JacksonJrDecoder is not able to deserialize generics

[yvasyliev]

Description

Although jackson-jr library is capable of generics deserialization, feign-jackson-jr module throws DecodeException when you try to decode the response.

How to reproduce

I created a simple demo to demonstrate this:

public class JacksonJrDemo {
    public static void main(String[] args) throws IOException {
        String json = "{\"data\":2024}";

        DataHolder<?> jrResult = JSON.std.beanFrom(DataHolder.class, json);
        System.out.println(jrResult); // works

        Object jrDecoderResult = new JacksonJrDecoder().decode(
            Response
                .builder()
                .request(Request.create(
                    Request.HttpMethod.GET,
                    "/v1/dummy",
                    Collections.emptyMap(),
                    Request.Body.empty(),
                    null
                ))
                .body(json, StandardCharsets.UTF_8)
                .build(),
            new TypeReference<DataHolder<Integer>>() {}.getType()
        );
        System.out.println(jrDecoderResult); // throws DecodeException
    }

    @Data
    private static class DataHolder<T> {
        private T data;
    }
}

Output:

JacksonJrDemo.DataHolder(data=2024)
Exception in thread "main" feign.codec.DecodeException: Cannot decode type: io.github.yvasyliev.JacksonJrDemo$DataHolder<java.lang.Integer>
	at feign.jackson.jr.JacksonJrDecoder.findTransformer(JacksonJrDecoder.java:109)
	at feign.jackson.jr.JacksonJrDecoder.decode(JacksonJrDecoder.java:67)
	at io.github.yvasyliev.JacksonJrDemo.main(JacksonJrDemo.java:21)

Tested with:

<dependency>
    <groupId>io.github.openfeign</groupId>
    <artifactId>feign-jackson-jr</artifactId>
    <version>13.1</version>
</dependency>

Initial investigation

I see two problems here:

1. feign.jackson.jr.JacksonJrDecoder#findTransformer always throws DecodeException unless type is instance of Class, List or Map.
2. This behevior is not possible to override since feign.jackson.jr.JacksonJrDecoder#findTransformer has private static modifiers. (why?)

Suggestion

1. In this issue I suggest jackson-jr to hand the control over generics deserialization. Because now JacksonJrDecoder is very limited.
2. I'm going to create another issue to make feign.jackson.jr.JacksonJrDecoder#findTransformer overridable.

Both problems requires very few code changes, and I'll try to handle both of them.

Actually, if problem 2 will be solved first, there is no urgent need to solve problem 1. Because in this case the user will be able to handle deserialization by extending JacksonJrDecoder. Yet both changes would be great to see.