There were several very small implementation of JVM over the years. One semi-popular one was leJOS for Legos Mindstorms. At one point, I recall even building it for DOS to test some things.
I was just looking at the Java bytecode and thinking "in theory, one could build a small JVM for this" [1]. But it still looks like an enormous effort to build, test and then use in a meaningful way.
[1] https://en.wikipedia.org/wiki/Java_bytecode_instruction_list...