深入 Go 中各个高性能 JSON 解析库

func Unmarshal(data []byte, v interface{})

func (d *decodeState) value(v reflect.Value) error {
	switch d.opcode {
	default:
		panic(phasePanicMsg)
	// 数组 
	case scanBeginArray:
		...
	// 结构体或map
	case scanBeginObject:
		...
	// 字面量，包括 int、string、float 等
	case scanBeginLiteral:
		...
	}
	return nil
}

func (d *decodeState) object(v reflect.Value) error {
	...  
	var fields structFields
	// 检验这个对象的类型是 map 还是 结构体
	switch v.Kind() {
	case reflect.Map: 
		...
	case reflect.Struct:
		// 缓存结构体的字段到 fields 对象中
		fields = cachedTypeFields(t)
		// ok
	default:
		d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
		d.skip()
		return nil
	}

	var mapElem reflect.Value
	origErrorContext := d.errorContext
	// 循环一个个解析JSON字符串中的 key value 值
	for {  
		start := d.readIndex()
		d.rescanLiteral()
		item := d.data[start:d.readIndex()]
		// 获取 key 值
		key, ok := unquoteBytes(item)
		if !ok {
			panic(phasePanicMsg)
		} 
		var subv reflect.Value
		destring := false   
		... 
		// 根据 value 的类型反射设置 value 值 
		if destring {
			// value 值是字面量会进入到这里
			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 {
			// 数组或对象会递归调用 value 方法
			if err := d.value(subv); err != nil {
				return err
			}
		}
		...
		// 直到遇到 } 最后退出循环
		if d.opcode == scanEndObject {
			break
		}
		if d.opcode != scanObjectValue {
			panic(phasePanicMsg)
		}
	}
	return nil
}

func main() {
	var p fastjson.Parser
	v, _ := p.Parse(`{
                "str": "bar",
                "int": 123,
                "float": 1.23,
                "bool": true,
                "arr": [1, "foo", {}]
        }`)
	fmt.Printf("foo=%s\n", v.GetStringBytes("str"))
	fmt.Printf("int=%d\n", v.GetInt("int"))
	fmt.Printf("float=%f\n", v.GetFloat64("float"))
	fmt.Printf("bool=%v\n", v.GetBool("bool"))
	fmt.Printf("arr.1=%s\n", v.GetStringBytes("arr", "1"))
}
// Output:
// foo=bar
// int=123
// float=1.230000
// bool=true
// arr.1=foo

type Value struct {
	o Object
	a []*Value
	s string
	t Type
}

type Object struct {
	kvs           []kv
	keysUnescaped bool
}

type kv struct {
	k string
	v *Value
}

func parseValue(s string, c *cache, depth int) (*Value, string, error) {
	if len(s) == 0 {
		return nil, s, fmt.Errorf("cannot parse empty string")
	}
	depth++
	// 最大深度的json串不能超过MaxDepth
	if depth > MaxDepth {
		return nil, s, fmt.Errorf("too big depth for the nested JSON; it exceeds %d", MaxDepth)
	}
	// 解析对象
	if s[0] == '{' {
		v, tail, err := parseObject(s[1:], c, depth)
		if err != nil {
			return nil, tail, fmt.Errorf("cannot parse object: %s", err)
		}
		return v, tail, nil
	}
	// 解析数组
	if s[0] == '[' {
		...
	}
	// 解析字符串
	if s[0] == '"' {
		...
	} 
	...
	return v, tail, nil
}

func parseObject(s string, c *cache, depth int) (*Value, string, error) {
	...
	o := c.getValue()
	o.t = TypeObject
	o.o.reset()
	for {
		var err error
		// 获取Ojbect结构体中的 kv 对象
		kv := o.o.getKV()
		... 
		// 解析 key 值
		
		kv.k, s, err = parseRawKey(s[1:])
		... 
		// 递归解析 value 值
		kv.v, s, err = parseValue(s, c, depth)
		...
		// 遇到 ，号继续往下解析
		if s[0] == ',' {
			s = s[1:]
			continue
		}
		// 解析完毕
		if s[0] == '}' {
			return o, s[1:], nil
		}
		return nil, s, fmt.Errorf("missing ',' after object value")
	}
}

json := `{"name":{"first":"li","last":"dj"},"age":18}`
lastName := gjson.Get(json, "name.last")

json := `{
	"name":{"first":"Tom", "last": "Anderson"},
	"age": 37,
	"children": ["Sara", "Alex", "Jack"]
}`
fmt.Println("third child*:", gjson.Get(json, "child*.2"))
fmt.Println("first c?ild:", gjson.Get(json, "c?ildren.0"))

json := `{
	"name":{"first":"Tom", "last": "Anderson"},
	"age": 37,
	"children": ["Sara", "Alex", "Jack"]
}`
fmt.Println("third child*:", gjson.Get(json, "children|@reverse"))

nestedJSON := `{"nested": ["one", "two", ["three", "four"]]}`
fmt.Println(gjson.Get(nestedJSON, "nested|@flatten"))

["one","two","three", "four"]

func Get(json, path string) Result {
	// 解析 path 
	if len(path) > 1 {
		...
	}
	var i int
	var c = &parseContext{json: json}
	if len(path) >= 2 && path[0] == '.' && path[1] == '.' {
		c.lines = true
		parseArray(c, 0, path[2:])
	} else {
		// 根据不同的对象进行解析,这里会一直循环，直到找到 '{' 或 '['
		for ; i < len(c.json); i++ {
			if c.json[i] == '{' {
				i++
				 
				parseObject(c, i, path)
				break
			}
			if c.json[i] == '[' {
				i++
				parseArray(c, i, path)
				break
			}
		}
	}
	if c.piped {
		res := c.value.Get(c.pipe)
		res.Index = 0
		return res
	}
	fillIndex(json, c)
	return c.value
}

func parseObject(c *parseContext, i int, path string) (int, bool) {
	var pmatch, kesc, vesc, ok, hit bool
	var key, val string
	rp := parseObjectPath(path)
	if !rp.more && rp.piped {
		c.pipe = rp.pipe
		c.piped = true
	}
	// 嵌套两个 for 循环 寻找 key 值
	for i < len(c.json) {
		for ; i < len(c.json); i++ {
			if c.json[i] == '"' { 
				i++
				var s = i
				for ; i < len(c.json); i++ {
					if c.json[i] > '\\' {
						continue
					}
					// 找到 key 值跳转到 parse_key_string_done
					if c.json[i] == '"' {
						i, key, kesc, ok = i+1, c.json[s:i], false, true
						goto parse_key_string_done
					}
					...
				}
				key, kesc, ok = c.json[s:], false, false
			// 直接break
			parse_key_string_done:
				break
			}
			if c.json[i] == '}' {
				return i + 1, false
			}
		}
		if !ok {
			return i, false
		}
		// 校验是否是模糊匹配
		if rp.wild {
			if kesc {
				pmatch = match.Match(unescape(key), rp.part)
			} else {
				pmatch = match.Match(key, rp.part)
			}
		} else {
			if kesc {
				pmatch = rp.part == unescape(key)
			} else {
				pmatch = rp.part == key
			}
		}
		// 解析 value
		hit = pmatch && !rp.more
		for ; i < len(c.json); i++ {
			switch c.json[i] {
			default:
				continue
			case '"':
				i++
				i, val, vesc, ok = parseString(c.json, i)
				if !ok {
					return i, false
				}
				if hit {
					if vesc {
						c.value.Str = unescape(val[1 : len(val)-1])
					} else {
						c.value.Str = val[1 : len(val)-1]
					}
					c.value.Raw = val
					c.value.Type = String
					return i, true
				}
			case '{':
				if pmatch && !hit {
					i, hit = parseObject(c, i+1, rp.path)
					if hit {
						return i, true
					}
				} else {
					i, val = parseSquash(c.json, i)
					if hit {
						c.value.Raw = val
						c.value.Type = JSON
						return i, true
					}
				}
			...
			break
		}
	}
	return i, false
}