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< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< /
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< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
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< h2 id = "127-获取结构体字段标签" > < a class = "header" href = "#127-获取结构体字段标签" > 12.7. 获取结构体字段标签< / a > < / h2 >
< p > 在4.5节我们使用构体成员标签用于设置对应JSON对应的名字。其中json成员标签让我们可以选择成员的名字和抑制零值成员的输出。在本节, 我们将看到如何通过反射机制类获取成员标签。< / p >
< p > 对于一个web服务, 大部分HTTP处理函数要做的第一件事情就是展开请求中的参数到本地变量中。我们定义了一个工具函数, 叫params.Unpack, 通过使用结构体成员标签机制来让HTTP处理函数解析请求参数更方便。< / p >
< p > 首先, 我们看看如何使用它。下面的search函数是一个HTTP请求处理函数。它定义了一个匿名结构体类型的变量, 用结构体的每个成员表示HTTP请求的参数。其中结构体成员标签指明了对于请求参数的名字, 为了减少URL的长度这些参数名通常都是神秘的缩略词。Unpack将请求参数填充到合适的结构体成员中, 这样我们可以方便地通过合适的类型类来访问这些参数。< / p >
< p > < u > < i > gopl.io/ch12/search< / i > < / u > < / p >
< pre > < code class = "language-Go" > import " gopl.io/ch12/params"
// search implements the /search URL endpoint.
func search(resp http.ResponseWriter, req *http.Request) {
var data struct {
Labels []string `http:" l" `
MaxResults int `http:" max" `
Exact bool `http:" x" `
}
data.MaxResults = 10 // set default
if err := params.Unpack(req, & data); err != nil {
http.Error(resp, err.Error(), http.StatusBadRequest) // 400
return
}
// ...rest of handler...
fmt.Fprintf(resp, " Search: %+v\n" , data)
}
< / code > < / pre >
< p > 下面的Unpack函数主要完成三件事情。第一, 它调用req.ParseForm()来解析HTTP请求。然后, req.Form将包含所有的请求参数, 不管HTTP客户端使用的是GET还是POST请求方法。< / p >
< p > 下一步, Unpack函数将构建每个结构体成员有效参数名字到成员变量的映射。如果结构体成员有成员标签的话, 有效参数名字可能和实际的成员名字不相同。reflect.Type的Field方法将返回一个reflect.StructField, 里面含有每个成员的名字、类型和可选的成员标签等信息。其中成员标签信息对应reflect.StructTag类型的字符串, 并且提供了Get方法用于解析和根据特定key提取的子串, 例如这里的http:" ..." 形式的子串。< / p >
< p > < u > < i > gopl.io/ch12/params< / i > < / u > < / p >
< pre > < code class = "language-Go" > // Unpack populates the fields of the struct pointed to by ptr
// from the HTTP request parameters in req.
func Unpack(req *http.Request, ptr interface{}) error {
if err := req.ParseForm(); err != nil {
return err
}
// Build map of fields keyed by effective name.
fields := make(map[string]reflect.Value)
v := reflect.ValueOf(ptr).Elem() // the struct variable
for i := 0; i < v.NumField(); i++ {
fieldInfo := v.Type().Field(i) // a reflect.StructField
tag := fieldInfo.Tag // a reflect.StructTag
name := tag.Get(" http" )
if name == " " {
name = strings.ToLower(fieldInfo.Name)
}
fields[name] = v.Field(i)
}
// Update struct field for each parameter in the request.
for name, values := range req.Form {
f := fields[name]
if !f.IsValid() {
continue // ignore unrecognized HTTP parameters
}
for _, value := range values {
if f.Kind() == reflect.Slice {
elem := reflect.New(f.Type().Elem()).Elem()
if err := populate(elem, value); err != nil {
return fmt.Errorf(" %s: %v" , name, err)
}
f.Set(reflect.Append(f, elem))
} else {
if err := populate(f, value); err != nil {
return fmt.Errorf(" %s: %v" , name, err)
}
}
}
}
return nil
}
< / code > < / pre >
< p > 最后, Unpack遍历HTTP请求的name/valu参数键值对, 并且根据更新相应的结构体成员。回想一下, 同一个名字的参数可能出现多次。如果发生这种情况, 并且对应的结构体成员是一个slice, 那么就将所有的参数添加到slice中。其它情况, 对应的成员值将被覆盖, 只有最后一次出现的参数值才是起作用的。< / p >
< p > populate函数小心用请求的字符串类型参数值来填充单一的成员v( 或者是slice类型成员中的单一的元素) 。目前, 它仅支持字符串、有符号整数和布尔型。其中其它的类型将留做练习任务。< / p >
< pre > < code class = "language-Go" > func populate(v reflect.Value, value string) error {
switch v.Kind() {
case reflect.String:
v.SetString(value)
case reflect.Int:
i, err := strconv.ParseInt(value, 10, 64)
if err != nil {
return err
}
v.SetInt(i)
case reflect.Bool:
b, err := strconv.ParseBool(value)
if err != nil {
return err
}
v.SetBool(b)
default:
return fmt.Errorf(" unsupported kind %s" , v.Type())
}
return nil
}
< / code > < / pre >
< p > 如果我们上上面的处理程序添加到一个web服务器, 则可以产生以下的会话: < / p >
< pre > < code > $ go build gopl.io/ch12/search
$ ./search &
$ ./fetch 'http://localhost:12345/search'
Search: {Labels:[] MaxResults:10 Exact:false}
$ ./fetch 'http://localhost:12345/search?l=golang& l=programming'
Search: {Labels:[golang programming] MaxResults:10 Exact:false}
$ ./fetch 'http://localhost:12345/search?l=golang& l=programming& max=100'
Search: {Labels:[golang programming] MaxResults:100 Exact:false}
$ ./fetch 'http://localhost:12345/search?x=true& l=golang& l=programming'
Search: {Labels:[golang programming] MaxResults:10 Exact:true}
$ ./fetch 'http://localhost:12345/search?q=hello& x=123'
x: strconv.ParseBool: parsing " 123" : invalid syntax
$ ./fetch 'http://localhost:12345/search?q=hello& max=lots'
max: strconv.ParseInt: parsing " lots" : invalid syntax
< / code > < / pre >
< p > < strong > 练习 12.11: < / strong > 编写相应的Pack函数, 给定一个结构体值, Pack函数将返回合并了所有结构体成员和值的URL。< / p >
< p > < strong > 练习 12.12: < / strong > 扩展成员标签以表示一个请求参数的有效值规则。例如, 一个字符串可以是有效的email地址或一个信用卡号码, 还有一个整数可能需要是有效的邮政编码。修改Unpack函数以检查这些规则。< / p >
< p > < strong > 练习 12.13: < / strong > 修改S表达式的编码器( §12.4) 和解码器( §12.6) , 采用和encoding/json包( §4.5) 类似的方式使用成员标签中的sexpr:" ..." 字串。< / p >
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