Serializing a pojo as a nested JSON dictionary

Question

Given the simple POJO:

public class SimplePojo {
    private String key ;
    private String value ;
    private int thing1 ;
    private boolean thing2;

    public String getKey() {
           return key;
    }
    ...
}

I have no issue in serializing into something like so (using Jackson):

 {
    "key": "theKey",
    "value": "theValue",
    "thing1": 123,
    "thing2": true
  }

but what would really make me happy is if I could serialize that object as such:

 {
    "theKey" {
           "value": "theValue",
            "thing1": 123,
            "thing2": true
     }
  }

I'm thinking I would need a custom serializer, but where I'm challenged is in inserting a new dictionary, e.g.:

@Override
public void serialize(SimplePojo value, JsonGenerator gen, SerializerProvider provider) throws IOException {
    gen.writeStartObject();
    gen.writeNumberField(value.getKey(), << Here be a new object with the remaining three properties >> );


}

Any suggestions?

score 1 · Accepted Answer · answered Jul 23 '19 at 07:09

You don't need a custom serializer. You can utilize @JsonAnyGetter annotation to produce a map that contains the desired output properties.
The code below takes the above example pojo and produces the desired json representation.
First, you have annotate all getter methods with @JsonIgnore in order for jackson to ignore them during serialization. the only method that will be called is the @JsonAnyGetter annotated one.

public class SimplePojo {
    private String key ;
    private String value ;
    private int thing1 ;
    private boolean thing2;

    // tell jackson to ignore all getter methods (and public attributes as well)
    @JsonIgnore
    public String getKey() {
        return key;
    }

    // produce a map that contains the desired properties in desired hierarchy 
    @JsonAnyGetter
    public Map<String, ?> getForJson() {
        Map<String, Object> map = new HashMap<>();
        Map<String, Object> attrMap = new HashMap<>();
        attrMap.put("value", value);
        attrMap.put("thing1", thing1);  // will autobox into Integer
        attrMap.put("thing2", thing2);  // will autobox into Boolean
        map.put(key, attrMap);
        return map;
    }
}

That would work- in fact I did try that (A fact I neglected to mention in my original post). I think I'm gonna stick with this, though I would like to explore the custom serializer option, as the @JsonAnyGetter for the sake of serialization has confused my fellow coders. — Eric, Jul 24 '19 at 19:00

score 0 · Answer 2 · answered Jul 22 '19 at 23:07

You need to use writeObjectFieldStart method to write field and open new JSON Object at the same type:

class SimplePojoJsonSerializer extends JsonSerializer<SimplePojo> {

    @Override
    public void serialize(SimplePojo value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeStartObject();

        gen.writeObjectFieldStart(value.getKey());
        gen.writeStringField("value", value.getValue());
        gen.writeNumberField("thing1", value.getThing1());
        gen.writeBooleanField("thing2", value.isThing2());
        gen.writeEndObject();

        gen.writeEndObject();
    }
}

Serializing a pojo as a nested JSON dictionary

2 Answers2