Wsrtjtyk

What I am trying to achieve:

to deserialize null values to empty lists

Note: I cannot change/annotate the entities, because they do come from a jar.

I have the following code:

import com.fasterxml.jackson.annotation.JsonInclude;

import com.fasterxml.jackson.annotation.JsonSetter;

import com.fasterxml.jackson.annotation.Nulls;

import com.fasterxml.jackson.core.JsonParser;

import com.fasterxml.jackson.core.JsonProcessingException;

import com.fasterxml.jackson.core.type.TypeReference;

import com.fasterxml.jackson.databind.*;

import com.fasterxml.jackson.databind.module.SimpleModule;



import java.io.IOException;

import java.util.Collections;

import java.util.List;



public class MyJsonMapperList extends ObjectMapper{



private static final MyJsonMapperList jsonMapper = new MyJsonMapperList();



public MyJsonMapperList() {

    SimpleModule sm =  new SimpleModule();

    sm.addDeserializer(List.class, new NullToEmptyListDeserializer());

    super.registerModule(sm);

    // setSerializationInclusion(JsonInclude.Include.NON_NULL);

    // disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

}



private static class NullToEmptyListDeserializer extends JsonDeserializer<List> {



    @Override

    public List deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {

        ObjectMapper mapper = new ObjectMapper();

        mapper.configOverride(List.class).setSetterInfo(JsonSetter.Value.forContentNulls(Nulls.AS_EMPTY));

        mapper.configOverride(List.class).setSetterInfo(JsonSetter.Value.forValueNulls(Nulls.AS_EMPTY));

        mapper.configOverride(List.class).setSetterInfo(JsonSetter.Value.forValueNulls(Nulls.AS_EMPTY, Nulls.AS_EMPTY));

        return mapper.readValue(jsonParser, new TypeReference<List>(){});

    }



    @Override

    public List getNullValue(DeserializationContext ctx) {

        return Collections.emptyList();

    }

}



public static <T> T fromJson(String json, Class<T> tClass) throws IOException {

    if (json == null) { return null; }

    return  jsonMapper.readerFor(tClass).readValue(json);



}



public static String toJson(Object bean) throws JsonProcessingException {

    if (bean == null) { return null; }

    return jsonMapper.writer().writeValueAsString(bean);

}

The source for this code is from here.
Did I interpret something incorrectly from the Kotlin code provided by shyiko?

*Note I've tried also directly with setDeserializerModifier like:

 public MyJsonMapperList () {

        SimpleModule sm = new SimpleModule();

        // sm.addDeserializer(CollectionType.class, new NullToEmptyListDeserializer ());

        sm.setDeserializerModifier(new BeanDeserializerModifier() {

            @Override

            public JsonDeserializer<?> modifyCollectionDeserializer(DeserializationConfig config, CollectionType type, BeanDescription beanDesc, JsonDeserializer<?> deserializer) {

                deserializer = new JsonDeserializer<List<?>>() {

                    @Override

                    public List<?> deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {

                        ObjectMapper mapper = new ObjectMapper();

                        return mapper.readValue(jsonParser, new TypeReference<List>(){});

                    }

                    @Override

                    public List getNullValue(DeserializationContext ctx) {

                        return Collections.emptyList();

                    }

                };

                return deserializer;

            }

        });

        super.registerModule(sm);

//        setSerializationInclusion(JsonInclude.Include.NON_NULL);

//        disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);



    }

The error which I get:

java.lang.IllegalArgumentException: Cannot handle managed/back reference 'defaultReference': type: value deserializer of type MyJsonMapperList$NullToEmptyListDeserializer does not support them



    at com.fasterxml.jackson.databind.JsonDeserializer.findBackReference(JsonDeserializer.java:365)

    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase._resolveManagedReferenceProperty(BeanDeserializerBase.java:764)

    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.resolve(BeanDeserializerBase.java:474)

    at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCache2(DeserializerCache.java:293)

    at com.fasterxml.jackson.databind.deser.DeserializerCache._createAndCacheValueDeserializer(DeserializerCache.java:244)

    at com.fasterxml.jackson.databind.deser.DeserializerCache.findValueDeserializer(DeserializerCache.java:142)

    at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:477)

    at com.fasterxml.jackson.databind.ObjectReader._prefetchRootDeserializer(ObjectReader.java:1935)

    at com.fasterxml.jackson.databind.ObjectReader.<init>(ObjectReader.java:189)

    at com.fasterxml.jackson.databind.ObjectMapper._newReader(ObjectMapper.java:658)

    at com.fasterxml.jackson.databind.ObjectMapper.readerFor(ObjectMapper.java:3517)

    at ...MyJsonMapperList.fromJson(MyJsonMapperList.java:47)

Entities do not have any @JsonManagedReference and @JsonBackReference annotations (even if they had, could not modify them)

Jackson dependency:

    <dependency>

        <groupId>com.fasterxml.jackson.core</groupId>

        <artifactId>jackson-databind</artifactId>

        <version>2.9.0</version>

    </dependency>

What am I missing?

Are there other possibilities which I ommitted?

This answer sounded promising, but you don't have access to objectMapperBuilder because of the protected access.