I tried multiple approaches here (check the commits if interested). Firstly I tried creating a MethodHandles.Lookup in the advice and passing it along. This allowed getting access to MethodHandles for findLoadedClass and defineClass. Unfortunately it didn't work on jdk8, apparently ClassLoader is a restricted class there for which you can't create a Lookup for. Didn't like that a special case was needed for jdk8. Next I tried writing the advice with asm. Since we need to call multiple methods it was a sizable chunk of asm so instead I tried what you see here. We use a modified stub ClassLoader class that has findLoadedClass and defineClass as public so we could use them in the advice. During the build we shade the stub to java.lang.ClassLoader. The bytecode works because this advice is inlined into subclasses of ClassLoader that can call these protected methods. It isn't ideal either as it required some build script hacking to make the shading work.

The following ClassInjector.UsingInstrumentation works without unsafe but it needs to put the classes in to a jar and append that jar to boot class path. We could use that as a fallback, but the thing is that besides the virtual field classes that we defined above using lookup we have only a couple of helper classes that we need to add to boot loader. Could consider moving those classes to a bootstrap module. With indy we would be loading those helpers into separate class loader and that would also get rid of the unsafe usage here.