Parts of it are written in Java. Over time, more of the JVM has been rewritten in Java. For example recently parts of the reflection subsystem were rewritten in

Parts of it are written in Java. Over time, more of the JVM has been rewritten in Java. For example recently parts of the reflection subsystem were rewritten in Java. Of course many libraries have been ported over time too. By the way, the .NET CLR is AFAIK written in C++, like with Hotspot. It's not fully self-hosting. It's a tricky process because HotSpot is highly performance sensitive code. People won't accept regressions just to convenience the JDK maintainers. Java meanwhile is deliberately a simple language to make it accessible for people, so you lose some low level techniques that are useful for performance. Nonetheless, GraalVM native image has proven that you can achieve HotSpot like performance with a JVM written in Java. However it requires a big change in the compilation model that isn't always appropriate.