从unmarshal带json字符串字段的json说起

事情是这样的,有一段json,里面有字段本应该是obj,但是encode的时候被当成string在,就成了这个样子:

{"body":"{\"sn\":\"aaaa\\\/bbbb\"}"}

json.Unmarshal来解析的话,显然要映射到这样的struct里:

	rawStr := `
{"body":"{\"sn\":\"aaaa\\\/bbbb\"}"}
`
	data := struct {
		Body string `json:"body"`
	}{}
	json.Unmarshal([]byte(rawStr), &data)

这样的话 我得再定义一个struct,然后把body的string解析出来:

	body := struct {
		Sn string
	}{}
	json.Unmarshal([]byte(data.Body), &body)

能不能一次到位 定义好结构体一次解析到位呢?

因为之前有通过实现encoding.TextMarshaler接口来完成结构体里string字段的自定义marshaler,所以理所当然地想到实现encoding.TextUnmarshaler接口来完成自定义的unmarshal

type dataEx struct {
	Body bodyEx
}

type bodyEx struct {
	Sn string
}

func (p *bodyEx) UnmarshalText(text []byte) error {
	return nil
}

func marshalEx(rawStr string) {
	data := &dataEx{}
	err := json.Unmarshal([]byte(rawStr), data)
	if err != nil {
		panic(err)
	}
}

先测试下,在unmarshaltext方法上打上断点,果然停住了。

实现unmarshaltext,如果直接用dataEx结构体去接收,是解析不了的,因为json解析器在扫描到body字段的value的时候 是当做 json的string处理的,那么我们在UnmarshalText方法里拿到的就是那段字符串,因此只要将这段字符串再解析到bodyEx里就好了:
本来预想的是这样就ok了:

func (p *bodyEx) UnmarshalText(text []byte) error {
	return json.Unmarshal(text, p)
}

实际运行发现报错:

json: cannot unmarshal object into Go struct field dataEx.Body of type *main.bodyEx

实际上 这段json解析到这样的结构体上应该是没问题的,现在报错 只能说是因为扩展了UnmarshalText方法导致的。因此暂时这样处理:

type dataEx struct {
	Body bodyEx
}

type bodyEx struct {
	Sn string
}
type bodyEx2 bodyEx

func (p *bodyEx) UnmarshalText(text []byte) error {
	t := bodyEx2{}
	err := json.Unmarshal(text, &t)
	if err != nil {
		return err
	}
	*p = bodyEx(t)
	return nil
}

至此,解决了json里被转义的json字符串一次解析到结构体里的问题。

因为上面使用bodyEx2这样的处理只是自己的猜测和尝试,我想看看到底为啥实现了UnmarshalText后就不能解析了。因此翻看json.Encode()源码

scanner

要实现对json字符串的解析,实际上就是对这段字符串进行词法分析,解析出json里的 obj、number、array、key、value等
json包里有一个scanner,它就是一个状态机:

// A scanner is a JSON scanning state machine.
// Callers call scan.reset() and then pass bytes in one at a time
// by calling scan.step(&scan, c) for each byte.
// The return value, referred to as an opcode, tells the
// caller about significant parsing events like beginning
// and ending literals, objects, and arrays, so that the
// caller can follow along if it wishes.
// The return value scanEnd indicates that a single top-level
// JSON value has been completed, *before* the byte that
// just got passed in.  (The indication must be delayed in order
// to recognize the end of numbers: is 123 a whole value or
// the beginning of 12345e+6?).

scanner的结构如下:

type scanner struct {
	// step 是遍历用的函数,它会随着状态的不同被赋予不同的实现方法
	step func(*scanner, byte) int
	// Reached end of top-level value.
	endTop bool
	// Stack of what we're in the middle of - array values, object keys, object values.
	parseState []int
	// Error that happened, if any.
	err error
	// total bytes consumed, updated by decoder.Decode
	bytes int64
}

简单看一下stateBeginValue状态函数


// stateBeginValue 是开始读取的状态
func stateBeginValue(s *scanner, c byte) int {
	if c <= ' ' && isSpace(c) {
		return scanSkipSpace
	}
	switch c {
	case '{':
		s.step = stateBeginStringOrEmpty
		s.pushParseState(parseObjectKey)
		return scanBeginObject
	case '[':
		s.step = stateBeginValueOrEmpty
		s.pushParseState(parseArrayValue)
		return scanBeginArray
	case '"':
		s.step = stateInString
		return scanBeginLiteral
	case '-':
		s.step = stateNeg
		return scanBeginLiteral
	case '0': // beginning of 0.123
		s.step = state0
		return scanBeginLiteral
	case 't': // beginning of true
		s.step = stateT
		return scanBeginLiteral
	case 'f': // beginning of false
		s.step = stateF
		return scanBeginLiteral
	case 'n': // beginning of null
		s.step = stateN
		return scanBeginLiteral
	}
	if '1' <= c && c <= '9' { // beginning of 1234.5
		s.step = state1
		return scanBeginLiteral
	}
	return s.error(c, "looking for beginning of value")
}

一段正常的json,开始读取的时候(跳过空格后),如果读到'{'name就意味着是一个obj,如果遇到'['就意味着是一个array,如果遇到其他的,都会返回scanBeginLiteral标记,而这个标记就决定着unmarshal的时候如何映射到对应的结构体里。
decodeStateliteralStore方法里,有各种处理:


// 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.
func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool) error {
	// Check for unmarshaler.
	if len(item) == 0 {
		//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 {
	case 'n': // null
		// 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:
			v.Set(reflect.Zero(v.Type()))
			// otherwise, ignore null for primitives/string
		}
	case 't', 'f': // true, false
		value := item[0] == 't'
		// 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:
			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:
			if v.Kind() == reflect.String && v.Type() == numberType {
				v.SetString(s)
				if !isValidNumber(s) {
					return fmt.Errorf("json: invalid number literal, trying to unmarshal %q into Number", item)
				}
				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
}

它会先判断 当前要映射的对象是否实现了 json.Unmarshaler接口和encoding.TextUnmarshaler接口,如果实现了前者,则直接调用前者的方法,否则,如果实现了后者,则针对引号开头的(quotedjson),会调用其UnmarshalText方法,也就是我们之前实现的自定义方法。

这里看到了为什么我们可以扩展,那为啥开始我们直接把字符串unmarshal到实现了UnmarshalText的对象上会报错呢?

我们在自定义方法里进行unmarshal的时候,这时候要解析的json是一段正常的json,而非quotedjson了,因此走的是decodeStateobject方法:

// object consumes an object from d.data[d.off-1:], decoding into v.
// The first byte ('{') of the object has been read already.
func (d *decodeState) object(v reflect.Value) error {
	// 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
	}
    ...//略去一堆
}

上面可以看出,针对obj的情况,若是实现了encoding.TextUnmarshaler接口,则直接返回错误了。

posted @ 2019-08-07 14:00  clannadxr  阅读(6344)  评论(1编辑  收藏  举报