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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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<main>
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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="58-deferred函数"><a class="header" href="#58-deferred函数">5.8. Deferred函数</a></h2>
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<p>在findLinks的例子中，我们用http.Get的输出作为html.Parse的输入。只有url的内容的确是HTML格式的，html.Parse才可以正常工作，但实际上，url指向的内容很丰富，可能是图片，纯文本或是其他。将这些格式的内容传递给html.parse，会产生不良后果。</p>
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<p>下面的例子获取HTML页面并输出页面的标题。title函数会检查服务器返回的Content-Type字段，如果发现页面不是HTML，将终止函数运行，返回错误。</p>
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<p><u><i>gopl.io/ch5/title1</i></u></p>
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<pre><code class="language-Go">func title(url string) error {
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resp, err := http.Get(url)
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if err != nil {
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return err
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}
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// Check Content-Type is HTML (e.g., "text/html;charset=utf-8").
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ct := resp.Header.Get("Content-Type")
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if ct != "text/html" && !strings.HasPrefix(ct,"text/html;") {
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resp.Body.Close()
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return fmt.Errorf("%s has type %s, not text/html",url, ct)
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}
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doc, err := html.Parse(resp.Body)
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resp.Body.Close()
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if err != nil {
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return fmt.Errorf("parsing %s as HTML: %v", url,err)
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}
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visitNode := func(n *html.Node) {
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if n.Type == html.ElementNode && n.Data == "title"&&n.FirstChild != nil {
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fmt.Println(n.FirstChild.Data)
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}
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}
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forEachNode(doc, visitNode, nil)
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return nil
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}
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</code></pre>
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<p>下面展示了运行效果：</p>
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<pre><code>$ go build gopl.io/ch5/title1
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$ ./title1 http://gopl.io
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The Go Programming Language
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$ ./title1 https://golang.org/doc/effective_go.html
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Effective Go - The Go Programming Language
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$ ./title1 https://golang.org/doc/gopher/frontpage.png
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title1: https://golang.org/doc/gopher/frontpage.png has type image/png, not text/html
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</code></pre>
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<p>resp.Body.close调用了多次，这是为了确保title在所有执行路径下（即使函数运行失败）都关闭了网络连接。随着函数变得复杂，需要处理的错误也变多，维护清理逻辑变得越来越困难。而Go语言独有的defer机制可以让事情变得简单。</p>
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<p>你只需要在调用普通函数或方法前加上关键字defer，就完成了defer所需要的语法。当执行到该条语句时，函数和参数表达式得到计算，但直到包含该defer语句的函数执行完毕时，defer后的函数才会被执行，不论包含defer语句的函数是通过return正常结束，还是由于panic导致的异常结束。你可以在一个函数中执行多条defer语句，它们的执行顺序与声明顺序相反。</p>
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<p>defer语句经常被用于处理成对的操作，如打开、关闭、连接、断开连接、加锁、释放锁。通过defer机制，不论函数逻辑多复杂，都能保证在任何执行路径下，资源被释放。释放资源的defer应该直接跟在请求资源的语句后。在下面的代码中，一条defer语句替代了之前的所有resp.Body.Close</p>
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<p><u><i>gopl.io/ch5/title2</i></u></p>
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<pre><code class="language-Go">func title(url string) error {
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resp, err := http.Get(url)
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if err != nil {
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return err
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}
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defer resp.Body.Close()
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ct := resp.Header.Get("Content-Type")
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if ct != "text/html" && !strings.HasPrefix(ct,"text/html;") {
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return fmt.Errorf("%s has type %s, not text/html",url, ct)
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}
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doc, err := html.Parse(resp.Body)
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if err != nil {
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return fmt.Errorf("parsing %s as HTML: %v", url,err)
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}
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// ...print doc's title element…
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return nil
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}
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</code></pre>
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<p>在处理其他资源时，也可以采用defer机制，比如对文件的操作：</p>
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<p><u><i>io/ioutil</i></u></p>
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<pre><code class="language-Go">package ioutil
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func ReadFile(filename string) ([]byte, error) {
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f, err := os.Open(filename)
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if err != nil {
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return nil, err
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}
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defer f.Close()
|
|||
|
|
return ReadAll(f)
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>或是处理互斥锁（9.2章）</p>
|
|||
|
|
<pre><code class="language-Go">var mu sync.Mutex
|
|||
|
|
var m = make(map[string]int)
|
|||
|
|
func lookup(key string) int {
|
|||
|
|
mu.Lock()
|
|||
|
|
defer mu.Unlock()
|
|||
|
|
return m[key]
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>调试复杂程序时，defer机制也常被用于记录何时进入和退出函数。下例中的bigSlowOperation函数，直接调用trace记录函数的被调情况。bigSlowOperation被调时，trace会返回一个函数值，该函数值会在bigSlowOperation退出时被调用。通过这种方式， 我们可以只通过一条语句控制函数的入口和所有的出口，甚至可以记录函数的运行时间，如例子中的start。需要注意一点：不要忘记defer语句后的圆括号，否则本该在进入时执行的操作会在退出时执行，而本该在退出时执行的，永远不会被执行。</p>
|
|||
|
|
<p><u><i>gopl.io/ch5/trace</i></u></p>
|
|||
|
|
<pre><code class="language-Go">func bigSlowOperation() {
|
|||
|
|
defer trace("bigSlowOperation")() // don't forget the extra parentheses
|
|||
|
|
// ...lots of work…
|
|||
|
|
time.Sleep(10 * time.Second) // simulate slow operation by sleeping
|
|||
|
|
}
|
|||
|
|
func trace(msg string) func() {
|
|||
|
|
start := time.Now()
|
|||
|
|
log.Printf("enter %s", msg)
|
|||
|
|
return func() {
|
|||
|
|
log.Printf("exit %s (%s)", msg,time.Since(start))
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>每一次bigSlowOperation被调用，程序都会记录函数的进入，退出，持续时间。（我们用time.Sleep模拟一个耗时的操作）</p>
|
|||
|
|
<pre><code>$ go build gopl.io/ch5/trace
|
|||
|
|
$ ./trace
|
|||
|
|
2015/11/18 09:53:26 enter bigSlowOperation
|
|||
|
|
2015/11/18 09:53:36 exit bigSlowOperation (10.000589217s)
|
|||
|
|
</code></pre>
|
|||
|
|
<p>我们知道，defer语句中的函数会在return语句更新返回值变量后再执行，又因为在函数中定义的匿名函数可以访问该函数包括返回值变量在内的所有变量，所以，对匿名函数采用defer机制，可以使其观察函数的返回值。</p>
|
|||
|
|
<p>以double函数为例：</p>
|
|||
|
|
<pre><code class="language-Go">func double(x int) int {
|
|||
|
|
return x + x
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>我们只需要首先命名double的返回值，再增加defer语句，我们就可以在double每次被调用时，输出参数以及返回值。</p>
|
|||
|
|
<pre><code class="language-Go">func double(x int) (result int) {
|
|||
|
|
defer func() { fmt.Printf("double(%d) = %d\n", x,result) }()
|
|||
|
|
return x + x
|
|||
|
|
}
|
|||
|
|
_ = double(4)
|
|||
|
|
// Output:
|
|||
|
|
// "double(4) = 8"
|
|||
|
|
</code></pre>
|
|||
|
|
<p>可能double函数过于简单，看不出这个小技巧的作用，但对于有许多return语句的函数而言，这个技巧很有用。</p>
|
|||
|
|
<p>被延迟执行的匿名函数甚至可以修改函数返回给调用者的返回值：</p>
|
|||
|
|
<pre><code class="language-Go">func triple(x int) (result int) {
|
|||
|
|
defer func() { result += x }()
|
|||
|
|
return double(x)
|
|||
|
|
}
|
|||
|
|
fmt.Println(triple(4)) // "12"
|
|||
|
|
</code></pre>
|
|||
|
|
<p>在循环体中的defer语句需要特别注意，因为只有在函数执行完毕后，这些被延迟的函数才会执行。下面的代码会导致系统的文件描述符耗尽，因为在所有文件都被处理之前，没有文件会被关闭。</p>
|
|||
|
|
<pre><code class="language-Go">for _, filename := range filenames {
|
|||
|
|
f, err := os.Open(filename)
|
|||
|
|
if err != nil {
|
|||
|
|
return err
|
|||
|
|
}
|
|||
|
|
defer f.Close() // NOTE: risky; could run out of file descriptors
|
|||
|
|
// ...process f…
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>一种解决方法是将循环体中的defer语句移至另外一个函数。在每次循环时，调用这个函数。</p>
|
|||
|
|
<pre><code class="language-Go">for _, filename := range filenames {
|
|||
|
|
if err := doFile(filename); err != nil {
|
|||
|
|
return err
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
func doFile(filename string) error {
|
|||
|
|
f, err := os.Open(filename)
|
|||
|
|
if err != nil {
|
|||
|
|
return err
|
|||
|
|
}
|
|||
|
|
defer f.Close()
|
|||
|
|
// ...process f…
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>下面的代码是fetch（1.5节）的改进版，我们将http响应信息写入本地文件而不是从标准输出流输出。我们通过path.Base提出url路径的最后一段作为文件名。</p>
|
|||
|
|
<p><u><i>gopl.io/ch5/fetch</i></u></p>
|
|||
|
|
<pre><code class="language-Go">// Fetch downloads the URL and returns the
|
|||
|
|
// name and length of the local file.
|
|||
|
|
func fetch(url string) (filename string, n int64, err error) {
|
|||
|
|
resp, err := http.Get(url)
|
|||
|
|
if err != nil {
|
|||
|
|
return "", 0, err
|
|||
|
|
}
|
|||
|
|
defer resp.Body.Close()
|
|||
|
|
local := path.Base(resp.Request.URL.Path)
|
|||
|
|
if local == "/" {
|
|||
|
|
local = "index.html"
|
|||
|
|
}
|
|||
|
|
f, err := os.Create(local)
|
|||
|
|
if err != nil {
|
|||
|
|
return "", 0, err
|
|||
|
|
}
|
|||
|
|
n, err = io.Copy(f, resp.Body)
|
|||
|
|
// Close file, but prefer error from Copy, if any.
|
|||
|
|
if closeErr := f.Close(); err == nil {
|
|||
|
|
err = closeErr
|
|||
|
|
}
|
|||
|
|
return local, n, err
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>对resp.Body.Close延迟调用我们已经见过了，在此不做解释。上例中，通过os.Create打开文件进行写入，在关闭文件时，我们没有对f.close采用defer机制，因为这会产生一些微妙的错误。许多文件系统，尤其是NFS，写入文件时发生的错误会被延迟到文件关闭时反馈。如果没有检查文件关闭时的反馈信息，可能会导致数据丢失，而我们还误以为写入操作成功。如果io.Copy和f.close都失败了，我们倾向于将io.Copy的错误信息反馈给调用者，因为它先于f.close发生，更有可能接近问题的本质。</p>
|
|||
|
|
<p>**练习5.18：**不修改fetch的行为，重写fetch函数，要求使用defer机制关闭文件。</p>
|
|||
|
|
|
|||
|
|
|
|||
|
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