在 Go 中解析 JSON 时如何指定默认值
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/30445479/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to specify default values when parsing JSON in Go
提问by JW.
I want to parse a JSON object in Go, but want to specify default values for fields that are not given. For example, I have the struct type:
我想在 Go 中解析一个 JSON 对象，但想为未给出的字段指定默认值。例如，我有结构类型：
type Test struct {
A string
B string
C string
}
The default values for A, B, and C, are "a", "b", and "c" respectively. This means that when I parse the json:
A、B 和 C 的默认值分别为“a”、“b”和“c”。这意味着当我解析 json 时：
{"A": "1", "C": 3}
I want to get the struct:
我想得到结构：
Test{A: "1", B: "b", C: "3"}
Is this possible using the built-in package encoding/json? Otherwise, is there any Go library that has this functionality?
这可以使用内置包encoding/json吗？否则，是否有任何具有此功能的 Go 库？
回答by JW.
This is possible using encoding/json: when calling json.Unmarshal, you do not need to give it an empty struct, you can give it one with default values.
这可以使用 encoding/json：调用时json.Unmarshal，您不需要给它一个空结构，您可以给它一个带有默认值的结构。
For your example:
对于您的示例：
var example []byte = []byte(`{"A": "1", "C": "3"}`)
out := Test{
A: "default a",
B: "default b",
// default for C will be "", the empty value for a string
}
err := json.Unmarshal(example, &out) // <--
if err != nil {
panic(err)
}
fmt.Printf("%+v", out)
Running this example in the Go playgroundreturns {A:1 B:default b C:3}.
在 Go 游乐场中运行此示例返回{A:1 B:default b C:3}.
As you can see, json.Unmarshal(example, &out)unmarshals the JSON into out, overwriting the values specified in the JSON, but leaving the other fields unchanged.
如您所见，json.Unmarshal(example, &out)将 JSON 解组为out，覆盖 JSON 中指定的值，但保持其他字段不变。
回答by Christian
In case u have a list or map of Teststructs the accepted answer is not possible anymore but it can easily be extended with a UnmarshalJSON method:
如果您有Test结构的列表或映射，则不再可能接受已接受的答案，但可以使用 UnmarshalJSON 方法轻松扩展它：
func (t *Test) UnmarshalJSON(data []byte) error {
type testAlias Test
test := &testAlias{
B: "default B",
}
_ = json.Unmarshal(data, test)
*t = Test(*test)
return nil
}
The testAlias is needed to prevent recursive calls to UnmarshalJSON. This works because a new type has no methods defined.
需要 testAlias 来防止对 UnmarshalJSON 的递归调用。这是有效的，因为新类型没有定义方法。
