Here are some rough notes on things observed in the codebase - not by the Espresso authors so may all be totally wrong.
Espresso is mostly written in Java but has some C components as well. These serve two purposes:
- The espresso engine can be compiled with native-image down to a shared library so it no longer needs hotspot and runs AOT. This is called “libespresso”
- Native code components from OpenJDK that use internal HotSpot APIs can be loaded. This allows a high degree of code re-use and means that unlike most JVM implementations, this one doesn’t need to implement the JDK class library, not even tricky parts in java.base or internal modules - in fact the classes you’d find in the java.base module in a regular OpenJDK should work as long as it’s of the right versions.
OpenJDK native code components include libjava, libjimage etc. These can be loaded as genuinely native components and also (it appears) via Sulong, which on GraalVM EE would give you sandboxing of these compon
