我写了其余的api，但主体在golang中返回nil。 为什么返回“ 0 {<nil> false [] <nil> false 0 <nil> 0}”

Question

func InsertApData(w http.ResponseWriter, r *http.Request) {

    decoder := json.NewDecoder(r.Body)
    fmt.Printf("Request Body : ", decoder)
    var apdata models.ApData
    err := decoder.Decode(&apdata)
    check(b.E(err))
    _, err2 := stmt.InsertApData.Exec(apdata.Mac, apdata.RssiMax, apdata.RssiMin, apdata.LocDefId)
    check(b.E(err2))
}

and data type for db

type ApData struct {
    ID       int    `db:"id"`
    Mac      string `db:"mac"`
    RssiMax  uint64 `db:"rssi_max"`
    RssiMin  uint16 `db:"rssi_min"`
    LocDefId uint64 `db:"loc_def_id"`
}

Why returning

Request Body : %!(EXTRA *json.Decoder=&{0xc820012b00 [] {[] 0 { false [] false 0 0} { false [] false 0 0} false} 0 { false [] false 0 0} 0 []})

also json :

{
"mac":"01:0a:95:9d:68:20",
"rssi_max":-73.50,
"rssi_min":-50.02,
"loc_def_id":1
}

Answer 1

If you examine the source code for json.Decoder,

...
type Decoder struct {
    r     io.Reader
    buf   []byte
    d     decodeState
    scanp int // start of unread data in buf
    scan  scanner
    err   error

    tokenState int
    tokenStack []int
}

func NewDecoder(r io.Reader) *Decoder {
    return &Decoder{r: r}
}
...

you can see that NewDecoder only sets the r field, and that is what you are observing in the output of fmt.Printf.

Until you execute err := decoder.Decode(&apdata), your structure will not be filled in. What you are seeing in the output is the content of the decoder, not the body or your structure.

If you print your structure after running err := decoder.Decode(&apdata), it contains proper data:

package main

import (
    "fmt"
    "encoding/json"
    "strings"
)

func main() {
    input := `{
        "mac":"01:0a:95:9d:68:20",
        "rssi_max":-73.50,
        "rssi_min":-50.02,
        "loc_def_id":1
    }`

    decoder := json.NewDecoder(strings.NewReader(input))
    var apdata struct {
            ID       int    `db:"id"`
            Mac      string `json:"mac" db:"mac"`
            RssiMax  uint64 `json:"rssi_max" db:"rssi_max"`
            RssiMin  uint16 `json:"rssi_min" db:"rssi_min"`
            LocDefId uint64 `json:"loc_def_id" db:"loc_def_id"`
    }
    _ = decoder.Decode(&apdata)
    fmt.Printf("appdata : %+v
", apdata)
}

Output:

appdata : {ID:0 Mac:01:0a:95:9d:68:20 RssiMax:0 RssiMin:0 LocDefId:1}

The RssiMax and RssiMin are zero because they are unsigned ints receiving negative values.