How to create a bytebuddy agent that intercepts a field setter from annotated field?

Question

What I am trying to do is basically match a class's field with an annotaion and them intercept the field's getter and setter.

 public class Foo {

    @Sensitive
    private String Blah;

Here is the code for my agent:

    private static AgentBuilder createAgent() {
        return new AgentBuilder
                .Default()
                .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
                .type(ElementMatchers.is(FieldTypeMatcher.class).and(ElementMatchers.isAnnotatedWith(Foo.class)))
                .transform(((builder, typeDescription, classLoader, module) ->
                        builder
                        .method(method -> method.getActualName().contains(typeDescription.getActualName()))
                        .intercept(Advice.to(Interceptor.class))
                ));
    }

I though I could match the field's name with the method's signature but I had no luck.

Answer 1

I assume that Foo has a getter and setter for Blah?

In this case, I recommend a custom ElementMatcher implementation such as:

class FieldMatcher implements ElementMatcher<MethodDescription> {
  @Override
  public boolean matches(MethodDescription target) {
    String fieldName;
    if (target.getName().startsWith("set") || target.getName().startsWith("get")) {
      fieldName = target.substring(3, 4).toLowerCase() + target.substring(4);
    } else if (target.getName().startsWith("is")) {
      fieldName = target.substring(2, 3).toLowerCase() + target.substring(3);
    } else {
      return false;
    }
    target.getDeclaringType()
      .getDeclaredFields()
      .filter(named)
      .getOnly()
      .getDeclaredAnnotations()
      .isAnnotationPresent(Sensitive.class);
  }
}

This matcher checks if a method is a getter or setter, locates the corresponding field and checks for the annotation being present on it.