JSON Schema for Go (google/jsonschema-go)

JSON Schema for Go

https://github.com/google/jsonschema-go
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This module implements the JSON Schema specification, supporting schema creation, JSON validation...

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# jsonschema-go

`jsonschema-go` is a Go implementation of the [JSON Schema specification](https://json-schema.org/), developed by Google. The library provides three core capabilities: constructing JSON Schema objects as Go structs, validating JSON values against a schema, and inferring a schema automatically from a Go type. It supports both the Draft 2020-12 and Draft-07 specifications and has no external dependencies outside the Go standard library.

The package is centered on the `Schema` struct, which maps directly to JSON Schema's object model. Schemas may be built programmatically, loaded from JSON via `encoding/json`, or derived from Go structs using the `For`/`ForType` inference functions. Before a schema can validate instances, it must first be resolved (via `Schema.Resolve`) to produce a `Resolved` value—this step validates the schema itself, resolves `$ref`/`$dynamicRef` pointers, and compiles regular expressions. The `Resolved.Validate` method then performs draft-compliant validation against any Go value that represents a JSON document.

---

## API Reference

### `Schema` struct — core JSON Schema object

`Schema` is the Go representation of a JSON Schema object, supporting all Draft-07 and Draft 2020-12 keywords. Fields map directly to their JSON counterparts. `Type` and `Types` are mutually exclusive. Unknown keywords are preserved in the `Extra` map to allow round-tripping.

```go
package main

import (
    "encoding/json"
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    // Build a schema programmatically
    minAge := 0.0
    maxAge := 150.0
    minName := 1
    s := &jsonschema.Schema{
        Schema:      "https://json-schema.org/draft/2020-12/schema",
        Title:       "Person",
        Description: "A person object",
        Type:        "object",
        Properties: map[string]*jsonschema.Schema{
            "name": {
                Type:      "string",
                MinLength: jsonschema.Ptr(minName),
            },
            "age": {
                Type:    "integer",
                Minimum: &minAge,
                Maximum: &maxAge,
            },
            "email": {
                Type:    "string",
                Pattern: `^[^@]+@[^@]+$`,
            },
        },
        Required: []string{"name", "age"},
        AdditionalProperties: &jsonschema.Schema{Not: &jsonschema.Schema{}}, // false
    }

    // Serialize to JSON
    data, err := json.MarshalIndent(s, "", "  ")
    if err != nil {
        panic(err)
    }
    fmt.Println(string(data))
    // Output (abbreviated):
    // {
    //   "$schema": "https://json-schema.org/draft/2020-12/schema",
    //   "title": "Person",
    //   "type": "object",
    //   "properties": { "name": {...}, "age": {...}, "email": {...} },
    //   "required": ["name","age"],
    //   "additionalProperties": false
    // }

    // Load a schema from JSON
    raw := `{"type":"string","minLength":3,"maxLength":20}`
    var loaded jsonschema.Schema
    if err := json.Unmarshal([]byte(raw), &loaded); err != nil {
        panic(err)
    }
    fmt.Println(loaded.Type) // string
}
```

---

### `Schema.Resolve` — resolve references and prepare for validation

`Resolve` validates the schema against its meta-schema, resolves all `$ref` and `$dynamicRef` pointers, compiles regular expressions, and optionally validates `default` values. It returns a `*Resolved` ready for use with `Validate`. Pass `nil` opts for defaults (no base URI, no loader, no default validation).

```go
package main

import (
    "encoding/json"
    "fmt"
    "net/url"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    // --- Simple resolution (no external refs) ---
    s := &jsonschema.Schema{
        Type: "object",
        Properties: map[string]*jsonschema.Schema{
            "id":    {Type: "integer"},
            "label": {Type: "string"},
        },
        Required: []string{"id"},
    }
    resolved, err := s.Resolve(nil) // nil opts = defaults
    if err != nil {
        panic(err) // schema is structurally invalid
    }
    _ = resolved

    // --- Resolution with a custom Loader for remote $refs ---
    schemaWithRef := &jsonschema.Schema{
        Schema: "https://json-schema.org/draft/2020-12/schema",
        Defs: map[string]*jsonschema.Schema{
            "address": {
                Type: "object",
                Properties: map[string]*jsonschema.Schema{
                    "street": {Type: "string"},
                    "city":   {Type: "string"},
                },
            },
        },
        Ref: "#/$defs/address",
    }
    opts := &jsonschema.ResolveOptions{
        BaseURI: "https://example.com/schemas/person.json",
        // Loader is called when a $ref points to an external URI
        Loader: func(u *url.URL) (*jsonschema.Schema, error) {
            // Fetch and unmarshal the schema at u
            var remote jsonschema.Schema
            // ... http.Get(u.String()) and json.Unmarshal ...
            return &remote, nil
        },
        ValidateDefaults: true, // verify "default" values pass schema validation
    }
    resolved2, err := schemaWithRef.Resolve(opts)
    if err != nil {
        fmt.Println("resolution error:", err)
        return
    }
    fmt.Println("schema:", resolved2.Schema().Title)
}
```

---

### `Resolved.Validate` — validate a JSON value against a resolved schema

`Validate` checks any Go value that represents JSON (the result of `json.Unmarshal` into `any`, or a `map[string]any`) against the resolved schema. Returns `nil` on success or a descriptive error on failure.

```go
package main

import (
    "encoding/json"
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    // Define and resolve a schema
    s := &jsonschema.Schema{
        Schema: "https://json-schema.org/draft/2020-12/schema",
        Type:   "object",
        Properties: map[string]*jsonschema.Schema{
            "username": {Type: "string", MinLength: jsonschema.Ptr(3)},
            "score":    {Type: "integer", Minimum: jsonschema.Ptr(0.0), Maximum: jsonschema.Ptr(100.0)},
            "tags":     {Types: []string{"null", "array"}, Items: &jsonschema.Schema{Type: "string"}},
        },
        Required: []string{"username", "score"},
    }
    resolved, err := s.Resolve(nil)
    if err != nil {
        panic(err)
    }

    // Valid instance (from JSON unmarshal)
    goodJSON := `{"username": "alice", "score": 95, "tags": ["go", "schema"]}`
    var good any
    json.Unmarshal([]byte(goodJSON), &good)
    if err := resolved.Validate(good); err != nil {
        fmt.Println("unexpected error:", err)
    } else {
        fmt.Println("valid!") // valid!
    }

    // Invalid: score out of range
    badJSON := `{"username": "bob", "score": 200}`
    var bad any
    json.Unmarshal([]byte(badJSON), &bad)
    if err := resolved.Validate(bad); err != nil {
        fmt.Println("validation error:", err)
        // validation error: validating root: maximum: 200 is greater than 100.000000
    }

    // Invalid: missing required field
    var missingField any
    json.Unmarshal([]byte(`{"username":"carol"}`), &missingField)
    if err := resolved.Validate(missingField); err != nil {
        fmt.Println("missing field error:", err)
        // missing field error: validating root: required: missing properties: ["score"]
    }
}
```

---

### `Resolved.ApplyDefaults` — apply schema defaults to an instance map

`ApplyDefaults` walks the schema's `properties` and fills in any missing optional properties in the instance with their `default` values. Only works on `map[string]any` instances (not structs). The argument must be a pointer to the instance.

```go
package main

import (
    "encoding/json"
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    schemaJSON := `{
        "type": "object",
        "properties": {
            "host":    {"type": "string", "default": "localhost"},
            "port":    {"type": "integer", "default": 8080},
            "debug":   {"type": "boolean", "default": false},
            "timeout": {"type": "integer"}
        },
        "required": ["timeout"]
    }`
    var s jsonschema.Schema
    if err := json.Unmarshal([]byte(schemaJSON), &s); err != nil {
        panic(err)
    }
    resolved, err := s.Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
    if err != nil {
        panic(err)
    }

    // Instance with only the required field present
    instance := map[string]any{"timeout": 30}

    if err := resolved.ApplyDefaults(&instance); err != nil {
        panic(err)
    }

    fmt.Println(instance["host"])  // localhost
    fmt.Println(instance["port"])  // 8080
    fmt.Println(instance["debug"]) // false
    fmt.Println(instance["timeout"]) // 30 (required; no default applied)

    // Validate after applying defaults
    if err := resolved.Validate(instance); err != nil {
        fmt.Println("error:", err)
    } else {
        fmt.Println("valid after defaults applied!")
    }
}
```

---

### `For[T]` — infer a schema from a Go generic type

`For[T]` uses reflection to construct a `*Schema` describing the Go type `T`. JSON tags control property names and optional fields; `jsonschema` tags set descriptions. Pointer types allow `null`. Standard library types like `time.Time` and `big.Int` map to their JSON serialization schemas.

```go
package main

import (
    "encoding/json"
    "fmt"
    "time"

    "github.com/google/jsonschema-go/jsonschema"
)

type Address struct {
    Street string `json:"street" jsonschema:"street address line"`
    City   string `json:"city"   jsonschema:"city name"`
    Zip    string `json:"zip"    jsonschema:"postal code"`
}

type User struct {
    ID        int64     `json:"id"                   jsonschema:"unique user identifier"`
    Username  string    `json:"username"             jsonschema:"login name, 3-20 chars"`
    Email     *string   `json:"email,omitempty"      jsonschema:"contact email"`
    CreatedAt time.Time `json:"created_at"           jsonschema:"creation timestamp"`
    Address   Address   `json:"address"              jsonschema:"mailing address"`
    Tags      []string  `json:"tags,omitempty"`
    Score     float64   `json:"score,omitzero"`
}

func main() {
    s, err := jsonschema.For[User](nil)
    if err != nil {
        panic(err)
    }

    data, _ := json.MarshalIndent(s, "", "  ")
    fmt.Println(string(data))
    // {
    //   "type": "object",
    //   "properties": {
    //     "id":         {"type": "integer", "description": "unique user identifier"},
    //     "username":   {"type": "string",  "description": "login name, 3-20 chars"},
    //     "email":      {"type": ["null","string"], "description": "contact email"},
    //     "created_at": {"type": "string",  "description": "creation timestamp"},
    //     "address":    {"type": "object",  "properties": {...}, "required": ["street","city","zip"], ...},
    //     "tags":       {"type": ["null","array"], "items": {"type":"string"}},
    //     "score":      {"type": "number"}
    //   },
    //   "required": ["id","username","created_at","address"],
    //   "additionalProperties": false
    // }

    // Resolve and validate
    resolved, err := s.Resolve(nil)
    if err != nil {
        panic(err)
    }
    instance := map[string]any{
        "id":         float64(42),
        "username":   "alice",
        "created_at": "2024-01-01T00:00:00Z",
        "address": map[string]any{
            "street": "123 Main St",
            "city":   "Springfield",
            "zip":    "12345",
        },
    }
    if err := resolved.Validate(instance); err != nil {
        fmt.Println("error:", err)
    } else {
        fmt.Println("valid!")
    }
}
```

---

### `ForType` — infer a schema from a `reflect.Type`

`ForType` is the reflection-based variant of `For`, accepting a `reflect.Type` at runtime. It supports the same `ForOptions` including `TypeSchemas` (overrides for specific types) and `IgnoreInvalidTypes` (skip unsupported types like `func` or `chan` rather than returning an error).

```go
package main

import (
    "encoding/json"
    "fmt"
    "reflect"

    "github.com/google/jsonschema-go/jsonschema"
)

type Status int

type Event struct {
    Name   string
    Status Status
    Meta   map[string]any
    Skip   func() // will be ignored
}

func main() {
    // Provide a custom schema for Status (otherwise it would be "integer")
    opts := &jsonschema.ForOptions{
        IgnoreInvalidTypes: true, // ignore func fields
        TypeSchemas: map[reflect.Type]*jsonschema.Schema{
            reflect.TypeFor[Status](): {
                Type: "string",
                Enum: []any{"pending", "active", "closed"},
            },
        },
    }

    t := reflect.TypeOf(Event{})
    s, err := jsonschema.ForType(t, opts)
    if err != nil {
        panic(err)
    }

    data, _ := json.MarshalIndent(s, "", "  ")
    fmt.Println(string(data))
    // {
    //   "type": "object",
    //   "properties": {
    //     "Name":   {"type": "string"},
    //     "Status": {"type": "string", "enum": ["pending","active","closed"]},
    //     "Meta":   {"type": "object", "additionalProperties": {}}
    //   },
    //   "required": ["Name","Status","Meta"],
    //   "additionalProperties": false
    // }
}
```

---

### `Schema.CloneSchemas` — deep-copy a schema tree

`CloneSchemas` returns a deep copy of a `Schema`, recursively cloning all sub-schemas. The clone is independent of the original: sub-schemas may be shared as fields of different parent schemas without violating the tree requirement imposed by `Resolve`.

```go
package main

import (
    "encoding/json"
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    base := &jsonschema.Schema{
        Type: "object",
        Properties: map[string]*jsonschema.Schema{
            "id":   {Type: "integer"},
            "name": {Type: "string"},
        },
        Required: []string{"id"},
    }

    // Clone and extend without affecting the original
    extended := base.CloneSchemas()
    extended.Properties["email"] = &jsonschema.Schema{Type: "string"}
    extended.Required = append(extended.Required, "name")

    origData, _ := json.Marshal(base)
    extData, _ := json.Marshal(extended)
    fmt.Println("original:", string(origData))
    // original: {"type":"object","properties":{"id":...,"name":...},"required":["id"]}
    fmt.Println("extended:", string(extData))
    // extended: {"type":"object","properties":{"id":...,"name":...,"email":...},"required":["id","name"]}

    // Use the same base in an allOf without sharing pointers (required by Resolve)
    parent := &jsonschema.Schema{
        AllOf: []*jsonschema.Schema{
            base.CloneSchemas(),
            base.CloneSchemas(),
        },
    }
    if _, err := parent.Resolve(nil); err != nil {
        fmt.Println("error:", err) // would error if sub-schemas were shared pointers
    } else {
        fmt.Println("resolved successfully")
    }
}
```

---

### `Equal` — JSON-semantic equality for Go values

`Equal` compares two Go values as JSON values according to the JSON Schema specification. Unlike `reflect.DeepEqual`, numbers are compared mathematically (so `int(1)` equals `float64(1.0)`).

```go
package main

import (
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    // Numeric equality across types
    fmt.Println(jsonschema.Equal(1, 1.0))          // true  (int == float64)
    fmt.Println(jsonschema.Equal(int8(42), 42.0))  // true
    fmt.Println(jsonschema.Equal(1, 2))            // false

    // Structural equality
    a := map[string]any{"x": 1, "y": []any{true, "hello"}}
    b := map[string]any{"x": 1.0, "y": []any{true, "hello"}}
    fmt.Println(jsonschema.Equal(a, b)) // true

    // Slice equality
    fmt.Println(jsonschema.Equal([]any{1, 2, 3}, []any{1.0, 2.0, 3.0})) // true
    fmt.Println(jsonschema.Equal([]any{1, 2}, []any{1, 2, 3}))           // false
}
```

---

### `Ptr[T]` — generic pointer helper

`Ptr` returns a pointer to a new variable holding the given value. Used when schema fields like `MinLength`, `MaxItems`, `Minimum`, etc., require `*int` or `*float64`.

```go
package main

import (
    "encoding/json"
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    s := &jsonschema.Schema{
        Type:      "string",
        MinLength: jsonschema.Ptr(5),
        MaxLength: jsonschema.Ptr(100),
    }

    numSchema := &jsonschema.Schema{
        Type:             "number",
        Minimum:          jsonschema.Ptr(0.0),
        Maximum:          jsonschema.Ptr(1.0),
        ExclusiveMaximum: jsonschema.Ptr(1.0),
        MultipleOf:       jsonschema.Ptr(0.01),
    }

    arrSchema := &jsonschema.Schema{
        Types:    []string{"null", "array"},
        Items:    &jsonschema.Schema{Type: "integer"},
        MinItems: jsonschema.Ptr(1),
        MaxItems: jsonschema.Ptr(10),
    }

    for _, sc := range []*jsonschema.Schema{s, numSchema, arrSchema} {
        data, _ := json.Marshal(sc)
        fmt.Println(string(data))
    }
    // {"type":"string","minLength":5,"maxLength":100}
    // {"type":"number","minimum":0,"maximum":1,"exclusiveMaximum":1,"multipleOf":0.01}
    // {"type":["null","array"],"items":{"type":"integer"},"minItems":1,"maxItems":10}
}
```

---

### `Resolved.Schema` — retrieve the original schema from a resolved value

`Schema()` returns the root `*Schema` that was resolved. It must not be modified after resolution.

```go
package main

import (
    "fmt"

    "github.com/google/jsonschema-go/jsonschema"
)

func main() {
    s := &jsonschema.Schema{
        Schema: "https://json-schema.org/draft/2020-12/schema",
        Title:  "Config",
        Type:   "object",
        Properties: map[string]*jsonschema.Schema{
            "debug": {Type: "boolean"},
        },
    }
    resolved, err := s.Resolve(nil)
    if err != nil {
        panic(err)
    }

    original := resolved.Schema()
    fmt.Println(original.Title)  // Config
    fmt.Println(original.Schema) // https://json-schema.org/draft/2020-12/schema
}
```

---

## Summary

`jsonschema-go` is well-suited for two primary use cases. The first is **input validation in APIs and services**: unmarshal or programmatically construct a schema once, resolve it, then call `Validate` concurrently against any number of incoming JSON payloads. The library's schema resolution is safe to share across goroutines, and `Validate` accepts plain `map[string]any` values from `json.Unmarshal`, making it trivial to slot into existing HTTP handlers or message processors. `ApplyDefaults` complements validation by filling in missing optional fields before processing, enabling schema-driven configuration patterns.

The second major use case is **code generation and schema tooling**: use `For[T]` or `ForType` to derive a JSON Schema from existing Go types—useful for generating OpenAPI specs, documentation, or validation schemas from a canonical Go data model. The `PropertyOrder` field preserves struct field ordering in the serialized JSON, which matters for schema registries and generated documentation. `CloneSchemas` enables safely combining or extending base schemas into composed schemas without pointer aliasing. Round-tripping through `json.Marshal`/`json.Unmarshal` is fully supported, and unknown keywords survive via the `Extra` map, so the library integrates cleanly with schema registries and external toolchains.