protocol buffers - Generic Parsing of PB in java

Question

Is it possible to parse protobuf in a generic fashion in Java?

I have looked into GeneratedMessage and could not find a way to parse any PB byte buffer into a GeneratedMessage.

Essentially, I am trying to parse a PB byte buffer into GeneratedMessage and then I would use reflection to detect fields inside it.

Answer 1

First of all, you can't parse PB data without knowing the schema. The schema originally comes from a ".proto" file and is typically embedded in the code generated by protoc. However, you can also tell protoc to store the schema in a format that's usable by the Java Protobuf library:

protoc --descriptor_set_out=mymessages.desc mymessages.proto

Then load it in your Java code:

FileInputStream fin = new FileInputStream("mymessages.desc");
Descriptors.FileDescriptorSet set =
  Descriptors.FileDescriptorSet.parseFrom(fin);
Descriptors.Descriptor md = set.getFile(0).getMessageType(0);

Once you have the schema for a message (Descriptor.Descriptor) parsing a message is easy:

byte[] data = ...;
DynamicMessage m = DynamicMessage.parseFrom(md, data);

DynamicMessage has a reflective API that lets you look through the fields.

The messy part is calling out to the protoc tool to convert the ".proto" file into a usable format. The C++ Protobuf library has a way to load ".proto" files directly, but unfortunately the Java Protobuf library does not.