Add metadata to fields in proto3 for Java

Question

Proto3 has been simplified such that the required and optional fields are no longer supported (See Why required and optional is removed in Protocol Buffers 3). Is there still a way to label a certain field as required? I've looked into FieldOptions, and tried something like this:

message MyMeta {
  bool isRequired = 1;
}

extend google.protobuf.FieldOptions {
   MyMeta meta = 1234;
}

message Person {
  string name = 1 [ (meta) = { isRequired: true }];
  string address = 2  [ (meta) = { isRequired: true }];
  string remarks = 3;
}

After compiling it into Java code, and as I was checking the compiled Java code I don't see any link between the fields and its metadata I specified in proto. Did I miss something here?

score 3 · Accepted Answer · answered Oct 27 '17 at 07:21

After a bit of tinkering and using @Eric Anderson's idea on using proto reflection, here is a way to retrieve MyMeta from the Person.name field:

    Descriptor rootDesc = PersonProto.getDescriptor();
    FieldDescriptor name = rootDesc.findFieldByName("name");
    FieldDescriptor ext = rootDesc.getFile().getExtensions().get(0);
    MyMeta meta = (MyMeta) name.getOptions().getField(ext);
    boolean isReq = meta.getIsRequired();

score 2 · Answer 2 · answered Oct 25 '17 at 22:21

2

No, that functionality was removed; use documentation instead. If you are trying to use FieldOptions for your own extensions, then you can make your own protoc plugin to generate supplemental code (like a verify utility) or use proto reflection at runtime (via FooMessage.getDescriptor() and Descriptors.FieldDescriptor.getOptions() in Java).

answered Oct 25 '17 at 22:21

Eric Anderson

24,057
5
55
76

Thanks! Though I felt we need to provide a more specific answer on how to retrieve the field metadata in Java. – Julius Delfino Oct 27 '17 at 07:16

Add metadata to fields in proto3 for Java

2 Answers2