http://d.hatena.ne.jp/fumokmm/20110226/1298729619

json.txt

[
  ["aaa", "bbb", "ccc"],
  {
    "key1" : "value1",
    "key2" : "value2",
    "key3" : "value3",
    "key4" : ""
  },
  ["ddd", "eee", "fff"]
]

jsonTestRead.groovy

import groovy.json.*

def slurper = new JsonSlurper()
def text = new File('json.txt').text
def root = slurper.parseText(text)

assert root == [
  ['aaa', 'bbb', 'ccc'],
  [key1: 'value1', key2: 'value2', key3: 'value3', key4: ''],
  ['ddd', 'eee', 'fff']
]

assert root instanceof ArrayList
assert root[0] instanceof ArrayList
assert root[1] instanceof HashMap
assert root[2] instanceof ArrayList

assert root[1].key3 == 'value3'
assert root.last().last() == 'fff'

jsonTestWrite.groovy

import groovy.json.*

def json = new JsonBuilder()
json (
  ['aaa', 'bbb', 'ccc'],
  [
    key1: 'value1',
    key2: 'value2',
    key3: 'value3',
    key4: ''
  ],
  ['ddd', 'eee', 'fff']
)

new File('json2.txt').text = json.toString()