JSON - Schema

JSON Schema is a specification for JSON based format for defining the structure of JSON data. It was written under IETF draft which expired in 2011. JSON Schema −

• Describes your existing data format.
• Clear, human- and machine-readable documentation.
• Complete structural validation, useful for automated testing.
• Complete structural validation, validating client-submitted data.

JSON Schema Validation Libraries

There are several validators currently available for different programming languages. Currently the most complete and compliant JSON Schema validator available is JSV.

Languages	Libraries
C	WJElement (LGPLv3)
Java	json-schema-validator (LGPLv3)
.NET	Json.NET (MIT)
ActionScript 3	Frigga (MIT)
Haskell	aeson-schema (MIT)
Python	Jsonschema
Ruby	autoparse (ASL 2.0); ruby-jsonschema (MIT)
PHP	php-json-schema (MIT). json-schema (Berkeley)
JavaScript	Orderly (BSD); JSV; json-schema; Matic (MIT); Dojo; Persevere (modified BSD or AFL 2.0); schema.js.

JSON Schema Example

Given below is a basic JSON schema, which covers a classical product catalog description −

{
   "$schema": "http://json-schema.org/draft-04/schema#",
   "title": "Product",
   "description": "A product from Acme's catalog",
   "type": "object",
 
   "properties": {
 
      "id": {
         "description": "The unique identifier for a product",
         "type": "integer"
      },
  
      "name": {
         "description": "Name of the product",
         "type": "string"
      },
  
      "price": {
         "type": "number",
         "minimum": 0,
         "exclusiveMinimum": true
      }
   },
 
   "required": ["id", "name", "price"]
}

Let's the check various important keywords that can be used in this schema −

Keywords	Description
$schema	The $schema keyword states that this schema is written according to the draft v4 specification.
title	You will use this to give a title to your schema.
description	A little description of the schema.
type	The type keyword defines the first constraint on our JSON data: it has to be a JSON Object.
properties	Defines various keys and their value types, minimum and maximum values to be used in JSON file.
required	This keeps a list of required properties.
minimum	This is the constraint to be put on the value and represents minimum acceptable value.
exclusiveMinimum	If "exclusiveMinimum" is present and has boolean value true, the instance is valid if it is strictly greater than the value of "minimum".
maximum	This is the constraint to be put on the value and represents maximum acceptable value.
exclusiveMaximum	If "exclusiveMaximum" is present and has boolean value true, the instance is valid if it is strictly lower than the value of "maximum".
multipleOf	A numeric instance is valid against "multipleOf" if the result of the division of the instance by this keyword's value is an integer.
maxLength	The length of a string instance is defined as the maximum number of its characters.
minLength	The length of a string instance is defined as the minimum number of its characters.
pattern	A string instance is considered valid if the regular expression matches the instance successfully.

You can check a http://json-schema.org for the complete list of keywords that can be used in defining a JSON schema. The above schema can be used to test the validity of the following JSON code −

[
   {
      "id": 2,
      "name": "An ice sculpture",
      "price": 12.50,
   },
 
   {
      "id": 3,
      "name": "A blue mouse",
      "price": 25.50,
   }
]