json-kotlin-schema

Kotlin implementation of JSON Schema (Draft-07)

NOTE: – from version 0.49, the underlying JSON and YAML libraries have been switched from jsonutil and yaml-simple to kjson-core and kjson-yaml. The change should be transparent to most users.

Quick Start

Given the following schema file (Taken from the Wikipedia article on JSON):

{
  "$schema": "http://json-schema.org/draft/2019-09/schema",
  "$id": "http://pwall.net/test",
  "title": "Product",
  "type": "object",
  "required": ["id", "name", "price"],
  "properties": {
    "id": {
      "type": "number",
      "description": "Product identifier"
    },
    "name": {
      "type": "string",
      "description": "Name of the product"
    },
    "price": {
      "type": "number",
      "minimum": 0
    },
    "tags": {
      "type": "array",
      "items": {
        "type": "string"
      }
    },
    "stock": {
      "type": "object",
      "properties": {
        "warehouse": {
          "type": "number"
        },
        "retail": {
          "type": "number"
        }
      }
    }
  }
}

and this JSON (from the same article):

{
  "id": 1,
  "name": "Foo",
  "price": 123,
  "tags": [
    "Bar",
    "Eek"
  ],
  "stock": {
    "warehouse": 300,
    "retail": 20
  }
}

the following code will validate that the JSON matches the schema:

    val schema = JSONSchema.parseFile("/path/to/example.schema.json")
    val json = File("/path/to/example.json").readText()
    require(schema.validate(json))

To see the detail of any errors found during validation:

    val schema = JSONSchema.parseFile("/path/to/example.schema.json")
    val json = File("/path/to/example.json").readText()
    val output = schema.validateBasic(json)
    output.errors?.forEach {
        println("${it.error} - ${it.instanceLocation}")
    }

The format of the error object produced by the validateBasic() function closely follows the Basic output specification.

It is also possible to read a schema from a string in memory:

    val str = File("/path/to/example.schema.json").readText()
    val schema = JSONSchema.parse(str)

An optional second parameter on the parse() function takes a URI, which will be used to construct the location in error objects.

YAML

While it may seem counter-intuitive to use a language other than JSON to express JSON Schema, YAML is a lot easier to work with, particularly when multi-line descriptions are required. This library functions equally well with schema representations in JSON or YAML.

For example, the above schema looks like this in YAML:

$schema: http://json-schema.org/draft/2019-09/schema
$id: http://pwall.net/test
title: Product
type: object
required:
- id
- name
- price
properties:
  id:
    type: number
    description: Product identifier
  name:
    type: string
    description: Name of the product
  price:
    type: number
    minimum: 0
  tags:
    type: array
    items:
      type: string
  stock:
    type: object
    properties:
      warehouse:
        type: number
      retail:
        type: number

To use this schema, simply specify a schema file with an extension of .yaml or .yml to the schema parser:

    val schema = JSONSchema.parseFile("/path/to/example.schema.yaml")

The YAML library used is this one.

References

At many points in a JSON Schema, the $ref construct allows a reference to schema information defined elsewhere. The reference takes the form of a URL, which may be internal to the current schema document (reference starts with a # character) or external - the reference points to a different document.

Internal references are resolved relative to the root of the schema document in which they appear, for example:

{
  "$ref": "#/$defs/Account"
}

This points to a schema named Account in the $defs section of the current schema document.

An external reference may be relative (in which case the URL will be resolved relative to the location of the document in which the reference appears) or absolute. The external reference may include a fragment (a JSON Pointer starting with #); if it does not the reference is taken as pointing to the root of the document. For example:

{
  "$ref": "common.schema.json#/$defs/Address"
}

This will look for a sibling (URL or file) to the current document and attempt to locate the Address schema in the $defs section of that document.

examples and default

The JSON Schema specification says, of examples and default: “It is RECOMMENDED that these values be valid against the associated schema.” The schema parser allows for the optional validation of examples and default entries; it requires the creation of a new Parser instance, and the setting of option flags:

    val parser = Parser()
    parser.options.validateExamples = true // to cause "examples" (and "example") entries to be validated
    parser.options.validateDefault = true  // to cause "default" entries to be validated
    parser.parseFile(filename)
    if (parser.examplesValidationErrors.isNotEmpty()) {
        // parser.examplesValidationErrors is a List of BasicOutput objects, one for each error found in examples
    }
    if (parser.defaultValidationErrors.isNotEmpty()) {
        // parser.examplesValidationErrors is a List of BasicOutput objects, one for each error found in default entries
    }

Implemented Subset

This implementation does not implement the full JSON Schema specification. It covers much of Draft 07 and a few features from Draft 2019-09.

The currently implemented subset includes:

Core

• $schema
• $id
• $ref (with some reservations)
• $defs
• $comment
• title
• description
• examples

Structure

• properties
• patternProperties
• additionalProperties
• propertyNames
• items
• additionalItems
• allOf
• anyOf
• oneOf
• if
• then
• else
• default

Validation

• type (null, boolean, object, array, number, string, integer)
• format (date-time, date, time, duration, email, hostname, uri, uri-reference, uri-template, uuid, ipv4, ipv6, json-pointer, relative-json-pointer, regex)
• enum
• const
• multipleOf
• maximum
• exclusiveMaximum
• minimum
• exclusiveMinimum
• minProperties
• maxProperties
• minItems
• maxItems
• uniqueItems
• maxLength
• minLength
• pattern
• required
• contains
• maxContains
• minContains

Not Currently Implemented

• $recursiveRef
• $recursiveAnchor
• $anchor
• $vocabulary
• unevaluatedProperties
• unevaluatedItems
• dependentcies
• dependentSchemas
• dependentRequired
• contentEncoding
• contentMediaType
• contentSchema
• deprecated
• readOnly
• writeOnly
• format (idn-email, idn-hostname, iri, iri-reference)

More documentation to follow.

Dependency Specification

The latest version of the library is 0.57, and it may be obtained from the Maven Central repository.

Maven

    <dependency>
      <groupId>net.pwall.json</groupId>
      <artifactId>json-kotlin-schema</artifactId>
      <version>0.57</version>
    </dependency>

Gradle

    implementation 'net.pwall.json:json-kotlin-schema:0.57'

Gradle (kts)

    implementation("net.pwall.json:json-kotlin-schema:0.57")

Peter Wall

2025-06-29