Unable to use custom JsonFormat.Printer for ProtobufResponseConverterFunction

Hello, we’re trying to use in a custom com.google.protobuf.util.JsonFormat.Printer for the ProtobufResponseConverterFunction , which we are using to convert the HttpResult from our endpoint into JSON

We’re doing this by passing in a custom instance of the converter function like so:

serverBuilder.annotatedService(myService, ProtobufResponseConverterFunction(JsonFormat.printer().includingDefaultValueFields()))) 

However, the custom Printer isn’t being used. It looks like a new converter is being instantiated along the way, which results in the Printer passed in to be ignored

[ikhoon]

Let’s say we have the following annotated service:

class MyService {
  @ProduceJson
  fun flowProtobufJson(...): Flow<MyProtobuf> {
      ...
  }

}

Server.buider()
      .annotatedService()
      .responseConverters(MyProtobufResponseConverter())
      .build(MyService())

We need two 2 response converters to completely convert Flow<MyProtobuf> into a JSON response. First, MyProtobufResponseConverter will be used to convert each MyProtobuf into a JSON object. Next, FlowResponseConverterFunction should be chosen to collect the JSON objects and convert them into a JSON array.

Currently, we provide two APIs for a custom response converter.

• ResponseConverterFunction defines how to convert an object.
• ResponseConverterFunctionProvider defines how to create a ResponseConverterFunction via SPI.

The downside of the APIs is that they don’t tell a ResponseConverterFunction is a primitive converter or a delegating ResponseConverterFunction. It is difficult to prioritize between them. So we’d like to change our API to show their explicit behavior. Note: As the API is @UnstableApi, we can still make a breaking change. 😅

Proposal:

• Split the original ResponseConverterFunctionProvider to two providers. DelegatingResponseConverterFunctionProvider requires a delegate to complete to convert a response like FlowResponseConverterFunction and the new ResponseConverterFunctionProvider does not take any parameters.
• Add ResponseConverterFunctionProvider that create a primitive ResponseConverterFunction which converters an object to an HttpResponse without depending on other ResponseConverterFunctions.
• A response converter created from DelegatingResponseConverterFunctionProvider has higher priority than primitive converters. Because it needs primitive converters to create a delegating response converter.

public interface ResponseConverterFunctionProvider {
    @Nullable
    ResponseConverterFunction newResponseConverterFunction();
}

public interface DelegatingResponseConverterFunctionProvider {

    @Nullable
    ResponseConverterFunction createResponseConverterFunction(
         Type responseType,
         ResponseConverterFunction responseConverter);
}

[jrhee17]

Just for clarification:

Change the ResponseConverterFunctionProvider to use the configured responseConverterFunctions or to be applied after the responseConverterFunctions

By this, I think we mean that we will define the behavior of armeria’s default ResponseConverterFunctionProvider implementation such that it returns a function:

1. Try to apply the passed responseConverterFunction
2. If the passed responseConverterFunction falls through, the ResponseConverterFunctionProvider will try to apply its own ResponseConverterFunction

so I guess the PR would look roughly like follows (the same would be applied to ScalaPbResponseConverterFunctionProvider):

public final class ProtobufResponseConverterFunctionProvider implements ResponseConverterFunctionProvider {
...
    public ResponseConverterFunction createResponseConverterFunction(
            Type returnType,
            ResponseConverterFunction responseConverter) {
        if (isSupportedType(returnType)) {
            return new responseConverter.orElse(ProtobufResponseConverterFunction);
        }
...

I wanted to make sure I also understood @minwoox 's comments correctly 😅