code: public java.lang.String getJavaClass() {
      java.lang.Object ref = javaClass_;
      if (ref instanceof java.lang.String) {
        return (java.lang.String) ref;
      } else {
        com.google.protobuf.ByteString bs = 
            (com.google.protobuf.ByteString) ref;
        java.lang.String s = bs.toStringUtf8();
        if (bs.isValidUtf8()) {
          javaClass_ = s;
        }
        return s;
      }
    }
docs: <pre>
  For "custom" requirements, the Java class used to enforce the requirement.
  Ignored otherwise.
  </pre>
  
  <code>optional string java_class = 8;</code>