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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.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</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-06.html"><strong aria-hidd
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<!-- 头部 -->
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<ul dir="auto">
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<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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</ul>
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<hr>
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<h2 id="77-httphandler接口"><a class="header" href="#77-httphandler接口">7.7. http.Handler接口</a></h2>
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<p>在第一章中，我们粗略的了解了怎么用net/http包去实现网络客户端（§1.5）和服务器（§1.7）。在这个小节中，我们会对那些基于http.Handler接口的服务器API做更进一步的学习：</p>
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<p><u><i>net/http</i></u></p>
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<pre><code class="language-go">package http
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type Handler interface {
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ServeHTTP(w ResponseWriter, r *Request)
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}
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func ListenAndServe(address string, h Handler) error
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</code></pre>
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<p>ListenAndServe函数需要一个例如“localhost:8000”的服务器地址，和一个所有请求都可以分派的Handler接口实例。它会一直运行，直到这个服务因为一个错误而失败（或者启动失败），它的返回值一定是一个非空的错误。</p>
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<p>想象一个电子商务网站，为了销售，将数据库中物品的价格映射成美元。下面这个程序可能是能想到的最简单的实现了。它将库存清单模型化为一个命名为database的map类型，我们给这个类型一个ServeHttp方法，这样它可以满足http.Handler接口。这个handler会遍历整个map并输出物品信息。</p>
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<p><u><i>gopl.io/ch7/http1</i></u></p>
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<pre><code class="language-go">func main() {
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db := database{"shoes": 50, "socks": 5}
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log.Fatal(http.ListenAndServe("localhost:8000", db))
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}
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type dollars float32
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func (d dollars) String() string { return fmt.Sprintf("$%.2f", d) }
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type database map[string]dollars
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func (db database) ServeHTTP(w http.ResponseWriter, req *http.Request) {
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for item, price := range db {
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fmt.Fprintf(w, "%s: %s\n", item, price)
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}
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}
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</code></pre>
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<p>如果我们启动这个服务，</p>
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<pre><code>$ go build gopl.io/ch7/http1
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$ ./http1 &
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</code></pre>
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<p>然后用1.5节中的获取程序（如果你更喜欢可以使用web浏览器）来连接服务器，我们得到下面的输出：</p>
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<pre><code>$ go build gopl.io/ch1/fetch
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$ ./fetch http://localhost:8000
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shoes: $50.00
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socks: $5.00
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</code></pre>
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<p>目前为止，这个服务器不考虑URL，只能为每个请求列出它全部的库存清单。更真实的服务器会定义多个不同的URL，每一个都会触发一个不同的行为。让我们使用/list来调用已经存在的这个行为并且增加另一个/price调用表明单个货品的价格，像这样/price?item=socks来指定一个请求参数。</p>
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<p><u><i>gopl.io/ch7/http2</i></u></p>
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<pre><code class="language-go">func (db database) ServeHTTP(w http.ResponseWriter, req *http.Request) {
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switch req.URL.Path {
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case "/list":
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for item, price := range db {
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fmt.Fprintf(w, "%s: %s\n", item, price)
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}
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case "/price":
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item := req.URL.Query().Get("item")
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price, ok := db[item]
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if !ok {
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w.WriteHeader(http.StatusNotFound) // 404
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fmt.Fprintf(w, "no such item: %q\n", item)
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return
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}
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fmt.Fprintf(w, "%s\n", price)
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default:
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w.WriteHeader(http.StatusNotFound) // 404
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fmt.Fprintf(w, "no such page: %s\n", req.URL)
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}
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}
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</code></pre>
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<p>现在handler基于URL的路径部分（req.URL.Path）来决定执行什么逻辑。如果这个handler不能识别这个路径，它会通过调用w.WriteHeader(http.StatusNotFound)返回客户端一个HTTP错误；这个检查应该在向w写入任何值前完成。（顺便提一下，http.ResponseWriter是另一个接口。它在io.Writer上增加了发送HTTP相应头的方法。）等效地，我们可以使用实用的http.Error函数：</p>
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<pre><code class="language-go">msg := fmt.Sprintf("no such page: %s\n", req.URL)
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http.Error(w, msg, http.StatusNotFound) // 404
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</code></pre>
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|
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<p>/price的case会调用URL的Query方法来将HTTP请求参数解析为一个map，或者更准确地说一个net/url包中url.Values(§6.2.1)类型的多重映射。然后找到第一个item参数并查找它的价格。如果这个货品没有找到会返回一个错误。</p>
|
|||
|
|
<p>这里是一个和新服务器会话的例子：</p>
|
|||
|
|
<pre><code>$ go build gopl.io/ch7/http2
|
|||
|
|
$ go build gopl.io/ch1/fetch
|
|||
|
|
$ ./http2 &
|
|||
|
|
$ ./fetch http://localhost:8000/list
|
|||
|
|
shoes: $50.00
|
|||
|
|
socks: $5.00
|
|||
|
|
$ ./fetch http://localhost:8000/price?item=socks
|
|||
|
|
$5.00
|
|||
|
|
$ ./fetch http://localhost:8000/price?item=shoes
|
|||
|
|
$50.00
|
|||
|
|
$ ./fetch http://localhost:8000/price?item=hat
|
|||
|
|
no such item: "hat"
|
|||
|
|
$ ./fetch http://localhost:8000/help
|
|||
|
|
no such page: /help
|
|||
|
|
</code></pre>
|
|||
|
|
<p>显然我们可以继续向ServeHTTP方法中添加case，但在一个实际的应用中，将每个case中的逻辑定义到一个分开的方法或函数中会很实用。此外，相近的URL可能需要相似的逻辑；例如几个图片文件可能有形如/images/*.png的URL。因为这些原因，net/http包提供了一个请求多路器ServeMux来简化URL和handlers的联系。一个ServeMux将一批http.Handler聚集到一个单一的http.Handler中。再一次，我们可以看到满足同一接口的不同类型是可替换的：web服务器将请求指派给任意的http.Handler
|
|||
|
|
而不需要考虑它后面的具体类型。</p>
|
|||
|
|
<p>对于更复杂的应用，一些ServeMux可以通过组合来处理更加错综复杂的路由需求。Go语言目前没有一个权威的web框架，就像Ruby语言有Rails和python有Django。这并不是说这样的框架不存在，而是Go语言标准库中的构建模块就已经非常灵活以至于这些框架都是不必要的。此外，尽管在一个项目早期使用框架是非常方便的，但是它们带来额外的复杂度会使长期的维护更加困难。</p>
|
|||
|
|
<p>在下面的程序中，我们创建一个ServeMux并且使用它将URL和相应处理/list和/price操作的handler联系起来，这些操作逻辑都已经被分到不同的方法中。然后我们在调用ListenAndServe函数中使用ServeMux为主要的handler。</p>
|
|||
|
|
<p><u><i>gopl.io/ch7/http3</i></u></p>
|
|||
|
|
<pre><code class="language-go">func main() {
|
|||
|
|
db := database{"shoes": 50, "socks": 5}
|
|||
|
|
mux := http.NewServeMux()
|
|||
|
|
mux.Handle("/list", http.HandlerFunc(db.list))
|
|||
|
|
mux.Handle("/price", http.HandlerFunc(db.price))
|
|||
|
|
log.Fatal(http.ListenAndServe("localhost:8000", mux))
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
type database map[string]dollars
|
|||
|
|
|
|||
|
|
func (db database) list(w http.ResponseWriter, req *http.Request) {
|
|||
|
|
for item, price := range db {
|
|||
|
|
fmt.Fprintf(w, "%s: %s\n", item, price)
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
func (db database) price(w http.ResponseWriter, req *http.Request) {
|
|||
|
|
item := req.URL.Query().Get("item")
|
|||
|
|
price, ok := db[item]
|
|||
|
|
if !ok {
|
|||
|
|
w.WriteHeader(http.StatusNotFound) // 404
|
|||
|
|
fmt.Fprintf(w, "no such item: %q\n", item)
|
|||
|
|
return
|
|||
|
|
}
|
|||
|
|
fmt.Fprintf(w, "%s\n", price)
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>让我们关注这两个注册到handlers上的调用。第一个db.list是一个方法值（§6.4），它是下面这个类型的值。</p>
|
|||
|
|
<pre><code class="language-go">func(w http.ResponseWriter, req *http.Request)
|
|||
|
|
</code></pre>
|
|||
|
|
<p>也就是说db.list的调用会援引一个接收者是db的database.list方法。所以db.list是一个实现了handler类似行为的函数，但是因为它没有方法（理解：该方法没有它自己的方法），所以它不满足http.Handler接口并且不能直接传给mux.Handle。</p>
|
|||
|
|
<p>语句http.HandlerFunc(db.list)是一个转换而非一个函数调用，因为http.HandlerFunc是一个类型。它有如下的定义：</p>
|
|||
|
|
<p><u><i>net/http</i></u></p>
|
|||
|
|
<pre><code class="language-go">package http
|
|||
|
|
|
|||
|
|
type HandlerFunc func(w ResponseWriter, r *Request)
|
|||
|
|
|
|||
|
|
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
|
|||
|
|
f(w, r)
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>HandlerFunc显示了在Go语言接口机制中一些不同寻常的特点。这是一个实现了接口http.Handler的方法的函数类型。ServeHTTP方法的行为是调用了它的函数本身。因此HandlerFunc是一个让函数值满足一个接口的适配器，这里函数和这个接口仅有的方法有相同的函数签名。实际上，这个技巧让一个单一的类型例如database以多种方式满足http.Handler接口：一种通过它的list方法，一种通过它的price方法等等。</p>
|
|||
|
|
<p>因为handler通过这种方式注册非常普遍，ServeMux有一个方便的HandleFunc方法，它帮我们简化handler注册代码成这样：</p>
|
|||
|
|
<p><u><i>gopl.io/ch7/http3a</i></u></p>
|
|||
|
|
<pre><code class="language-go">mux.HandleFunc("/list", db.list)
|
|||
|
|
mux.HandleFunc("/price", db.price)
|
|||
|
|
</code></pre>
|
|||
|
|
<p>从上面的代码很容易看出应该怎么构建一个程序：由两个不同的web服务器监听不同的端口，并且定义不同的URL将它们指派到不同的handler。我们只要构建另外一个ServeMux并且再调用一次ListenAndServe（可能并行的）。但是在大多数程序中，一个web服务器就足够了。此外，在一个应用程序的多个文件中定义HTTP handler也是非常典型的，如果它们必须全部都显式地注册到这个应用的ServeMux实例上会比较麻烦。</p>
|
|||
|
|
<p>所以为了方便，net/http包提供了一个全局的ServeMux实例DefaultServerMux和包级别的http.Handle和http.HandleFunc函数。现在，为了使用DefaultServeMux作为服务器的主handler，我们不需要将它传给ListenAndServe函数；nil值就可以工作。</p>
|
|||
|
|
<p>然后服务器的主函数可以简化成：</p>
|
|||
|
|
<p><u><i>gopl.io/ch7/http4</i></u></p>
|
|||
|
|
<pre><code class="language-go">func main() {
|
|||
|
|
db := database{"shoes": 50, "socks": 5}
|
|||
|
|
http.HandleFunc("/list", db.list)
|
|||
|
|
http.HandleFunc("/price", db.price)
|
|||
|
|
log.Fatal(http.ListenAndServe("localhost:8000", nil))
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>最后，一个重要的提示：就像我们在1.7节中提到的，web服务器在一个新的协程中调用每一个handler，所以当handler获取其它协程或者这个handler本身的其它请求也可以访问到变量时，一定要使用预防措施，比如锁机制。我们后面的两章中将讲到并发相关的知识。</p>
|
|||
|
|
<p><strong>练习 7.11：</strong> 增加额外的handler让客户端可以创建，读取，更新和删除数据库记录。例如，一个形如 <code>/update?item=socks&price=6</code> 的请求会更新库存清单里一个货品的价格并且当这个货品不存在或价格无效时返回一个错误值。（注意：这个修改会引入变量同时更新的问题）</p>
|
|||
|
|
<p><strong>练习 7.12：</strong> 修改/list的handler让它把输出打印成一个HTML的表格而不是文本。html/template包（§4.6）可能会对你有帮助。</p>
|
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