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("!");
    }
  }
}

Try it on JDoodle.

The console log:

Hi there!
Hello WORLD!
null
heLLo AGAIN!

As you can see, the advice converts the method argument for concat to upper case, but only if the String it is called upon matches "hello " (case-insensitive). Furthermore, it proceeds to the original method call, if the method argument ends with "!". Otherwise, it will cancel method execution, yielding a return value of null.

In connection with @OnMethodExit, more things would be possible, e.g. altering the method call result, passing information from the "enter" to the "exit" advice, handling all arguments as an array etc.

Thank you very much. It works.