Different classloader for Java8 and >Java9

[NingZhang-e]

@chibash, just saw some changes happened 3 years ago: https://github.com/jboss-javassist/javassist/commit/778c463e5aa1795591e56916c6c1c3205317fc3e https://github.com/jboss-javassist/javassist/commit/e41e0790c0cb073e9e2e30071afecfcdc4621d42

There are several changes regarding controlling classloader. Not clear why there is different behavior. Currently, we can not change classloader easily to load class on Java8. e.g. load class dynamically from one URL. But this could be achieved on >Java9. Currently, we have to switch to Java11 to load class dynamically. We still have some code still running on Java8. Could you give some tips about this? If no better way, we have to step to Java11.