An implementation of JSON Schema for the Go programming language. Supports draft-04, draft-06 and draft-07.

References :

• http://json-schema.org
• http://json-schema.org/latest/json-schema-core.html
• http://json-schema.org/latest/json-schema-validation.html

go get github.com/xeipuuv/gojsonschema

Dependencies :

• github.com/xeipuuv/gojsonpointer
• github.com/xeipuuv/gojsonreference
• github.com/stretchr/testify/assert

package main

import (
    "fmt"
    "github.com/xeipuuv/gojsonschema"
)

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 {
        panic(err.Error())
    }

    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("http://www.some_host.com/schema.json")

• 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)

And

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)
        }
    }

By default file and http(s) references to external schemas are loaded automatically via the file system or via http(s). An external schema can also be loaded using a SchemaLoader.

	sl := gojsonschema.NewSchemaLoader()
	loader1 := gojsonschema.NewStringLoader(`{ "type" : "string" }`)
	err := sl.AddSchema("http://some_host.com/string.json", loader1)

Alternatively if your schema already has an $id you can use the AddSchemas function

	loader2 := gojsonschema.NewStringLoader(`{
			"$id" : "http://some_host.com/maxlength.json",
			"maxLength" : 5
		}`)
	err = sl.AddSchemas(loader2)

The main schema should be passed to the Compile function. This main schema can then directly reference the added schemas without needing to download them.

	loader3 := gojsonschema.NewStringLoader(`{
		"$id" : "http://some_host.com/main.json",
		"allOf" : [
			{ "$ref" : "http://some_host.com/string.json" },
			{ "$ref" : "http://some_host.com/maxlength.json" }
		]
	}`)

	schema, err := sl.Compile(loader3)

	documentLoader := gojsonschema.NewStringLoader(`"hello world"`)

	result, err := schema.Validate(documentLoader)

It's also possible to pass a ReferenceLoader to the Compile function that references a loaded schema.

err = sl.AddSchemas(loader3)
schema, err := sl.Compile(gojsonschema.NewReferenceLoader("http://some_host.com/main.json"))

Schemas added by AddSchema and AddSchemas are only validated when the entire schema is compiled, unless meta-schema validation is used.

By default gojsonschema will try to detect the draft of a schema by using the $schema keyword and parse it in a strict draft-04, draft-06 or draft-07 mode. If $schema is missing, or the draft version is not explicitely set, a hybrid mode is used which merges together functionality of all drafts into one mode.

Autodectection can be turned off with the AutoDetect property. Specific draft versions can be specified with the Draft property.

sl := gojsonschema.NewSchemaLoader()
sl.Draft = gojsonschema.Draft7
sl.AutoDetect = false

If autodetection is on (default), a draft-07 schema can savely reference draft-04 schemas and vice-versa, as long as $schema is specified in all schemas.

Schemas that are added using the AddSchema, AddSchemas and Compile can be validated against their meta-schema by setting the Validate property.

The following example will produce an error as multipleOf must be a number. If Validate is off (default), this error is only returned at the Compile step.

sl := gojsonschema.NewSchemaLoader()
sl.Validate = true
err := sl.AddSchemas(gojsonschema.NewStringLoader(`{
     "$id" : "http://some_host.com/invalid.json",
    "$schema": "http://json-schema.org/draft-07/schema#",
    "multipleOf" : true
}`))

Errors returned by meta-schema validation are more readable and contain more information, which helps significantly if you are developing a schema.

Meta-schema validation also works with a custom $schema. In case $schema is missing, or AutoDetect is set to false, the meta-schema of the used draft is used.

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.

Newer versions of gojsonschema may have new additional errors, so code that uses a custom locale will need to be updated when this happens.

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
"const": ConstEror
"enum": EnumError
"array_no_additional_items": ArrayNoAdditionalItemsError
"array_min_items": ArrayMinItemsError
"array_max_items": ArrayMaxItemsError
"unique": ItemsMustBeUniqueError
"contains" : ArrayContainsError
"array_min_properties": ArrayMinPropertiesError
"array_max_properties": ArrayMaxPropertiesError
"additional_property_not_allowed": AdditionalPropertyNotAllowedError
"invalid_property_pattern": InvalidPropertyPatternError
"invalid_property_name":  InvalidPropertyNameError
"string_gte": StringLengthGTEError
"string_lte": StringLengthLTEError
"pattern": DoesNotMatchPatternError
"multiple_of": MultipleOfError
"number_gte": NumberGTEError
"number_gt": NumberGTError
"number_lte": NumberLTEError
"number_lt": NumberLTError
"condition_then" : ConditionThenError
"condition_else" : ConditionElseError

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"}

Not all formats defined in draft-07 are available. Implemented formats are:

• date
• time
• date-time
• hostname. Subdomains that start with a number are also supported, but this means that it doesn't strictly follow RFC1034 and has the implication that ipv4 addresses are also recognized as valid hostnames.
• email. Go's email parser deviates slightly from RFC5322. Includes unicode support.
• idn-email. Same caveat as email.
• ipv4
• ipv6
• uri. Includes unicode support.
• uri-reference. Includes unicode support.
• iri
• iri-reference
• uri-template
• uuid
• regex. Go uses the RE2 engine and is not ECMA262 compatible.
• json-pointer
• relative-json-pointer

email, uri and uri-reference use the same validation code as their unicode counterparts idn-email, iri and iri-reference. If you rely on unicode support you should use the specific unicode enabled formats for the sake of interoperability as other implementations might not support unicode in the regular formats.

The validation code for uri, idn-email and their relatives use mostly standard library code.

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{})

Formats can also be removed, for example if you want to override one of the formats that is defined by default.

gojsonschema.FormatCheckers.Remove("hostname")

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 {
    gojsonschema.ResultErrorFields
}

func newAnswerInvalidError(context *gojsonschema.JsonContext, value interface{}, details gojsonschema.ErrorDetails) *AnswerInvalidError {
    err := AnswerInvalidError{}
    err.SetContext(context)
    err.SetType("custom_invalid_error")
    // 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}}")
    err.SetValue(value)
    err.SetDetails(details)

    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,
        }
        result.AddError(
            newAnswerInvalidError(
                gojsonschema.NewJsonContext("answer", jsonContext),
                52,
                errDetail,
            ),
            errDetail,
        )
    }

    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 :

https://github.com/json-schema/JSON-Schema-Test-Suite