How to deal with unknown protobuf fields in Java?

问题

I have a Java application that reads some protobuf data from another computer and can then modify some values and write it back. It is very likely that a user could read the data using an outdated .proto file, so there would be some fields it doesn't understand in this case. I would ultimately like to preserve the uknown data when writing back the changes made; however, I could settle for just detecting that there is unknown data (to prompt the user to upgrade his/her application). It is not clear to me how to deal with unknown fields in Java.

If it helps, I am using a version 2 .proto file because I need it to be compatible with nanopb on the remote computer.

This question gets me part of the way, but my question has nothing to do with JSON.

回答1:

First please pay attention when you say unknown fields. In protobuf you can have unknown fields by definition but on the other hand - and I suppose this is your case - you can have fields that you not having in your current proto file.

In both situation you can easily access the values. Let say you have a proto message named foo.

You have to access the descriptor and get the fields from there by name, and lastly get the values exemplified like below:

Builder builder = foo.toBuilder();
FieldDescriptor field = builder.getDescriptorForType().findFieldByName("whatever field");
Object obj = builder.getField(field);

// if your field is int32 cast to int
int value = (int) obj

If you wish to write the 'unknown' value you could proceed the other way around:

Builder builder = foo.toBuilder();
FieldDescriptor field = builder.getDescriptorForType().findFieldByName("whatever field");
builder.setField(field, 100); // 100 is an example int value
Foo foo = builder.build();

In case if you really want to insert proto defined unknown fields you have to do something like:

 UnknownFieldSet.Field seqField = UnknownFieldSet.Field
            .newBuilder()
            .addFixed32(100) // 100 is an example int value
            .build();

 UnknownFieldSet unkFieldSet = UnknownFieldSet
            .newBuilder()
            .addField(99, seqField) // 99 is a proto index number chosen by me
            .build();

 Foo message = foo.toBuilder().setUnknownFields(unkFieldSet).build();

Reading the defined unknown fields are again is done by the:

 foo.toBuilder().getUnknownFields()....

I hope this helps.

来源：https://stackoverflow.com/questions/56090867/how-to-deal-with-unknown-protobuf-fields-in-java

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java

protocol-buffers

protobuf-java