Golang中 json.Decoder vs json.Unmarshal
json的反序列化方式有两种：
- Use json.Unmarshal passing the entire response string
// func Unmarshal(data []byte, v interface{}) error data, err := ioutil.ReadAll(resp.Body) if err == nil && data != nil { err = json.Unmarshal(data, value) } - using json.NewDecoder.Decode
// func NewDecoder(r io.Reader) *Decoder // func (dec *Decoder) Decode(v interface{}) error err = json.NewDecoder(resp.Body).Decode(value)
这两种方法看似差不多，但有不同的应用场景
-
Use json.Decoder if your data is coming from an io.Reader stream, or you need to decode multiple values from a stream of data.
For the case of reading from an HTTP request, I’d pick json.Decoder since you’re obviously reading from a stream.
-
Use json.Unmarshal if you already have the JSON data in memory.
例子
从文件中读入一个巨大的json数组用json.Decoder
json.Decoder会一个一个元素进行加载，不会把整个json数组读到内存里面
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)
}
// t的类型是json.Delim
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)
}
// t的类型是json.Delim
fmt.Printf("%T: %v\n", t, t)
}
结果：
json.Delim: [
Ed: Knock knock.
Sam: Who's there?
Ed: Go fmt.
Sam: Go fmt who?
Ed: Go fmt yourself!
json.Delim: ]
从文件中读入json流用json.Decoder
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)
}
}
结果：
Ed: Knock knock.
Sam: Who's there?
Ed: Go fmt.
Sam: Go fmt who?
Ed: Go fmt yourself!
本来就以[]byte存在于内存中的用json.Unmarshal
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)
}
结果：
[{Name:Platypus Order:Monotremata} {Name:Quoll Order:Dasyuromorphia}]
直接读入json数组的两种方式
data.json
[
{
"site" : "npr",
"link" : "http://www.npr.org/rss/rss.php?id=1001",
"type" : "rss"
},
{
"site" : "npr",
"link" : "http://www.npr.org/rss/rss.php?id=1008",
"type" : "rss"
},
{
"site" : "npr",
"link" : "http://www.npr.org/rss/rss.php?id=1006",
"type" : "rss"
}
]
1. 得到数组元素的地址
[]*Feed
package main
import (
//"helloworld/hello"
"encoding/json"
"fmt"
"os"
)
const dataFile = "data.json"
type Feed struct {
Name string `json:"site"`
URI string `json:"link"`
Type string `json:"type"`
}
func main() {
//hello.Hello()
file, err := os.Open(dataFile)
if err != nil {
os.Exit(1)
}
defer file.Close()
var feeds []*Feed
err = json.NewDecoder(file).Decode(&feeds)
fmt.Printf("%T | %v\n", feeds, feeds)
for i := range feeds {
fmt.Println(*feeds[i])
}
}
结果：
[]*main.Feed | [0xc0420861e0 0xc0420862a0 0xc0420862d0]
{npr http://www.npr.org/rss/rss.php?id=1001 rss}
{npr http://www.npr.org/rss/rss.php?id=1008 rss}
{npr http://www.npr.org/rss/rss.php?id=1006 rss}
2. 得到数组元素的拷贝
[]Feed
package main
import (
//"helloworld/hello"
"encoding/json"
"fmt"
"os"
)
const dataFile = "data.json"
type Feed struct {
Name string `json:"site"`
URI string `json:"link"`
Type string `json:"type"`
}
func main() {
//hello.Hello()
file, err := os.Open(dataFile)
if err != nil {
os.Exit(1)
}
defer file.Close()
var feeds []Feed
err = json.NewDecoder(file).Decode(&feeds)
fmt.Printf("%T | %v\n", feeds, feeds)
// for i := range feeds {
// fmt.Println(*feeds[i])
// }
}
结果：
[]main.Feed | [{npr http://www.npr.org/rss/rss.php?id=1001 rss} {npr http://www.npr.org/rss/rss.php?id=1008 rss} {npr http://www.npr.org/rss/rss.php?id=1006 rss}]