Document is hard to read

1. Missing is a description of each component and its common methods (like TypePool, DynamicType, ClassLoadingStrategy etc.)
2. Class calls may vary from version to version, such as the current version is:

package foo;
class Bar { }
class MyApplication {
  public static void main(String[] args) {
    TypePool typePool = TypePool.Default.ofClassPath();
    new ByteBuddy()
      .redefine(typePool.describe("foo.Bar").resolve(), // do not use 'Bar.class'
                ClassFileLocator.ForClassLoader.ofClassPath())
      .defineField("qux", String.class) // we learn more about defining fields later
      .make()
      .load(ClassLoader.getSystemClassLoader());
    assertThat(Bar.class.getDeclaredField("qux"), notNullValue());
  }
}

It was not compiled in 1.9.13, but the 1.9.13 version of the document could not be found. Try to change it to:

package foo;
class Bar { }
class MyApplication {
  public static void main(String[] args) {
    TypePool typePool = TypePool.Default.of(Thread.currentThread().getContextClassLoader());
    new ByteBuddy()
      .redefine(typePool.describe("foo.Bar").resolve(), // do not use 'Bar.class'
                ClassFileLocator.ForClassLoader.of(Thread.currentThread().getContextClassLoader()))
      .defineField("qux", String.class) // we learn more about defining fields later
      .make()
      .load(ClassLoader.getSystemClassLoader());
     assertThat(Bar.class.getDeclaredField("qux"), notNullValue());
  }
}

But in the end:

Exception in thread "main" java.lang.IllegalStateException: Class already loaded: foo.Bar

Much more complex than using javassist