service MyDataService {
  rpc UpateMyData (UpdateMyDataRequest)
     returns (UpdateMyDataResponse);
}
message MyData {
  int32 id = 1;
  string stringValue = 2;
  SubData subData = 3;
}
message SubData {
int64 bigValue = 1;
}
message UpdateMyDataRequest {
  MyData update = 1;
}

UpdateMyDataRequest request = UpdateMyDataRequest.newBuilder()
  .setUpdate(MyData.newBuilder()
    .setId(id)
    .setStringValue(null)
  )
serviceFutureStub.update(request)

UpdateMyDataRequest.newBuilder().build().getStringValue() == ""

{
  "id": 123
  "stringValue": null
}

syntax = "proto3";
package io.github.efenglu.protobuf.examples.oneof;
option java_multiple_files = true;
import "google/protobuf/struct.proto";
service MyDataService {
  rpc UpateMyData (UpdateMyDataRequest)
     returns (UpdateMyDataResponse);
}
message MyData {
  int32 intValue = 1;
  NullableString stringValue = 2;
  NullableSubData subData = 3;
}
message SubData {
  int64 bigValue = 1;
}
message NullableSubData {
  oneof kind {
    google.protobuf.NullValue null = 1;
    SubData data = 2;
  }
}
message NullableString {
  oneof kind {
    google.protobuf.NullValue null = 1;
    string data = 2;
  }
}
message UpdateMyDataRequest {
MyData data = 1;
}
message UpdateMyDataResponse {
}

UpdateMyDataRequest request = UpdateMyDataRequest.newBuilder()
  .setData(MyData.newBuilder()
    .setStringValue(NullableString.newBuilder()
      .setNull(NullValue.NULL_VALUE)
      .build()
    )
    .setSubData(NullableSubData.newBuilder()
      .setNull(NullValue.NULL_VALUE)
      .build()
    ).build()
).build();
service.upateMyData(request);

UpdateMyDataRequest request = UpdateMyDataRequest.newBuilder()
  .setData(MyData.newBuilder()
    .setStringValue(NullableString.newBuilder()
      .setData("hello")
      .build()
    )
    .setSubData(NullableSubData.newBuilder()
      .setData(SubData.newBuilder()
        .setBigValue(1234567)
      .build()
    ).build()
  ).build()
).build();
service.upateMyData(request);

if (request.hasData()) {
  if (request.getData().hasStringValue()) {
    final String nullableString;
    if (request.getData().getStringValue().hasNull()) {
      nullableString = null;
    } else {
      nullableString = request.getData()
        .getStringValue()
        .getData();
      }
  }
  if (request.getData().hasSubData()) {
    final SubData nullableSubData;
    if (request.getData().getSubData().hasNull()) {
      nullableSubData = null;
    } else {
      nullableSubData = request.getData()
       .getSubData()
       .getData();
    }
  }
}

{
"id": 123
-- ommited "stringValue" --
}

service MyDataService {
  rpc Update (UpdateMyDataRequest) returns (UpdateMyDataResponse);
  rpc List (ListMyDataRequest) returns (ListMyDataResponse);
}
message MyData {
  int32 id = 1;
  string stringValue = 2;
  SubData subData = 3;
}
message SubData {
  int64 bigValue = 1;
}
message UpdateMyDataRequest {
  MyData update = 1;
  google.protobuf.FieldMask field_mask = 2;
}
message UpdateMyDataResponse {
  MyData new_data = 1;
}
message ListMyDataRequest {
  int32 id = 1;
  google.protobuf.FieldMask field_mask = 2;
}
message ListMyDataResponse {
  repeated MyData data = 1;
}

MyData sendUpdate(int id, String value) {
  UpdateMyDataRequest request = UpdateMyDataRequest.newBuilder()
    .setUpdate(MyData.newBuilder()
      .setId(id)
      .setStringValue(value)
    )
    .setFieldMask(FieldMaskUtil.fromFieldNumbers(
      MyData.class,
      MyData.STRINGVALUE_FIELD_NUMBER)
    )
    .build();
  return serviceFutureStub.update(request).getNewData();
}
List<MyData> listOnlySubData(int id) {
  ListMyDataRequest request = ListMyDataRequest.newBuilder()
    .setId(id)
    .setFieldMask(FieldMaskUtil.fromFieldNumbers(
      MyData.class,
      MyData.SUBDATA_FIELD_NUMBER)
    )
    .build();
  return serviceFutureStub.list(request).getDataList();
}

@Override
public void update(
  UpdateMyDataRequest request,
  StreamObserver<UpdateMyDataResponse> responseObserver
) {
  MyData updateData = request.getUpdate();
  FieldMask fieldMask = request.getFieldMask();
  // Fetch exiting Values
  MyData existing = repo.readData(updateData.getId());
  MyData.Builder builder = existing.toBuilder();
  // Update only the fields listed in the fieldmask
  FieldMaskUtil.merge(fieldMask, updateData, builder);
  // Store the result
  repo.writeData(builder.build());
  // Send the new state back
  responseObserver.onNext(UpdateMyDataResponse.newBuilder()
    .setNewData(builder)
    .build()
  );
}

@Override
public void list(
  ListMyDataRequest request,
  StreamObserver<ListMyDataResponse> responseObserver
) {
  int id = request.getId();
  FieldMask fieldMask = request.getFieldMask();
  // Fetch the list
  List<MyData> result = repo.listData(id);
  ListMyDataResponse.Builder response =
    ListMyDataRespons.newBuilder();
  MyData.Builder builder = MyData.newBuilder();
  for (MyData data : result) {
    builder.clear();
    // Use the field mask to send back ONLY the data requested
    FieldMaskUtil.merge(fieldMask, data, builder);
    response.addData(builder);
  }
  // Send the filtered list back
  responseObserver.onNext(response.build());
}

...
import "google/protobuf/wrappers.proto";
service MyDataService {
  rpc Update (UpdateMyDataRequest) returns (UpdateMyDataResponse);
}
...
message UpdateMyDataRequest {
  int32 id = 1;
  google.protobuf.StringValue stringValue = 2;
  UpdateSubData subData = 3;
}
message UpdateSubData {
  google.protobuf.Int64Value bigValue = 1;
}
...

void update() {
  service.update(UpdateMyDataRequest.newBuilder()
    .setStringValue(StringValue.of("customValue"))
    .build()
  );
}

@Override
public void update(
  UpdateMyDataRequest request,
  StreamObserver<UpdateMyDataResponse> responseObserver) {
  // Fetch exiting Values
  MyData existing = repo.readData(request.getId());
  MyData.Builder builder = existing.toBuilder();
  // Update Fields as necessary
  if (request.hasStringValue()) {
    builder.setStringValue(request.getStringValue().getValue());
  }
  if (request.hasSubData()) {
    if (request.getSubData().hasBigValue()) {
      builder.setSubData(
        builder.getSubData().toBuilder()
          .setBigValue(request.getSubData()
            .getBigValue()
            .getValue()
          )
        );
      }
  }
  repo.writeData(builder.build());
  responseObserver.onNext(UpdateMyDataResponse.newBuilder()
    .setNewData(builder)
    .build()
  );
}

String value;
if (value != null) {
  // insert value into database
}

String value;
if (StringUtils.isNotBlank(value)) {
  // insert value into database
}