OS within an OS

Would it be possible to make my own mini OS inside a java application? Over the pass few days I've been working on UI stuff and then I realized I basically implemented my own blackbox environment which made me start to think that with a few modifications i could easily make it into a mini OS

you could, instead, build a simple computer emulator with the register banks, mmu, alu and so on, to get a feel of what actually is going on, developing your own architecture, assembly and so on is actually incredible practice

or, you could implement some kind of session and window managers and implement a layer that encapsulates system calls maybe you can do some kind of OS on that java machine but really... kernel building is much better practise