An implementation of JSON Schema, draft v4 - Go language
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format_checkers.go Removed check for String type when running FormatChecker Sep 6, 2017
format_checkers_test.go feat) add json schema v6 uri-reference format #126 May 12, 2017
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internalLog.go Wraps internalLog calls with internalLogEnabled Sep 22, 2015
jsonContext.go make jsonContext public + improve result.AddError Feb 1, 2018
jsonLoader.go updated May 23, 2017
locales.go Merge errors for then/else + fix error description Jan 26, 2018
result.go PR Changes Feb 7, 2018
schema.go Merge pull request #177 from hoshsadiq/master Jan 27, 2018
schemaPool.go Implement id parsing in conjuction with $ref fixes Dec 21, 2017
schemaReferencePool.go Implement id parsing in conjuction with $ref fixes Dec 21, 2017
schemaType.go Error messaging for Schema May 23, 2017
schema_test.go Merge pull request #177 from hoshsadiq/master Jan 27, 2018
subSchema.go Merge pull request #177 from hoshsadiq/master Jan 27, 2018
types.go restructured, moved missing locales, shortened names, allows loading Jan 1, 2015
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utils_test.go Use a decoder instead of Unmarshall for JSON parsing. Sep 21, 2015
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GoDoc Build Status



An implementation of JSON Schema, based on IETF's draft v4 - Go language

References :


go get

Dependencies :



package main

import (

func main() {

    schemaLoader := gojsonschema.NewReferenceLoader("file:///home/me/schema.json")
    documentLoader := gojsonschema.NewReferenceLoader("file:///home/me/document.json")

    result, err := gojsonschema.Validate(schemaLoader, documentLoader)
    if err != nil {

    if result.Valid() {
        fmt.Printf("The document is valid\n")
    } else {
        fmt.Printf("The document is not valid. see errors :\n")
        for _, desc := range result.Errors() {
            fmt.Printf("- %s\n", desc)



There are various ways to load your JSON data. In order to load your schemas and documents, first declare an appropriate loader :

  • Web / HTTP, using a reference :
loader := gojsonschema.NewReferenceLoader("")
  • Local file, using a reference :
loader := gojsonschema.NewReferenceLoader("file:///home/me/schema.json")

References use the URI scheme, the prefix (file://) and a full path to the file are required.

  • JSON strings :
loader := gojsonschema.NewStringLoader(`{"type": "string"}`)
  • Custom Go types :
m := map[string]interface{}{"type": "string"}
loader := gojsonschema.NewGoLoader(m)


type Root struct {
	Users []User `json:"users"`

type User struct {
	Name string `json:"name"`


data := Root{}
data.Users = append(data.Users, User{"John"})
data.Users = append(data.Users, User{"Sophia"})
data.Users = append(data.Users, User{"Bill"})

loader := gojsonschema.NewGoLoader(data)


Once the loaders are set, validation is easy :

result, err := gojsonschema.Validate(schemaLoader, documentLoader)

Alternatively, you might want to load a schema only once and process to multiple validations :

schema, err := gojsonschema.NewSchema(schemaLoader)
result1, err := schema.Validate(documentLoader1)
result2, err := schema.Validate(documentLoader2)
// etc ...

To check the result :

    if result.Valid() {
    	fmt.Printf("The document is valid\n")
    } else {
        fmt.Printf("The document is not valid. see errors :\n")
        for _, err := range result.Errors() {
        	// Err implements the ResultError interface
            fmt.Printf("- %s\n", err)

Working with Errors

The library handles string error codes which you can customize by creating your own gojsonschema.locale and setting it

gojsonschema.Locale = YourCustomLocale{}

However, each error contains additional contextual information.

err.Type(): string Returns the "type" of error that occurred. Note you can also type check. See below

Note: An error of RequiredType has an err.Type() return value of "required"

"required": RequiredError
"invalid_type": InvalidTypeError
"number_any_of": NumberAnyOfError
"number_one_of": NumberOneOfError
"number_all_of": NumberAllOfError
"number_not": NumberNotError
"missing_dependency": MissingDependencyError
"internal": InternalError
"enum": EnumError
"array_no_additional_items": ArrayNoAdditionalItemsError
"array_min_items": ArrayMinItemsError
"array_max_items": ArrayMaxItemsError
"unique": ItemsMustBeUniqueError
"array_min_properties": ArrayMinPropertiesError
"array_max_properties": ArrayMaxPropertiesError
"additional_property_not_allowed": AdditionalPropertyNotAllowedError
"invalid_property_pattern": InvalidPropertyPatternError
"string_gte": StringLengthGTEError
"string_lte": StringLengthLTEError
"pattern": DoesNotMatchPatternError
"multiple_of": MultipleOfError
"number_gte": NumberGTEError
"number_gt": NumberGTError
"number_lte": NumberLTEError
"number_lt": NumberLTError

err.Value(): interface{} Returns the value given

err.Context(): gojsonschema.JsonContext Returns the context. This has a String() method that will print something like this: (root).firstName

err.Field(): string Returns the fieldname in the format firstName, or for embedded properties, person.firstName. This returns the same as the String() method on err.Context() but removes the (root). prefix.

err.Description(): string The error description. This is based on the locale you are using. See the beginning of this section for overwriting the locale with a custom implementation.

err.DescriptionFormat(): string The error description format. This is relevant if you are adding custom validation errors afterwards to the result.

err.Details(): gojsonschema.ErrorDetails Returns a map[string]interface{} of additional error details specific to the error. For example, GTE errors will have a "min" value, LTE will have a "max" value. See errors.go for a full description of all the error details. Every error always contains a "field" key that holds the value of err.Field()

Note in most cases, the err.Details() will be used to generate replacement strings in your locales, and not used directly. These strings follow the text/template format i.e.

{{.field}} must be greater than or equal to {{.min}}

The library allows you to specify custom template functions, should you require more complex error message handling.

gojsonschema.ErrorTemplateFuncs = map[string]interface{}{
	"allcaps": func(s string) string {
		return strings.ToUpper(s)

Given the above definition, you can use the custom function "allcaps" in your localization templates:

{{allcaps .field}} must be greater than or equal to {{.min}}

The above error message would then be rendered with the field value in capital letters. For example:

"PASSWORD must be greater than or equal to 8"

Learn more about what types of template functions you can use in ErrorTemplateFuncs by referring to Go's text/template FuncMap type.


JSON Schema allows for optional "format" property to validate instances against well-known formats. gojsonschema ships with all of the formats defined in the spec that you can use like this:

{"type": "string", "format": "email"}

Available formats: date-time, hostname, email, ipv4, ipv6, uri, uri-reference.

For repetitive or more complex formats, you can create custom format checkers and add them to gojsonschema like this:

// Define the format checker
type RoleFormatChecker struct {}

// Ensure it meets the gojsonschema.FormatChecker interface
func (f RoleFormatChecker) IsFormat(input interface{}) bool {

    asString, ok := input.(string)
    if ok == false {
        return false

    return strings.HasPrefix("ROLE_", asString)

// Add it to the library
gojsonschema.FormatCheckers.Add("role", RoleFormatChecker{})

Now to use in your json schema:

{"type": "string", "format": "role"}

Another example would be to check if the provided integer matches an id on database:

JSON schema:

{"type": "integer", "format": "ValidUserId"}
// Define the format checker
type ValidUserIdFormatChecker struct {}

// Ensure it meets the gojsonschema.FormatChecker interface
func (f ValidUserIdFormatChecker) IsFormat(input interface{}) bool {

    asFloat64, ok := input.(float64) // Numbers are always float64 here
    if ok == false {
        return false

    // XXX
    // do the magic on the database looking for the int(asFloat64)

    return true

// Add it to the library
gojsonschema.FormatCheckers.Add("ValidUserId", ValidUserIdFormatChecker{})

Additional custom validation

After the validation has run and you have the results, you may add additional errors using Result.AddError. This is useful to maintain the same format within the resultset instead of having to add special exceptions for your own errors. Below is an example.

type AnswerInvalidError struct {

func newAnswerInvalidError(context *gojsonschema.JsonContext, value interface{}, details gojsonschema.ErrorDetails) *AnswerInvalidError {
    err := AnswerInvalidError{}
    // it is important to use SetDescriptionFormat() as this is used to call SetDescription() after it has been parsed
    // using the description of err will be overridden by this.
    err.SetDescriptionFormat("Answer to the Ultimate Question of Life, the Universe, and Everything is {{.answer}}")

    return &err

func main() {
    // ...
    schema, err := gojsonschema.NewSchema(schemaLoader)
    result, err := gojsonschema.Validate(schemaLoader, documentLoader)

    if true { // some validation
        jsonContext := gojsonschema.NewJsonContext("question", nil)
        errDetail := gojsonschema.ErrorDetails{
            "answer": 42,
                gojsonschema.NewJsonContext("answer", jsonContext),

    return result, err


This is especially useful if you want to add validation beyond what the json schema drafts can provide such business specific logic.


gojsonschema uses the following test suite :