Jackson

Jackson is a library to serialize and deserialize JSON in the JVM world.

Sane Settings #

After working a while with this I want to register these sane defaults:

configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, true)
configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)

FAIL_ON_NULL_FOR_PRIMITIVES #

Setting FAIL_ON_NULL_FOR_PRIMITIVES forces clients to explicitely provide all values including primitives. Consider the following POJO:

data class Foo(bar: Boolean)

Without the setting, a payload such as { } would render ~Foo(bar=false)~ despite the lack of default value. Given this, to guarantee consistency between the source-code and the external contract, I advise enabling this.

FAIL_ON_UNKNOWN_PROPERTIES #

Setting FAIL_ON_UNKNOWN_PROPERTIES is useful when working on two systems in paralel. Settings this to false enables clients to send fields to the server that are not yet supported but will be. The alternative would be:

1. Server includes those fields as optional fields (to avoid breaking current clients).
2. Server roll-out.
3. Clients update their HTTP clients to include the new fields.
4. Once all-known clients support the new fields, make the same fields mandatory.

By setting this to true, this whole orchestration is not required.

TODO Sub-Types #

Consider the following example that attempts to model a DSL that supports + and - operations.

sealed class Expression
data class Sum(val a: Int, val b: Int): Expression()
data class Sub(val a: Int, val b: Int): Expression()

data class Request(
    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "type", visible = true)
    @JsonSubTypes(
	JsonSubTypes.Type(value = Sum::class, name = "+")
	JsonSubTypes.Type(value = Sub::class, name = "-")
    )
    val operation: Expression
)

In short, this is saying if the JSON contains a field name type with value + it would deserialize to Sum and to Sub if type had value -. I.e., a sum operation between 1 and 2 would require the following JSON payload:

{
    "type": "+",
    "operation": {
	 "a": 1,
	 "b": 2
    }
}