Injecting helper classes into parent classloader

[rupinder10]

I have a URLClassloader that runs the code in the class I am trying to instrument. The instrumented code uses reflection to call methods in another classloader that is one of the descendent in the class loader hierarchy, lets call it X. It works fine for me. But I would like to cache these into another class called CachedMethods and then call the instrumentation and have the instrumented code do CacheMethods.get("myMethod").invoke(). But I get a ClassNotFoundException at runtime. How do I load the cache in the classloader X and then make it available in the upstream URL Classloader ?

[raphw]

Have a look at ClassInjector which offers utilities. Since Java 9, UsingLookup would be the prefered way to do this. Simply inject the classes before starting. If you can create the intermediate class loader, ByteArrayClassLoader can be used for injecting through API, too.

[rupinder10]

Thanks. What is the downside of using ClassInjector.UsingReflection in Java 9+. Looks like UsingLookup requires a bridge class that supplies the lookup in the target classloader. I dont have the ability to add that to the target classloader.

ClassInjector.UsingReflection works but I am not sure what the downside is.

[raphw]

Reflection will no longer work once unsafe is sealed off. If you want to support Java 8 and below, you will need to use a dynamic strategy. You can check the class file version using ClassFileVersion.ofCurrentVm().

[rupinder10]

I don’t have to support Java 8 or below. So if I don’t have the ability to add any class in the target classloader how do I use UsingLookup. The examples I have seen require the MethodHandles.Lookup to be provided by the target classloader. Or is there some other way to achieve that ?