Golang: JSON 综合案例(天气预报)

we create a tiny JSON client/server app in Go.

JSON 数据格式&改编版就不说了，这里主要记录读写，简单小案例，练习。

我现在有一个 JSON 数据文件，请给我生成一个自动生成的 struct 结构体。(手写也可以，但是太麻烦)
免得我自己去写 backquote 映射。可以借助 josn2go工具 ，

但是对于嵌入式结构，不要嵌入定义，带出来单独定义:

type weatherData struct {
  LocationName string   `json: locationName`
  Weather      string   `json: weather`
  Temperature  int      `json: temperature`
  Celsius      bool     `json: celsius`
  TempForecast []int    `json: temp_forecast`
  Wind         windData `json: wind`   //here
}

type windData struct {
  Direction string `json: direction`
  Speed     int    `json: speed`
}

然后读写代码如下:( ioutil.ReadAll 方法可能会占用大量机器内存)

type weatherData struct {
  LocationName string   `json: locationName`
  Weather      string   `json: weather`
  Temperature  int      `json: temperature`
  Celsius      bool     `json: celsius`
  TempForecast []int    `json: temp_forecast`
  Wind         windData `json: wind`   //here
}

type windData struct {
  Direction string `json: direction`
  Speed     int    `json: speed`
}

唯一可能需要注意的是， JSON Field 只映射 struct 中 exported，公有的字段。(指定的映射默认此规则)

然后读写代码如下:( ioutil.ReadAll 方法可能会占用大量机器内存)

package main

import (
  "bytes"
  "encoding/json"
  "fmt"
  "io/ioutil"
  "log"
  "net/http"
)

type weatherData struct {
  LocationName string   `json: locationName`
  Weather      string   `json: weather`
  Temperature  int      `json: temperature`
  Celsius      bool     `json: celsius`
  TempForecast []int    `json: temp_forecast`
  Wind         windData `json: wind`
}

type windData struct {
  Direction string `json: direction`
  Speed     int    `json: speed`
}

type loc struct {
  Lat float32 `json: lat`
  Lon float32 `json: lon`
}

func weatherHandler(w http.ResponseWriter, r *http.Request) {
  location := loc{}

  log.Println(r.Method)
  jsn, err := ioutil.ReadAll(r.Body)
  if err != nil {
    log.Fatal("Error reading the body", err)
  }

  err = json.Unmarshal(jsn, &location)
  if err != nil {
    log.Fatal("Decoding error: ", err)
  }

  log.Printf("Received: %v\n", location)
  weather := weatherData{
    LocationName: "Zzyzx",
    Weather:      "cloudy",
    Temperature:  31,
    Celsius:      true,
    TempForecast: []int{30, 32, 29},
    Wind: windData{
      Direction: "S",
      Speed:     20,
    },
  }

  weatherJson, err := json.Marshal(weather)
  if err != nil {
    fmt.Fprintf(w, "Error: %s", err)
  }
  w.Header().Set("Content-Type", "application/json")
  w.Write(weatherJson)

}

func server() {
  http.HandleFunc("/", weatherHandler)
  http.ListenAndServe(":8080", nil)
}

func client() {
  locJson, err := json.Marshal(loc{Lat: 35.14326, Lon: -116.104})
  req, err := http.NewRequest("POST", "http://localhost:8080", bytes.NewBuffer(locJson))
  req.Header.Set("Content-Type", "application/json")
  client := &http.Client{}
  resp, err := client.Do(req)
  if err != nil {
    log.Fatal(err)
  }
  body, err := ioutil.ReadAll(resp.Body)
  fmt.Println("Response: ", string(body))
  resp.Body.Close()
}

func main() {
  go server()
  client()
}

代码和简单，跑一个客户端发送 location 给 服务端，服务端解析请求JSON段，然后打印；之后服务端回写一些JSON给client。专门用 client := &http.Client{} 建立一个客户端实例的原因是，既要发送，又要等着接收服务端的回写。

然后运行结果如下：

$ go run main.go
2018/03/16 15:27:34 POST
2018/03/16 15:27:34 Received: {35.14326 -116.104}
Response:  {"LocationName":"Zzyzx","Weather":"cloudy","Temperature":31,"Celsius":true,"TempForecast":[30,32,29],"Wind":{"Direction":"S","Speed":20}}

结合 net/http, 练习了一下 encoding/json 感觉不错。(虽然数据都是 Mock 的)

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Merlin 2018.3 更复杂的案例，可以参考 blog.golang.org/json-and-go