Get custom file option Value from descriptor file (Java)

[Moustafa Nassar]

The file I have is:

test.proto

syntax = "proto3"
import "google/protobuf/descriptor.proto"
extend google.protobuf.FileOptions{
    string myOption = 50000;
}
option (myOption) = "foo";

I create a descriptor file by using: protoc --include_imports --descriptor_set_out="test.desc" test.proto

I then get the FileDescriptor from test.desc and do System.out.println(FileDescriptor.getOptions()). It prints out 50000: "foo" but I don't know how to get that in a string or something that I can work with.

[Nadav Samet]

Hi Moustafa,

To parse the FileDescriptor:

1. create an ExtensionRegistry using ExtensionRegistry.newInstance, 

2. Use YourProtoFile.registerAllExtensions(registry)  (look for that generated method)

3. Parse using parseFrom(data, registry)

4. Access using result.getExtension(TestProto.myOption)  // look for the exact location in the generated code.

There's more documentation on this here: https://developers.google.com/protocol-buffers/docs/reference/java-generated

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-Nadav

[Moustafa Nassar]

So the issue I am having is that I am parsing the descriptor files at runtime, I don't have access to the proto class therefore I can't call YourProtoFile.registerAllExtensions. Here is how I am reading the descriptor file and getting the FileDescriptor:

// Open test.desc and get the FileDescriptorSet
FileInputStream fin = new FileInputStream("/path/to/test.desc");
FileDescriptorSet set = FileDescriptorSet.parseFrom(fin);

// This loop handles my imports
List<FileDescriptor> fileDescriptors = new ArrayList<>();
for(int i=0; i<set.getFileCount;i++){
    fileDescriptors.add(FileDescriptor.buildFrom(set.getFile(i), new FileDescriptor[] {})));
}

// Get the FileDescriptor
FileDescriptor descriptor = FileDescriptor.buildFrom(set.getFile(set.getFileCount()-1), fileDescriptors.toArray(new FileDescriptor[0]));

[Nadav Samet]

In that case, I believe you should be able to find the data in the UnknownFieldSet:

fileDescriptor.getOptions().getUnknownFields().getField(50000).getLengthDelimitedList()

That should give you a list with your string represented as a ByteString (UTF-8 encoded).

To view this discussion on the web visit https://groups.google.com/d/msgid/protobuf/366a6f6d-098c-4aba-8fc1-28db932fbd8cn%40googlegroups.com.

--

-Nadav

[Moustafa Nassar]

Thank you this should work for my purposes, I think the first approach you presented to me is preferred if possible. There seems to be a solution here:  https://groups.google.com/g/protobuf/c/zKYLsr9xE90  I am not skilled enough in protobuf to understand what the solution was though...