An example agent that intercepts bootstrap class method String::concat. Tested with Byte Buddy 1.14.11 on JDKs 8 to 21. Based on a question about https://gist.github.com/kriegaex/0f4dd4c9d05f3a68e3a8e1ed75359c3b.

BootstrapAdviceAgent.java

import net.bytebuddy.agent.ByteBuddyAgent;
import net.bytebuddy.agent.builder.AgentBuilder;
import net.bytebuddy.agent.builder.ResettableClassFileTransformer;
import net.bytebuddy.asm.Advice;
import net.bytebuddy.asm.Advice.*;
import net.bytebuddy.dynamic.loading.ClassInjector;

import java.io.IOException;
import java.lang.instrument.Instrumentation;
import java.lang.instrument.UnmodifiableClassException;

import static net.bytebuddy.implementation.bytecode.assign.Assigner.Typing.DYNAMIC;
import static net.bytebuddy.matcher.ElementMatchers.*;

public class BootstrapAdviceAgent {
  public static void main(String[] args) throws IOException, UnmodifiableClassException {
    premain(null, ByteBuddyAgent.install());
    System.out.println("Hi ".concat("there!"));
    System.out.println("Hello ".concat("world!"));
    System.out.println("Say ".concat("something."));
    System.out.println("heLLo ".concat("again!"));
  }

  public static void premain(String arg, Instrumentation instrumentation) throws IOException, UnmodifiableClassException {
    ClassInjector.UsingUnsafe.Factory factory = ClassInjector.UsingUnsafe.Factory.resolve(instrumentation);
    AgentBuilder agentBuilder = new AgentBuilder.Default();
    agentBuilder = agentBuilder.with(new AgentBuilder.InjectionStrategy.UsingUnsafe.OfFactory(factory));

    ResettableClassFileTransformer transformer = agentBuilder
      .disableClassFormatChanges()
      .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
      .ignore(none())
      .ignore(nameStartsWith("net.bytebuddy."))
      .type(is(String.class))
      .transform((builder, typeDescription, classLoader, module, protectionDomain) -> builder
        .visit(Advice.to(MyAdvice.class).on(named("concat")))
      )
      .installOn(instrumentation);
  }

  public static class MyAdvice {
    @OnMethodEnter(skipOn = OnDefaultValue.class)
    public static boolean before(
      @This(typing = DYNAMIC, optional = true) String target,
      @Argument(value = 0, readOnly = false) String arg
    ) {
      if (target.equalsIgnoreCase("Hello "))
        arg = arg.toUpperCase();
      return arg.endsWith("!");
    }
  }
}

Thank you very much. It works.