Jackson custom serializer by field name?

Question

I have a POJO to be serialized which has field of type Object named representation and I have a custom serializer written for it.

POJO:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "user", propOrder = {
    //......
})
public class User {

    protected User.Event userEvent;
    protected Boolean isValid;
    protected Boolean isPrivleged;

    // getter/ setters

     // Inner static class
     public static class Event {

        protected Object representation;
        protected User.Event.Monitor userMonitor;

        // getter setters and Monitor static class

     }
}

Now, I cannot edit my POJO for some reason, so I want all configurations of Jackson in code via ObjectMapper. I am not able to register my custom serializer for field Object representation as it handles type Object which is a super class for all.

    public class CustomSerializer extends JsonSerializer<Object>{

        @Override
        public void serialize(Object obj, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {



            jgen.writeObject(content);
            // ...........      
        }

        @Override
        public Class<Object> handledType() {
            return Object.class;
        }

    }

This gives exception:

java.lang.IllegalArgumentException: JsonSerializer of type CustomSerializer does not define valid handledType() -- must either register with method that takes type argument  or make serializer extend 'com.fasterxml.jackson.databind.ser.std.StdSerializer'
    at com.fasterxml.jackson.databind.module.SimpleSerializers.addSerializer(SimpleSerializers.java:80)
    at com.fasterxml.jackson.databind.module.SimpleModule.addSerializer(SimpleModule.java:240)

So I guess since every field has superclass Object, thus it's saying invalid handleType().

Is there a way to register a Serializer programtically via the fieled name or something. Eg When field name is representation register my serializer for it ??

How to handle the above case ??

cassiomolin · Accepted Answer · 2016-08-11T09:25:56.187

2

Have you ever considered Jackson mix-in annotations?

Jackson mix-in annotations

It's a great alternative when modifying the classes is not an option. You can think of it as kind of aspect-oriented way of adding more annotations during runtime, to augment statically defined ones.

Define a mix-in annotation interface (class would do as well):

public interface EventMixIn {

    @JsonProperty("representation")
    @JsonSerialize(using = CustomSerializer.class)
    Object getRepresentation();
}

Then configure ObjectMapper to use the defined interface as a mix-in for your POJO:

ObjectMapper mapper = new ObjectMapper().enable(SerializationFeature.INDENT_OUTPUT)
                                        .addMixIn(User.Event.class, EventMixIn.class);

Usage considerations

Here are some usage considerations:

All annotation sets that Jackson recognizes can be mixed in.
All kinds of annotations (member method, static method, field, constructor annotations) can be mixed in.
Only method (and field) name and signature are used for matching annotations: access definitions (private, protected, ...) and method implementations are ignored.

For more details, you can have a look at this page.

edited Aug 11 '16 at 09:25

answered Aug 11 '16 at 07:19

cassiomolin

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Thanks for the detailed answer. i tried as it is in above, but somehow my custom serializer is not getting called (I added an SOP in it, it was not printed). PS: I am using Jackson 2.x so to add MixIn i use: `SimpleModule simpleModule = new SimpleModule("SimpleModule"); simpleModule.setMixInAnnotation(User.Event.class, EventMixIn.class); objectMapper.registerModule(simpleModule);` – Siddharth Trikha Aug 11 '16 at 09:24
@SiddharthTrikha I've just tried both approaches to add the mix-in and both worked for me. The custom serializer is invoked as expected here. – cassiomolin Aug 11 '16 at 09:30
1

I was also using `JaxbAnnotationIntrospector`, thus my serializer was not being called. Thanks – Siddharth Trikha Aug 11 '16 at 09:57
@Casio: Can I add one MixIn class for the above case ?? Like UserMixIn which also holds annotations for it's subclass Event ?? Or do I have to add separate MixIns ? – Siddharth Trikha Aug 11 '16 at 11:27
1

@SiddharthTrikha According to the [documentation](https://github.com/FasterXML/jackson-docs/wiki/JacksonMixInAnnotations), *mix-ins work as expected within inheritance hierarchy: it is feasible (and useful) to attach mix-in annotations to super-classes -- if so, mix-in annotations can further be overridden by annotations sub-classes (of target) provide*. – cassiomolin Aug 11 '16 at 11:48

score 1 · Answer 2 · answered Sep 12 '20 at 12:53

Just to add to the excellent answer by cassiomolin if you're trying to use this with Spring Boot and Kotlin to use your custom Jackson ObjectMapper:

// the custom serializer
class CustomSerializer : JsonSerializer<Any>() {
  override fun serialize(value: Any?, 
                         gen: JsonGenerator?, 
                         serializers: SerializerProvider?) {
    gen?.let { 
      gen.writeObject(content) 
     
      // ...
    }
  }
}

// the mixin
interface EventMixIn {
  @JsonProperty("representation")
  @JsonSerialize(using = CustomSerializer::class)
  fun getRepresentation(): Any?
}

// the config with the bean
@Configuration
class AppConfig {
  @Bean
  fun createObjectMapper(): MappingJackson2HttpMessageConverter {
    val objectMapper = ObjectMapper()
    objectMapper.addMixIn(Event::class.java, EventMixIn::class.java)
    return MappingJackson2HttpMessageConverter(objectMapper)
  }
}

Jackson custom serializer by field name?

2 Answers2

Jackson mix-in annotations

Usage considerations

Linked