Skip to content
A JSON to Fantom object mapping library
Fantom HTML
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.

Json v1.0.2

Written in: Fantom pod: v1.0.2 Licence: ISC


Json is a support library that aids Alien-Factory in the development of other libraries, frameworks and applications. Though you are welcome to use it, you may find features are missing and the documentation incomplete.

Json is a customisable Fantom to Javascript Object Notation (JSON) mapping library.

It goes far beyond the usual JsonInStream and JsonOutStream classes by mapping and instantiating fully fledged Fantom domain objects.


  • JSON pretty printing
  • Converts all core Fantom types
  • Converts nested / embedded objects
  • Runs on Javascript platforms
  • IoC enabled
  • Simple to use

Just annotate fields with @JsonProperty then read / write JSON with readJson(...) and writeJson(...) - couldn't be easier!


Install Json with the Fantom Pod Manager ( FPM ):

C:\> fpm install afJson

Or install Json with fanr:

C:\> fanr install -r afJson

To use in a Fantom project, add a dependency to

depends = ["sys 1.0", ..., "afJson 1.0"]


Full API & fandocs are available on the Eggbox - the Fantom Pod Repository.

Quick Start

  1. Create a text file called

     using afJson
     class Example {
         Void main() {
             jsonService := Json()
             // write some JSON...
             json := """{
                            "name"  : "Emma",
                            "sex"   : "female",
                            "likes" : ["Cakes","Adventure"],
                            "car"   : {
                                "name"  : "Golf",
                                "brand" : "VW"
                            "score" : 9
             // ...and WHAM! A fully inflated domain object!
             friend := (Friend) jsonService.readJson(json, Friend#)
             echo(     // --> Emma
             echo( // --> Golf
             friend.score = 11
      = null
             // we can even convert the other way!
             moarJson := jsonService.writeJson(friend, Friend#)
             // --> {"name":"Emma","sex":"female","score":11,"likes":["Cakes","Adventure"]}
     class Friend {
         @JsonProperty Str    name
         @JsonProperty Sex    sex
         @JsonProperty Int    score
         @JsonProperty Str[]  likes
         @JsonProperty Car?   car    // embedded objects!
         new make(|This| f) { f(this) }
     class Car {
         @JsonProperty Str    name
         @JsonProperty Str    brand
         new make(|This| f) { f(this) }
     enum class Sex {
         male, female;
  2. Run as a Fantom script from the command line:

     C:\> fan


JSON is the string representation of a Javascript object.

JsonObj is the Fantom representation of a JSON object. It only contains Maps, Lists, Bools, Nums, Strs, and null.

Entity is a Fantom object from your problem domain.

All conversion of Entities to and from JSON goes through an intermediary JsonObj stage:

Entity <--> JsonObj <--> JSON

EntityConverter converts between Entities and JsonObjs.

JsonReader and JsonWriter convert between JsonObjs and JSON.

Json has methods to convert between all:

JSON Methods


Any Fantom object may be converted to and from JSON. Just make sure that all fields to be converted are annotated with the @JsonProperty facet.

Supported Types

The JSON Spec only defines types for Bool, List, Null, Number, Object, and String. As such, this library provides the following mappings:

 Fantom              JSON
--------------      --------
 sys::Bool     <-->  Bool
 sys::Decimal  <-->  Number
 sys::Enum     <-->  String
 sys::Field    <-->  String
 sys::Float    <-->  Number
 sys::Int      <-->  Number
 sys::List     <-->  List
 sys::Map      <-->  Object
 sys::Method   <-->  String
      null     <-->  Null
 sys::Obj      <-->  Object
 sys::Slot     <-->  String
 sys::Str      <-->  String
 sys::Type     <-->  String

Plus any Type annotated with @Serializable { simple = true } is converted to and from a Str. Combined that accounts for all Fantom literals and core types.

Const vs Non-Const

This library can instantiate any Fantom object, both const and non-const. But if the type is const, or if it has non-null fields, then it must have an it-block ctor like the one below. That is the only way fields can be set during construction.

const class User {

    const Str name

    // the it-block ctor
    new make(|This| f) {

Null vs Non-Null

Nullable fields are optional, that is, they do not require a JSON value.

class User {
    Str? name  // name is optional because it is nullable

    new make(|This| f) { f(this) }

json := "{}"
user := Json().readJson(json, User#) as User

echo(  // --> null

Similarly, when converting an entity to JSON, null values are not written out:

class User {
    Str? name

    new make(|This| f) { f(this) }

user := User { name = null }
json := Json().writeJson(user, User#)

echo(json)  // --> {}

If you want null values to be written, then set storeNullValues = true on the desired field:

class User {
    @JsonProperty { storeNullValues = true }
    Str? name

    new make(|This| f) { f(this) }

user := User { name = null }
json := Json().writeEntity(user)

echo(json)  // --> {"name":null}

Property Names

Sometimes you want the JSON name to be different to the field names. To facilitate this, set the @JsonProperty.propertyName attribute:

class User {
    @JsonProperty { propertyName = "judge" }
    Str? name

    new make(|This| f) { f(this) }

user := User { name = "Dredd" }
json := Json().writeEntity(user, User#)

echo(json)  // --> {"judge":"Dredd"}


When Json is added as a dependency to an IoC enabled application, such as BedSheet or Reflux, then the following services are automatically made available to IoC:

The above makes use of the non-invasive module feature of IoC 3.

It is useful if your converted Fantom objects are built by IoC. That way they may contain services and perform operations on themselves, such as persisting to a database. To enable this, create an IocObjConverter class that extends from Json's @NoDoc ObjConverter class:

using afIoc
using afJson

const class IocObjConverter : ObjConverter {
    @Inject const |->Scope| scope

    new make(|This|in) { in(this) }

    override Obj? createEntity(Type type, Field:Obj? fieldVals) {
        scope().build(type, null, fieldVals)

Contribute this with the following:

@Contribute { serviceType=JsonTypeInspectors# }
Void contributeJsonTypeInspectors(Configuration config) {
    config.overrideValue("afJson.obj", ObjInspector { it.converter = })

Custom Conversion

The Json library is extremely configurable at all levels. Because of that, object conversion may not be as straight forward as you may think. This explains how the process works...

Every Fantom type to be converted, either a top level entity or an embedded object, is inspected by JsonTypeInspectors which produces a JsonTypeMeta instance. JsonTypeMeta describes how the Fantom type will be converted to / and from JSON. It also holds the JsonConverter instance that will do the converting.

Because each Fantom Type is mapped to a cached JsonTypeMeta instance, conversion is fast because types don't have to be re-inspected. Also, if an object is not being converted as you expect, you can inspect the nested hierarchy of JsonTypeMeta objects to see exactly what will happen.

JsonTypeInspectors holds a list of JsonTypeInspector instances. During inspection, each inspector is called in turn until one of them returns a JsonTypeMeta instance. This makes the order of the inspectors important.

To go the full hog with respect to custom conversion, you should create your own JsonTypeInspector and add / contribute it to JsonTypeInspectors. Your inspector should create JsonTypeMeta, complete with a custom converter, that completely describes how the type should be converted.

For example, there's no reason why fields must be annotated with @JsonProperty, you could easily create an inspector that returns JsonTypeMeta for every field!

If you did not want to go as far as creating an inspector, you could instead create a JsonTypeMeta instance. This could either be set on JsonTypeInspectors to be used for all given types, or passed to Json methods for ad hoc conversions.

Or the easiest, but most limited way, would be to set the @JsonProperty.converterType attribute on the affected fields.

If you are contemplating implementating custom conversion then you are encouraged to look at the Json library source for help and examples. It may be preferable to simply extend one the current @NoDoc Inspectors or Converters.

JSON and Dates

JSON does not define a Date object. As such, when writing Dates, they are serialised as ISO strings. At the other end, presumably in Javascript land, something must walk your object and de-serialise all your date strings back into Date objects.

But sometimes you want a quick hack and some people advocate inserting Javascript statements directly into the JSON. It may not be the best idea, but it's a good example of custom inspectors and converters...

using afJson

class User {
    @JsonProperty Str?      name
    @JsonProperty DateTime? timestamp

class Example {
    Void main() {
        jsonService := Json(JsonTypeInspectors(
            JsonTypeInspectors.defaultInspectors.insert(0, JsDateInspector())

        user := User { name = "Judge Dredd"; timestamp = }
        jsonService.writeJson(user, User#)

        echo(json) // --> {"name":"Judge Dredd","timestamp":new Date(1456178248297)}

const class JsDateInspector : JsonTypeInspector {
    const JsonConverter converter := JsDateConverter()

    override JsonTypeMeta? inspect(Type type, JsonTypeInspectors inspectors) {
        type.toNonNullable == DateTime# ? JsonTypeMeta { it.type = type; it.converter = this.converter } : null

const class JsDateConverter : JsonConverter {
    override Obj? toFantom(JsonConverterCtx ctx, Obj? jsonObj) { throw UnsupportedErr() }

    override Obj? toJsonObj(JsonConverterCtx ctx, Obj? fantomObj) {
        fantomObj == null ? null : JsLiteral("new Date(${((DateTime) fantomObj).toJava})")


If using IoC then you wouldn't create a new Json() service, instead you would contribute JsDateInspector to JsonTypeInspectors:

@Contribute { serviceType=JsonTypeInspectors# }
Void contributeJsonTypeInspectors(Configuration config) {
    config.set("jsDates", JsDateInspector()).before("afJson.literal")

Putting your inspector before afJson.literal ensures it is first in the inspector list.

You can’t perform that action at this time.