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

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

Answer 1

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

Answer 2

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.