Geting list of protobuf message objects from the SpringBoot

I want to get a list of protobuf message objects from the Spring boot app.
I did manage to get a single protobuf message object from the app but getting a list of them throws exception.

...
2020-01-24 14:57:02.359 ERROR 15883 --- [nio-8081-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.http.converter.HttpMessageConversionException: Type definition error: [simple type, class com.google.protobuf.UnknownFieldSet$Parser]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class com.google.protobuf.UnknownFieldSet$Parser and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ImmutableCollections$ListN[0]->com.example.demo.Lecture["unknownFields"]->com.google.protobuf.UnknownFieldSet["parserForType"])] with root cause

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class com.google.protobuf.UnknownFieldSet$Parser and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ImmutableCollections$ListN[0]->com.example.demo.Lecture["unknownFields"]->com.google.protobuf.UnknownFieldSet["parserForType"])
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.10.2.jar:2.10.2]
...

My code (simplified).

tl;dr

• create Spring boot app
• generate class from proto file
• try return List of generated class objects (RESTful)

My code (simplified).
Controler

import lombok.extern.slf4j.Slf4j;
import org.springframework.web.bind.annotation.*;
import java.util.List;

@Slf4j
@org.springframework.web.bind.annotation.RestController
@RequestMapping("/timetable")
public class RestController {

    @PostMapping("/single")      // Works
    private Lecture getLecture(@RequestBody Lecture lecture) {
        log.info("Single2 got: {}", lecture);
        return Lecture.newBuilder(lecture)
                .setDuration(lecture.getDuration() +1)
                .build();
    }

    @GetMapping("/list")       // Does not work
    private @ResponseBody List<Lecture> getLectures() {
        return List.of(
                Lecture.newBuilder()
                        .setDuration(1)
                        .setWeekDay(Lecture.WeekDay.MONDAY)
                        .setModule(Module.newBuilder().setName("Math1").build())
                        .build()
                // ... 
        );
    }
}

App

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.protobuf.ProtobufHttpMessageConverter;
import org.springframework.http.converter.protobuf.ProtobufJsonFormatHttpMessageConverter;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Bean
    @Primary
    ProtobufHttpMessageConverter protobufHttpMessageConverter() {
        return new ProtobufJsonFormatHttpMessageConverter();
    }
}

pom.xml

<!-- ... -->
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <java.version>11</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <!--        https://dzone.com/articles/exposing-microservices-over-rest-protocol-buffers-->
        <dependency>
            <groupId>com.google.protobuf</groupId>
            <artifactId>protobuf-java</artifactId>
            <version>3.11.1</version>
        </dependency>
        <dependency>
            <groupId>com.googlecode.protobuf-java-format</groupId>
            <artifactId>protobuf-java-format</artifactId>
            <version>1.4</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/com.google.protobuf/protobuf-java-util -->
        <dependency>
            <groupId>com.google.protobuf</groupId>
            <artifactId>protobuf-java-util</artifactId>
            <version>3.11.1</version>
        </dependency>
    </dependencies>
<!-- ... -->

I generate message objects using:

#!/bin/bash
SRC_DIR=../proto
DST_DIR=../../../target/
mkdir -p $DST_DIR
protoc -I=$SRC_DIR --java_out=$DST_DIR $SRC_DIR/college.proto

proto file

syntax = "proto3";

package my.college;

option java_multiple_files = true;
option java_package = "com.example.demo";

message Module {
    string name = 1;
    // ... other
}

message Lecture {
    WeekDay weekDay = 1;
    Module module = 2;
    uint32 duration = 3;
    // ... other

    enum WeekDay {
        SUNDAY = 0;
        MONDAY = 1;
        // ...
    }
}

I did found simmilar issue but it had no solution.

Answer

Workaround
I couldn’t find a solution to the problem so came up with a workaround.
Instead of returning generated protobuf message objects I returned wrappers for those objects. Using Lombok annotation it could be done:

import lombok.Data;

@Data // Lombok magic
public class Module {
    private String name;
    // ...

    public Module(ie.gmit.proto.Module moduleProto){
        this.name = moduleProto.getName();
        // ...
    }
}

This workaround doesn’t feel very bad as it uses standard Spring boot dependencies.