Can I redefine a private method from a parent class using Byte Buddy?

Question

Based on the idea of Can I redefine private methods using Byte Buddy?, I would like to redefine a private method from a parent class. Is that possible? Or it's like the chicken and egg problem ?

Thanks!

Answer 1

Private methods are not dispatched virtually, therefore, you cannot change the dispatch of the private method within the subclass. The only way to do this, would be to hardcode a conditional dispatch into the private method directly.

You can do so, by using Byte Buddy's Advice class in combination with a Java agent. The Java agent would look like the following:

new AgentBuilder.Default()
  .disableClassFormatChanges()
  .with(RedefinitionStrategy.REDEFINITION)
  .type(is(ClassWithPrivateMethod.class))
  .transform((builder, type, cl, module) -> {
     return builder.visitor(Advice.to(MyAdvice.class)
                                  .on(named("privateMethod")));
   }).install(inst);

where the code of MyAdvice is inlined in the befinning of the method named privateMethod. The conditional dispatch would look like the following:

class Adv {
  @OnMethodEnter(skipOn = OnNonDefaultValue.class)
  static boolean intercept(@This ClassWithPrivateMethod self) {
    if (self instanceof ParentClass) {
      // Your code
      return true;
    } else {
      return false;
    }
  }
}

By returning true and by using the skip coniditon, you only execute the actual code if false is returned.