[proto3] Default value for message is null?

[biran0079]

From this doc, default value for message field is null, while from the generated code, it seems the DEFAULT_INSTANCE of the message is returned, which is not null.
Is the doc wrong?

[biran0079]

According to protocolbuffers/protobuf#359, message field can be null.
It seems to me that protobuf for java is not implemented correctly?

[jhspaybar]

For what it's worth, this caught me off guard as well. I just use the hasX() messages now when I want to test if a message was set or not. It may be better to open this issue against the protobuf library as well as gRPC just creates a compiler plugin but isn't the core project for how Java protobuf works.

[biran0079]

@jhspaybar: I actually meant proto version 3, sorry for confusion.
hasX() method is not even generated in proto3.
It should work correctly for proto2 though.

[ejona86]

Requests and responses themselves will be non-null, as zero bytes is actually a valid encoding for a message. Nested messages can be null though, because field presence is possible in the wire format (since nested messages a length-prefixed). Field presence (hasx()) is gone in proto3 for primitives but still exists for messages, to align with and gain efficiency in many programming languages (C++, Java, Go, C#).

[biran0079]

@ejona86: thanks for the detailed explanation!
I didn't notice that hasX() is still present for nested messages.
There is still one thing I am confused: getX() seems never return null even when the field "is null".

message TestBool {
  bool inner = 1;
}
message TestWrapper {
  TestBool outer = 1;
}

generates

public TestBool getOuter() {
  return outer_ == null ? TestBool.getDefaultInstance() : outer_;
}

Is that intended?

[ejona86]

@biran0079, hmmm... I would have expected it to be null. But it does appear to be on purpose. That'd be a question for protobuf.

[biran0079]

I filed protocolbuffers/protobuf#1718, and it seems the documentation is wrong.

So in proto3, hasX() can tell if a message field is set or not, and getX() will return the default instance of the proto if unset.