Adding a property using Jackson MixIn's? - jackson

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:
A constant (such as a version number)
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)
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
}