Java JIT compiler that plugs into existing monolithic JVMs, and offer competitive performance to JITs typically written in C or C . We propose an architecture for a reflective JIT compiler on a monolithic VM, and describe the details of its frontend system. And we demonstrate how reflective JITs could be useful class- or application specific customization and optimization by providing an important reflective "hook" into a Java system.