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Sven Van Caekenberghe

June 2012

(This is a draft)

JSON (JavaScript Object Notation) is a popular data-interchange format. NeoJSON is an elegant and efficient standalone Smalltalk framework to read and write JSON converting to or from Smalltalk objects.

An introduction to JSON

JSON is a lightweight text-based open standard designed for human-readable data interchange. It was derived from the JavaScript scripting language for representing simple data structures and associative arrays, called objects. Despite its relationship to JavaScript, it is language-independent, with parsers available for many languages.

Here are some relevant links:

There are only a couple of primitive types in JSON:

  • numbers (integer or floating point)
  • strings
  • the boolean constants true and false
  • null

Only two composite types exist:

  • lists (an ordered sequenece of values)
  • maps (an unordered associative array, mapping string property names to values)

That is really all there is to it. No options or additions are defined in the standard.


The NeoJSON framework contains a reader (NeoJSONReader) and a writer (NeoJSONWriter) to parse respectively generate JSON to or from Smalltalk objects. The goals of this project are:

  • to be standalone (have no dependencies and have little requirements)
  • to be small, elegant and understandable
  • to be efficient (both in time and space)
  • to be flexible and non-intrusive

Compared to other Smalltalk JSON frameworks, NeoJSON has

  • less dependencies and little requirements
  • can be more efficient (be faster and use less memory)
  • allows for the use of schemas and mappings


Obviously, the primitive types are mapped to corresponding Smalltalk classes. While reading:

  • numbers become Integers or Floats
  • strings become Strings
  • booleans become Booleans
  • null become nil

While writing

  • Numbers are converted to floats, except for Integers that become integers
  • Strings and subclasses become strings
  • Booleans become booleans
  • nil becomes null

Generic Mode

NeoJSON can operate in a generic mode that requires no further configuration. While reading:

  • maps become instances of mapClass, Dictionary by default
  • lists become instance of listClass, Array by default

The reader can be customized to use a different mapClass or listClass. There is also an option to convert all map keys to symbols, which is off by default. While writing:

  • Dictionary and SmallDictionary become maps
  • all other Collection classes become lists
  • all other Objects are rejected

Here are some examples writing in generic mode:

NeoJSONWriter toString: #(1 2 3).

NeoJSONWriter toString: { Float pi. true. false. 'string' }.

NeoJSONWriter toStringPretty: (Dictionary new at: #x put: 1; at: #y put: 2; yourself).

NeoJSONWriter can output either in a compact format (the default) or in a pretty printed format. And these are some examples reading in generic mode:

NeoJSONReader fromString: ' [ 1,2,3 ] '.

NeoJSONReader fromString: ' [ 3.14159, true, false, null, "string" ] '.

NeoJSONReader fromString: ' { "x" : 1, "y" : 2 } '.

In order to use the generic mode, you have to convert your domain objects to and from Dictionaries and SequenceableCollections. This is realatively easy but not very efficient, depending on the use case.

Schemas and Mappings

NeoJSON allows for the optional specification of schemas and mappings to be used when writing and/or when reading. A NeoJSONMapper holds a number of schemas. Each schema is identified by either a class or a symbol. Each schema specifies a mapping, an object that will help in doing the actual reading or writing.

The most common mapping deals with objects that define a number of named properties or attributes. These can be defined based on instance variables (optionally derived by reflection), accessors (getter/setter pairs) or even blocks. Such an object mapping is identified by a Smalltalk class, which is also used to create new instances. Each property mapping can have an optional value schema to be used recursively when reading and/or writing property values.

The less common custom mapping holds a generic reader and/or writer block to deal with special cases such as specific collection types with an optional schema for the elements, or a direct mapping of semi primitive types such as Date or DateAndTime.

A mapping can be specified explicitely on a mapper, or can be resolved using the #neoJsonMapping: class method.

Here are some examples of mappings:

mapper mapAllInstVarsFor: Point.

mapper for: TestObject do: [ :mapping |
    mapping mapInstVars: #(id name).
    (mapping mapInstVar: #timestamp to: 'created-at') valueSchema: DateAndTime.
    (mapping mapInstVar: #points) valueSchema: #ArrayOfPoints.
    (mapping mapInstVar: #bytes) valueSchema: ByteArray ].

mapper for: DateAndTime customDo: [ :mapping |
    mapping decoder: [ :string | DateAndTime fromString: string ].
    mapping encoder: [ :dateAndTime | dateAndTime printString ] ].

mapper for: #ArrayOfPoints customDo: [ :mapping |
    mapping listOfElementSchema: Point ].

mapper for: #DictionaryOfPoints customDo: [ :mapping |
    mapping mapWithValueSchema: Point ].

mapper for: ByteArray customDo: [ :mapping |
    mapping listOfType: ByteArray ]

The classes NeoJSONReader and NeoJSONWriter are subclasses of NeoJSONMapper. When writing, mappings are used when arbitrary objects are seen. For example, in order to be able to write an array of points, you could do as follows:

String streamContents: [ :stream |
    (NeoJSONWriter on: stream)
        prettyPrint: true;
        mapInstVarsFor: Point;
        nextPut: (Array with: 1@3 with: -1@3) ].

Collections are handled automatically, like in the generic case. When reading, a mapping is used as a binding or an explicit type specifying what Smalltalk objects that you want to read. Here is a very simple case, reading a map as a point:

(NeoJSONReader on: ' { "x" : 1, "y" : 2 } ' readStream)
    mapInstVarsFor: Point;
    nextAs: Point.  

Since JSON lacks a universal way to specify the class of an object/map, we have to specify the target schema that we want to use as an argument to #nextAs:.

With custom mappings, it is possible to

  • define the schema of the elements of a list
  • define the schema of the elements of a list as well as the class of the list
  • define the schema of the values of a map In fact, NeoJSONCustomMapping can be extended to implement even more specialized mappings.

Finally, here is a more complex example, reading a list of maps as an array of points:

(NeoJSONReader on: '[ { "x" : 1, "y" : 2 }, { "x" : 3, "y" : 4 } ]' readStream)
    mapInstVarsFor: Point;
    for: #ArrayOfPoints customDo: [ :mapping |
        mapping listOfElementSchema: Point ];
    nextAs: #ArrayOfPoints.

NeoJSON deals efficiently with mappings: the minimal amount of intermediary structures are created, which is quite different from the generic case.


On modern hardware, NeoJSON can write or read in the tens of thousands of small objects per second. Several benchmarks are included in the unit tests package.