Protobuf: raw binary data to json

Question

All I have is raw protobuf binary data, I don't have access to .proto file, and I need to convert it to json string in Java. So is there is a way to do that in java? i.e. something similar to protoc tool

to be honest, my information about protobuf is limited. I tried several things I found on different tutorials on internet, but I couldn't find any easy way to directly convert say byte array to json. like what protoc --decode_raw is doing — Anthony J.
I don't think you can decode the protobuf without having the .proto schema. You need to know which fields it contains. — marstran
@AnthonyJ. In such case, take a look here: developers.google.com/protocol-buffers/docs/reference/java/com/…. This implements the majority of the logic from the Python answer you mentioned before. If that answer is correct, you will need to switch on the last 3 bits of the field tag to learn the type. However, be aware of the limitations of this approach as listed here: stackoverflow.com/a/7343928/2946480. After all, protobuf is a schemaless data format. — Michał

Marc Gravell Marc Gravell · Accepted Answer · 2018-12-11T14:53:05

This won't be possible without the .proto schema. For multiple reasons:

the raw binary doesn't include field names, just numbers; you can of course create JSON with integer-looking properties, but:
the data format is ambiguous without a schema:
- "varint" (simple integers) can mean multiple different things in different contexts, including signed, unsigned, zig-zag signed, or Boolean
- ditto for fixed length, which could be integers (signed or unsigned) or floating-point
- "length prefix" could be a utf-8 string, a packed array, or a sub-message

So: there is simply no good and reliable way of understanding data without the schema, let alone choosing how to display it as JSON.

Protobuf: raw binary data to json

2 Answers