serializing annotations as well as fields to JSON

Question

I have a spring boot app, and I want to send DTO validation constraints as well as field value to the client.

Having DTO

class PetDTO {
  @Length(min=5, max=15)
  String name;
}

where name happens to be 'Leviathan', should result in this JSON being sent to client:

{
    name: 'Leviathan'
    name_constraint: { type: 'length', min:5, max: 15},
}

Reasoning is to have single source of truth for validations. Can this be done with reasonable amount of work?

Answer 1

To extend Frederik's answer I'll show a little sample code that convers an object to map and serializes it.

So here is the User pojo:

import org.hibernate.validator.constraints.Length;

public class User {

    private String name;

    public User(String name) {
        this.name = name;
    }

    @Length(min = 5, max = 15)
    public String getName() {
        return name;
    }
}

Then the actual serializer:

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.std.StdSerializer;
import org.springframework.util.ReflectionUtils;

import java.beans.IntrospectionException;
import java.beans.Introspector;
import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.*;
import java.util.stream.Stream;

import static java.util.stream.Collectors.toMap;

public class UserSerializer extends StdSerializer<User> {

    public UserSerializer(){
        this(User.class);
    }

    private UserSerializer(Class t) {
        super(t);
    }

    @Override
    public void serialize(User bean, JsonGenerator gen, SerializerProvider provider) throws IOException {
        Map<String, Object> properties = beanProperties(bean);
        gen.writeStartObject();
        for (Map.Entry<String, Object> entry : properties.entrySet()) {
            gen.writeObjectField(entry.getKey(), entry.getValue());
        }
        gen.writeEndObject();
    }

    private static Map<String, Object> beanProperties(Object bean) {
        try {
            return Arrays.stream(Introspector.getBeanInfo(bean.getClass(), Object.class).getPropertyDescriptors())
                    .filter(descriptor -> Objects.nonNull(descriptor.getReadMethod()))
                    .flatMap(descriptor -> {
                        String name = descriptor.getName();
                        Method getter = descriptor.getReadMethod();
                        Object value = ReflectionUtils.invokeMethod(getter, bean);
                        Property originalProperty = new Property(name, value);

                        Stream<Property> constraintProperties = Stream.of(getter.getAnnotations())
                                .map(anno -> new Property(name + "_constraint", annotationProperties(anno)));

                        return Stream.concat(Stream.of(originalProperty), constraintProperties);
                    })
                    .collect(toMap(Property::getName, Property::getValue));
        } catch (Exception e) {
            return Collections.emptyMap();
        }
    }

    // Methods from Annotation.class
    private static List<String> EXCLUDED_ANNO_NAMES = Arrays.asList("toString", "equals", "hashCode", "annotationType");

    private static Map<String, Object> annotationProperties(Annotation anno) {
        try {
            Stream<Property> annoProps = Arrays.stream(Introspector.getBeanInfo(anno.getClass(), Proxy.class).getMethodDescriptors())
                    .filter(descriptor -> !EXCLUDED_ANNO_NAMES.contains(descriptor.getName()))
                    .map(descriptor -> {
                        String name = descriptor.getName();
                        Method method = descriptor.getMethod();
                        Object value = ReflectionUtils.invokeMethod(method, anno);
                        return new Property(name, value);
                    });
            Stream<Property> type = Stream.of(new Property("type", anno.annotationType().getName()));
            return Stream.concat(type, annoProps).collect(toMap(Property::getName, Property::getValue));
        } catch (IntrospectionException e) {
            return Collections.emptyMap();
        }
    }

    private static class Property {
        private String name;
        private Object value;

        public Property(String name, Object value) {
            this.name = name;
            this.value = value;
        }

        public String getName() {
            return name;
        }

        public Object getValue() {
            return value;
        }
    }
}

And finally we need to register this serializer to be used by Jackson:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication(scanBasePackages = "sample.spring.serialization")
public class SerializationApp {

    @Bean
    public Jackson2ObjectMapperBuilder mapperBuilder(){
        Jackson2ObjectMapperBuilder jackson2ObjectMapperBuilder = new Jackson2ObjectMapperBuilder();
        jackson2ObjectMapperBuilder.serializers(new UserSerializer());
        return jackson2ObjectMapperBuilder;
    }

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

@RestController
class SerializationController {
    @GetMapping("/user")
    public User user() {
        return new User("sample");
    }
}

The Json that will be emitted:

{  
   "name_constraint":{  
      "min":5,
      "max":15,
      "payload":[],
      "groups":[],
      "message":"{org.hibernate.validator.constraints.Length.message}",
      "type":"org.hibernate.validator.constraints.Length"
   },
   "name":"sample"
}

Hope this helps. Good luck.