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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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<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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<h2 id="510-recover捕获异常"><a class="header" href="#510-recover捕获异常">5.10. Recover捕获异常</a></h2>
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<p>通常来说,不应该对panic异常做任何处理,但有时,也许我们可以从异常中恢复,至少我们可以在程序崩溃前,做一些操作。举个例子,当web服务器遇到不可预料的严重问题时,在崩溃前应该将所有的连接关闭;如果不做任何处理,会使得客户端一直处于等待状态。如果web服务器还在开发阶段,服务器甚至可以将异常信息反馈到客户端,帮助调试。</p>
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<p>如果在deferred函数中调用了内置函数recover,并且定义该defer语句的函数发生了panic异常,recover会使程序从panic中恢复,并返回panic value。导致panic异常的函数不会继续运行,但能正常返回。在未发生panic时调用recover,recover会返回nil。</p>
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<p>让我们以语言解析器为例,说明recover的使用场景。考虑到语言解析器的复杂性,即使某个语言解析器目前工作正常,也无法肯定它没有漏洞。因此,当某个异常出现时,我们不会选择让解析器崩溃,而是会将panic异常当作普通的解析错误,并附加额外信息提醒用户报告此错误。</p>
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<pre><code class="language-Go">func Parse(input string) (s *Syntax, err error) {
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defer func() {
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if p := recover(); p != nil {
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err = fmt.Errorf("internal error: %v", p)
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}
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}()
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// ...parser...
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}
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</code></pre>
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<p>deferred函数帮助Parse从panic中恢复。在deferred函数内部,panic value被附加到错误信息中;并用err变量接收错误信息,返回给调用者。我们也可以通过调用runtime.Stack往错误信息中添加完整的堆栈调用信息。</p>
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<p>不加区分的恢复所有的panic异常,不是可取的做法;因为在panic之后,无法保证包级变量的状态仍然和我们预期一致。比如,对数据结构的一次重要更新没有被完整完成、文件或者网络连接没有被关闭、获得的锁没有被释放。此外,如果写日志时产生的panic被不加区分的恢复,可能会导致漏洞被忽略。</p>
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<p>虽然把对panic的处理都集中在一个包下,有助于简化对复杂和不可以预料问题的处理,但作为被广泛遵守的规范,你不应该试图去恢复其他包引起的panic。公有的API应该将函数的运行失败作为error返回,而不是panic。同样的,你也不应该恢复一个由他人开发的函数引起的panic,比如说调用者传入的回调函数,因为你无法确保这样做是安全的。</p>
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<p>有时我们很难完全遵循规范,举个例子,net/http包中提供了一个web服务器,将收到的请求分发给用户提供的处理函数。很显然,我们不能因为某个处理函数引发的panic异常,杀掉整个进程;web服务器遇到处理函数导致的panic时会调用recover,输出堆栈信息,继续运行。这样的做法在实践中很便捷,但也会引起资源泄漏,或是因为recover操作,导致其他问题。</p>
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<p>基于以上原因,安全的做法是有选择性的recover。换句话说,只恢复应该被恢复的panic异常,此外,这些异常所占的比例应该尽可能的低。为了标识某个panic是否应该被恢复,我们可以将panic value设置成特殊类型。在recover时对panic value进行检查,如果发现panic value是特殊类型,就将这个panic作为error处理,如果不是,则按照正常的panic进行处理(在下面的例子中,我们会看到这种方式)。</p>
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<p>下面的例子是title函数的变形,如果HTML页面包含多个<code><title></code>,该函数会给调用者返回一个错误(error)。在soleTitle内部处理时,如果检测到有多个<code><title></code>,会调用panic,阻止函数继续递归,并将特殊类型bailout作为panic的参数。</p>
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<pre><code class="language-Go">// soleTitle returns the text of the first non-empty title element
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// in doc, and an error if there was not exactly one.
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func soleTitle(doc *html.Node) (title string, err error) {
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type bailout struct{}
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defer func() {
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switch p := recover(); p {
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case nil: // no panic
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case bailout{}: // "expected" panic
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err = fmt.Errorf("multiple title elements")
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default:
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panic(p) // unexpected panic; carry on panicking
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}
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}()
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// Bail out of recursion if we find more than one nonempty title.
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forEachNode(doc, func(n *html.Node) {
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if n.Type == html.ElementNode && n.Data == "title" &&
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n.FirstChild != nil {
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if title != "" {
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panic(bailout{}) // multiple titleelements
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}
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title = n.FirstChild.Data
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}
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}, nil)
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if title == "" {
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return "", fmt.Errorf("no title element")
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}
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return title, nil
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}
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</code></pre>
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<p>在上例中,deferred函数调用recover,并检查panic value。当panic value是bailout{}类型时,deferred函数生成一个error返回给调用者。当panic value是其他non-nil值时,表示发生了未知的panic异常,deferred函数将调用panic函数并将当前的panic value作为参数传入;此时,等同于recover没有做任何操作。(请注意:在例子中,对可预期的错误采用了panic,这违反了之前的建议,我们在此只是想向读者演示这种机制。)</p>
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<p>有些情况下,我们无法恢复。某些致命错误会导致Go在运行时终止程序,如内存不足。</p>
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<p><strong>练习5.19:</strong> 使用panic和recover编写一个不包含return语句但能返回一个非零值的函数。</p>
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