Jackson JSON List with Object Type

Question

I have to serialize JSON from a list of Objects. The resulting JSON has to look like this:

{
    \"status\": \"success\",
    \"models\": [
        {
                


        

Answer 1

There's no built-in way to do this. You'll have to write your own JsonSerializer. Something like

class ModelSerializer extends JsonSerializer<List<Model>> {

    @Override
    public void serialize(List<Model> value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException {
        jgen.writeStartArray();
        for (Model model : value) {
            jgen.writeStartObject();
            jgen.writeObjectField("model", model);
            jgen.writeEndObject();    
        }
        jgen.writeEndArray();
    }

}

and then annotate the models field so that it uses it

@JsonSerialize(using = ModelSerializer.class)
private List<Model> models;

This would serialize as

{
    "status": "success",
    "models": [
        {
            "model": {
                "id": 1,
                "color": "red"
            }
        },
        {
            "model": {
                "id": 2,
                "color": "green"
            }
        }
    ]
}

If you're both serializing and deserializing this, you'll need a custom deserializer as well.

Answer 2

Another approach is using StdConverter class. Here is a working (abbreviated) example:

// MyParentObject.java
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;

public class MyParentObject {
  @JsonSerialize(converter = ChildListToString.class)
  @JsonDeserialize(converter = StringToChildList.class)
  @JsonProperty
  public List<AChildObject> myChildren;
}

// ChildListToString.java

import com.fasterxml.jackson.databind.util.StdConverter;

import java.util.List;
import java.util.stream.Collectors;

public class ChildListToString extends StdConverter<List<AChildObject>, String> {
  @Override
  public String convert(List<IntakeModuleUrn> value) {
    // this is just as effective as using Jackson "write array"
    // Try-Catch omitted for brevity
    StringBuilder builder = new StringBuilder("[");
    value.stream().map(value -> new ObjectMapper().writeValue(value)).forEach(urnStr -> {
      if(builder.length() > 1) {
        builder.append(", ");
      }

      builder.append("\"").append(urnStr).append("\"");
    });

    builder.append("]");

    return builder.toString();
  }
}

// StringToChildList.java

import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.util.StdConverter;


import java.io.IOException;
import java.net.URISyntaxException;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class StringToChildList extends StdConverter<String, List<AChildObject>> {

  @Override
  public List<AChildObject> convert(String value) {
    // try - catch omitted here for brevity
      List<String> strings = new ObjectMapper().readValue(value, new TypeReference<List<String>>() {});
      return strings.stream()
          .map(string -> {
            return new ObjectMapper().readValue(string, AChildObject.class)
          }).collect(Collectors.toList());

  }
}

I like this because it gives you control of serialization and deserialization separately.

Answer 3

This is an oldish question, But there is an arguably more idiomatic way of implementing this (I'm using jackson-databind:2.8.8):

Define a ModelSerializer (That extends StdSerializer as recommended by Jackson) that prints your model how you like and use the @JsonSerialize(contentUsing = ...) over your collection type:

class ModelSerializer extends StdSerializer<Model> {

    public ModelSerializer(){this(null);}
    public ModelSerializer(Class<Model> t){super(t);} // sets `handledType` to the provided class

    @Override
    public void serialize(List<Model> value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {
        jgen.writeStartObject();
        jgen.writeObjectField("model", value);
        jgen.writeEndObject();
    }
}

Meanwhile, in another file:

class SomethingWithModels {
    // ...
    @JsonSerialize(contentUsing = ModelSerializer.class)
    private Collection<Model> models;
    // ...
}

Now you aren't bound to just Lists of models but may apply this to Collections, Sets, Native []s and even the values of Maps.