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 = "<The Go Programming Language>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" >
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< /
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" >
2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
< / ul >
< 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(" internal error: %v" , p)
}
}()
// ...parser...
}
< / code > < / pre >
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 >
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 > < 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 >
< 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{}: // " expected" panic
err = fmt.Errorf(" multiple title elements" )
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 & & n.Data == " title" & &
n.FirstChild != nil {
if title != " " {
panic(bailout{}) // multiple titleelements
}
title = n.FirstChild.Data
}
}, nil)
if title == " " {
return " " , fmt.Errorf(" no title element" )
}
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 >