Julia Language JSON


  • using JSON
  • JSON.parse(str)
  • JSON.json(obj)
  • JSON.print(io, obj, indent)


Since neither Julia Dict nor JSON objects are inherently ordered, it's best not to rely on the order of key-value pairs in a JSON object.

Installing JSON.jl

JSON is a popular data interchange format. The most popular JSON library for Julia is JSON.jl. To install this package, use the package manager:

julia> Pkg.add("JSON")

The next step is to test whether the package is working on your machine:

julia> Pkg.test("JSON")

If all tests passed, then the library is ready for use.

Parsing JSON

JSON that has been encoded as a string can easily be parsed into a standard Julia type:

julia> using JSON

julia> JSON.parse("""{
           "this": ["is", "json"],
           "numbers": [85, 16, 12.0],
           "and": [true, false, null]
Dict{String,Any} with 3 entries:
  "this"    => Any["is","json"]
  "numbers" => Any[85,16,12.0]
  "and"     => Any[true,false,nothing]

There are a few immediate properties of JSON.jl of note:

  • JSON types map to sensible types in Julia: Object becomes Dict, array becomes Vector, number becomes Int64 or Float64, boolean becomes Bool, and null becomes nothing::Void.
  • JSON is an untyped container format: Thus returned Julia vectors are of type Vector{Any}, and returned dictionaries are of type Dict{String, Any}.
  • JSON standard does not distinguish between integers and decimal numbers, but JSON.jl does. A number without a decimal point or scientific notation is parsed into Int64, whereas a number with a decimal point is parsed into Float64. This matches closely with the behavior of JSON parsers in many other languages.

Serializing JSON

The JSON.json function serializes a Julia object into a Julia String containing JSON:

julia> using JSON

julia> JSON.json(Dict(:a => :b, :c => [1, 2, 3.0], :d => nothing))

julia> println(ans)

If a string is not desired, JSON can be printed directly to an IO stream:

julia> JSON.print(STDOUT, [1, 2, true, false, "x"])

Note that STDOUT is the default, and can be omitted in the above call.

Prettier printing can be achieved by passing the optional indent parameter:

julia> JSON.print(STDOUT, Dict(:a => :b, :c => :d), 4)
    "c": "d",
    "a": "b"

There is a sane default serialization for complex Julia types:

julia> immutable Point3D

julia> JSON.print(Point3D(1.0, 2.0, 3.0), 4)
    "y": 2.0,
    "z": 3.0,
    "x": 1.0