Schema validation (Draft 7)

JsonFlow includes a full Draft 7 JSON Schema validator. The primary class is TSchemaValidator (unit JsonFlow.SchemaValidator), which implements IJSONSchemaValidator.

Basic validation

uses
  JsonFlow.SchemaValidator,
  JsonFlow.Reader,
  JsonFlow.Interfaces;

var
  LReader: TJSONReader;
  LValidator: TSchemaValidator;
  LSchema, LData: IJSONElement;
begin
  LReader := TJSONReader.Create;
  LValidator := TSchemaValidator.Create;
  try
    LSchema := LReader.Read(
      '{"type":"object",' +
      '"properties":{"name":{"type":"string","minLength":2}},' +
      '"required":["name"]}'
    );
    LData := LReader.Read('{"name":"A"}'); // will fail minLength

    LValidator.ParseSchema(LSchema);

    if not LValidator.Validate(LData) then
    begin
      for var LErr in LValidator.GetErrors do
        WriteLn(LErr.Path + ': ' + LErr.Message);
    end;
  finally
    LValidator.Free;
    LReader.Free;
  end;
end;

Validate from strings

// Both schema and data as strings
if not LValidator.Validate('{"name":"A"}', '{"type":"object","required":["name"]}') then
  WriteLn(LValidator.GetLastError);

TValidationError fields

Field	Type	Description
Path	string	JSON Pointer to the failing data location
SchemaPath	string	JSON Pointer to the failing schema keyword
Message	string	Human-readable error description
FoundValue	string	Actual value that failed
ExpectedValue	string	Expected value or constraint
Keyword	string	Schema keyword that triggered the error
LineNumber	Integer	Source line (when available)
ColumnNumber	Integer	Source column (when available)
Context	string	Additional context

Supported Draft 7 keywords

Type validation

type — string, number, integer, boolean, null, array, object

String keywords

minLength, maxLength, pattern, format

Number keywords

minimum, maximum, exclusiveMinimum, exclusiveMaximum, multipleOf

Array keywords

items, minItems, maxItems, uniqueItems, contains

Object keywords

properties, required, additionalProperties, patternProperties, propertyNames, minProperties, maxProperties

Composition keywords

allOf, anyOf, oneOf, not, if / then / else

Reference

$ref — resolves internal (#/definitions/...) and external references

Enumeration

enum

Built-in format validators

JsonFlow ships format validators for:

Format	Unit
date	JsonFlow.FormatValidators.Date
date-time	JsonFlow.FormatValidators.DateTime
time	JsonFlow.FormatValidators.Time
email	JsonFlow.FormatValidators.Email
ipv4	JsonFlow.FormatValidators.Ipv4
ipv6	JsonFlow.FormatValidators.Ipv6
uri	JsonFlow.FormatValidators.Uri
uuid	JsonFlow.FormatValidators.Uuid
cpf (Brazil)	JsonFlow.FormatValidators.CPF
cnpj (Brazil)	JsonFlow.FormatValidators.CNPJ
cep (Brazil)	JsonFlow.FormatValidators.CEP
phone-br (Brazil)	JsonFlow.FormatValidators.BrazilianPhone
license-plate-br	JsonFlow.FormatValidators.BrazilianLicensePlate

Loading schemas from files

Use TJSONSchemaReader (unit JsonFlow.SchemaReader) to load a schema from a file:

uses
  JsonFlow.SchemaReader;

var
  LSchemaReader: TJSONSchemaReader;
begin
  LSchemaReader := TJSONSchemaReader.Create;
  try
    LSchemaReader.LoadFromFile('path/to/schema.json');
    // or: LSchemaReader.LoadFromString('{"type":"object",...}');

    if not LSchemaReader.Validate('{"name":"Alice"}') then
    begin
      for var LErr in LSchemaReader.GetErrors do
        WriteLn(LErr.Path + ': ' + LErr.Message);
    end;
  finally
    LSchemaReader.Free;
  end;
end;

TJSONSchemaReader public API:

Member	Description
Create	Default constructor (uses Draft 7 validator internally)
LoadFromFile(AFileName)	Load and parse schema from a file; returns Boolean
LoadFromString(AJsonString)	Load and parse schema from a string; returns Boolean
Validate(AJson)	Validate JSON string against the loaded schema; returns Boolean
Validate(AElement)	Validate IJSONElement against the loaded schema; returns Boolean
Validate(AJson, AJsonSchema)	Validate JSON string with schema string in one call; returns Boolean
GetErrors	Returns TArray<TValidationError>
GetVersion	Returns the detected TJsonSchemaVersion
GetSchema	Returns the loaded schema as IJSONElement

External $ref resolution

Implement IJSONSchemaRef and register it with the validator to resolve external $ref URIs:

uses
  JsonFlow.Interfaces;

type
  TMySchemaRef = class(TInterfacedObject, IJSONSchemaRef)
    function FetchReference(const ARef: string): IJSONElement;
  end;

Built-in HTTP resolvers are provided for Indy (JsonFlow.SchemaRefIndy) and Synapse (JsonFlow.SchemaRefSynapse).

Logging

LValidator.OnLog(
  procedure(const AMsg: string) begin WriteLn(AMsg); end
);

Async validation