enum MyEnum {
    @MyAnnotation("123")
    WITH_ANNOTATION,

    WITHOUT_ANNOTATION;
}

In order to retrieve the annotation of an enum constant, one should remember that an enum constant is a field of the concrete enum class. The field name is the name of the enum constant.

MyEnum enumConstant = ...
Class<MyAnnotation> annotation = ...

MyAnnotation result = enumConstant.getClass().getField(enumConstant.name()).getAnnotation(annotation);