Source: decode.go in package encoding/json

Represents JSON data structure using native Go types: booleans, floats, strings, arrays, and maps.


package json

import (
	"encoding"
	"encoding/base64"
	"fmt"
	"reflect"
	"strconv"
	"strings"
	"unicode"
	"unicode/utf16"
	"unicode/utf8"
)

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 the Unmarshaler interface, Unmarshal calls that value's 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 that value's UnmarshalText method 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 []interface{}, for JSON arrays map[string]interface{}, 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, implement json.Unmarshaler, or implement encoding.TextUnmarshaler. 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.

Check for well-formedness. Avoids filling out half a data structure before discovering a JSON syntax error.

	var d decodeState
	err := checkValid(data, &d.scan)
	if err != nil {
		return err
	}

	d.init(data)
	return d.unmarshal(v)
}

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. By convention, to approximate the behavior of Unmarshal itself, Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.

type Unmarshaler interface {
	UnmarshalJSON([]byte) error
}

An UnmarshalTypeError describes a JSON value that was not appropriate for a value of a specific Go type.

type UnmarshalTypeError struct {
	Value  string       // description of JSON value - "bool", "array", "number -5"
	Type   reflect.Type // type of Go value it could not be assigned to
	Offset int64        // error occurred after reading Offset bytes
	Struct string       // name of the struct type containing the field
	Field  string       // the full path from root node to the field
}

func (e *UnmarshalTypeError) Error() string {
	if e.Struct != "" || e.Field != "" {
		return "json: cannot unmarshal " + e.Value + " into Go struct field " + e.Struct + "." + e.Field + " of type " + e.Type.String()
	}
	return "json: cannot unmarshal " + e.Value + " into Go value of type " + e.Type.String()
}

An UnmarshalFieldError describes a JSON object key that led to an unexported (and therefore unwritable) struct field. Deprecated: No longer used; kept for compatibility.

type UnmarshalFieldError struct {
	Key   string
	Type  reflect.Type
	Field reflect.StructField
}

func (e *UnmarshalFieldError) Error() string {
	return "json: cannot unmarshal object key " + strconv.Quote(e.Key) + " into unexported field " + e.Field.Name + " of type " + e.Type.String()
}

An InvalidUnmarshalError describes an invalid argument passed to Unmarshal. (The argument to Unmarshal must be a non-nil pointer.)

type InvalidUnmarshalError struct {
	Type reflect.Type
}

func (e *InvalidUnmarshalError) Error() string {
	if e.Type == nil {
		return "json: Unmarshal(nil)"
	}

	if e.Type.Kind() != reflect.Ptr {
		return "json: Unmarshal(non-pointer " + e.Type.String() + ")"
	}
	return "json: Unmarshal(nil " + e.Type.String() + ")"
}

func (d *decodeState) unmarshal(v interface{}) error {
	rv := reflect.ValueOf(v)
	if rv.Kind() != reflect.Ptr || rv.IsNil() {
		return &InvalidUnmarshalError{reflect.TypeOf(v)}
	}

	d.scan.reset()

We decode rv not rv.Elem because the Unmarshaler interface test must be applied at the top level of the value.

	err := d.value(rv)
	if err != nil {
		return d.addErrorContext(err)
	}
	return d.savedError
}

A Number represents a JSON number literal.

type Number string

String returns the literal text of the number.

func (n Number) String() string { return string(n) }

Float64 returns the number as a float64.

func (n Number) Float64() (float64, error) {
	return strconv.ParseFloat(string(n), 64)
}

Int64 returns the number as an int64.

func (n Number) Int64() (int64, error) {
	return strconv.ParseInt(string(n), 10, 64)
}

decodeState represents the state while decoding a JSON value.

type decodeState struct {
	data         []byte
	off          int // next read offset in data
	opcode       int // last read result
	scan         scanner
	errorContext struct { // provides context for type errors
		Struct     reflect.Type
		FieldStack []string
	}
	savedError            error
	useNumber             bool
	disallowUnknownFields bool
}

readIndex returns the position of the last byte read.

func (d *decodeState) readIndex() int {
	return d.off - 1
}

phasePanicMsg is used as a panic message when we end up with something that shouldn't happen. It can indicate a bug in the JSON decoder, or that something is editing the data slice while the decoder executes.

const phasePanicMsg = "JSON decoder out of sync - data changing underfoot?"

func (d *decodeState) init(data []byte) *decodeState {
	d.data = data
	d.off = 0
	d.savedError = nil
	d.errorContext.Struct = nil

Reuse the allocated space for the FieldStack slice.

	d.errorContext.FieldStack = d.errorContext.FieldStack[:0]
	return d
}

saveError saves the first err it is called with, for reporting at the end of the unmarshal.

func (d *decodeState) saveError(err error) {
	if d.savedError == nil {
		d.savedError = d.addErrorContext(err)
	}
}

addErrorContext returns a new error enhanced with information from d.errorContext

func (d *decodeState) addErrorContext(err error) error {
	if d.errorContext.Struct != nil || len(d.errorContext.FieldStack) > 0 {
		switch err := err.(type) {
		case *UnmarshalTypeError:
			err.Struct = d.errorContext.Struct.Name()
			err.Field = strings.Join(d.errorContext.FieldStack, ".")
			return err
		}
	}
	return err
}

skip scans to the end of what was started.

func (d *decodeState) skip() {
	s, data, i := &d.scan, d.data, d.off
	depth := len(s.parseState)
	for {
		op := s.step(s, data[i])
		i++
		if len(s.parseState) < depth {
			d.off = i
			d.opcode = op
			return
		}
	}
}

scanNext processes the byte at d.data[d.off].

func (d *decodeState) scanNext() {
	if d.off < len(d.data) {
		d.opcode = d.scan.step(&d.scan, d.data[d.off])
		d.off++
	} else {
		d.opcode = d.scan.eof()
		d.off = len(d.data) + 1 // mark processed EOF with len+1
	}
}

scanWhile processes bytes in d.data[d.off:] until it receives a scan code not equal to op.

func (d *decodeState) scanWhile(op int) {
	s, data, i := &d.scan, d.data, d.off
	for i < len(data) {
		newOp := s.step(s, data[i])
		i++
		if newOp != op {
			d.opcode = newOp
			d.off = i
			return
		}
	}

	d.off = len(data) + 1 // mark processed EOF with len+1
	d.opcode = d.scan.eof()
}

rescanLiteral is similar to scanWhile(scanContinue), but it specialises the common case where we're decoding a literal. The decoder scans the input twice, once for syntax errors and to check the length of the value, and the second to perform the decoding. Only in the second step do we use decodeState to tokenize literals, so we know there aren't any syntax errors. We can take advantage of that knowledge, and scan a literal's bytes much more quickly.

func (d *decodeState) rescanLiteral() {
	data, i := d.data, d.off
Switch:
	switch data[i-1] {
	case '"': // string
		for ; i < len(data); i++ {
			switch data[i] {
			case '\\':
				i++ // escaped char
			case '"':
				i++ // tokenize the closing quote too
				break Switch
			}
		}
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-': // number
		for ; i < len(data); i++ {
			switch data[i] {
			case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
				'.', 'e', 'E', '+', '-':
			default:
				break Switch
			}
		}
	case 't': // true
		i += len("rue")
	case 'f': // false
		i += len("alse")
	case 'n': // null
		i += len("ull")
	}
	if i < len(data) {
		d.opcode = stateEndValue(&d.scan, data[i])
	} else {
		d.opcode = scanEnd
	}
	d.off = i + 1
}

value consumes a JSON value from d.data[d.off-1:], decoding into v, and reads the following byte ahead. If v is invalid, the value is discarded. The first byte of the value has been read already.

func (d *decodeState) value(v reflect.Value) error {
	switch d.opcode {
	default:
		panic(phasePanicMsg)

	case scanBeginArray:
		if v.IsValid() {
			if err := d.array(v); err != nil {
				return err
			}
		} else {
			d.skip()
		}
		d.scanNext()

	case scanBeginObject:
		if v.IsValid() {
			if err := d.object(v); err != nil {
				return err
			}
		} else {
			d.skip()
		}
		d.scanNext()

All bytes inside literal return scanContinue op code.

		start := d.readIndex()
		d.rescanLiteral()

		if v.IsValid() {
			if err := d.literalStore(d.data[start:d.readIndex()], v, false); err != nil {
				return err
			}
		}
	}
	return nil
}

type unquotedValue struct{}

valueQuoted is like value but decodes a quoted string literal or literal null into an interface value. If it finds anything other than a quoted string literal or null, valueQuoted returns unquotedValue{}.

func (d *decodeState) valueQuoted() interface{} {
	switch d.opcode {
	default:
		panic(phasePanicMsg)

	case scanBeginArray, scanBeginObject:
		d.skip()
		d.scanNext()

	case scanBeginLiteral:
		v := d.literalInterface()
		switch v.(type) {
		case nil, string:
			return v
		}
	}
	return unquotedValue{}
}

indirect walks down v allocating pointers as needed, until it gets to a non-pointer. If it encounters an Unmarshaler, indirect stops and returns that. If decodingNull is true, indirect stops at the first settable pointer so it can be set to nil.

Issue #24153 indicates that it is generally not a guaranteed property that you may round-trip a reflect.Value by calling Value.Addr().Elem() and expect the value to still be settable for values derived from unexported embedded struct fields. The logic below effectively does this when it first addresses the value (to satisfy possible pointer methods) and continues to dereference subsequent pointers as necessary. After the first round-trip, we set v back to the original value to preserve the original RW flags contained in reflect.Value.

	v0 := v
	haveAddr := false

If v is a named type and is addressable, start with its address, so that if the type has pointer methods, we find them.

	if v.Kind() != reflect.Ptr && v.Type().Name() != "" && v.CanAddr() {
		haveAddr = true
		v = v.Addr()
	}

Load value from interface, but only if the result will be usefully addressable.

		if v.Kind() == reflect.Interface && !v.IsNil() {
			e := v.Elem()
			if e.Kind() == reflect.Ptr && !e.IsNil() && (!decodingNull || e.Elem().Kind() == reflect.Ptr) {
				haveAddr = false
				v = e
				continue
			}
		}

		if v.Kind() != reflect.Ptr {
			break
		}

		if decodingNull && v.CanSet() {
			break
		}

Prevent infinite loop if v is an interface pointing to its own address: var v interface{} v = &v

		if v.Elem().Kind() == reflect.Interface && v.Elem().Elem() == v {
			v = v.Elem()
			break
		}
		if v.IsNil() {
			v.Set(reflect.New(v.Type().Elem()))
		}
		if v.Type().NumMethod() > 0 && v.CanInterface() {
			if u, ok := v.Interface().(Unmarshaler); ok {
				return u, nil, reflect.Value{}
			}
			if !decodingNull {
				if u, ok := v.Interface().(encoding.TextUnmarshaler); ok {
					return nil, u, reflect.Value{}
				}
			}
		}

		if haveAddr {
			v = v0 // restore original value after round-trip Value.Addr().Elem()
			haveAddr = false
		} else {
			v = v.Elem()
		}
	}
	return nil, nil, v
}

array consumes an array from d.data[d.off-1:], decoding into v. The first byte of the array ('[') has been read already.

Check for unmarshaler.

	u, ut, pv := indirect(v, false)
	if u != nil {
		start := d.readIndex()
		d.skip()
		return u.UnmarshalJSON(d.data[start:d.off])
	}
	if ut != nil {
		d.saveError(&UnmarshalTypeError{Value: "array", Type: v.Type(), Offset: int64(d.off)})
		d.skip()
		return nil
	}
	v = pv

Check type of target.

	switch v.Kind() {
	case reflect.Interface:

Decoding into nil interface? Switch to non-reflect code.

			ai := d.arrayInterface()
			v.Set(reflect.ValueOf(ai))
			return nil

Otherwise it's invalid.

		fallthrough
	default:
		d.saveError(&UnmarshalTypeError{Value: "array", Type: v.Type(), Offset: int64(d.off)})
		d.skip()
		return nil
	case reflect.Array, reflect.Slice:
		break
	}

	i := 0

Look ahead for ] - can only happen on first iteration.

		d.scanWhile(scanSkipSpace)
		if d.opcode == scanEndArray {
			break
		}

Get element of array, growing if necessary.

Grow slice if necessary

			if i >= v.Cap() {
				newcap := v.Cap() + v.Cap()/2
				if newcap < 4 {
					newcap = 4
				}
				newv := reflect.MakeSlice(v.Type(), v.Len(), newcap)
				reflect.Copy(newv, v)
				v.Set(newv)
			}
			if i >= v.Len() {
				v.SetLen(i + 1)
			}
		}

Decode into element.

			if err := d.value(v.Index(i)); err != nil {
				return err
			}

Ran out of fixed array: skip.

			if err := d.value(reflect.Value{}); err != nil {
				return err
			}
		}
		i++

Next token must be , or ].

		if d.opcode == scanSkipSpace {
			d.scanWhile(scanSkipSpace)
		}
		if d.opcode == scanEndArray {
			break
		}
		if d.opcode != scanArrayValue {
			panic(phasePanicMsg)
		}
	}

	if i < v.Len() {

Array. Zero the rest.

			z := reflect.Zero(v.Type().Elem())
			for ; i < v.Len(); i++ {
				v.Index(i).Set(z)
			}
		} else {
			v.SetLen(i)
		}
	}
	if i == 0 && v.Kind() == reflect.Slice {
		v.Set(reflect.MakeSlice(v.Type(), 0, 0))
	}
	return nil
}

var nullLiteral = []byte("null")
var textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()

object consumes an object from d.data[d.off-1:], decoding into v. The first byte ('{') of the object has been read already.

Check for unmarshaler.

	u, ut, pv := indirect(v, false)
	if u != nil {
		start := d.readIndex()
		d.skip()
		return u.UnmarshalJSON(d.data[start:d.off])
	}
	if ut != nil {
		d.saveError(&UnmarshalTypeError{Value: "object", Type: v.Type(), Offset: int64(d.off)})
		d.skip()
		return nil
	}
	v = pv
	t := v.Type()

Decoding into nil interface? Switch to non-reflect code.

	if v.Kind() == reflect.Interface && v.NumMethod() == 0 {
		oi := d.objectInterface()
		v.Set(reflect.ValueOf(oi))
		return nil
	}

	var fields structFields

Check type of target: struct or map[T1]T2 where T1 is string, an integer type, or an encoding.TextUnmarshaler

	switch v.Kind() {

Map key must either have string kind, have an integer kind, or be an encoding.TextUnmarshaler.

		switch t.Key().Kind() {
		case reflect.String,
			reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
			reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		default:
			if !reflect.PtrTo(t.Key()).Implements(textUnmarshalerType) {
				d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
				d.skip()
				return nil
			}
		}
		if v.IsNil() {
			v.Set(reflect.MakeMap(t))
		}
	case reflect.Struct:

	default:
		d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
		d.skip()
		return nil
	}

	var mapElem reflect.Value
	origErrorContext := d.errorContext

Read opening " of string key or closing }.

		d.scanWhile(scanSkipSpace)

closing } - can only happen on first iteration.

			break
		}
		if d.opcode != scanBeginLiteral {
			panic(phasePanicMsg)
		}

Read key.

		start := d.readIndex()
		d.rescanLiteral()
		item := d.data[start:d.readIndex()]
		key, ok := unquoteBytes(item)
		if !ok {
			panic(phasePanicMsg)
		}

Figure out field corresponding to key.

		var subv reflect.Value
		destring := false // whether the value is wrapped in a string to be decoded first

		if v.Kind() == reflect.Map {
			elemType := t.Elem()
			if !mapElem.IsValid() {
				mapElem = reflect.New(elemType).Elem()
			} else {
				mapElem.Set(reflect.Zero(elemType))
			}
			subv = mapElem
		} else {
			var f *field

Found an exact name match.

				f = &fields.list[i]

Fall back to the expensive case-insensitive linear search.

				for i := range fields.list {
					ff := &fields.list[i]
					if ff.equalFold(ff.nameBytes, key) {
						f = ff
						break
					}
				}
			}
			if f != nil {
				subv = v
				destring = f.quoted
				for _, i := range f.index {
					if subv.Kind() == reflect.Ptr {

If a struct embeds a pointer to an unexported type, it is not possible to set a newly allocated value since the field is unexported. See https://golang.org/issue/21357

							if !subv.CanSet() {

Invalidate subv to ensure d.value(subv) skips over the JSON value without assigning it to subv.

								subv = reflect.Value{}
								destring = false
								break
							}
							subv.Set(reflect.New(subv.Type().Elem()))
						}
						subv = subv.Elem()
					}
					subv = subv.Field(i)
				}
				d.errorContext.FieldStack = append(d.errorContext.FieldStack, f.name)
				d.errorContext.Struct = t
			} else if d.disallowUnknownFields {
				d.saveError(fmt.Errorf("json: unknown field %q", key))
			}
		}

Read : before value.

		if d.opcode == scanSkipSpace {
			d.scanWhile(scanSkipSpace)
		}
		if d.opcode != scanObjectKey {
			panic(phasePanicMsg)
		}
		d.scanWhile(scanSkipSpace)

		if destring {
			switch qv := d.valueQuoted().(type) {
			case nil:
				if err := d.literalStore(nullLiteral, subv, false); err != nil {
					return err
				}
			case string:
				if err := d.literalStore([]byte(qv), subv, true); err != nil {
					return err
				}
			default:
				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal unquoted value into %v", subv.Type()))
			}
		} else {
			if err := d.value(subv); err != nil {
				return err
			}
		}

Write value back to map; if using struct, subv points into struct already.

		if v.Kind() == reflect.Map {
			kt := t.Key()
			var kv reflect.Value
			switch {
			case reflect.PtrTo(kt).Implements(textUnmarshalerType):
				kv = reflect.New(kt)
				if err := d.literalStore(item, kv, true); err != nil {
					return err
				}
				kv = kv.Elem()
			case kt.Kind() == reflect.String:
				kv = reflect.ValueOf(key).Convert(kt)
			default:
				switch kt.Kind() {
				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
					s := string(key)
					n, err := strconv.ParseInt(s, 10, 64)
					if err != nil || reflect.Zero(kt).OverflowInt(n) {
						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
						break
					}
					kv = reflect.ValueOf(n).Convert(kt)
				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
					s := string(key)
					n, err := strconv.ParseUint(s, 10, 64)
					if err != nil || reflect.Zero(kt).OverflowUint(n) {
						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
						break
					}
					kv = reflect.ValueOf(n).Convert(kt)
				default:
					panic("json: Unexpected key type") // should never occur
				}
			}
			if kv.IsValid() {
				v.SetMapIndex(kv, subv)
			}
		}

Next token must be , or }.

		if d.opcode == scanSkipSpace {
			d.scanWhile(scanSkipSpace)

Reset errorContext to its original state. Keep the same underlying array for FieldStack, to reuse the space and avoid unnecessary allocs.

		d.errorContext.FieldStack = d.errorContext.FieldStack[:len(origErrorContext.FieldStack)]
		d.errorContext.Struct = origErrorContext.Struct
		if d.opcode == scanEndObject {
			break
		}
		if d.opcode != scanObjectValue {
			panic(phasePanicMsg)
		}
	}
	return nil
}

convertNumber converts the number literal s to a float64 or a Number depending on the setting of d.useNumber.

func (d *decodeState) convertNumber(s string) (interface{}, error) {
	if d.useNumber {
		return Number(s), nil
	}
	f, err := strconv.ParseFloat(s, 64)
	if err != nil {
		return nil, &UnmarshalTypeError{Value: "number " + s, Type: reflect.TypeOf(0.0), Offset: int64(d.off)}
	}
	return f, nil
}

var numberType = reflect.TypeOf(Number(""))

literalStore decodes a literal stored in item into v. fromQuoted indicates whether this literal came from unwrapping a string from the ",string" struct tag option. this is used only to produce more helpful error messages.

Check for unmarshaler.

Empty string given

		d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
		return nil
	}
	isNull := item[0] == 'n' // null
	u, ut, pv := indirect(v, isNull)
	if u != nil {
		return u.UnmarshalJSON(item)
	}
	if ut != nil {
		if item[0] != '"' {
			if fromQuoted {
				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
				return nil
			}
			val := "number"
			switch item[0] {
			case 'n':
				val = "null"
			case 't', 'f':
				val = "bool"
			}
			d.saveError(&UnmarshalTypeError{Value: val, Type: v.Type(), Offset: int64(d.readIndex())})
			return nil
		}
		s, ok := unquoteBytes(item)
		if !ok {
			if fromQuoted {
				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
			}
			panic(phasePanicMsg)
		}
		return ut.UnmarshalText(s)
	}

	v = pv

	switch c := item[0]; c {

The main parser checks that only true and false can reach here, but if this was a quoted string input, it could be anything.

		if fromQuoted && string(item) != "null" {
			d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
			break
		}
		switch v.Kind() {
		case reflect.Interface, reflect.Ptr, reflect.Map, reflect.Slice:

otherwise, ignore null for primitives/string

		}
	case 't', 'f': // true, false

The main parser checks that only true and false can reach here, but if this was a quoted string input, it could be anything.

		if fromQuoted && string(item) != "true" && string(item) != "false" {
			d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
			break
		}
		switch v.Kind() {
		default:
			if fromQuoted {
				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
			} else {
				d.saveError(&UnmarshalTypeError{Value: "bool", Type: v.Type(), Offset: int64(d.readIndex())})
			}
		case reflect.Bool:
			v.SetBool(value)
		case reflect.Interface:
			if v.NumMethod() == 0 {
				v.Set(reflect.ValueOf(value))
			} else {
				d.saveError(&UnmarshalTypeError{Value: "bool", Type: v.Type(), Offset: int64(d.readIndex())})
			}
		}

	case '"': // string
		s, ok := unquoteBytes(item)
		if !ok {
			if fromQuoted {
				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
			}
			panic(phasePanicMsg)
		}
		switch v.Kind() {
		default:
			d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
		case reflect.Slice:
			if v.Type().Elem().Kind() != reflect.Uint8 {
				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
				break
			}
			b := make([]byte, base64.StdEncoding.DecodedLen(len(s)))
			n, err := base64.StdEncoding.Decode(b, s)
			if err != nil {
				d.saveError(err)
				break
			}
			v.SetBytes(b[:n])
		case reflect.String:
			if v.Type() == numberType && !isValidNumber(string(s)) {
				return fmt.Errorf("json: invalid number literal, trying to unmarshal %q into Number", item)
			}
			v.SetString(string(s))
		case reflect.Interface:
			if v.NumMethod() == 0 {
				v.Set(reflect.ValueOf(string(s)))
			} else {
				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
			}
		}

	default: // number
		if c != '-' && (c < '0' || c > '9') {
			if fromQuoted {
				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
			}
			panic(phasePanicMsg)
		}
		s := string(item)
		switch v.Kind() {
		default:

s must be a valid number, because it's already been tokenized.

				v.SetString(s)
				break
			}
			if fromQuoted {
				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
			}
			d.saveError(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
		case reflect.Interface:
			n, err := d.convertNumber(s)
			if err != nil {
				d.saveError(err)
				break
			}
			if v.NumMethod() != 0 {
				d.saveError(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
				break
			}
			v.Set(reflect.ValueOf(n))

		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
			n, err := strconv.ParseInt(s, 10, 64)
			if err != nil || v.OverflowInt(n) {
				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
				break
			}
			v.SetInt(n)

		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
			n, err := strconv.ParseUint(s, 10, 64)
			if err != nil || v.OverflowUint(n) {
				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
				break
			}
			v.SetUint(n)

		case reflect.Float32, reflect.Float64:
			n, err := strconv.ParseFloat(s, v.Type().Bits())
			if err != nil || v.OverflowFloat(n) {
				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
				break
			}
			v.SetFloat(n)
		}
	}
	return nil
}

The xxxInterface routines build up a value to be stored in an empty interface. They are not strictly necessary, but they avoid the weight of reflection in this common case.

valueInterface is like value but returns interface{}

func (d *decodeState) valueInterface() (val interface{}) {
	switch d.opcode {
	default:
		panic(phasePanicMsg)
	case scanBeginArray:
		val = d.arrayInterface()
		d.scanNext()
	case scanBeginObject:
		val = d.objectInterface()
		d.scanNext()
	case scanBeginLiteral:
		val = d.literalInterface()
	}
	return
}

arrayInterface is like array but returns []interface{}.

func (d *decodeState) arrayInterface() []interface{} {
	var v = make([]interface{}, 0)

Look ahead for ] - can only happen on first iteration.

		d.scanWhile(scanSkipSpace)
		if d.opcode == scanEndArray {
			break
		}

		v = append(v, d.valueInterface())

Next token must be , or ].

		if d.opcode == scanSkipSpace {
			d.scanWhile(scanSkipSpace)
		}
		if d.opcode == scanEndArray {
			break
		}
		if d.opcode != scanArrayValue {
			panic(phasePanicMsg)
		}
	}
	return v
}

objectInterface is like object but returns map[string]interface{}.

func (d *decodeState) objectInterface() map[string]interface{} {
	m := make(map[string]interface{})

Read opening " of string key or closing }.

		d.scanWhile(scanSkipSpace)

closing } - can only happen on first iteration.

			break
		}
		if d.opcode != scanBeginLiteral {
			panic(phasePanicMsg)
		}

Read string key.

		start := d.readIndex()
		d.rescanLiteral()
		item := d.data[start:d.readIndex()]
		key, ok := unquote(item)
		if !ok {
			panic(phasePanicMsg)
		}

Read : before value.

		if d.opcode == scanSkipSpace {
			d.scanWhile(scanSkipSpace)
		}
		if d.opcode != scanObjectKey {
			panic(phasePanicMsg)
		}
		d.scanWhile(scanSkipSpace)

Read value.

		m[key] = d.valueInterface()

Next token must be , or }.

		if d.opcode == scanSkipSpace {
			d.scanWhile(scanSkipSpace)
		}
		if d.opcode == scanEndObject {
			break
		}
		if d.opcode != scanObjectValue {
			panic(phasePanicMsg)
		}
	}
	return m
}

literalInterface consumes and returns a literal from d.data[d.off-1:] and it reads the following byte ahead. The first byte of the literal has been read already (that's how the caller knows it's a literal).

All bytes inside literal return scanContinue op code.

	start := d.readIndex()
	d.rescanLiteral()

	item := d.data[start:d.readIndex()]

	switch c := item[0]; c {
	case 'n': // null
		return nil

	case 't', 'f': // true, false
		return c == 't'

	case '"': // string
		s, ok := unquote(item)
		if !ok {
			panic(phasePanicMsg)
		}
		return s

	default: // number
		if c != '-' && (c < '0' || c > '9') {
			panic(phasePanicMsg)
		}
		n, err := d.convertNumber(string(item))
		if err != nil {
			d.saveError(err)
		}
		return n
	}
}

getu4 decodes \uXXXX from the beginning of s, returning the hex value, or it returns -1.

func getu4(s []byte) rune {
	if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
		return -1
	}
	var r rune
	for _, c := range s[2:6] {
		switch {
		case '0' <= c && c <= '9':
			c = c - '0'
		case 'a' <= c && c <= 'f':
			c = c - 'a' + 10
		case 'A' <= c && c <= 'F':
			c = c - 'A' + 10
		default:
			return -1
		}
		r = r*16 + rune(c)
	}
	return r
}

unquote converts a quoted JSON string literal s into an actual string t. The rules are different than for Go, so cannot use strconv.Unquote.

func unquote(s []byte) (t string, ok bool) {
	s, ok = unquoteBytes(s)
	t = string(s)
	return
}

func unquoteBytes(s []byte) (t []byte, ok bool) {
	if len(s) < 2 || s[0] != '"' || s[len(s)-1] != '"' {
		return
	}
	s = s[1 : len(s)-1]

Check for unusual characters. If there are none, then no unquoting is needed, so return a slice of the original bytes.

	r := 0
	for r < len(s) {
		c := s[r]
		if c == '\\' || c == '"' || c < ' ' {
			break
		}
		if c < utf8.RuneSelf {
			r++
			continue
		}
		rr, size := utf8.DecodeRune(s[r:])
		if rr == utf8.RuneError && size == 1 {
			break
		}
		r += size
	}
	if r == len(s) {
		return s, true
	}

	b := make([]byte, len(s)+2*utf8.UTFMax)
	w := copy(b, s[0:r])

Out of room? Can only happen if s is full of malformed UTF-8 and we're replacing each byte with RuneError.

		if w >= len(b)-2*utf8.UTFMax {
			nb := make([]byte, (len(b)+utf8.UTFMax)*2)
			copy(nb, b[0:w])
			b = nb
		}
		switch c := s[r]; {
		case c == '\\':
			r++
			if r >= len(s) {
				return
			}
			switch s[r] {
			default:
				return
			case '"', '\\', '/', '\'':
				b[w] = s[r]
				r++
				w++
			case 'b':
				b[w] = '\b'
				r++
				w++
			case 'f':
				b[w] = '\f'
				r++
				w++
			case 'n':
				b[w] = '\n'
				r++
				w++
			case 'r':
				b[w] = '\r'
				r++
				w++
			case 't':
				b[w] = '\t'
				r++
				w++
			case 'u':
				r--
				rr := getu4(s[r:])
				if rr < 0 {
					return
				}
				r += 6
				if utf16.IsSurrogate(rr) {
					rr1 := getu4(s[r:])

A valid pair; consume.

						r += 6
						w += utf8.EncodeRune(b[w:], dec)
						break

Invalid surrogate; fall back to replacement rune.

					rr = unicode.ReplacementChar
				}
				w += utf8.EncodeRune(b[w:], rr)
			}

Quote, control characters are invalid.

		case c == '"', c < ' ':
			return

ASCII

		case c < utf8.RuneSelf:
			b[w] = c
			r++
			w++

Coerce to well-formed UTF-8.

		default:
			rr, size := utf8.DecodeRune(s[r:])
			r += size
			w += utf8.EncodeRune(b[w:], rr)
		}
	}
	return b[0:w], true