ehlxr/gopl-zh.github.com
1
0
Fork 0
mirror of https://github.com/gopl-zh/gopl-zh.github.com.git synced 2024-11-24 15:18:57 +00:00
Code Issues Projects Releases Wiki Activity
15f2a4c650
gopl-zh.github.com/ch5/ch5-10.html

289 lines
34 KiB
HTML
Raw Normal View History

deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<!DOCTYPE HTML>
<html lang="zh" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Recover捕获异常 - Go语言圣经</title>
<!-- Custom HTML head -->
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<meta name="description" content="&lt;The Go Programming Language&gt;中文版">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<link rel="icon" href="../favicon.svg">
<link rel="shortcut icon" href="../favicon.png">
<link rel="stylesheet" href="../css/variables.css">
<link rel="stylesheet" href="../css/general.css">
<link rel="stylesheet" href="../css/chrome.css">
<link rel="stylesheet" href="../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="../highlight.css">
<link rel="stylesheet" href="../tomorrow-night.css">
<link rel="stylesheet" href="../ayu-highlight.css">
<!-- Custom theme stylesheets -->
<link rel="stylesheet" href="../style.css">
</head>
<body>
<!-- Provide site root to javascript -->
<script type="text/javascript">
var path_to_root = "../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script type="text/javascript">
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
deploy: 863c1951eaf46b8ebba056e50fac7bed767ffc39
2023-06-26 17:30:52 +00:00
<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-zh.html">译者序</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</
deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky bordered">
<div class="left-buttons">
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</button>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Go语言圣经</h1>
<div class="right-buttons">
<a href="../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/gopl-zh/gopl-zh.github.com" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
<a href="https://github.com/gopl-zh/gopl-zh.github.com/edit/master/./ch5/ch5-10.md" title="Suggest an edit" aria-label="Suggest an edit">
<i id="git-edit-button" class="fa fa-edit"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script type="text/javascript">
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<!-- Page table of contents -->
<div class="sidetoc"><nav class="pagetoc"></nav></div>
<main>
<!-- 头部 -->
<ul dir="auto">
deploy: d14d1d0a9cf38240bfa93c6f2a5d961f306145a6
2024-06-12 07:20:07 +00:00
<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>
<li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
deploy: f4a7cd48b47cf9fc8efd37402a01bbfc55ff01c7
2023-06-26 17:46:27 +00:00
</ul>
deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<hr>
<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>
<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>
<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>
<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>
<pre><code class="language-Go">func Parse(input string) (s *Syntax, err error) {
defer func() {
if p := recover(); p != nil {
err = fmt.Errorf(&quot;internal error: %v&quot;, p)
}
}()
// ...parser...
}
</code></pre>
deploy: 1597ed7bcd228dd4c8bf769e72bd32eebb6d19ba
2022-10-06 07:02:50 +00:00
<p>recover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>
deploy: 06a1bdf7353d7ab131c932b22e155c41f763dde3
2022-08-04 06:59:33 +00:00
<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>
<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>
<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>
<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>
<p>下面的例子是title函数的变形，如果HTML页面包含多个<code>&lt;title&gt;</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>&lt;title&gt;</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>
<pre><code class="language-Go">// soleTitle returns the text of the first non-empty title element
// in doc, and an error if there was not exactly one.
func soleTitle(doc *html.Node) (title string, err error) {
type bailout struct{}
defer func() {
switch p := recover(); p {
case nil: // no panic
case bailout{}: // &quot;expected&quot; panic
err = fmt.Errorf(&quot;multiple title elements&quot;)
default:
panic(p) // unexpected panic; carry on panicking
}
}()
// Bail out of recursion if we find more than one nonempty title.
forEachNode(doc, func(n *html.Node) {
if n.Type == html.ElementNode &amp;&amp; n.Data == &quot;title&quot; &amp;&amp;
n.FirstChild != nil {
if title != &quot;&quot; {
panic(bailout{}) // multiple titleelements
}
title = n.FirstChild.Data
}
}, nil)
if title == &quot;&quot; {
return &quot;&quot;, fmt.Errorf(&quot;no title element&quot;)
}
return title, nil
}
</code></pre>
<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>
<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>
<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>
<!-- 公众号 -->
<hr>
<table>
<tr>
<td>
<img width="222px" src="https://chai2010.cn/advanced-go-programming-book/css.png">
</td>
<td>
<img width="222px" src="https://chai2010.cn/advanced-go-programming-book/cch.png">
</td>
</tr>
</table>
<div id="giscus-container"></div>
<footer class="page-footer">
<span>© 2015-2016 | <a href="https://github.com/gopl-zh"> Go语言圣经中文版</a>, 仅学习交流使用</span>
</footer>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../ch5/ch5-09.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../ch6/ch6.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../ch5/ch5-09.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../ch6/ch6.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script type="text/javascript">
window.playground_copyable = true;
</script>
<script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../mark.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../searcher.js" type="text/javascript" charset="utf-8"></script>
<script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script>
<script src="../highlight.js" type="text/javascript" charset="utf-8"></script>
<script src="../book.js" type="text/javascript" charset="utf-8"></script>
<script type="text/javascript" charset="utf-8">
var pagePath = "ch5/ch5-10.md"
</script>
<!-- Custom JS scripts -->
<script type="text/javascript" src="../js/custom.js"></script>
<script type="text/javascript" src="../js/bigPicture.js"></script>
</body>
</html>
Reference in New Issue Copy Permalink