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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-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</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-06.html"><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a></li></ol></li><li class="chapter-item expanded "><a href="../ch5/ch5.html"><strong aria-hidden="true">5.</strong> 函数</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch5/ch5-01.html"><strong aria-hidden="true">5.1.</strong> 函数声明</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-02.html"><strong aria-hidden="true">5.2.</strong> 递归</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-03.html"><strong aria-hidden="true">5.3.</strong> 多返回值</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-04.html"><strong aria-hidden="true">5.4.</strong> 错误</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-05.html"><strong aria-hidden="true">5.5.</strong> 函数值</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-06.html"><strong aria-hidden="true">5.6.</strong> 匿名函数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-07.html"><strong aria-hidden="true">5.7.</strong> 可变参数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-08.html"><strong aria-hidden="true">5.8.</strong> Deferred函数</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-09.html"><strong aria-hidden="true">5.9.</strong> Panic异常</a></li><li class="chapter-item expanded "><a href="../ch5/ch5-10.html"><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a></li></ol></li><li class="chapter-item expanded "><a href="../ch6/ch6.html"><strong aria-hidden="true">6.</strong> 方法</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch6/ch6-01.html"><strong aria-hidden="true">6.1.</strong> 方法声明</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-02.html"><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-03.html"><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-04.html"><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-05.html"><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a></li><li class="chapter-item expanded "><a href="../ch6/ch6-06.html"><strong aria-hidden="true">6.6.</strong> 封装</a></li></ol></li><li class="chapter-item expanded "><a href="../ch7/ch7.html"><strong aria-hidden="true">7.</strong> 接口</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch7/ch7-01.html"><strong aria-hidden="true">7.1.</strong> 接口是合约</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-02.html"><strong aria-hidden="true">7.2.</strong> 接口类型</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-03.html"><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-04.html"><strong aria-hidden="true">7.4.</strong> flag.Value接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-05.html"><strong aria-hidden="true">7.5.</strong> 接口值</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-06.html"><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-07.html"><strong aria-hidden="true">7.7.</strong> http.Handler接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-08.html"><strong aria-hidden="true">7.8.</strong> error接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-09.html"><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-10.html"><strong aria-hidden="true">7.10.</strong> 类型断言</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-11.html"><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-12.html"><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-13.html"><strong aria-hidden="true">7.13.</strong> 类型分支</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-14.html"><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a></li><li class="chapter-item expanded "><a href="../ch7/ch7-15.html"><strong aria-hidden="true">7.15.</strong> 补充几点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch8/ch8.html"><strong aria-hidden="true">8.</strong> Goroutines和Channels</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch8/ch8-01.html"><strong aria-hidden="true">8.1.</strong> Goroutines</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-02.html"><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-03.html"><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-04.html"><strong aria-hidden="true">8.4.</strong> Channels</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-05.html"><strong aria-hidden="true">8.5.</strong> 并发的循环</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-06.html"><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-07.html"><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-08.html"><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-09.html"><strong aria-hidden="true">8.9.</strong> 并发的退出</a></li><li class="chapter-item expanded "><a href="../ch8/ch8-10.html"><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a></li></ol></li><li class="chapter-item expanded "><a href="../ch9/ch9.html"><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch9/ch9-01.html"><strong aria-hidden="true">9.1.</strong> 竞争条件</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-02.html"><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-03.html" class="active"><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-04.html"><strong aria-hidden="true">9.4.</strong> 内存同步</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-05.html"><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-06.html"><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-07.html"><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a></li><li class="chapter-item expanded "><a href="../ch9/ch9-08.html"><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a></li></ol></li><li class="chapter-item expanded "><a href="../ch10/ch10.html"><strong aria-hidden="true">10.</strong> 包和工具</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch10/ch10-01.html"><strong aria-hidden="true">10.1.</strong> 包简介</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-02.html"><strong aria-hidden="true">10.2.</strong> 导入路径</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-03.html"><strong aria-hidden="true">10.3.</strong> 包声明</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-04.html"><strong aria-hidden="true">10.4.</strong> 导入声明</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-05.html"><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-06.html"><strong aria-hidden="true">10.6.</strong> 包和命名</a></li><li class="chapter-item expanded "><a href="../ch10/ch10-07.html"><strong aria-hidden="true">10.7.</strong> 工具</a></li></ol></li><li class="chapter-item expanded "><a href="../ch11/ch11.html"><strong aria-hidden="true">11.</strong> 测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch11/ch11-01.html"><strong aria-hidden="true">11.1.</strong> go test</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-02.html"><strong aria-hidden="true">11.2.</strong> 测试函数</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-03.html"><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-04.html"><strong aria-hidden="true">11.4.</strong> 基准测试</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-05.html"><strong aria-hidden="true">11.5.</strong> 剖析</a></li><li class="chapter-item expanded "><a href="../ch11/ch11-06.html"><strong aria-hidden="true">11.6.</strong> 示例函数</a></li></ol></li><li class="chapter-item expanded "><a href="../ch12/ch12.html"><strong aria-hidden="true">12.</strong> 反射</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch12/ch12-01.html"><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-02.html"><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-03.html"><strong aria-hidden="true">12.3.</strong> Display递归打印</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-04.html"><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-05.html"><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-06.html"><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-07.html"><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-08.html"><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a></li><li class="chapter-item expanded "><a href="../ch12/ch12-09.html"><strong aria-hidden="true">12.9.</strong> 几点忠告</a></li></ol></li><li class="chapter-item expanded "><a href="../ch13/ch13.html"><strong aria-hidden="true">13.</strong> 底层编程</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch13/ch13-01.html"><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-02.html"><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-03.html"><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-04.html"><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a></li><li class="chapter-item expanded "><a href="../ch13/ch13-05.html"><strong aria-hidden="true">13.5.</strong> 几点忠告</a></li></ol></li><li class="chapter-item expanded "><a href="../appendix/appendix.html"><strong aria-hidden="true">14.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../appendix/appendix-a-errata.html"><strong aria-hidden="true">14.1.</strong> 附录A：原文勘误</a></li><li class="chapter-item expanded "><a href="../appendix/appendix-b-author.html"><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a></li><li class="chapter-item expanded "><a href="../appendix/appendix-c-cpoyright.html"><strong aria-hidden="true">14.3.</strong> 附录C：译文授权</a></li><li class="chapter-item expanded "><a href="../appendix/appendix-d-translations.html"><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a></li></ol></li></ol> </div>
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<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>
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<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>
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<hr>
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<h2 id="93-syncrwmutex读写锁"><a class="header" href="#93-syncrwmutex读写锁">9.3. sync.RWMutex读写锁</a></h2>
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<p>在100刀的存款消失时不做记录多少还是会让我们有一些恐慌，Bob写了一个程序，每秒运行几百次来检查他的银行余额。他会在家，在工作中，甚至会在他的手机上来运行这个程序。银行注意到这些陡增的流量使得存款和取款有了延时，因为所有的余额查询请求是顺序执行的，这样会互斥地获得锁，并且会暂时阻止其它的goroutine运行。</p>
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<p>由于Balance函数只需要读取变量的状态，所以我们同时让多个Balance调用并发运行事实上是安全的，只要在运行的时候没有存款或者取款操作就行。在这种场景下我们需要一种特殊类型的锁，其允许多个只读操作并行执行，但写操作会完全互斥。这种锁叫作“多读单写”锁（multiple readers, single writer lock），Go语言提供的这样的锁是sync.RWMutex：</p>
|
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<pre><code class="language-go">var mu sync.RWMutex
|
||
var balance int
|
||
func Balance() int {
|
||
mu.RLock() // readers lock
|
||
defer mu.RUnlock()
|
||
return balance
|
||
}
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</code></pre>
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<p>Balance函数现在调用了RLock和RUnlock方法来获取和释放一个读取或者共享锁。Deposit函数没有变化，会调用mu.Lock和mu.Unlock方法来获取和释放一个写或互斥锁。</p>
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<p>在这次修改后，Bob的余额查询请求就可以彼此并行地执行并且会很快地完成了。锁在更多的时间范围可用，并且存款请求也能够及时地被响应了。</p>
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<p>RLock只能在临界区共享变量没有任何写入操作时可用。一般来说，我们不应该假设逻辑上的只读函数/方法也不会去更新某一些变量。比如一个方法功能是访问一个变量，但它也有可能会同时去给一个内部的计数器+1（译注：可能是记录这个方法的访问次数啥的），或者去更新缓存——使即时的调用能够更快。如果有疑惑的话，请使用互斥锁。</p>
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<p>RWMutex只有当获得锁的大部分goroutine都是读操作，而锁在竞争条件下，也就是说，goroutine们必须等待才能获取到锁的时候，RWMutex才是最能带来好处的。RWMutex需要更复杂的内部记录，所以会让它比一般的无竞争锁的mutex慢一些。</p>
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