Groovy: JSON - reading and writing

JSON is one of the most popular language-independent formats for data serialization. It is also heavily used for configuration files.

Groovy comes with a module called json to handle various use-cases with JSON.

Parse JSON string

We can use the JsonSlurper class to parse JSON strings.

If we have JSON string in a Groovy variable we can parse it to become a Groovy map:

import groovy.json.JsonSlurper

def json_str = '''{
   "name": "Foo Bar",
   "year": 2018,
   "timestamp": "2018-03-08T00:00:00",
   "tags": [ "person", "employee" ],
   "grade": 3.14 }'''


def jsonSlurper = new JsonSlurper()
cfg = jsonSlurper.parseText(json_str)
println(cfg)          // [name:Foo Bar, year:2018, timestamp:2018-03-08T00:00:00, tags:[person, employee], grade:3.14]
println(cfg['name'])  // Foo Bar
println(cfg.name)     // Foo Bar

Creating JSON string

JsonOutput has several methods. toJson returns a JSON string in one line. prettyPrint returns a nicely formatted JSON string. The latter takes up more space, but it is also human-readable.

import groovy.json.JsonOutput

def data = [
    name: "Foo Bar",
    year: "2018",
    timestamp: "2018-03-08T00:00:00",
    tags: [ "person", "employee"],
    grade: 3.14
]

def json_str = JsonOutput.toJson(data)
println(json_str)

def json_beauty = JsonOutput.prettyPrint(json_str)
println(json_beauty)

In the output you can see both the result of toString and the result of prettyPrint.

{"name":"Foo Bar","year":"2018","timestamp":"2018-03-08T00:00:00","tags":["person","employee"],"grade":3.14}

{
    "name": "Foo Bar",
    "year": "2018",
    "timestamp": "2018-03-08T00:00:00",
    "tags": [
        "person",
        "employee"
    ],
    "grade": 3.14
}

Read JSON from file

import groovy.json.JsonSlurper

if (args.size() < 1) {
    println("Missing filename")
    System.exit(1)
}

filename = args[0]

def jsonSlurper = new JsonSlurper()
data = jsonSlurper.parse(new File(filename))

println(data)

The parse method accepts a File object, reads in the content of the file and then parses it.

Write JSON file

In order to write a JSON file, you need to create the JSON string (either as a plain string or as a beautified string) and then use the File class to save it.

import groovy.json.JsonOutput

if (args.size() < 1) {
    println("Missing filename")
    System.exit(1)
}

filename = args[0]


def data = [
    name: "Foo Bar",
    year: "2018",
    timestamp: "2018-03-08T00:00:00",
    tags: [ "person", "employee"],
    grade: 3.14
]

def json_str = JsonOutput.toJson(data)
def json_beauty = JsonOutput.prettyPrint(json_str)
File file = new File(filename)
file.write(json_beauty)

More JSON

Groovy JSON module

timestamp: 2018-08-02T08:30:01 tags: