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

Jackson 2.5 introduced the @JsonAppend annotation, which can be used to add "virtual" properties during serialization. It can be used with the mixin functionality to avoid modifying the original POJO.

The following example adds an ApprovalState property during serialization:

@JsonAppend(     attrs = {         @JsonAppend.Attr(value = "ApprovalState")     } ) public static class ApprovalMixin {} 

Register the mixin with the ObjectMapper:

mapper.addMixIn(POJO.class, ApprovalMixin.class); 

Use an ObjectWriter to set the attribute during serialization:

ObjectWriter writer = mapper.writerFor(POJO.class)                           .withAttribute("ApprovalState", "Pending"); 

Using the writer for serialization will add the ApprovalState field to the ouput.