Jackson: How to add custom property to the JSON without modifying the POJO

Question

I am developing a REST interface for my app using Jackson to serialize my POJO domain objects to JSON representation. I want to customize the serialization for some types to add additional properties to the JSON representation that do not exist in POJOs (e.g. add some metadata, reference data, etc). I know how to write my own JsonSerializer, but in that case I would need to explicitly call JsonGenerator.writeXXX(..) methods for each property of my object while all I need is just to add an additional property. In other words I would like to be able to write something like:

@Override
public void serialize(TaxonomyNode value, JsonGenerator jgen, SerializerProvider provider) {
    jgen.writeStartObject();
    jgen.writeAllFields(value); // <-- The method I'd like to have
    jgen.writeObjectField("my_extra_field", "some data");
    jgen.writeEndObject();
}

or (even better) to somehow intercept the serialization before the jgen.writeEndObject() call, e.g.:

@Override void beforeEndObject(....) {
    jgen.writeObjectField("my_extra_field", "some data");
}

I thought I could extend BeanSerializer and override its serialize(..) method but it's declared final and also I couldn't find an easy way to create a new instance of BeanSerializer without providing it with all the type metadata details practically duplicating a good portion of Jackson. So I've given up on doing that.

My question is - how to customize Jackson's serialization to add additional stuff to the JSON output for particular POJOs without introducing too much of the boilerplate code and reusing as much as possible of the default Jackson behaviour.

Answer 1

Since (I think) Jackson 1.7 you can do this with a BeanSerializerModifier and extending BeanSerializerBase. I've tested the example below with Jackson 2.0.4.

import java.io.IOException;

import org.junit.Test;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.module.SimpleModule;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;
import com.fasterxml.jackson.databind.ser.impl.ObjectIdWriter;
import com.fasterxml.jackson.databind.ser.std.BeanSerializerBase;


public class JacksonSerializeWithExtraField {

    @Test
    public void testAddExtraField() throws Exception
    {
        ObjectMapper mapper = new ObjectMapper();

        mapper.registerModule(new SimpleModule() {

            public void setupModule(SetupContext context) {
                super.setupModule(context);

                context.addBeanSerializerModifier(new BeanSerializerModifier() {

                    public JsonSerializer<?> modifySerializer(
                            SerializationConfig config,
                            BeanDescription beanDesc,
                            JsonSerializer<?> serializer) {
                        if (serializer instanceof BeanSerializerBase) { 
                              return new ExtraFieldSerializer(
                                   (BeanSerializerBase) serializer);
                        } 
                        return serializer; 

                    }                   
                });
            }           
        });

        mapper.writeValue(System.out, new MyClass());       
        //prints {"classField":"classFieldValue","extraField":"extraFieldValue"}
    }


    class MyClass {

        private String classField = "classFieldValue";

        public String getClassField() { 
            return classField; 
        }
        public void setClassField(String classField) { 
            this.classField = classField; 
        }
    }


    class ExtraFieldSerializer extends BeanSerializerBase {

        ExtraFieldSerializer(BeanSerializerBase source) {
            super(source);
        }

        ExtraFieldSerializer(ExtraFieldSerializer source, 
                ObjectIdWriter objectIdWriter) {
            super(source, objectIdWriter);
        }

        ExtraFieldSerializer(ExtraFieldSerializer source, 
                String[] toIgnore) {
            super(source, toIgnore);
        }

        protected BeanSerializerBase withObjectIdWriter(
                ObjectIdWriter objectIdWriter) {
            return new ExtraFieldSerializer(this, objectIdWriter);
        }

        protected BeanSerializerBase withIgnorals(String[] toIgnore) {
            return new ExtraFieldSerializer(this, toIgnore);
        }

        public void serialize(Object bean, JsonGenerator jgen,
                SerializerProvider provider) throws IOException,
                JsonGenerationException {           
            jgen.writeStartObject();                        
            serializeFields(bean, jgen, provider);
            jgen.writeStringField("extraField", "extraFieldValue"); 
            jgen.writeEndObject();
        }
    }
}