I can't use protobuf compile a specifity java enum, and i don't known how to solve this problem

[Dongzhao Chen]

     now，I got a problem about protobuf, when  I use protobuf (version3) compile a java enum accoring to offical website，It is work well,  

But when I try to use protobuf compile a specifity java enum, It is always throw a Exception，and I find more document，but i don't got a

idea,plz help me.

    syntax = "proto2";

    option java_package = "io.demo.protobuf";

    option java_outer_classname = "Qos";

    enum qosBuf{

  

       AT_MOST_ONCE = 1;

       AT_LEAST_ONCE = 2;

       EXACTLY_ONCE = 3;

      required int32 val = AT_MOST_ONCE.value;   // here always can not compile

  

   }