I would like to be able to implement a method at runtime that is called before an object runs the initializers. This will allow me to set fields that are used during initia
What you are trying to do would entail creating a new anonymous inner class that is a subclass of some class that you only know at runtime. Reflection cannot solve this problem.
Dynamic proxies might ... if you were trying to implement a dynamically loaded interface.
The only other approaches I can think of are to use BCEL or dynamic source code generation / compilation. And both are going to be very complicated. And neither count as pure Java.
My advice would be to take a fresh look at what it is you are trying to achieve. (Your question gives no hints as to what that might be, so it is hard to offer any practical suggestions.)