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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 &quot;gopl.io/ch12/params&quot;
// search implements the /search URL endpoint.
func search(resp http.ResponseWriter, req *http.Request) {
var data struct {
Labels []string `http:&quot;l&quot;`
MaxResults int `http:&quot;max&quot;`
Exact bool `http:&quot;x&quot;`
}
data.MaxResults = 10 // set default
if err := params.Unpack(req, &amp;data); err != nil {
http.Error(resp, err.Error(), http.StatusBadRequest) // 400
return
}
// ...rest of handler...
fmt.Fprintf(resp, &quot;Search: %+v\n&quot;, 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:&quot;...&quot;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 &lt; v.NumField(); i++ {
fieldInfo := v.Type().Field(i) // a reflect.StructField
tag := fieldInfo.Tag // a reflect.StructTag
name := tag.Get(&quot;http&quot;)
if name == &quot;&quot; {
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(&quot;%s: %v&quot;, name, err)
}
f.Set(reflect.Append(f, elem))
} else {
if err := populate(f, value); err != nil {
return fmt.Errorf(&quot;%s: %v&quot;, 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(&quot;unsupported kind %s&quot;, 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 &amp;
$ ./fetch 'http://localhost:12345/search'
Search: {Labels:[] MaxResults:10 Exact:false}
$ ./fetch 'http://localhost:12345/search?l=golang&amp;l=programming'
Search: {Labels:[golang programming] MaxResults:10 Exact:false}
$ ./fetch 'http://localhost:12345/search?l=golang&amp;l=programming&amp;max=100'
Search: {Labels:[golang programming] MaxResults:100 Exact:false}
$ ./fetch 'http://localhost:12345/search?x=true&amp;l=golang&amp;l=programming'
Search: {Labels:[golang programming] MaxResults:10 Exact:true}
$ ./fetch 'http://localhost:12345/search?q=hello&amp;x=123'
x: strconv.ParseBool: parsing &quot;123&quot;: invalid syntax
$ ./fetch 'http://localhost:12345/search?q=hello&amp;max=lots'
max: strconv.ParseInt: parsing &quot;lots&quot;: 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:&quot;...&quot;字串。</p>
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