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    Documentation for the Axiell Schema Project

    Documentation for the Axiell Schema Project

    Overview

    The Axiell Schema Project provides a small library for validating JSON documents against a JSON Schema. The library conforms to the JSON Schema Draft #7 specification.

    The library is based on a few simple principles, namely:

    • referential transparency. Each of the methods in the library is referentially transparent, that is if the method is called with the same arguments it will always return the same value and the method does not have any side effect (e.g. update globals, change the operating environment, etc).
    • minimal functionality. The library is designed to be small incorporating a small number of dependencies. The number of methods associated with a class/object is restricted to those that provide a base level of functionality.
    • support multiple JSON libraries. It should be possible to use the library with any number of different JSON library packages (e.g json4s, jackson, etc). The current library only supports json4s, but a facade approach should be added to hide individual library implementations.
    • generic "format" implementations. The JSON Schema specification mandates a standard set of "format" types that can be used to validate string values. Each "format" type is implemented as a class with the components of the type available.
    • full error handling. Error resulting from the validation process are returned as a list of validation failures. All possible errors should be returned for a single call, eliminating the need to fix a single error and then recall a method.

    Package structure

    The com.axiell.schema packages consists of a number of utilities, where each utility provides a single piece of functionality. Since the library validates JSON structures two main components are provided. The first component (com.axiell.schema.Schema) allows JSON documents conforming to the JSON Schema specification to be read. Once read a resolution process may be required to resolve any external references defined in the document.

    The second component (com.axiell.schema.SchemaValidator) takes a resolved schema and allows JSON documents to be validated against that schema. When creating a validator it is also possible to incorporate your own "format" types for validation.

    A set of auxiliary components implement parsers for many of the "format" types and can be used independently of the validator itself.

    Notable utilities are:

    • Schema wrapper around a JSON document containing the document itself, its $id value and resolution mappings for all $id and $ref directives.
    • SchemaValidator takes a Schema and an optional handler for "format" types and provides a validation method for arbitrary JSON documents.
    • Implicits implements implicit methods and classes used to convert one class to another or pimp methods onto existing classes (e.g. same() method to JValue class).
    • various auxiliary classes for handling "format" types including:

    Dependencies

    The list of dependencies are:

    • Axiell Util Library provides error handling classes and methods that allow multilingual error messages to be generated.
    • JSON for Scala Library provides mthods for dealing with JSON structures. The routines work around an AST (Abstract Syntax Tree) used to represent a JSON document.
    • Cats Functional Programming Library provides various traits, classes and methods that simplify functional programming in Scala. Also provides the Validated class used to return lists of validation errors.
    Definition Classes
    root
  • package com
    Definition Classes
    root
  • package axiell
    Definition Classes
    com
  • package schema

    Contains types used by the library.

    Contains types used by the library.

    In order to simplify method signatures a series of types are defined. The types are used throughout the library.

    Definition Classes
    axiell
  • package instances
    Definition Classes
    schema
  • package syntax
    Definition Classes
    schema
  • Assertions
  • Context
  • EmailAddress
  • Formats
  • Hostname
  • Implicits
  • Ipv4Address
  • Ipv6Address
  • JsonPointer
  • RelativeJsonPointer
  • Resolver
  • Schema
  • SchemaValidator
  • SchemaWalker
  • Validator
c

com.axiell.schema

SchemaWalker

abstract class SchemaWalker[A] extends AnyRef

Provides a "visitor" pattern to walk a JSON Schema document.

The walk method visits each node in a JSON Schema document. For each JSON Object encountered it invokes the abstract process method for each property defined within the JSON object. Along with each property name and value it passes in the JSON object containing the property and the base URI for the object. The base URI is determined by processing $$id properties found in the JSON Schema.

Users should implement the abstract process method if they want to handle properties stored in a JSON Schema document. The method should return either an error in Error or a value defined by the A type generic. Since process accumulates results the type A must implement a Semigroup implicit value to combine A elements.

Example:

val walker = new SchemaWalker[Unit] {
  def process((path: JsonPointer, baseUri: URI, parent: JObject,
      name: String, value: JValue): Validated[Error, Unit] =
    (name, value) match {
      case ("$ref", JString(uri)) =>
        println(s"Found reference ${baseUri.resolve(uri)}")
      case _ =>
    }
}

walker.walk(new Uri(), parse("""{ <JSON Schema Document> }"""))
A

type returned from process method

Source
SchemaWalker.scala
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Instance Constructors

  1. new SchemaWalker()

Abstract Value Members

  1. abstract def process(path: JsonPointer, baseUri: URI, parent: JObject, name: String, value: JValue): Validated[Error, A]

    Invoked for each property in a JSON Schema document.

    Invoked for each property in a JSON Schema document.

    The method is called when each property is visited in each JSON object in a JSON Schema document. The calling order is top-down. The method should return a Error error if an error results while processing the node, otherwise a value of generic type A should be given.

    path

    JSON Pointer containing the path for the current property

    baseUri

    for the current property

    parent

    object containing the current property

    name

    of the current property

    value

    of the current property

    returns

    an A otherwise an error

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  1. final def !=(arg0: Any): Boolean
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  20. def walk(baseUri: URI, jsonSchema: JObject)(implicit semigroup: Semigroup[Option[A]]): Result[Option[A]]

    Walks the supplied JSON Schema calling process for each property.

    Walks the supplied JSON Schema calling process for each property.

    The JSON Schema Document is traversed in a top-down fashion with process called for each property found in the document. The calculation of the base URI is handled by the walk method by scanning for $id properties and applying them before processing any properties in the enclosing JSON object.

    baseUri

    to use as the root URI (can be e an empty URI)

    jsonSchema

    JSON Schema document to walk

    semigroup

    used to combine return values from process method

    returns

    optional A value. scala.None is returned if the schema consists of a boolean value only

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