Class<? extends Foo> dynamicType = new ByteBuddy() .subclass(Foo.class) .method(named("sayHello")) .intercept(FixedValue.value("Hello Byte Buddy Agent!")) .make() .load(Foo.class.getClassLoader()) .getLoaded(); Foo foo = dynamicType.getDeclaredConstructor().newInstance(); System.out.println(foo.sayHello()); public static void premain(String agentArgs, Instrumentation instrumentation) { new AgentBuilder.Default() .type(ElementMatchers.any()) .transform((builder, type, classLoader, module) -> builder.method(named("sayHello")) .intercept(FixedValue.value("Hello Byte Buddy Agent!"))) .installOn(instrumentation); }