How to construct json using JsonBuilder with key having the name of a variable and value having its value?

How to construct json using JsonBuilder with key and value having same name?

import groovy.json.JsonBuilder

def userId = 12 // some user id obtained from else where.

def json = new JsonBuilder()
def root = json {
    userId userId
}
print json.toString()

Which produces the error

groovy.lang.MissingMethodException: No signature of method: java.lang.Integer.call() is applicable for argument types: (java.lang.Integer) values: [12] Possible solutions: wait(), any(), abs(), wait(long), wait(long, int), and(java.lang.Number)

Quoting the key does has no effect. Any idea how to make this work.

Edit:

I want the JSON to be like { userId: 12 }. Also, why does writing the key as string not work?

long userId = 12   
def json = new JsonBuilder()
def root = json {
    "userId" userId
}

The example provided is just a snippet. The situation is that I have a lot of controller actions, which has various variables already. Now I am adding a part where I am trying to create a JSON string with various values the variables hold. So it's not very practical to change existing variable names and if I could construct the JSON string with the same name, it would be more consistent. Writing accessor methods for all the variables I wanted is also not an elegant method. What I did at present is to use different naming scheme like user_id for userId but again, it's not consistent with rest of the conventions I follow. So I am looking for an elegant approach and the reason why JsonBuilder behaves in this manner.

In case of JavaScript,

var a = 1
JSON.stringify({a: a})    // gives "{"a":1}"

which is the expected result.

• Declare accessors for the variable userId, if you need the JSON to look like {userId:12}

as

import groovy.json.JsonBuilder

def getUserId(){
    def userId = 12 // some user id obtained from else where.
}

def json = new JsonBuilder()
def root = json{
    userId userId
}
print json.toString()

• If you need the JSON to look like {12:12} which is the simplest case:

then

import groovy.json.JsonBuilder

def userId = 12 // some user id obtained from else where.

def json = new JsonBuilder()
def root = json{
    "$userId" userId
}
print json.toString()

• Just for the sake of the groovy script you can remove def from userId to get the first behavior. :)

as

import groovy.json.JsonBuilder

userId = 12

def json = new JsonBuilder()
def root = json{
    userId userId
}
print json.toString()

UPDATE

Local variables can also be used as map keys (which is String by default) while building JSON.

import groovy.json.JsonBuilder

def userId = 12 
def age = 20 //For example
def email = "abc@xyz.com"

def json = new JsonBuilder()
def root = json userId: userId, age: age, email: email

print json.toString() //{"userId":12,"age":20,"email":"abc@xyz.com"}