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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
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<hr>
<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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