Involved Source Filesv2_decode.go Package json implements encoding and decoding of JSON as defined in
RFC 7159. The mapping between JSON and Go values is described
in the documentation for the Marshal and Unmarshal functions.
See "JSON and Go" for an introduction to this package:
https://golang.org/doc/articles/json_and_go.htmlv2_indent.gov2_inject.go Migrating to v2
This package (i.e., [encoding/json]) is now formally known as the v1 package
since a v2 package now exists at [encoding/json/v2].
All the behavior of the v1 package is implemented in terms of
the v2 package with the appropriate set of options specified that
preserve the historical behavior of v1.
The [jsonv2.Marshal] function is the newer equivalent of v1 [Marshal].
The [jsonv2.Unmarshal] function is the newer equivalent of v1 [Unmarshal].
The v2 functions have the same calling signature as the v1 equivalent
except that they take in variadic [Options] arguments that can be specified
to alter the behavior of marshal or unmarshal. Both v1 and v2 generally
behave in similar ways, but there are some notable differences.
The following is a list of differences between v1 and v2:
- In v1, JSON object members are unmarshaled into a Go struct using a
case-insensitive name match with the JSON name of the fields.
In contrast, v2 matches fields using an exact, case-sensitive match.
The [jsonv2.MatchCaseInsensitiveNames] and [MatchCaseSensitiveDelimiter]
options control this behavior difference. To explicitly specify a Go struct
field to use a particular name matching scheme, either the `case:ignore`
or the `case:strict` field option can be specified.
Field-specified options take precedence over caller-specified options.
- In v1, when marshaling a Go struct, a field marked as `omitempty`
is omitted if the field value is an "empty" Go value, which is defined as
false, 0, a nil pointer, a nil interface value, and
any empty array, slice, map, or string. In contrast, v2 redefines
`omitempty` to omit a field if it encodes as an "empty" JSON value,
which is defined as a JSON null, or an empty JSON string, object, or array.
The [OmitEmptyWithLegacyDefinition] option controls this behavior difference.
Note that `omitempty` behaves identically in both v1 and v2 for a
Go array, slice, map, or string (assuming no user-defined MarshalJSON method
overrides the default representation). Existing usages of `omitempty` on a
Go bool, number, pointer, or interface value should migrate to specifying
`omitzero` instead (which is identically supported in both v1 and v2).
- In v1, a Go struct field marked as `string` can be used to quote a
Go string, bool, or number as a JSON string. It does not recursively
take effect on composite Go types. In contrast, v2 restricts
the `string` option to only quote a Go number as a JSON string.
It does recursively take effect on Go numbers within a composite Go type.
The [StringifyWithLegacySemantics] option controls this behavior difference.
- In v1, a nil Go slice or Go map is marshaled as a JSON null.
In contrast, v2 marshals a nil Go slice or Go map as
an empty JSON array or JSON object, respectively.
The [jsonv2.FormatNilSliceAsNull] and [jsonv2.FormatNilMapAsNull] options
control this behavior difference. To explicitly specify a Go struct field
to use a particular representation for nil, either the `format:emitempty`
or `format:emitnull` field option can be specified.
Field-specified options take precedence over caller-specified options.
- In v1, a Go array may be unmarshaled from a JSON array of any length.
In contrast, in v2 a Go array must be unmarshaled from a JSON array
of the same length, otherwise it results in an error.
The [UnmarshalArrayFromAnyLength] option controls this behavior difference.
- In v1, a Go byte array is represented as a JSON array of JSON numbers.
In contrast, in v2 a Go byte array is represented as a Base64-encoded JSON string.
The [FormatBytesWithLegacySemantics] option controls this behavior difference.
To explicitly specify a Go struct field to use a particular representation,
either the `format:array` or `format:base64` field option can be specified.
Field-specified options take precedence over caller-specified options.
- In v1, MarshalJSON methods declared on a pointer receiver are only called
if the Go value is addressable. In contrast, in v2 a MarshalJSON method
is always callable regardless of addressability.
The [CallMethodsWithLegacySemantics] option controls this behavior difference.
- In v1, MarshalJSON and UnmarshalJSON methods are never called for Go map keys.
In contrast, in v2 a MarshalJSON or UnmarshalJSON method is eligible for
being called for Go map keys.
The [CallMethodsWithLegacySemantics] option controls this behavior difference.
- In v1, a Go map is marshaled in a deterministic order.
In contrast, in v2 a Go map is marshaled in a non-deterministic order.
The [jsonv2.Deterministic] option controls this behavior difference.
- In v1, JSON strings are encoded with HTML-specific or JavaScript-specific
characters being escaped. In contrast, in v2 JSON strings use the minimal
encoding and only escape if required by the JSON grammar.
The [jsontext.EscapeForHTML] and [jsontext.EscapeForJS] options
control this behavior difference.
- In v1, bytes of invalid UTF-8 within a string are silently replaced with
the Unicode replacement character. In contrast, in v2 the presence of
invalid UTF-8 results in an error. The [jsontext.AllowInvalidUTF8] option
controls this behavior difference.
- In v1, a JSON object with duplicate names is permitted.
In contrast, in v2 a JSON object with duplicate names results in an error.
The [jsontext.AllowDuplicateNames] option controls this behavior difference.
- In v1, when unmarshaling a JSON null into a non-empty Go value it will
inconsistently either zero out the value or do nothing.
In contrast, in v2 unmarshaling a JSON null will consistently and always
zero out the underlying Go value. The [MergeWithLegacySemantics] option
controls this behavior difference.
- In v1, when unmarshaling a JSON value into a non-zero Go value,
it merges into the original Go value for array elements, slice elements,
struct fields (but not map values),
pointer values, and interface values (only if a non-nil pointer).
In contrast, in v2 unmarshal merges into the Go value
for struct fields, map values, pointer values, and interface values.
In general, the v2 semantic merges when unmarshaling a JSON object,
otherwise it replaces the value. The [MergeWithLegacySemantics] option
controls this behavior difference.
- In v1, a [time.Duration] is represented as a JSON number containing
the decimal number of nanoseconds. In contrast, in v2 a [time.Duration]
is represented as a JSON string containing the formatted duration
(e.g., "1h2m3.456s") according to [time.Duration.String].
The [FormatTimeWithLegacySemantics] option controls this behavior difference.
To explicitly specify a Go struct field to use a particular representation,
either the `format:nano` or `format:units` field option can be specified.
Field-specified options take precedence over caller-specified options.
- In v1, errors are never reported at runtime for Go struct types
that have some form of structural error (e.g., a malformed tag option).
In contrast, v2 reports a runtime error for Go types that are invalid
as they relate to JSON serialization. For example, a Go struct
with only unexported fields cannot be serialized.
The [ReportErrorsWithLegacySemantics] option controls this behavior difference.
As mentioned, the entirety of v1 is implemented in terms of v2,
where options are implicitly specified to opt into legacy behavior.
For example, [Marshal] directly calls [jsonv2.Marshal] with [DefaultOptionsV1].
Similarly, [Unmarshal] directly calls [jsonv2.Unmarshal] with [DefaultOptionsV1].
The [DefaultOptionsV1] option represents the set of all options that specify
default v1 behavior.
For many of the behavior differences, there are Go struct field options
that the author of a Go type can specify to control the behavior such that
the type is represented identically in JSON under either v1 or v2 semantics.
The availability of [DefaultOptionsV1] and [jsonv2.DefaultOptionsV2],
where later options take precedence over former options allows for
a gradual migration from v1 to v2. For example:
- jsonv1.Marshal(v)
uses default v1 semantics.
- jsonv2.Marshal(v, jsonv1.DefaultOptionsV1())
is semantically equivalent to jsonv1.Marshal
and thus uses default v1 semantics.
- jsonv2.Marshal(v, jsonv1.DefaultOptionsV1(), jsontext.AllowDuplicateNames(false))
uses mostly v1 semantics, but opts into one particular v2-specific behavior.
- jsonv2.Marshal(v, jsonv1.CallMethodsWithLegacySemantics(true))
uses mostly v2 semantics, but opts into one particular v1-specific behavior.
- jsonv2.Marshal(v, ..., jsonv2.DefaultOptionsV2())
is semantically equivalent to jsonv2.Marshal since
jsonv2.DefaultOptionsV2 overrides any options specified earlier
and thus uses default v2 semantics.
- jsonv2.Marshal(v)
uses default v2 semantics.
All new usages of "json" in Go should use the v2 package,
but the v1 package will forever remain supported.v2_scanner.gov2_stream.go
Code Examples
package main
import (
"encoding/json"
"fmt"
"io"
"log"
"strings"
)
func main() {
const jsonStream = `
{"Name": "Ed", "Text": "Knock knock."}
{"Name": "Sam", "Text": "Who's there?"}
{"Name": "Ed", "Text": "Go fmt."}
{"Name": "Sam", "Text": "Go fmt who?"}
{"Name": "Ed", "Text": "Go fmt yourself!"}
`
type Message struct {
Name, Text string
}
dec := json.NewDecoder(strings.NewReader(jsonStream))
for {
var m Message
if err := dec.Decode(&m); err == io.EOF {
break
} else if err != nil {
log.Fatal(err)
}
fmt.Printf("%s: %s\n", m.Name, m.Text)
}
}
package main
import (
"encoding/json"
"fmt"
"log"
"strings"
)
func main() {
const jsonStream = `
[
{"Name": "Ed", "Text": "Knock knock."},
{"Name": "Sam", "Text": "Who's there?"},
{"Name": "Ed", "Text": "Go fmt."},
{"Name": "Sam", "Text": "Go fmt who?"},
{"Name": "Ed", "Text": "Go fmt yourself!"}
]
`
type Message struct {
Name, Text string
}
dec := json.NewDecoder(strings.NewReader(jsonStream))
// read open bracket
t, err := dec.Token()
if err != nil {
log.Fatal(err)
}
fmt.Printf("%T: %v\n", t, t)
// while the array contains values
for dec.More() {
var m Message
// decode an array value (Message)
err := dec.Decode(&m)
if err != nil {
log.Fatal(err)
}
fmt.Printf("%v: %v\n", m.Name, m.Text)
}
// read closing bracket
t, err = dec.Token()
if err != nil {
log.Fatal(err)
}
fmt.Printf("%T: %v\n", t, t)
}
package main
import (
"encoding/json"
"fmt"
"io"
"log"
"strings"
)
func main() {
const jsonStream = `
{"Message": "Hello", "Array": [1, 2, 3], "Null": null, "Number": 1.234}
`
dec := json.NewDecoder(strings.NewReader(jsonStream))
for {
t, err := dec.Token()
if err == io.EOF {
break
}
if err != nil {
log.Fatal(err)
}
fmt.Printf("%T: %v", t, t)
if dec.More() {
fmt.Printf(" (more)")
}
fmt.Printf("\n")
}
}
package main
import (
"bytes"
"encoding/json"
"os"
)
func main() {
var out bytes.Buffer
json.HTMLEscape(&out, []byte(`{"Name":"<b>HTML content</b>"}`))
out.WriteTo(os.Stdout)
}
package main
import (
"bytes"
"encoding/json"
"log"
"os"
)
func main() {
type Road struct {
Name string
Number int
}
roads := []Road{
{"Diamond Fork", 29},
{"Sheep Creek", 51},
}
b, err := json.Marshal(roads)
if err != nil {
log.Fatal(err)
}
var out bytes.Buffer
json.Indent(&out, b, "=", "\t")
out.WriteTo(os.Stdout)
}
package main
import (
"encoding/json"
"fmt"
"os"
)
func main() {
type ColorGroup struct {
ID int
Name string
Colors []string
}
group := ColorGroup{
ID: 1,
Name: "Reds",
Colors: []string{"Crimson", "Red", "Ruby", "Maroon"},
}
b, err := json.Marshal(group)
if err != nil {
fmt.Println("error:", err)
}
os.Stdout.Write(b)
}
package main
import (
"encoding/json"
"fmt"
"log"
)
func main() {
data := map[string]int{
"a": 1,
"b": 2,
}
b, err := json.MarshalIndent(data, "<prefix>", "<indent>")
if err != nil {
log.Fatal(err)
}
fmt.Println(string(b))
}
package main
import (
"encoding/json"
"fmt"
"os"
)
func main() {
h := json.RawMessage(`{"precomputed": true}`)
c := struct {
Header *json.RawMessage `json:"header"`
Body string `json:"body"`
}{Header: &h, Body: "Hello Gophers!"}
b, err := json.MarshalIndent(&c, "", "\t")
if err != nil {
fmt.Println("error:", err)
}
os.Stdout.Write(b)
}
package main
import (
"encoding/json"
"fmt"
"log"
)
func main() {
type Color struct {
Space string
Point json.RawMessage // delay parsing until we know the color space
}
type RGB struct {
R uint8
G uint8
B uint8
}
type YCbCr struct {
Y uint8
Cb int8
Cr int8
}
var j = []byte(`[
{"Space": "YCbCr", "Point": {"Y": 255, "Cb": 0, "Cr": -10}},
{"Space": "RGB", "Point": {"R": 98, "G": 218, "B": 255}}
]`)
var colors []Color
err := json.Unmarshal(j, &colors)
if err != nil {
log.Fatalln("error:", err)
}
for _, c := range colors {
var dst any
switch c.Space {
case "RGB":
dst = new(RGB)
case "YCbCr":
dst = new(YCbCr)
}
err := json.Unmarshal(c.Point, dst)
if err != nil {
log.Fatalln("error:", err)
}
fmt.Println(c.Space, dst)
}
}
package main
import (
"encoding/json"
"fmt"
)
func main() {
var jsonBlob = []byte(`[
{"Name": "Platypus", "Order": "Monotremata"},
{"Name": "Quoll", "Order": "Dasyuromorphia"}
]`)
type Animal struct {
Name string
Order string
}
var animals []Animal
err := json.Unmarshal(jsonBlob, &animals)
if err != nil {
fmt.Println("error:", err)
}
fmt.Printf("%+v", animals)
}
package main
import (
"encoding/json"
"fmt"
)
func main() {
goodJSON := `{"example": 1}`
badJSON := `{"example":2:]}}`
fmt.Println(json.Valid([]byte(goodJSON)), json.Valid([]byte(badJSON)))
}
//go:build !goexperiment.jsonv2
package main
import (
"encoding/json"
"fmt"
"log"
"strings"
)
type Animal int
const (
Unknown Animal = iota
Gopher
Zebra
)
func (a *Animal) UnmarshalJSON(b []byte) error {
var s string
if err := json.Unmarshal(b, &s); err != nil {
return err
}
switch strings.ToLower(s) {
default:
*a = Unknown
case "gopher":
*a = Gopher
case "zebra":
*a = Zebra
}
return nil
}
func (a Animal) MarshalJSON() ([]byte, error) {
var s string
switch a {
default:
s = "unknown"
case Gopher:
s = "gopher"
case Zebra:
s = "zebra"
}
return json.Marshal(s)
}
func main() {
blob := `["gopher","armadillo","zebra","unknown","gopher","bee","gopher","zebra"]`
var zoo []Animal
if err := json.Unmarshal([]byte(blob), &zoo); err != nil {
log.Fatal(err)
}
census := make(map[Animal]int)
for _, animal := range zoo {
census[animal] += 1
}
fmt.Printf("Zoo Census:\n* Gophers: %d\n* Zebras: %d\n* Unknown: %d\n",
census[Gopher], census[Zebra], census[Unknown])
}
//go:build !goexperiment.jsonv2
package main
import (
"encoding/json"
"fmt"
"log"
"strings"
)
type Size int
const (
Unrecognized Size = iota
Small
Large
)
func (s *Size) UnmarshalText(text []byte) error {
switch strings.ToLower(string(text)) {
default:
*s = Unrecognized
case "small":
*s = Small
case "large":
*s = Large
}
return nil
}
func (s Size) MarshalText() ([]byte, error) {
var name string
switch s {
default:
name = "unrecognized"
case Small:
name = "small"
case Large:
name = "large"
}
return []byte(name), nil
}
func main() {
blob := `["small","regular","large","unrecognized","small","normal","small","large"]`
var inventory []Size
if err := json.Unmarshal([]byte(blob), &inventory); err != nil {
log.Fatal(err)
}
counts := make(map[Size]int)
for _, size := range inventory {
counts[size] += 1
}
fmt.Printf("Inventory Counts:\n* Small: %d\n* Large: %d\n* Unrecognized: %d\n",
counts[Small], counts[Large], counts[Unrecognized])
}
Package-Level Type Names (total 17)
/* sort by: | */
A Decoder reads and decodes JSON values from an input stream. Buffered returns a reader of the data remaining in the Decoder's
buffer. The reader is valid until the next call to [Decoder.Decode]. Decode reads the next JSON-encoded value from its
input and stores it in the value pointed to by v.
See the documentation for [Unmarshal] for details about
the conversion of JSON into a Go value. DisallowUnknownFields causes the Decoder to return an error when the destination
is a struct and the input contains object keys which do not match any
non-ignored, exported fields in the destination. InputOffset returns the input stream byte offset of the current decoder position.
The offset gives the location of the end of the most recently returned token
and the beginning of the next token. More reports whether there is another element in the
current array or object being parsed. Token returns the next JSON token in the input stream.
At the end of the input stream, Token returns nil, [io.EOF].
Token guarantees that the delimiters [ ] { } it returns are
properly nested and matched: if Token encounters an unexpected
delimiter in the input, it will return an error.
The input stream consists of basic JSON values—bool, string,
number, and null—along with delimiters [ ] { } of type [Delim]
to mark the start and end of arrays and objects.
Commas and colons are elided. UseNumber causes the Decoder to unmarshal a number into an
interface value as a [Number] instead of as a float64.
func NewDecoder(r io.Reader) *Decoder
A Delim is a JSON array or object delimiter, one of [ ] { or }.( Delim) String() string
Delim : expvar.Var
Delim : fmt.Stringer
An Encoder writes JSON values to an output stream. Encode writes the JSON encoding of v to the stream,
followed by a newline character.
See the documentation for [Marshal] for details about the
conversion of Go values to JSON. SetEscapeHTML specifies whether problematic HTML characters
should be escaped inside JSON quoted strings.
The default behavior is to escape &, <, and > to \u0026, \u003c, and \u003e
to avoid certain safety problems that can arise when embedding JSON in HTML.
In non-HTML settings where the escaping interferes with the readability
of the output, SetEscapeHTML(false) disables this behavior. SetIndent instructs the encoder to format each subsequent encoded
value as if indented by the package-level function Indent(dst, src, prefix, indent).
Calling SetIndent("", "") disables indentation.
func NewEncoder(w io.Writer) *Encoder
An InvalidUnmarshalError describes an invalid argument passed to [Unmarshal].
(The argument to [Unmarshal] must be a non-nil pointer.)Typereflect.Type(*InvalidUnmarshalError) Error() string
*InvalidUnmarshalError : error
Before Go 1.2, an InvalidUTF8Error was returned by [Marshal] when
attempting to encode a string value with invalid UTF-8 sequences.
As of Go 1.2, [Marshal] instead coerces the string to valid UTF-8 by
replacing invalid bytes with the Unicode replacement rune U+FFFD.
Deprecated: No longer used; kept for compatibility. // the whole string value that caused the error(*InvalidUTF8Error) Error() string
*InvalidUTF8Error : error
Marshaler is the interface implemented by types that
can marshal themselves into valid JSON.
A Number represents a JSON number literal. Float64 returns the number as a float64. Int64 returns the number as an int64. MarshalJSONTo implements [jsonv2.MarshalerTo]. String returns the literal text of the number. UnmarshalJSONFrom implements [jsonv2.UnmarshalerFrom].
Number : encoding/json/v2.MarshalerTo
*Number : encoding/json/v2.UnmarshalerFrom
Number : expvar.Var
Number : fmt.Stringer
Options are a set of options to configure the v2 "json" package
to operate with v1 semantics for particular features.
Values of this type can be passed to v2 functions like
[jsonv2.Marshal] or [jsonv2.Unmarshal].
Instead of referencing this type, use [jsonv2.Options].
See the "Migrating to v2" section for guidance on how to migrate usage
of "json" from using v1 to using v2 instead.
RawMessage is a raw encoded JSON value.
It implements [Marshaler] and [Unmarshaler] and can
be used to delay JSON decoding or precompute a JSON encoding.
A SyntaxError is a description of a JSON syntax error.
[Unmarshal] will return a SyntaxError if the JSON can't be parsed. // error occurred after reading Offset bytes(*SyntaxError) Error() string
*SyntaxError : error
A Token holds a value of one of these types:
- [Delim], for the four JSON delimiters [ ] { }
- bool, for JSON booleans
- float64, for JSON numbers
- [Number], for JSON numbers
- string, for JSON string literals
- nil, for JSON null
func (*Decoder).Token() (Token, error)
Unmarshaler is the interface implemented by types
that can unmarshal a JSON description of themselves.
The input can be assumed to be a valid encoding of
a JSON value. UnmarshalJSON must copy the JSON data
if it wishes to retain the data after returning.
An UnmarshalFieldError describes a JSON object key that
led to an unexported (and therefore unwritable) struct field.
Deprecated: No longer used; kept for compatibility.Fieldreflect.StructFieldKeystringTypereflect.Type(*UnmarshalFieldError) Error() string
*UnmarshalFieldError : error
An UnmarshalTypeError describes a JSON value that was
not appropriate for a value of a specific Go type. // may be nil // the full path from root node to the value // error occurred after reading Offset bytes // name of the root type containing the field // type of Go value it could not be assigned to // description of JSON value - "bool", "array", "number -5"(*UnmarshalTypeError) Error() string(*UnmarshalTypeError) Unwrap() error
*UnmarshalTypeError : error
An UnsupportedTypeError is returned by [Marshal] when attempting
to encode an unsupported value type.Typereflect.Type(*UnsupportedTypeError) Error() string
*UnsupportedTypeError : error
CallMethodsWithLegacySemantics specifies that calling of type-provided
marshal and unmarshal methods follow legacy semantics:
- When marshaling, a marshal method declared on a pointer receiver
is only called if the Go value is addressable.
Values obtained from an interface or map element are not addressable.
Values obtained from a pointer or slice element are addressable.
Values obtained from an array element or struct field inherit
the addressability of the parent. In contrast, the v2 semantic
is to always call marshal methods regardless of addressability.
- When marshaling or unmarshaling, the [Marshaler] or [Unmarshaler]
methods are ignored for map keys. However, [encoding.TextMarshaler]
or [encoding.TextUnmarshaler] are still callable.
In contrast, the v2 semantic is to serialize map keys
like any other value (with regard to calling methods),
which may include calling [Marshaler] or [Unmarshaler] methods,
where it is the implementation's responsibility to represent the
Go value as a JSON string (as required for JSON object names).
- When marshaling, if a map key value implements a marshal method
and is a nil pointer, then it is serialized as an empty JSON string.
In contrast, the v2 semantic is to report an error.
- When marshaling, if an interface type implements a marshal method
and the interface value is a nil pointer to a concrete type,
then the marshal method is always called.
In contrast, the v2 semantic is to never directly call methods
on interface values and to instead defer evaluation based upon
the underlying concrete value. Similar to non-interface values,
marshal methods are not called on nil pointers and
are instead serialized as a JSON null.
This affects either marshaling or unmarshaling.
The v1 default is true.
Compact appends to dst the JSON-encoded src with
insignificant space characters elided.
DefaultOptionsV1 is the full set of all options that define v1 semantics.
It is equivalent to the following boolean options being set to true:
- [CallMethodsWithLegacySemantics]
- [EscapeInvalidUTF8]
- [FormatBytesWithLegacySemantics]
- [FormatTimeWithLegacySemantics]
- [MatchCaseSensitiveDelimiter]
- [MergeWithLegacySemantics]
- [OmitEmptyWithLegacyDefinition]
- [ReportErrorsWithLegacySemantics]
- [StringifyWithLegacySemantics]
- [UnmarshalArrayFromAnyLength]
- [jsonv2.Deterministic]
- [jsonv2.FormatNilMapAsNull]
- [jsonv2.FormatNilSliceAsNull]
- [jsonv2.MatchCaseInsensitiveNames]
- [jsontext.AllowDuplicateNames]
- [jsontext.AllowInvalidUTF8]
- [jsontext.EscapeForHTML]
- [jsontext.EscapeForJS]
- [jsontext.PreserveRawString]
All other boolean options are set to false.
All non-boolean options are set to the zero value,
except for [jsontext.WithIndent], which defaults to "\t".
The [Marshal] and [Unmarshal] functions in this package are
semantically identical to calling the v2 equivalents with this option:
jsonv2.Marshal(v, jsonv1.DefaultOptionsV1())
jsonv2.Unmarshal(b, v, jsonv1.DefaultOptionsV1())
EscapeInvalidUTF8 specifies that when encoding a [jsontext.String]
with bytes of invalid UTF-8, such bytes are escaped as
a hexadecimal Unicode codepoint (i.e., \ufffd).
In contrast, the v2 default is to use the minimal representation,
which is to encode invalid UTF-8 as the Unicode replacement rune itself
(without any form of escaping).
This only affects encoding and is ignored when decoding.
The v1 default is true.
FormatBytesWithLegacySemantics specifies that handling of
[]~byte and [N]~byte types follow legacy semantics:
- A Go [N]~byte is always treated as as a normal Go array
in contrast to the v2 default of treating [N]byte as
using some form of binary data encoding (RFC 4648).
- A Go []~byte is to be treated as using some form of
binary data encoding (RFC 4648) in contrast to the v2 default
of only treating []byte as such. In particular, v2 does not
treat slices of named byte types as representing binary data.
- When marshaling, if a named byte implements a marshal method,
then the slice is serialized as a JSON array of elements,
each of which call the marshal method.
- When unmarshaling, if the input is a JSON array,
then unmarshal into the []~byte as if it were a normal Go slice.
In contrast, the v2 default is to report an error unmarshaling
a JSON array when expecting some form of binary data encoding.
- When unmarshaling, '\r' and '\n' characters are ignored
within the encoded "base32" and "base64" data.
In contrast, the v2 default is to report an error in order to be
strictly compliant with RFC 4648, section 3.3,
which specifies that non-alphabet characters must be rejected.
This affects either marshaling or unmarshaling.
The v1 default is true.
FormatTimeWithLegacySemantics specifies that [time] types are formatted
with legacy semantics:
- When marshaling or unmarshaling, a [time.Duration] is formatted as
a JSON number representing the number of nanoseconds.
In contrast, the default v2 behavior uses a JSON string
with the duration formatted with [time.Duration.String].
If a duration field has a `format` tag option,
then the specified formatting takes precedence.
- When unmarshaling, a [time.Time] follows loose adherence to RFC 3339.
In particular, it permits historically incorrect representations,
allowing for deviations in hour format, sub-second separator,
and timezone representation. In contrast, the default v2 behavior
is to strictly comply with the grammar specified in RFC 3339.
This affects either marshaling or unmarshaling.
The v1 default is true.
HTMLEscape appends to dst the JSON-encoded src with <, >, &, U+2028 and U+2029
characters inside string literals changed to \u003c, \u003e, \u0026, \u2028, \u2029
so that the JSON will be safe to embed inside HTML <script> tags.
For historical reasons, web browsers don't honor standard HTML
escaping within <script> tags, so an alternative JSON encoding must be used.
Indent appends to dst an indented form of the JSON-encoded src.
Each element in a JSON object or array begins on a new,
indented line beginning with prefix followed by one or more
copies of indent according to the indentation nesting.
The data appended to dst does not begin with the prefix nor
any indentation, to make it easier to embed inside other formatted JSON data.
Although leading space characters (space, tab, carriage return, newline)
at the beginning of src are dropped, trailing space characters
at the end of src are preserved and copied to dst.
For example, if src has no trailing spaces, neither will dst;
if src ends in a trailing newline, so will dst.
Marshal returns the JSON encoding of v.
Marshal traverses the value v recursively.
If an encountered value implements [Marshaler]
and is not a nil pointer, Marshal calls [Marshaler.MarshalJSON]
to produce JSON. If no [Marshaler.MarshalJSON] method is present but the
value implements [encoding.TextMarshaler] instead, Marshal calls
[encoding.TextMarshaler.MarshalText] and encodes the result as a JSON string.
The nil pointer exception is not strictly necessary
but mimics a similar, necessary exception in the behavior of
[Unmarshaler.UnmarshalJSON].
Otherwise, Marshal uses the following type-dependent default encodings:
Boolean values encode as JSON booleans.
Floating point, integer, and [Number] values encode as JSON numbers.
NaN and +/-Inf values will return an [UnsupportedValueError].
String values encode as JSON strings coerced to valid UTF-8,
replacing invalid bytes with the Unicode replacement rune.
So that the JSON will be safe to embed inside HTML <script> tags,
the string is encoded using [HTMLEscape],
which replaces "<", ">", "&", U+2028, and U+2029 are escaped
to "\u003c","\u003e", "\u0026", "\u2028", and "\u2029".
This replacement can be disabled when using an [Encoder],
by calling [Encoder.SetEscapeHTML](false).
Array and slice values encode as JSON arrays, except that
[]byte encodes as a base64-encoded string, and a nil slice
encodes as the null JSON value.
Struct values encode as JSON objects.
Each exported struct field becomes a member of the object, using the
field name as the object key, unless the field is omitted for one of the
reasons given below.
The encoding of each struct field can be customized by the format string
stored under the "json" key in the struct field's tag.
The format string gives the name of the field, possibly followed by a
comma-separated list of options. The name may be empty in order to
specify options without overriding the default field name.
The "omitempty" option specifies that the field should be omitted
from the encoding if the field has an empty value, defined as
false, 0, a nil pointer, a nil interface value, and any array,
slice, map, or string of length zero.
As a special case, if the field tag is "-", the field is always omitted.
Note that a field with name "-" can still be generated using the tag "-,".
Examples of struct field tags and their meanings:
// Field appears in JSON as key "myName".
Field int `json:"myName"`
// Field appears in JSON as key "myName" and
// the field is omitted from the object if its value is empty,
// as defined above.
Field int `json:"myName,omitempty"`
// Field appears in JSON as key "Field" (the default), but
// the field is skipped if empty.
// Note the leading comma.
Field int `json:",omitempty"`
// Field is ignored by this package.
Field int `json:"-"`
// Field appears in JSON as key "-".
Field int `json:"-,"`
The "omitzero" option specifies that the field should be omitted
from the encoding if the field has a zero value, according to rules:
1) If the field type has an "IsZero() bool" method, that will be used to
determine whether the value is zero.
2) Otherwise, the value is zero if it is the zero value for its type.
If both "omitempty" and "omitzero" are specified, the field will be omitted
if the value is either empty or zero (or both).
The "string" option signals that a field is stored as JSON inside a
JSON-encoded string. It applies only to fields of string, floating point,
integer, or boolean types. This extra level of encoding is sometimes used
when communicating with JavaScript programs:
Int64String int64 `json:",string"`
The key name will be used if it's a non-empty string consisting of
only Unicode letters, digits, and ASCII punctuation except quotation
marks, backslash, and comma.
Embedded struct fields are usually marshaled as if their inner exported fields
were fields in the outer struct, subject to the usual Go visibility rules amended
as described in the next paragraph.
An anonymous struct field with a name given in its JSON tag is treated as
having that name, rather than being anonymous.
An anonymous struct field of interface type is treated the same as having
that type as its name, rather than being anonymous.
The Go visibility rules for struct fields are amended for JSON when
deciding which field to marshal or unmarshal. If there are
multiple fields at the same level, and that level is the least
nested (and would therefore be the nesting level selected by the
usual Go rules), the following extra rules apply:
1) Of those fields, if any are JSON-tagged, only tagged fields are considered,
even if there are multiple untagged fields that would otherwise conflict.
2) If there is exactly one field (tagged or not according to the first rule), that is selected.
3) Otherwise there are multiple fields, and all are ignored; no error occurs.
Handling of anonymous struct fields is new in Go 1.1.
Prior to Go 1.1, anonymous struct fields were ignored. To force ignoring of
an anonymous struct field in both current and earlier versions, give the field
a JSON tag of "-".
Map values encode as JSON objects. The map's key type must either be a
string, an integer type, or implement [encoding.TextMarshaler]. The map keys
are sorted and used as JSON object keys by applying the following rules,
subject to the UTF-8 coercion described for string values above:
- keys of any string type are used directly
- keys that implement [encoding.TextMarshaler] are marshaled
- integer keys are converted to strings
Pointer values encode as the value pointed to.
A nil pointer encodes as the null JSON value.
Interface values encode as the value contained in the interface.
A nil interface value encodes as the null JSON value.
Channel, complex, and function values cannot be encoded in JSON.
Attempting to encode such a value causes Marshal to return
an [UnsupportedTypeError].
JSON cannot represent cyclic data structures and Marshal does not
handle them. Passing cyclic structures to Marshal will result in
an error.
MarshalIndent is like [Marshal] but applies [Indent] to format the output.
Each JSON element in the output will begin on a new line beginning with prefix
followed by one or more copies of indent according to the indentation nesting.
MatchCaseSensitiveDelimiter specifies that underscores and dashes are
not to be ignored when performing case-insensitive name matching which
occurs under [jsonv2.MatchCaseInsensitiveNames] or the `case:ignore` tag option.
Thus, case-insensitive name matching is identical to [strings.EqualFold].
Use of this option diminishes the ability of case-insensitive matching
to be able to match common case variants (e.g, "foo_bar" with "fooBar").
This affects either marshaling or unmarshaling.
The v1 default is true.
MergeWithLegacySemantics specifies that unmarshaling into a non-zero
Go value follows legacy semantics:
- When unmarshaling a JSON null, this preserves the original Go value
if the kind is a bool, int, uint, float, string, array, or struct.
Otherwise, it zeros the Go value.
In contrast, the default v2 behavior is to consistently and always
zero the Go value when unmarshaling a JSON null into it.
- When unmarshaling a JSON value other than null, this merges into
the original Go value for array elements, slice elements,
struct fields (but not map values),
pointer values, and interface values (only if a non-nil pointer).
In contrast, the default v2 behavior is to merge into the Go value
for struct fields, map values, pointer values, and interface values.
In general, the v2 semantic merges when unmarshaling a JSON object,
otherwise it replaces the original value.
This only affects unmarshaling and is ignored when marshaling.
The v1 default is true.
NewDecoder returns a new decoder that reads from r.
The decoder introduces its own buffering and may
read data from r beyond the JSON values requested.
NewEncoder returns a new encoder that writes to w.
OmitEmptyWithLegacyDefinition specifies that the `omitempty` tag option
follows a definition of empty where a field is omitted if the Go value is
false, 0, a nil pointer, a nil interface value,
or any empty array, slice, map, or string.
This overrides the v2 semantic where a field is empty if the value
marshals as a JSON null or an empty JSON string, object, or array.
The v1 and v2 definitions of `omitempty` are practically the same for
Go strings, slices, arrays, and maps. Usages of `omitempty` on
Go bools, ints, uints floats, pointers, and interfaces should migrate to use
the `omitzero` tag option, which omits a field if it is the zero Go value.
This only affects marshaling and is ignored when unmarshaling.
The v1 default is true.
ReportErrorsWithLegacySemantics specifies that Marshal and Unmarshal
should report errors with legacy semantics:
- When marshaling or unmarshaling, the returned error values are
usually of types such as [SyntaxError], [MarshalerError],
[UnsupportedTypeError], [UnsupportedValueError],
[InvalidUnmarshalError], or [UnmarshalTypeError].
In contrast, the v2 semantic is to always return errors as either
[jsonv2.SemanticError] or [jsontext.SyntacticError].
- When marshaling, if a user-defined marshal method reports an error,
it is always wrapped in a [MarshalerError], even if the error itself
is already a [MarshalerError], which may lead to multiple redundant
layers of wrapping. In contrast, the v2 semantic is to
always wrap an error within [jsonv2.SemanticError]
unless it is already a semantic error.
- When unmarshaling, if a user-defined unmarshal method reports an error,
it is never wrapped and reported verbatim. In contrast, the v2 semantic
is to always wrap an error within [jsonv2.SemanticError]
unless it is already a semantic error.
- When marshaling or unmarshaling, if a Go struct contains type errors
(e.g., conflicting names or malformed field tags), then such errors
are ignored and the Go struct uses a best-effort representation.
In contrast, the v2 semantic is to report a runtime error.
- When unmarshaling, the syntactic structure of the JSON input
is fully validated before performing the semantic unmarshaling
of the JSON data into the Go value. Practically speaking,
this means that JSON input with syntactic errors do not result
in any mutations of the target Go value. In contrast, the v2 semantic
is to perform a streaming decode and gradually unmarshal the JSON input
into the target Go value, which means that the Go value may be
partially mutated when a syntactic error is encountered.
- When unmarshaling, a semantic error does not immediately terminate the
unmarshal procedure, but rather evaluation continues.
When unmarshal returns, only the first semantic error is reported.
In contrast, the v2 semantic is to terminate unmarshal the moment
an error is encountered.
This affects either marshaling or unmarshaling.
The v1 default is true.
StringifyWithLegacySemantics specifies that the `string` tag option
may stringify bools and string values. It only takes effect on fields
where the top-level type is a bool, string, numeric kind, or a pointer to
such a kind. Specifically, `string` will not stringify bool, string,
or numeric kinds within a composite data type
(e.g., array, slice, struct, map, or interface).
When marshaling, such Go values are serialized as their usual
JSON representation, but quoted within a JSON string.
When unmarshaling, such Go values must be deserialized from
a JSON string containing their usual JSON representation.
A JSON null quoted in a JSON string is a valid substitute for JSON null
while unmarshaling into a Go value that `string` takes effect on.
This affects either marshaling or unmarshaling.
The v1 default is true.
Unmarshal parses the JSON-encoded data and stores the result
in the value pointed to by v. If v is nil or not a pointer,
Unmarshal returns an [InvalidUnmarshalError].
Unmarshal uses the inverse of the encodings that
[Marshal] uses, allocating maps, slices, and pointers as necessary,
with the following additional rules:
To unmarshal JSON into a pointer, Unmarshal first handles the case of
the JSON being the JSON literal null. In that case, Unmarshal sets
the pointer to nil. Otherwise, Unmarshal unmarshals the JSON into
the value pointed at by the pointer. If the pointer is nil, Unmarshal
allocates a new value for it to point to.
To unmarshal JSON into a value implementing [Unmarshaler],
Unmarshal calls that value's [Unmarshaler.UnmarshalJSON] method, including
when the input is a JSON null.
Otherwise, if the value implements [encoding.TextUnmarshaler]
and the input is a JSON quoted string, Unmarshal calls
[encoding.TextUnmarshaler.UnmarshalText] with the unquoted form of the string.
To unmarshal JSON into a struct, Unmarshal matches incoming object
keys to the keys used by [Marshal] (either the struct field name or its tag),
preferring an exact match but also accepting a case-insensitive match. By
default, object keys which don't have a corresponding struct field are
ignored (see [Decoder.DisallowUnknownFields] for an alternative).
To unmarshal JSON into an interface value,
Unmarshal stores one of these in the interface value:
- bool, for JSON booleans
- float64, for JSON numbers
- string, for JSON strings
- []any, for JSON arrays
- map[string]any, for JSON objects
- nil for JSON null
To unmarshal a JSON array into a slice, Unmarshal resets the slice length
to zero and then appends each element to the slice.
As a special case, to unmarshal an empty JSON array into a slice,
Unmarshal replaces the slice with a new empty slice.
To unmarshal a JSON array into a Go array, Unmarshal decodes
JSON array elements into corresponding Go array elements.
If the Go array is smaller than the JSON array,
the additional JSON array elements are discarded.
If the JSON array is smaller than the Go array,
the additional Go array elements are set to zero values.
To unmarshal a JSON object into a map, Unmarshal first establishes a map to
use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
reuses the existing map, keeping existing entries. Unmarshal then stores
key-value pairs from the JSON object into the map. The map's key type must
either be any string type, an integer, or implement [encoding.TextUnmarshaler].
If the JSON-encoded data contain a syntax error, Unmarshal returns a [SyntaxError].
If a JSON value is not appropriate for a given target type,
or if a JSON number overflows the target type, Unmarshal
skips that field and completes the unmarshaling as best it can.
If no more serious errors are encountered, Unmarshal returns
an [UnmarshalTypeError] describing the earliest such error. In any
case, it's not guaranteed that all the remaining fields following
the problematic one will be unmarshaled into the target object.
The JSON null value unmarshals into an interface, map, pointer, or slice
by setting that Go value to nil. Because null is often used in JSON to mean
“not present,” unmarshaling a JSON null into any other Go type has no effect
on the value and produces no error.
When unmarshaling quoted strings, invalid UTF-8 or
invalid UTF-16 surrogate pairs are not treated as an error.
Instead, they are replaced by the Unicode replacement
character U+FFFD.
UnmarshalArrayFromAnyLength specifies that Go arrays can be unmarshaled
from input JSON arrays of any length. If the JSON array is too short,
then the remaining Go array elements are zeroed. If the JSON array
is too long, then the excess JSON array elements are skipped over.
This only affects unmarshaling and is ignored when marshaling.
The v1 default is true.
Valid reports whether data is a valid JSON encoding.
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PR and bug reports are welcome and can be submitted to the issue list.
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