How to add extra fields to an Object during Jackson's json serialization? - json

Question

I need to add new property to an object, when serializing to JSON. The value for the property is calculated on runtime and does not exist in the object. Also the same object can be used for creation of different JSON with different set ot fields (kind of having a base class with subclasses, but I don't want to create ones just for JSON generation).
What is the best way of doing that, which doesn't involve creation of custom serializer class, which will take care of serializing of whole set of object's fields? Or may be it is possible to inherit some "basic" serializer, and simply take it's output and add new field to it somehow?
I learned about mixins, and looks like it is possible to rename/hide some fields, however it seems not be possible to add an extra one.

Answer 1

Can you not just add a method in value class? Note that it does not have to be either public, or use getter naming convention; you could do something like:
public class MyStuff {
// ... the usual fields, getters and/or setters
#JsonProperty("sum") // or whatever name you need in JSON
private int calculateSumForJSON() {
return 42; // calculate somehow
}
}
Otherwise you could convert POJO into JSON Tree value:
JsonNode tree = mapper.valueToTree(value);
and then modify it by adding properties etc.

Answer 2

2021 calling...
Simplest way I found to do this is #JsonUnwrapped:
public class Envelope<T> {
#JsonUnwrapped // content's fields are promoted alongside the envelope's
public T content;
// Transmission specific fields
public String url;
public long timestamp;
}
This works (bi-directionally) so long as Envelope's fieldnames do not clash with those of content. Also has a nice feature of keeping the transmission properties at the end of the serialised JSON.

Answer 3

One option is to add a field for this property and set it on the object before writing to JSON. A second option, if the property can be computed from other object properties you could just add a getter for it, for example:
public String getFullName() {
return getFirstName() + " " + getLastName();
}
And even though there's no matching field Jackson will automatically call this getter while writing the JSON and it will appear as fullName in the JSON output. If that won't work a third option is to convert the object to a map and then manipulate it however you need:
ObjectMapper mapper //.....
MyObject o //.....
long specialValue //.....
Map<String, Object> map = mapper.convertValue(o, new TypeReference<Map<String, Object>>() { });
map.put("specialValue", specialValue);
You're question didn't mention unmarshalling but if you need to do that as well then the first option would work fine but the second two would need some tweaking.
And as for writing different fields of the same object it sounds like a job for #JsonView