JSON

pklim101 · web-flow · commit af44e54d00de · 2018-04-28T15:41:05.000+08:00
diff --git a/go/note.md b/go/note.md
@@ -609,3 +609,147 @@ if err != nil {
 }
 fmt.Printf("%d, %s\n", rint, bs)
 ```
+
+## JSON
+```go
+package main
+import (
+    "encoding/json"
+    "fmt"
+    "os"
+    "reflect"
+)
+type Response1 struct {
+    Page   int
+    Fruits []string
+}
+type Response2 struct {
+    Page   int      `json:"page"`
+    Fruits []string `json:"fruits"`
+}
+
+func main() {
+    //encoding basic data types to JSON strings。
+    /**
+    :!go run a.go
+    []uint8
+    [116 114 117 101]
+
+    []uint8
+    1
+
+    []uint8
+    3.12
+    [51 46 49 50]
+
+    []uint8
+    "hello"
+    [34 104 101 108 108 111 34]
+    */
+    bolB, _ := json.Marshal(true)
+    fmt.Println(reflect.TypeOf(bolB))
+    fmt.Println(bolB)
+
+    intB, _ := json.Marshal(1)
+    fmt.Println(reflect.TypeOf(intB))
+    fmt.Println(string(intB))
+
+    fltB, _ := json.Marshal(3.12)
+    fmt.Println(reflect.TypeOf(fltB))
+    fmt.Println(string(fltB))
+    fmt.Println(fltB)
+
+    strB, _ := json.Marshal("hello")
+    fmt.Println(reflect.TypeOf(strB))
+    fmt.Println(string(strB))
+    fmt.Println(strB)
+    
+    //encoding slices and maps to JSON arrays and objects.
+    /** slice
+    []uint8
+    ["apple","orange"]
+    [91 34 97 112 112 108 101 34 44 34 111 114 97 110 103 101 34 93] 
+    */  
+    slcD := []string{"apple", "orange"}
+    slcB, _ := json.Marshal(slcD)
+    fmt.Println(reflect.TypeOf(slcB))
+    fmt.Println(string(slcB))
+    fmt.Println(slcB)
+    /** map 
+    []uint8
+    {"apple":5,"orange":6}
+    [123 34 97 112 112 108 101 34 58 53 44 34 111 114 97 110 103 101 34 58 54 125]
+    */  
+    mapD := map[string]int{"apple": 5, "orange": 6}
+    mapB, _ := json.Marshal(mapD)
+    fmt.Println(reflect.TypeOf(mapB))
+    fmt.Println(string(mapB))
+    fmt.Println(mapB)
+
+    //encoding automatically your custom data types.
+    //{"Page":1,"Fruits":["apple","orange"]}
+    res1D := &Response1{
+        Page:   1,  
+        Fruits: []string{"apple", "orange"},
+    }   
+    res1B, _ := json.Marshal(res1D)
+    fmt.Println(string(res1B))
+    //use tags on struct field declarations to customize the encoded JSON key names.
+    //{"page":1,"fruits":["apple","orange"]}     [add tags to keys]
+    res2D := &Response2{
+        Page:   1,  
+        Fruits: []string{"apple", "orange"},
+    }   
+    res2B, _ := json.Marshal(res2D)
+    fmt.Println(string(res2B))
+
+    fmt.Println("DECODING JSON")
+    /***************************DECODING JSON**************************/
+    /** 
+    1. provide a variable where the JSON package can put the decoded data.
+    2. map[string]interface()/ will hold/ a map of/ strings to arbitray data types.
+    */  
+    var dat map[string]interface{}
+    fmt.Println(reflect.TypeOf(dat)) //map[string]interface {}
+
+    byt := []byte(`{"num":6.13,"strs":["apple","orange"]}`)
+    //check for associated errors
+    /** 
+    map[num:6.13 strs:[a b]] 
+    map[string]interface {}
+    
+    */  
+    if err := json.Unmarshal(byt, &dat); err != nil {
+        panic(err)
+    }   
+    fmt.Println(dat)                 //map[num:6.13 strs:[a b]] 
+    fmt.Println(reflect.TypeOf(dat)) //map[string]interface {}
+
+    // situation1: none-nested.  get value to get directly.
+    num := dat["num"].(float64)
+    fmt.Println(num)                 //6.13
+    fmt.Println(reflect.TypeOf(num)) //float64
+    // situation2: nested.  requires a series of casts.
+    strs := dat["strs"].([]interface{})
+    str1 := strs[0].(string)
+    fmt.Println(reflect.TypeOf(dat["strs"])) //[]interface {}
+    fmt.Println(reflect.TypeOf(strs))        //[]interface {}
+    fmt.Println(reflect.TypeOf(str1))        //string
+    fmt.Println(str1)
+
+    // decode JSON into custom data types.
+    str := `{"page": 1, "fruits": ["apple", "peach"]}`
+    res := Response2{}
+    json.Unmarshal([]byte(str), &res)
+    fmt.Println(res)           //{1 [apple peach]}
+    fmt.Println(res.Page)      //1 
+    fmt.Println(res.Fruits[0]) //apple
+
+    /** 
+    Except used bytes and strings as imtermediates between the data and JSON represation on standard out above.
+    We can also stream JSON encoding to os.Writer like os.Stdout or HTTP response bodies.
+    */  
+    enc := json.NewEncoder(os.Stdout)
+    d := map[string]int{"apple": 5, "orange": 7}
+    enc.Encode(d) //{"apple":5,"orange":7}
+```
