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();
        }
    }
}

You probably have the class included in your agent jar and also attach it to the boot loader. This way it will be loaded in the system loader before it is attached to boot and therefore be loaded twice. With different class loaders, the two classes are different. Try not including it as a ".class" file in the boot loader.

how can I try not to including it as a ".class" file in the boot loader ?

Have a look at ClassInjector and Instrumentation which both offer APIs for that.

Have a look at ClassInjector and Instrumentation which both offer APIs for that.

OK, thanks raphw.