code: public com.google.protobuf.ByteString
        getJavaClassBytes() {
      java.lang.Object ref = javaClass_;
      if (ref instanceof java.lang.String) {
        com.google.protobuf.ByteString b = 
            com.google.protobuf.ByteString.copyFromUtf8(
                (java.lang.String) ref);
        javaClass_ = b;
        return b;
      } else {
        return (com.google.protobuf.ByteString) ref;
      }
    }
docs: <pre>
  For "custom" requirements, the Java class used to enforce the requirement.
  Ignored otherwise.
  </pre>
  
  <code>optional string java_class = 8;</code>