Comment by TeaVMFan

While you wait for CheerpJ, please check out TeaVM:

https://teavm.org/

It has a permissive Apache license and numerous success stories:

https://frequal.com/TeaVM/TeaVmBasedSites.html

There is also a full SPA toolkit called Flavour Plus for making pure Java, web-native, full-stack web apps:

https://frequal.com/FlavourPlus/