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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.

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

Resolver

object Resolver

Resolves $$ref references against a given JSON Schema.

JSON Schema documents may contain $$ref properties. The property allows another part of a JSON Schema to be referenced from the current location. It is in essence an import or include mechanism. The value of a $$ref property is a URI referring to a schema and a location within the schema. When a JSON Schema document is loaded the $$ref references need to be resolved to ensure that the validation process can follow references without error.

The resolve method performs reference resolution against a given JSON Schema. Each Schema has a table listing all references in the Schema document. The resolution routine updates that table based on the contents of a supplied schema. Such an approach allows references to external schemas to exist. To resolve such external references the resolution method must be called with the external schema. As more schemas are resolved against the references table the number of unresolved references should diminish. Once all references are resolved validation can be performed without an undefined reference error being generated.

Source: Resolver.scala

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def resolve(schema: Schema, refs: Refs): Result[Refs]
Resolves any applicable references against the supplied Schema.
Resolves any applicable references against the supplied Schema.
The supplied schema is checked against any unresolved entries in the refs table. Any unresolved reference that can be resolved against the schema is updated with the resolved location. A new instance of the references table is returned with any updated references.
schema
against which to resolve references
refs
table containing mapping between references and their resolved locations
returns
updated reference map otherwise a list of errors (generally caused by invalid URIs)
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Packages

Overview

Package structure

Dependencies

Resolver

object Resolver

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Packages

Overview

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Resolver

object Resolver

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Resolver