TomSpencerLondon / ObjectMapperJSON

Practice with Jackson

ObjectMapperJSON

Problem

Given input:

public record PlayerWonGame(int payout, PlayerOutcome playerOutcome) { }

Fetch Output:

 {"type": "PlayerWonGame", "payout":100,"playerOutcome":"WON"}

First Attempt

Using ObjectMapper:

// only gives {"PlayerWonGame":{"payout":100,"playerOutcome":"WON"}}
@JsonTypeName("PlayerWonGame")
@JsonTypeInfo(include = As.WRAPPER_OBJECT, use = Id.NAME)
public record PlayerWonGame(int payout, PlayerOutcome playerOutcome) { }

Correct Solution

// gives {"type":"PlayerWonGame","payout":100,"playerOutcome":"WON"}
@JsonTypeName("PlayerWonGame")
@JsonTypeInfo(include = As.PROPERTY, use = Id.NAME, property = "type")
public record PlayerWonGame(int payout, PlayerOutcome playerOutcome) { }

Explanation

We are using Object Mapper to convert Java record objects into Json string but what if I need to add another type into Json without changing my record object. The above annotations help us to solve this problem. I have used two annotations @JsonTypeName() and @JsonTypeInfo(). Using these annotations we can specify which attribute to add at what level. Firstly, I added the type attribute on the class level with the name as PlayerWonGame for this I used include = As.WRAPPER_OBJECT. Hence I got the following output:

{
  "PlayerWonGame": {
    "payout": 100,
    "playerOutcome": "WON"
  }
}

I wanted PlayerWonGame as the value for a type key inside the object not wrapping it. Hence I used include = As.PROPERTY. This provided me with the correct output which I wanted. Here @JsonTypeName("PlayerWonGame") specifies the value for the key which needs to be added and use = Id.NAME maps it and the name of the key is provided by property = "type". Hence I got type: "PlayerWonGame". The final output is below:

{
  "type": "PlayerWonGame",
  "payout": 100,
  "playerOutcome": "WON"
}