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/*
Title: 47.JSON.go
Author: OpenSource
Date: 2017-05-23
Description: For Study
Go offers built-in support for JSON encoding and decoding,
including to and from built-in and custom data types.
*/
package main
import (
"encoding/json"
"fmt"
"os"
)
// We’ll use these two structs to demonstrate encoding and decoding of custom types below.
type Response1 struct{
Page int
Fruits []string
}
type Response2 struct{
Page int `json: "page"`
Fruits []string `json: "fruits"`
}
func main(){
fmt.Println("47.JSON.go---------Start------------\n\n")
// First we’ll look at encoding basic data types to JSON strings.
// Here are some examples for atomic values
bolB, _ := json.Marshal(true)
fmt.Println(string(bolB))
intB, _ := json.Marshal(1)
fmt.Println(string(intB))
fltB, _ := json.Marshal(2.34)
fmt.Println(string(fltB))
strB, _ := json.Marshal("gopher")
fmt.Println(string(strB))
// And here are some for slices and maps,
// which encode to JSON arrays and objects as you’d expect.
slcD := []string{"apple", "peach", "pear"}
slcB, _ := json.Marshal(slcD)
fmt.Println(string(slcB))
mapD := map[string]int{"apple": 5, "lecture":7}
mapB, _ := json.Marshal(mapD)
fmt.Println(string(mapB))
// The JSON package can automatically encode your custom data types.
// It will only include exported fields in the encoded output and will
// by default use those names as the JSON keys.
res1D := &Response1{
page: 1,
Fruits: []string{"apple", "peach", "pear"}}
reslB, _ json.Marshal(res1D)
fmt.Println(string(reslB))
// You can use tags on struct field declarations to customize the encoded JSON key names.
// Check the definition of Response2 above to see an example of such tags.
res2D := &Response2{
page: 1,
Fruits: []string{"apple", "peach", "pear"}}
res2B, _ := json.Marshal(res2D)
fmt.Println(string(res2B))
// Now let’s look at decoding JSON data into Go values.
// Here’s an example for a generic data structure.
byt := []byte(`{"num": 6.13, "strs": ["a","b"]}`)
// We need to provide a variable where the JSON package can put the decoded data.
// This map[string]interface{} will hold a map of strings to arbitrary data types.
var dat map[string]interface()
// Here’s the actual decoding, and a check for associated errors.
if err := json.Unmarshal(byt, &dat); err != nil{
panic(err)
}
fmt.Println(dat)
// In order to use the values in the decoded map,
// we’ll need to cast them to their appropriate type.
// For example here we cast the value in num to the expected float64 type.
num := dat["num"].(float64)
fmt.Prinln(num)
// Accessing nested data requires a series of casts.
strs := dat["strs"].([]interface{})
str1 := strs[0].(string)
fmt.Println(str1)
// We can also decode JSON into custom data types.
// This has the advantages of adding additional type-safety to our programs
// and eliminating the need for type assertions when accessing the decoded data.
str := `{"page": 1, "fruits": ["apple", "peach"]}`
res := Response2{}
json.Unmarshal([]byte(str), &res)
fmt.Println(res)
fmt.Println(res.Fruits[0])
// In the examples above we always used bytes and strings as intermediates
// between the data and JSON representation on standard out.
// We can also stream JSON encodings directly to os.Writers like os.
// Stdout or even HTTP response bodies.
enc := json.NewEncoder(os.Stdout)
d := map[string]int{"apple": 5, "lettuce": 7}
enc.Encode(d)
fmt.Println("\n\n47.JSON.go-----------End------------")
// We’ve covered the basic of JSON in Go here,
// but check out the JSON and Go blog post and JSON package docs for more.
}
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