戈朗 : How can i access json decoded field?

Question

我有下一个 JSON 数据:http://jsonblob.com/532d537ce4b0f2fd20c517a4

所以我试图迭代的(就像 PHP 中的 foreach 一样)是:invoices -> invoice(是一个数组)

所以，我想做的是:

package main
import (
    "fmt"
    "reflect"
    "encoding/json"
)

func main() {
    json_string := `{"result":"success","totalresults":"494","startnumber":0,"numreturned":2,"invoices":{"invoice":[{"id":"10660","userid":"126","firstname":"Warren","lastname":"Tapiero","companyname":"ONETIME","invoicenum":"MT-453","date":"2014-03-20","duedate":"2014-03-25","datepaid":"2013-07-20 15:51:48","subtotal":"35.00","credit":"0.00","tax":"0.00","tax2":"0.00","total":"35.00","taxrate":"0.00","taxrate2":"0.00","status":"Paid","paymentmethod":"paypalexpress","notes":"","currencycode":"USD","currencyprefix":"$","currencysuffix":" USD"},{"id":"10661","userid":"276","firstname":"koffi","lastname":"messigah","companyname":"Altech France","invoicenum":"","date":"2014-03-21","duedate":"2014-03-21","datepaid":"0000-00-00 00:00:00","subtotal":"440.00","credit":"0.00","tax":"0.00","tax2":"0.00","total":"440.00","taxrate":"0.00","taxrate2":"0.00","status":"Unpaid","paymentmethod":"paypal","notes":"","currencycode":"USD","currencyprefix":"$","currencysuffix":" USD"}]}}`

    var dat map[string]interface{}
    if err := json.Unmarshal([]byte(json_string), &dat); err != nil {
        panic(err)
    }

    invoices := dat["invoices"]

    fmt.Println("\nJSON-VALUE:",json_string)
    fmt.Println("\nVar type:",invoices)
    fmt.Println("\nVar type using REFLECT:",reflect.TypeOf(invoices))

    for index,value := range invoices.invoice {
        fmt.Println(index,value)
    }

}

但我收到如下错误:invoices.invoice 未定义(类型接口(interface) {} 没有字段或方法 invoice)

http://play.golang.org/p/8uTtN6KtTq

所以，我需要一些帮助，这是我使用 Go 的第一天。

非常感谢。

Answer 1

当您执行 invoices := dat["invoices"] 时，invoices 的类型是 interface{}，它代表 could是任何东西。

实际上它是一个map[string]interface{}。要将 interface{} 转换为具体类型，您需要使用 type assertion像这样。

for index,value := range invoices.(map[string]interface{}) {
    fmt.Println(index,value)
}

参见 the playground完整的例子。

但是，如果这个结构定义明确(并非所有 json 都是)，那么我会定义一个 struct and unmarshal to that这将使您的代码更易于阅读。不要忘记为结构名称使用大写字母，然后使用 json:"name" 标签命名它们(参见 tags in the Marshal section of the docs )