Example of how to serialize and deserialize polymorphic messages, discovered at run time, with Jackson.

This uses reflection on startup to identify all the message requests and responses tagged with "@CustomRequest" or "@CustomResponse", and populates ObjectMapper so that the mapper understands these polymorphic types.

GameMessages, consisting of a Header and a (polymorphic) Body, can be serialized and deserialized in a type-specific way via ObjectMapper.

A GameMessage consists of a GameMessageHeader and a Body. The GameMessageHeader is a plain old POJO without any interesting features, so we won't talk about it any more.

We have to declare to ObjectMapper what types are valid for getBody() and how ObjectMapper should store that information so that the resulting JSON can be deserialized. One way to do this is to enumerate all of the possible types at compile time - this is the easiest where you know the list of types is small and doesn't change very frequently.

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "@messageType")
@JsonSubTypes(value = {
    @JsonSubTypes.Type(value = ExampleRequest.class)
    // other types get listed here
})
public Object getBody() { ... }

However, for this example we want to discover the possible values for the JsonSubType when the application starts. Perhaps we want to scan other JARs that might be present at runtime.

A serialized ExampleRequest looks like this:

{
  "header": {
    "messageId":"headerId"
  },
  "body":{
    "@messageType":"ExampleRequest",
    "name":"foo",
    "number":12345
  }
}

where the identifier "ExampleRequest" is embedded in the body as the property we asked for in the GameMessage definition.