[SOLVED] How to hook into protobuf java code generation?

How to hook into protobuf java code generation?

Is there any way to hook into the generation of java classes when invoking protoc? I know there are plugins which can be created for protoc but as far as I understood it is only possible to generate additional files there. What I would like to do is adding additional code into the java classes based on some options in the protobuf schemas.

So for the following schema:

syntax = "proto3";

import "google/protobuf/descriptor.proto";

enum DateFormat {
  LOCAL_DATE = 0;
  LOCAL_DATE_TIME = 1;
}

extend google.protobuf.FieldOptions  {
  DateFormat format = 95765;
}

message MyMessage {
  string name = 1;
  sint64 id = 2;
  string dateTimeField = 3 [(format) = LOCAL_DATE_TIME];
}

I would like to have the dateTimeField in the generated java class of type LocalDateTime which contains the already parsed date time.

Solution

I finally found a solution for this. Plugins can not only create new files, but also use insertion points to extend classes generated by the java code generator, see https://protobuf.dev/reference/java/java-generated/#plugins.

There is a great article which describes exactly how to create a plugin which uses those insertion points.