substratevm

The SubstrateVM is the AOT compiler for Graal.

Espresso), which is a JVM written in Java designed to be compiled to run on top of SubstrateVM.

It's a JIT for HotSpot or an AOT compiler for SubstrateVM which is a separate JVM altogether.

Now Oracle has SubstrateVM, and there are plans for AOT compilation at least in Java 10 timeframe.

If you want to go research, there is Graal/SubstrateVM from Oracle Labs and JikesRVM, a meta-circular JVM.

There are already JVMs written in Java like JikesRVM, Graal/SubstrateVM and a few others.

Also that Oracle Labs have SubstrateVM, which is an AOT compiler based on Graal.

A nice code base to study if you're new to GCs is the GenScavange module of SubstrateVM, a JVM written entirely in Java.

The Oracle Hotspot replacement project, Graal allows for AOT compilation via SubstrateVM.

SubstrateVM is done by the same Oracle Labs working on Graal.

There are plans to eventually replace Hotspot with Graal and SubstrateVM, in some future version of the OpenJDK, thus reducing even more the C++ surface.

But SubstrateVM, Graal, Truffle, isn't ready for another few years.

That's why AOT compilation for Java is a thing now, with SubstrateVM (aka "GraalVM native-image"), precisely to eliminate startup overhead.

I can't remember what exactly happens with just "mx build" but chances are this only gives you a bare GraalVM without anything else, which means also no SubstrateVM ("native-image").

For a more realistic example of such a JVM, look at SubstrateVM (written in "SystemJava" and compiled to native code ahead of time along with the app it runs), and "Java on Truffle" (a.k.a.