Injecting byte code using class loader

 Annotation Processor Tool (APT)
   source -(compile and performs modifications at model level)-> bytecode already modified - regular class loader -> loads class into memory
   Post-Compiling processing
   source -compile -> bytecode -post-compile-> modified bytecode - regular class loader -> loads class into memory
   Runtime modifications
   source -compile -> bytecode -post-compile-> modified bytecode - regular class loader -> loaded into memory - javassist proxy -> modified class - javassist hot swapper -> re-modified class

Source :stackoverflow.com