Adding a property using Jackson MixIn's?

Question

I know we can use Jackson MixIn's to rename a property or to ignore a property (see examples here). But is it possible to add a property?

The added property can be:

1. A constant (such as a version number)
2. A computed value (e.g. if the source class has properties for getWidth() and getHeight(), but we want to ignore both and export a getArea() instead)
3. Flattened information from nested members (e.g. a class has a member Information which in turn has a member Description, and we want to have a new property for description and skipping the nesting structure of Information)

Answer 1

From documentation:

"Mix-in" annotations are a way to associate annotations with classes, without modifying (target) classes themselves, originally intended to help support 3rd party datatypes where user can not modify sources to add annotations.

With mix-ins you can:
1. Define that annotations of a '''mix-in class''' (or interface)
2. will be used with a '''target class''' (or interface) such that it appears
3. as if the ''target class'' had all annotations that the ''mix-in'' class has (for purposes of configuring serialization / deserialization)

To solve your problems you can:

• Create new POJO which has all required fields.
• Implement custom serialiser.
• Before serialisation convert POJO to Map and add/remove nodes.
• Use com.fasterxml.jackson.databind.ser.BeanSerializerModifier to extend custom serialisers. See: Jackson custom serialization and deserialization.

For example, to add a constant version to each object you can wrap it in Verisoned class:

class Versioned {

    private final String version;

    @JsonUnwrapped
    private final Object pojo;

    public Versioned(String version, Object pojo) {
        this.version = version;
        this.pojo = pojo;
    }

    public String getVersion() {
        return version;
    }

    public Object getPojo() {
        return pojo;
    }
}

Now, if you wrap an Arae(width, height) object:

Area area = new Area(11, 12);
String json = mapper.writeValueAsString(new Versioned("1.1", area));

output will be:

{
  "version" : "1.1",
  "width" : 11,
  "height" : 12
}