How to Read and Write JSON Files in Julia (JSON.jl) 📂Julia

How to Read and Write JSON Files in Julia (JSON.jl)

Overview

Packages that help handle JSON files in Julia include JSON.jl, JSON3.jl, and Serde.jl. This document will explain JSON.jl.

JSON.jl is a package written in pure Julia that provides functionality for parsing and outputting JSON files. Although it is not part of Julia’s standard library, since the package is grouped under the JuliaIO group on GitHub and managed by a Julia developer, it can be considered almost an official package.

Code

Writing

Dictionary to JSON File

The code below saves a dictionary to a JSON file. By providing an indent as the third argument to JSON.print, it is saved nicely formatted.

using JSON

dict_data = Dict("name"      => "Jang, Wonyoung",
                 "age"       => 19,
                 "group"     => "IVE",
                 "nicknames" => ["Wonnyo", "Gatgi", "Lucky-Vicky"])

open("wonnyo.json", "w") do io
    JSON.print(io, dict_data)
end

open("pretty_wonnyo.json", "w") do io
    JSON.print(io, dict_data, 4)
end

String to JSON File

The following code saves a string to a JSON file. The basic syntax is similar to that of a dictionary, but the string must be parsed once using JSON.parse(). Similarly, providing an indent as the third argument results in a nicely formatted file.

str_data = """
{
    "name": "Jang, Wonyoung",
    "age": 19,
    "group": "IVE",
    "nicknames": ["Wonnyo", "Gatgi", "Lucky-Vicky"]
}
"""

open("wonnyo.json", "w") do io
    JSON.print(io, JSON.parse(str_data))
end

open("pretty_wonnyo.json", "w") do io
    JSON.print(io, JSON.parse(str_data), 4)
end

Alternatively, you can directly save a string to a JSON file using the JSON.write function as shown below. In this case, spaces and newlines are preserved.

str_data = """
{
    "name": "Jang, Wonyoung",
    "age": 19,
    "group": "IVE",
    "nicknames": ["Wonnyo", "Gatgi", "Lucky-Vicky"]
}
"""

unpretty_str_data = """
{"name": "Jang, Wonyoung", "age": 19, "group": "IVE", "nicknames": ["Wonnyo", "Gatgi", "Lucky-Vicky"]}
"""

open("str2json.json", "w") do io
    JSON.write(io, str_data)
end

open("unpretty_str2json.json", "w") do io
    JSON.write(io, unpretty_str_data)
end

Reading

You can load a JSON file with the code below.

julia> using JSON

julia> dcit_from_json = JSON.parsefile("wonnyo.json")
Dict{String, Any} with 4 entries:
  "name"      => "Jang, Wonyoung"
  "nicknames" => Any["Wonnyo", "Gatgi", "Lucky-Vicky"]
  "group"     => "IVE"
  "age"       => 19

Alternatively, you can read it directly using an IO stream as shown:

julia> open("wonnyo.json", "r") do io
           str_from_json = read(io, String)
           data_from_str = JSON.parse(str_from_json)
       end
Dict{String, Any} with 4 entries:
  "name"      => "Jang, Wonyoung"
  "nicknames" => Any["Wonnyo", "Gatgi", "Lucky-Vicky"]
  "group"     => "IVE"
  "age"       => 19

Environment

OS: Windows11
Version: Julia 1.11.3, JSON v0.21.4