An example agent that intercepts a method of the bootstrap class loader.

BootstrapAgent.java

package net.bytebuddy;

import net.bytebuddy.agent.ByteBuddyAgent;
import net.bytebuddy.agent.builder.AgentBuilder;
import net.bytebuddy.description.type.TypeDescription;
import net.bytebuddy.dynamic.ClassFileLocator;
import net.bytebuddy.dynamic.DynamicType;
import net.bytebuddy.dynamic.loading.ClassInjector;
import net.bytebuddy.implementation.MethodDelegation;
import net.bytebuddy.implementation.bind.annotation.SuperCall;
import net.bytebuddy.matcher.ElementMatchers;

import java.io.File;
import java.lang.instrument.Instrumentation;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.file.Files;
import java.util.Collections;
import java.util.concurrent.Callable;

public class BootstrapAgent {

    public static void main(String[] args) throws Exception {
        premain(null, ByteBuddyAgent.install());
        HttpURLConnection urlConnection = (HttpURLConnection) new URL("http://www.google.com").openConnection();
        System.out.println(urlConnection.getRequestMethod());
    }

    public static void premain(String arg, Instrumentation inst) throws Exception {
        File temp = Files.createTempDirectory("tmp").toFile();
        ClassInjector.UsingInstrumentation.of(temp, ClassInjector.UsingInstrumentation.Target.BOOTSTRAP, inst).inject(Collections.singletonMap(
                new TypeDescription.ForLoadedType(MyInterceptor.class),
                ClassFileLocator.ForClassLoader.read(MyInterceptor.class).resolve()));
        new AgentBuilder.Default()
                .ignore(ElementMatchers.nameStartsWith("net.bytebuddy."))
                .enableBootstrapInjection(temp, inst)
                .type(ElementMatchers.nameEndsWith(".HttpURLConnection"))
                .transform(new AgentBuilder.Transformer() {
                    @Override
                    public DynamicType.Builder<?> transform(DynamicType.Builder<?> builder, TypeDescription typeDescription, ClassLoader classLoader) {
                        return builder.method(ElementMatchers.named("getRequestMethod")).intercept(MethodDelegation.to(MyInterceptor.class));
                    }
                }).installOn(inst);
    }

    public static class MyInterceptor {

        public static String intercept(@SuperCall Callable<String> zuper) throws Exception {
            System.out.println("Intercepted!");
            return zuper.call();
        }
    }
}

Thanks for the advice,

My main issue now regards to SpringBoot and classes that I defined using bybuddy, I trying to mitigate the problem,
SpringData uses a class to scan the class for Resources to locate the classes that extends the Repository Interface,

Using the classloader getResource i locate the bytebuddy defined class so, on my understanding from class construction perspectiveall is fine.

I`ve already did some experiments (with sucess) using byte-buddy to construct an EntityModel for Hibernate and a Repository Model for spring data with success, but i had to define the SpringData beans manually because the AutoConfiguration does not locate the classes defined by ByteByddy. I can locate the Classes using Class.forName(bytebuddyClass) but Spring does not locate the classes and both Spring classes and ByteBuddy defined classes are in the classloader.. Anyhow if i don't find the solution will continue to work with what is already working.

Regards.

Found the problem
The Class.forName works fine.
but both methods bellow don`t

Object clazz = org.nanotek.config.spring.BootstrapAgent.class.getClassLoader()
.getResourceAsStream("org/nanotek/config/spring/repositories/SimpleTableRepository.class");
System.err.println("clazz "+clazz);

Object resource = org.nanotek.config.spring.BootstrapAgent.class.getClassLoader()
.getResource("org/nanotek/config/spring/repositories/SimpleTableRepository.class");
System.err.println("resource "+resource);

Now, I understood that a "ClassLoader" need to be used to override the methods that SpringData uses to scan the packages,
and also "register" the instances of DynamicTypes manually but, using the sequence of registration used by the framework.