Jackson: Serialize and deserialize enum values as integers

Question

Consider the following enum and class:

public enum State {
    OFF,
    ON,
    UNKNOWN
}

public class Machine {
    String name;
    int numCores;
    State state;

    public Machine(String name, int numCores, State state) {
        this.name = name;
        this.numCores = numCores;
        this.state = state;
    }
}

And consider the following main function:

public static void main(String args[]) {
    Machine m = new Machine("Machine 1", 8, State.OFF);
    ObjectMapper mapper = new ObjectMapper();
    String machineAsJsonString = mapper.writeValueAsString(m);
    System.out.println(machineAsJsonString);
}

Currently, the output of this main is:

{"name" : "Machine 1", "numCores" : 8, "state" : "OFF"}

This output is not good for me, as instead of the string "OFF" for state, I would like it to be 0, which is the ordinal value of OFF in the enum State.

So the actual result I want to get is:

{"name" : "Machine 1", "numCores" : 8, "state" : 0}

Is there some elegant way to make it behave this way?

Answer 1

It should work by specifying JsonValue mapper.

public enum State {
    OFF,
    ON,
    UNKNOWN;

    @JsonValue
    public int toValue() {
        return ordinal();
    }
}  

This works for deserialization also, as noted in Javadoc of @JsonValue annotation:

NOTE: when use for Java enums, one additional feature is that value returned by annotated method is also considered to be the value to deserialize from, not just JSON String to serialize as. This is possible since set of Enum values is constant and it is possible to define mapping, but can not be done in general for POJO types; as such, this is not used for POJO deserialization