Create Json Strings from Groovy Variables in Jenkins Pipeline

JSON is a format that uses human-readable text to transmit data objects consisting of attribute–value pairs and array data types. So, in general json is a formatted text.

In groovy json object is just a sequence of maps/arrays.

parsing json using JsonSlurperClassic

//use JsonSlurperClassic because it produces HashMap that could be serialized by pipeline
import groovy.json.JsonSlurperClassic

node{
    def json = readFile(file:'message2.json')
    def data = new JsonSlurperClassic().parseText(json)
    echo "color: ${data.attachments[0].color}"
}

parsing json using pipeline

node{
    def data = readJSON file:'message2.json'
    echo "color: ${data.attachments[0].color}"
}

building json from code and write it to file

import groovy.json.JsonOutput
node{
    //to create json declare a sequence of maps/arrays in groovy
    //here is the data according to your sample
    def data = [
        attachments:[
            [
                fallback: "New open task [Urgent]: < out Slack message attachments>",
                pretext : "New open task [Urgent]: < out Slack message attachments>",
                color   : "#D00000",
                fields  :[
                    [
                        title: "Notes",
                        value: "This is much easier than I thought it would be.",
                        short: false
                    ]
                ]
            ]
        ]
    ]
    //two alternatives to write

    //native pipeline step:
    writeJSON(file: 'message1.json', json: data)

    //but if writeJSON not supported by your version:
    //convert maps/arrays to json formatted string
    def json = JsonOutput.toJson(data)
    //if you need pretty print (multiline) json
    json = JsonOutput.prettyPrint(json)

    //put string into the file:
    writeFile(file:'message2.json', text: json)

}