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</script><!-- hexo injector head_end end --><meta name="generator" content="Hexo 7.3.0"></head><body class="is-2-column"><nav class="navbar navbar-main"><div class="container navbar-container"><div class="navbar-brand justify-content-center"><a class="navbar-item navbar-logo" href="/"><img src="/img/logo.svg" alt="w1m's Blog" height="28"></a></div><div class="navbar-menu"><div class="navbar-start"><a class="navbar-item is-active" href="/">Home</a><a class="navbar-item" href="/archives">Archives</a><a class="navbar-item" href="/categories">Categories</a><a class="navbar-item" href="/tags">Tags</a><a class="navbar-item" href="/about">About</a></div><div class="navbar-end"><a class="navbar-item" target="_blank" rel="noopener" title="Download on GitHub" href="https://github.com/ppoffice/hexo-theme-icarus"><i class="fab fa-github"></i></a><a class="navbar-item search" title="搜索" href="javascript:;"><i class="fas fa-search"></i></a></div></div></div></nav><section class="section"><div class="container"><div class="columns"><div class="column order-2 column-main is-8-tablet is-8-desktop is-8-widescreen"><div class="card"><article class="card-content article" role="article"><div class="article-meta is-size-7 is-uppercase level is-mobile"><div class="level-left"><span class="level-item"><time dateTime="2025-06-20T08:08:54.000Z" title="2025/6/20 16:08:54">2025-06-20</time>发表</span><span class="level-item"><time dateTime="2025-06-20T08:11:40.286Z" title="2025/6/20 16:11:40">2025-06-20</time>更新</span><span class="level-item">10 分钟读完 (大约1558个字)</span></div></div><p class="title is-3 is-size-4-mobile"><a class="link-muted" href="/2025/06/20/%E9%81%93%E6%99%AE%E4%BA%91%E9%9D%A2%E8%AF%95/">道普云面试</a></p><div class="content"><ul>
<li>自我介绍</li>
</ul>
<hr>
<ul>
<li>java基础： List了解，ArrayList底层实现，延伸出扩容机制；Hashmap底层实现；<br>答出了ArrayList动态数组实现，扩容机制（Arrays.copyOf创建新数组并迁移）。问到具体的容量和扩容倍数没有答上来。<br>Hashmap底层什么实现的提示后说了数组加链表；实际上是数组+链表，链表长度≥8且数组长度≥64时，链表转红黑树。没有问Hashmap扩容，看来是看这个样子我也答不上来。</li>
</ul>
<blockquote>
<p> <strong>HashMap扩容机制</strong></p>
</blockquote>
<p> <strong>触发条件：当 <code>HashMap</code> 中的元素数量（<code>size</code>）超过 <code>capacity * loadFactor</code> 时触发扩容。</strong></p>
<ul>
<li><code>capacity</code>：当前数组（<code>Node<K,V>[] table</code>）的长度（默认初始 <code>16</code>）</li>
<li><code>loadFactor</code>：负载因子（默认 <code>0.75</code>）<br> <strong>扩容过程</strong></li>
</ul>
<ol>
<li><strong>计算新容量：</strong><code>新容量 = 旧容量 << 1</code>（最大容量<code>1 << 30</code>）</li>
<li><strong>数据迁移：</strong> 遍历旧数组，重新计算每个键值对的哈希值，并分配到新数组的对应位置 。由于新容量是 <code>2 倍</code>，元素在新数组的位置要么是 <code>原位置</code>，要么是 <code>原位置 + 旧容量</code>。</li>
<li><strong>链表拆分：</strong> 如果某个桶是链表，会拆分成两个链表（<code>low</code> 和 <code>high</code>），分别放在新数组的 <code>原位置</code> 和 <code>原位置 + 旧容量</code></li>
<li><strong>树形优化：</strong> 如果某个桶已经是红黑树，会检查是否需要拆分成两棵树或退化为链表。</li>
</ol>
<hr>
<ul>
<li>商城项目为什么要用redis缓存商品数据和营业状态数据，被纠正不能把Redis当作DB，因为持久化。讲了一些RDBAOF持久化的机制，询问有没有了解过mysql和Redis如何保持数据一致性；<blockquote>
<p><strong>为什么用Redis</strong></p>
</blockquote>
</li>
<li><strong>商品数据</strong>：高频读取，缓存减轻MySQL压力，响应速度提升。</li>
<li><strong>营业状态</strong>：需要快速切换（如打烊状态），Redis的原子操作（如<code>SETNX</code>）更高效。<blockquote>
<p><strong>持久化问题</strong></p>
</blockquote>
</li>
<li><strong>RDB</strong>：定时快照，适合备份但可能丢失最近数据。</li>
<li><strong>AOF</strong>：记录写命令，可配置为<code>everysec</code>平衡性能与安全。<blockquote>
<p><strong>数据一致性方案</strong></p>
</blockquote>
</li>
<li><strong>双写</strong>：先更新DB再删缓存（存在短暂不一致）。</li>
<li><strong>监听Binlog</strong>：通过Canal将MySQL变更同步到Redis（最终一致性）</li>
</ul>
<hr>
<ul>
<li>问mysql索引底层实现的数据结构，有哪些情况会导致索引失效，mysql调优相关<br>mysql索引答描述性的了B+树，突然想不起来链接叶子节点的叫双向链表，描述了很久。索引失效、调优都说了一点《数据库原理》课上讲的内容。<br>参考：<br><a target="_blank" rel="noopener" href="https://www.cnblogs.com/dgp-zjz/p/18743672">mysql 索引失效及其解决办法 - 凡人半睁眼 - 博客园</a><br><a target="_blank" rel="noopener" href="https://www.cnblogs.com/lgx211/p/18499524">一文彻底弄懂MySQL的优化 - lgx211 - 博客园</a></li>
</ul>
<hr>
<ul>
<li>问Spring常用注解，配置类用哪个注解<br>答得很磕绊，所以被直接问配置类注解是什么，答了@Bean，提示着说出了@Configuration。</li>
</ul>
<blockquote>
<p>Spring常用注解</p>
</blockquote>
<ul>
<li><strong>常用注解</strong>：<ul>
<li><code>@Controller</code>/<code>@RestController</code>：定义Web层。</li>
<li><code>@Service</code>：业务逻辑层。</li>
<li><code>@Autowired</code>：依赖注入。</li>
<li><code>@Transactional</code>：声明事务。</li>
</ul>
</li>
<li><strong>配置类注解</strong>：<code>@Configuration</code> + <code>@Bean</code>。<br><code>@Configuration</code> 是一个类级别的注解，用于标识一个类作为 Spring 应用上下文的配置源。<code>@Bean</code> 是一个方法级别的注解，用于指示方法返回一个由 Spring 容器管理的 bean。</li>
</ul>
<hr>
<ul>
<li><p>询问rvos是微内核还是宏内核；具体实现了哪些功能；有没有设计虚拟内存还是直接映射的物理内存；进程调度有没有优先级；<br>实际上根本不知道微内核和宏内核的区别，被教了很久。其他就是照常交代情况。</p>
<blockquote>
<p>微内核与宏内核</p>
</blockquote>
</li>
<li><p><strong>宏内核：</strong> 简单来说，就是把很多东西都集成进内核，例如linux内核，除了最基本的进程、线程管理、内存管理外，文件系统，驱动，网络协议等等都在内核里面。优点是效率高。缺点是稳定性差，开发过程中的bug经常会导致整个系统挂掉。</p>
</li>
<li><p><strong>微内核：</strong> 内核中只有最基本的调度、内存管理。驱动、文件系统等都是用户态的守护进程去实现的。优点是超级稳定，驱动等的错误只会导致相应进程死掉，不会导致整个系统都崩溃，做驱动开发时，发现错误，只需要kill掉进程，修正后重启进程就行了，比较方便。缺点是效率低。典型代表QNX，QNX的文件系统是跑在用户态的进程，称为resmgr的东西，是订阅发布机制，文件系统的错误只会导致这个守护进程挂掉。不过数据吞吐量就比较不乐观了。</p>
</li>
</ul>
<hr>
<ul>
<li>OpenEuler适配的时候是否需要修改Doris源码；实习的时候TPC-H数据集怎么生成的；如何对比在x86和RISC-V上的表现；base64优化具体做了什么；<br>照着简历答了些东西，似乎感觉有点浅了。</li>
</ul>
<hr>
<ul>
<li>有没有了解过HTTPS加密算法<br>没有了解过。<blockquote>
<p>HTTPS加密原理<br><a target="_blank" rel="noopener" href="https://cloud.tencent.com/developer/article/2523484">【网络编程】九、详解 HTTPS 加密原理-腾讯云开发者社区-腾讯云</a></p>
</blockquote>
</li>
</ul>
<hr>
<ul>
<li>对shell了解如何<br>说偶尔会用到shell脚本配置环境。</li>
</ul>
<hr>
<ul>
<li>AI专业作业一般做的是什么相关的。<br>没多讲，就说了下一些课堂任务。</li>
</ul>
<hr>
<ul>
<li>有没有用过matlab<br>说自己数学建模比赛用过。</li>
</ul>
</div></article></div><div class="card"><article class="card-content article" role="article"><div class="article-meta is-size-7 is-uppercase level is-mobile"><div class="level-left"><span class="level-item"><time dateTime="2025-06-08T08:51:36.000Z" title="2025/6/8 16:51:36">2025-06-08</time>发表</span><span class="level-item"><time dateTime="2025-06-08T09:30:30.823Z" title="2025/6/8 17:30:30">2025-06-08</time>更新</span><span class="level-item">9 分钟读完 (大约1401个字)</span></div></div><p class="title is-3 is-size-4-mobile"><a class="link-muted" href="/2025/06/08/%E6%9B%B2%E9%80%9F%E7%A7%91%E6%8A%80%E7%AC%94%E8%AF%95/">曲速科技笔试</a></p><div class="content"><h2><span id="a-电话系统设计">A 电话系统设计</span></h2><p>Q:设计一个简单电话，0-9数字键，拨号/挂机键；拨号键接通电话后会变成挂机键。考虑到扩展性，如追加*按键，添加按键音（类图或伪代码）<br>A：<br>以下是AI给出的答案。把电话的不同状态抽象成独立类确实比起设置按键状态更易拓展。</p>
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class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br><span class="line">138</span><br><span class="line">139</span><br><span class="line">140</span><br><span class="line">141</span><br><span class="line">142</span><br><span class="line">143</span><br><span class="line">144</span><br><span class="line">145</span><br><span class="line">146</span><br><span class="line">147</span><br><span class="line">148</span><br><span class="line">149</span><br><span class="line">150</span><br><span class="line">151</span><br><span class="line">152</span><br><span class="line">153</span><br><span class="line">154</span><br><span class="line">155</span><br><span class="line">156</span><br><span class="line">157</span><br><span class="line">158</span><br><span class="line">159</span><br><span class="line">160</span><br><span class="line">161</span><br><span class="line">162</span><br><span class="line">163</span><br><span class="line">164</span><br><span class="line">165</span><br><span class="line">166</span><br><span class="line">167</span><br><span class="line">168</span><br><span class="line">169</span><br><span class="line">170</span><br><span class="line">171</span><br><span class="line">172</span><br><span class="line">173</span><br><span class="line">174</span><br><span class="line">175</span><br><span class="line">176</span><br><span class="line">177</span><br><span class="line">178</span><br><span class="line">179</span><br><span class="line">180</span><br><span class="line">181</span><br><span class="line">182</span><br><span class="line">183</span><br><span class="line">184</span><br><span class="line">185</span><br><span class="line">186</span><br><span class="line">187</span><br><span class="line">188</span><br><span class="line">189</span><br><span class="line">190</span><br><span class="line">191</span><br><span class="line">192</span><br><span class="line">193</span><br><span class="line">194</span><br><span class="line">195</span><br><span class="line">196</span><br><span class="line">197</span><br><span class="line">198</span><br><span class="line">199</span><br><span class="line">200</span><br><span class="line">201</span><br><span class="line">202</span><br><span class="line">203</span><br><span class="line">204</span><br><span class="line">205</span><br><span class="line">206</span><br><span class="line">207</span><br><span class="line">208</span><br><span class="line">209</span><br><span class="line">210</span><br><span class="line">211</span><br><span class="line">212</span><br><span class="line">213</span><br><span class="line">214</span><br><span class="line">215</span><br><span class="line">216</span><br><span class="line">217</span><br><span class="line">218</span><br><span class="line">219</span><br><span class="line">220</span><br><span class="line">221</span><br><span class="line">222</span><br><span class="line">223</span><br><span class="line">224</span><br><span class="line">225</span><br><span class="line">226</span><br><span class="line">227</span><br><span class="line">228</span><br><span class="line">229</span><br><span class="line">230</span><br><span class="line">231</span><br><span class="line">232</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> java.util.HashMap; </span><br><span class="line"><span class="keyword">import</span> java.util.Map; </span><br><span class="line"> </span><br><span class="line"><span class="comment">// 状态接口 </span></span><br><span class="line"><span class="keyword">interface</span> <span class="title class_">PhoneState</span> { </span><br><span class="line"> <span class="keyword">void</span> <span class="title function_">handleDigit</span><span class="params">(<span class="type">char</span> digit)</span>; </span><br><span class="line"> <span class="keyword">void</span> <span class="title function_">handleDial</span><span class="params">()</span>; </span><br><span class="line"> <span class="keyword">void</span> <span class="title function_">handleStar</span><span class="params">()</span>; <span class="comment">// 扩展特殊键 </span></span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="comment">// 具体状态实现 </span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">ReadyState</span> <span class="keyword">implements</span> <span class="title class_">PhoneState</span> { </span><br><span class="line"> <span class="keyword">private</span> Phone phone; </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">ReadyState</span><span class="params">(Phone phone)</span> { </span><br><span class="line"> <span class="built_in">this</span>.phone = phone; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleDigit</span><span class="params">(<span class="type">char</span> digit)</span> { </span><br><span class="line"> phone.setCurrentNumber(<span class="string">""</span> + digit); </span><br><span class="line"> phone.getSoundService().playTone(digit); </span><br><span class="line"> phone.setState(<span class="keyword">new</span> <span class="title class_">DialingState</span>(phone)); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleDial</span><span class="params">()</span> { </span><br><span class="line"> <span class="comment">// 待机状态按拨号键无响应 </span></span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleStar</span><span class="params">()</span> { </span><br><span class="line"> phone.getSoundService().playTone(<span class="string">'*'</span>); </span><br><span class="line"> <span class="comment">// 可扩展*键功能 </span></span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="keyword">class</span> <span class="title class_">DialingState</span> <span class="keyword">implements</span> <span class="title class_">PhoneState</span> { </span><br><span class="line"> <span class="keyword">private</span> Phone phone; </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">DialingState</span><span class="params">(Phone phone)</span> { </span><br><span class="line"> <span class="built_in">this</span>.phone = phone; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleDigit</span><span class="params">(<span class="type">char</span> digit)</span> { </span><br><span class="line"> phone.setCurrentNumber(phone.getCurrentNumber() + digit); </span><br><span class="line"> phone.getSoundService().playTone(digit); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleDial</span><span class="params">()</span> { </span><br><span class="line"> System.out.println(<span class="string">"Dialing: "</span> + phone.getCurrentNumber()); </span><br><span class="line"> phone.setState(<span class="keyword">new</span> <span class="title class_">ConnectedState</span>(phone)); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleStar</span><span class="params">()</span> { </span><br><span class="line"> phone.getSoundService().playTone(<span class="string">'*'</span>); </span><br><span class="line"> <span class="comment">// 可扩展*键功能 </span></span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="keyword">class</span> <span class="title class_">ConnectedState</span> <span class="keyword">implements</span> <span class="title class_">PhoneState</span> { </span><br><span class="line"> <span class="keyword">private</span> Phone phone; </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">ConnectedState</span><span class="params">(Phone phone)</span> { </span><br><span class="line"> <span class="built_in">this</span>.phone = phone; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleDigit</span><span class="params">(<span class="type">char</span> digit)</span> { </span><br><span class="line"> <span class="comment">// 通话中输入数字（如DTMF） </span></span><br><span class="line"> phone.getSoundService().playTone(digit); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleDial</span><span class="params">()</span> { </span><br><span class="line"> System.out.println(<span class="string">"Hanging up"</span>); </span><br><span class="line"> phone.setCurrentNumber(<span class="string">""</span>); </span><br><span class="line"> phone.setState(<span class="keyword">new</span> <span class="title class_">ReadyState</span>(phone)); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">handleStar</span><span class="params">()</span> { </span><br><span class="line"> phone.getSoundService().playTone(<span class="string">'*'</span>); </span><br><span class="line"> <span class="comment">// 可扩展*键功能 </span></span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="comment">// 按键抽象 </span></span><br><span class="line"><span class="keyword">abstract</span> <span class="keyword">class</span> <span class="title class_">Button</span> { </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">abstract</span> <span class="keyword">void</span> <span class="title function_">press</span><span class="params">()</span>; </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="keyword">class</span> <span class="title class_">DigitButton</span> <span class="keyword">extends</span> <span class="title class_">Button</span> { </span><br><span class="line"> <span class="keyword">private</span> <span class="type">char</span> digit; </span><br><span class="line"> <span class="keyword">private</span> Phone phone; </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">DigitButton</span><span class="params">(<span class="type">char</span> digit, Phone phone)</span> { </span><br><span class="line"> <span class="built_in">this</span>.digit = digit; </span><br><span class="line"> <span class="built_in">this</span>.phone = phone; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">press</span><span class="params">()</span> { </span><br><span class="line"> phone.getState().handleDigit(digit); </span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="keyword">class</span> <span class="title class_">StarButton</span> <span class="keyword">extends</span> <span class="title class_">Button</span> { </span><br><span class="line"> <span class="keyword">private</span> Phone phone; </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">StarButton</span><span class="params">(Phone phone)</span> { </span><br><span class="line"> <span class="built_in">this</span>.phone = phone; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">press</span><span class="params">()</span> { </span><br><span class="line"> phone.getState().handleStar(); </span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="keyword">class</span> <span class="title class_">DialButton</span> <span class="keyword">extends</span> <span class="title class_">Button</span> { </span><br><span class="line"> <span class="keyword">private</span> Phone phone; </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">DialButton</span><span class="params">(Phone phone)</span> { </span><br><span class="line"> <span class="built_in">this</span>.phone = phone; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">press</span><span class="params">()</span> { </span><br><span class="line"> phone.getState().handleDial(); </span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="comment">// 声音服务 </span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">SoundService</span> { </span><br><span class="line"> <span class="keyword">private</span> Map<Character, String> tones = <span class="keyword">new</span> <span class="title class_">HashMap</span><>(); </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">SoundService</span><span class="params">()</span> { </span><br><span class="line"> <span class="comment">// 初始化音调映射 </span></span><br><span class="line"> <span class="keyword">for</span> (<span class="type">char</span> <span class="variable">c</span> <span class="operator">=</span> <span class="string">'0'</span>; c <= <span class="string">'9'</span>; c++) { </span><br><span class="line"> tones.put(c, <span class="string">"DTMF-"</span> + c); </span><br><span class="line"> } </span><br><span class="line"> tones.put(<span class="string">'*'</span>, <span class="string">"DTMF-star"</span>); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">playTone</span><span class="params">(<span class="type">char</span> digit)</span> { </span><br><span class="line"> <span class="type">String</span> <span class="variable">tone</span> <span class="operator">=</span> tones.getOrDefault(digit, <span class="string">"default-tone"</span>); </span><br><span class="line"> System.out.println(<span class="string">"Playing: "</span> + tone); </span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="comment">// 电话主体 </span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Phone</span> { </span><br><span class="line"> <span class="keyword">private</span> PhoneState state; </span><br><span class="line"> <span class="keyword">private</span> SoundService soundService; </span><br><span class="line"> <span class="keyword">private</span> <span class="type">String</span> <span class="variable">currentNumber</span> <span class="operator">=</span> <span class="string">""</span>; </span><br><span class="line"> <span class="keyword">private</span> Map<String, Button> buttons = <span class="keyword">new</span> <span class="title class_">HashMap</span><>(); </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="title function_">Phone</span><span class="params">()</span> { </span><br><span class="line"> <span class="built_in">this</span>.soundService = <span class="keyword">new</span> <span class="title class_">SoundService</span>(); </span><br><span class="line"> <span class="built_in">this</span>.state = <span class="keyword">new</span> <span class="title class_">ReadyState</span>(<span class="built_in">this</span>); </span><br><span class="line"> initButtons(); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">void</span> <span class="title function_">initButtons</span><span class="params">()</span> { </span><br><span class="line"> <span class="comment">// 数字键 </span></span><br><span class="line"> <span class="keyword">for</span> (<span class="type">int</span> <span class="variable">i</span> <span class="operator">=</span> <span class="number">0</span>; i <= <span class="number">9</span>; i++) { </span><br><span class="line"> <span class="type">char</span> <span class="variable">digit</span> <span class="operator">=</span> (<span class="type">char</span>)(<span class="string">'0'</span> + i); </span><br><span class="line"> buttons.put(<span class="string">"DIGIT_"</span> + digit, <span class="keyword">new</span> <span class="title class_">DigitButton</span>(digit, <span class="built_in">this</span>)); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 特殊键 </span></span><br><span class="line"> buttons.put(<span class="string">"STAR"</span>, <span class="keyword">new</span> <span class="title class_">StarButton</span>(<span class="built_in">this</span>)); </span><br><span class="line"> buttons.put(<span class="string">"DIAL"</span>, <span class="keyword">new</span> <span class="title class_">DialButton</span>(<span class="built_in">this</span>)); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 状态管理方法 </span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">setState</span><span class="params">(PhoneState state)</span> { </span><br><span class="line"> <span class="built_in">this</span>.state = state; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> PhoneState <span class="title function_">getState</span><span class="params">()</span> { </span><br><span class="line"> <span class="keyword">return</span> state; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> SoundService <span class="title function_">getSoundService</span><span class="params">()</span> { </span><br><span class="line"> <span class="keyword">return</span> soundService; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> String <span class="title function_">getCurrentNumber</span><span class="params">()</span> { </span><br><span class="line"> <span class="keyword">return</span> currentNumber; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">setCurrentNumber</span><span class="params">(String number)</span> { </span><br><span class="line"> <span class="built_in">this</span>.currentNumber = number; </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 按键触发 </span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">pressButton</span><span class="params">(String buttonId)</span> { </span><br><span class="line"> <span class="type">Button</span> <span class="variable">button</span> <span class="operator">=</span> buttons.get(buttonId); </span><br><span class="line"> <span class="keyword">if</span> (button != <span class="literal">null</span>) { </span><br><span class="line"> button.press(); </span><br><span class="line"> } </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 扩展新按钮 </span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">addButton</span><span class="params">(String id, Button button)</span> { </span><br><span class="line"> buttons.put(id, button); </span><br><span class="line"> } </span><br><span class="line">} </span><br><span class="line"> </span><br><span class="line"><span class="comment">// 使用示例 </span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> { </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> { </span><br><span class="line"> <span class="type">Phone</span> <span class="variable">phone</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Phone</span>(); </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 拨号示例 </span></span><br><span class="line"> phone.pressButton(<span class="string">"DIGIT_1"</span>); </span><br><span class="line"> phone.pressButton(<span class="string">"DIGIT_2"</span>); </span><br><span class="line"> phone.pressButton(<span class="string">"DIGIT_3"</span>); </span><br><span class="line"> phone.pressButton(<span class="string">"DIAL"</span>); <span class="comment">// 拨号 </span></span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 特殊键使用 </span></span><br><span class="line"> phone.pressButton(<span class="string">"STAR"</span>); </span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 挂断 </span></span><br><span class="line"> phone.pressButton(<span class="string">"DIAL"</span>); <span class="comment">// 此时变为挂机键 </span></span><br><span class="line"> } </span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2><span id="b-字符串处理算法题">B 字符串处理算法题</span></h2><p>Q:算法题，找出字符串中出现次数最多的字符</p>
<p>A:<br>比较简单，时间复杂度$O(n)$。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">MostFrequentChar</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> List<Character> <span class="title function_">findMostFrequentChar</span><span class="params">(String str)</span> {</span><br><span class="line"> <span class="keyword">if</span> (str == <span class="literal">null</span> || str.isEmpty()) {</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">ArrayList</span><>();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> Map<Character, Integer> charCount = <span class="keyword">new</span> <span class="title class_">HashMap</span><>();</span><br><span class="line"> <span class="type">int</span> <span class="variable">maxCount</span> <span class="operator">=</span> <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 统计字符频率并记录最大出现次数</span></span><br><span class="line"> <span class="keyword">for</span> (<span class="type">char</span> c : str.toCharArray()) {</span><br><span class="line"> <span class="type">int</span> <span class="variable">count</span> <span class="operator">=</span> charCount.getOrDefault(c, <span class="number">0</span>) + <span class="number">1</span>;</span><br><span class="line"> charCount.put(c, count);</span><br><span class="line"> <span class="keyword">if</span> (count > maxCount) {</span><br><span class="line"> maxCount = count;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 找出所有出现次数等于maxCount的字符</span></span><br><span class="line"> List<Character> result = <span class="keyword">new</span> <span class="title class_">ArrayList</span><>();</span><br><span class="line"> <span class="keyword">for</span> (Map.Entry<Character, Integer> entry : charCount.entrySet()) {</span><br><span class="line"> <span class="keyword">if</span> (entry.getValue() == maxCount) {</span><br><span class="line"> result.add(entry.getKey());</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> result;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> {</span><br><span class="line"> <span class="type">String</span> <span class="variable">str</span> <span class="operator">=</span> <span class="string">"aabbc"</span>;</span><br><span class="line"> List<Character> result = findMostFrequentChar(str);</span><br><span class="line"> System.out.println(<span class="string">"Most frequent characters: "</span> + result);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2><span id="c-sql查询语句">C SQL查询语句</span></h2><p>Q: sql查询语句，查询2022年入职的所有用户信息并按照公司ID排序，包含用户表、用户关系表（用户和上级）、公司表<br>A: 作答的时候好像都没有好好看要求，回来后才想起来没排序。</p>
<h2><span id="d-多线程文件下载实现">D 多线程文件下载实现</span></h2><p>Q：给了<code>String[] url, String dir</code>下载N个文件到某个目录，文件名截取url后面一部分，完全下载完成后返回true；用不超过3个线程处理。<br>下载文件的方法不需要实现<code>publci byte[] getBytesFromUrl(String url);</code></p>
<p>A:<br>Java多线程部分需要进一步学习。<br>查看<a href="./java%E5%A4%9A%E7%BA%BF%E7%A8%8B.md">Java多线程笔记</a></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">FileDownloader</span> {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="type">int</span> <span class="variable">MAX_THREADS</span> <span class="operator">=</span> <span class="number">3</span>; <span class="comment">// 最大线程数</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="type">boolean</span> <span class="title function_">downloadFiles</span><span class="params">(String[] urls, String dir)</span> {</span><br><span class="line"> <span class="comment">// 检查目录是否存在，不存在则创建</span></span><br><span class="line"> <span class="type">File</span> <span class="variable">directory</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">File</span>(dir);</span><br><span class="line"> <span class="keyword">if</span> (!directory.exists() && !directory.mkdirs()) {</span><br><span class="line"> System.err.println(<span class="string">"无法创建目录: "</span> + dir);</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 创建线程池（固定大小为3）</span></span><br><span class="line"> <span class="type">ExecutorService</span> <span class="variable">executor</span> <span class="operator">=</span> Executors.newFixedThreadPool(MAX_THREADS);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 创建异步任务列表</span></span><br><span class="line"> List<CompletableFuture<Boolean>> futures = </span><br><span class="line"> Arrays.stream(urls)</span><br><span class="line"> .map(url -> downloadAsync(url, dir, executor))</span><br><span class="line"> .collect(Collectors.toList());</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 等待所有任务完成并收集结果</span></span><br><span class="line"> <span class="type">boolean</span> <span class="variable">allSuccess</span> <span class="operator">=</span> futures.stream()</span><br><span class="line"> .map(CompletableFuture::join)</span><br><span class="line"> .allMatch(success -> success);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 关闭线程池</span></span><br><span class="line"> executor.shutdown();</span><br><span class="line"> <span class="keyword">return</span> allSuccess;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> CompletableFuture<Boolean> <span class="title function_">downloadAsync</span><span class="params">(String url, String dir, ExecutorService executor)</span> {</span><br><span class="line"> <span class="keyword">return</span> CompletableFuture.supplyAsync(() -> {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="comment">// 获取文件名（URL最后一个'/'后的部分）</span></span><br><span class="line"> <span class="type">String</span> <span class="variable">fileName</span> <span class="operator">=</span> url.substring(url.lastIndexOf(<span class="string">'/'</span>) + <span class="number">1</span>);</span><br><span class="line"> <span class="type">File</span> <span class="variable">outputFile</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">File</span>(dir, fileName);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 获取文件内容（题目提供的方法）</span></span><br><span class="line"> <span class="type">byte</span>[] data = getBytesFromUrl(url);</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 写入文件</span></span><br><span class="line"> <span class="keyword">try</span> (<span class="type">FileOutputStream</span> <span class="variable">fos</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">FileOutputStream</span>(outputFile)) {</span><br><span class="line"> fos.write(data);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span>;</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> System.err.println(<span class="string">"下载失败: "</span> + url + <span class="string">" | 错误: "</span> + e.getMessage());</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">false</span>;</span><br><span class="line"> }</span><br><span class="line"> }, executor);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 题目提供的下载方法（实际实现应处理网络请求）</span></span><br><span class="line"> <span class="keyword">private</span> <span class="type">byte</span>[] getBytesFromUrl(String url) <span class="keyword">throws</span> Exception {</span><br><span class="line"> <span class="comment">// 这里应包含实际的下载逻辑</span></span><br><span class="line"> <span class="comment">// 示例中我们返回空数组</span></span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">byte</span>[<span class="number">0</span>];</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h2><span id="e-服务器性能问题排查">E 服务器性能问题排查</span></h2><p>Q:你开发的功能模块，发布到生产后，服务器压力激增，怎么排查是什么原因造成的？<br>A:<br>参考以下文章：<br><a target="_blank" rel="noopener" href="https://www.cnblogs.com/yanggb/p/10846806.html">服务器压力大的解决方案 - yanggb - 博客园</a><br><a target="_blank" rel="noopener" href="https://cloud.tencent.com/developer/article/1879120">服务器压力过大？CPU打满？我来帮你快速检查Linux服务器性能-腾讯云开发者社区-腾讯云</a></p>
</div></article></div><div class="card"><article class="card-content article" role="article"><div class="article-meta is-size-7 is-uppercase level is-mobile"><div class="level-left"><span class="level-item"><time dateTime="2025-06-08T07:11:55.000Z" title="2025/6/8 15:11:55">2025-06-08</time>发表</span><span class="level-item"><time dateTime="2025-06-08T08:50:57.092Z" title="2025/6/8 16:50:57">2025-06-08</time>更新</span><span class="level-item">34 分钟读完 (大约5075个字)</span></div></div><p class="title is-3 is-size-4-mobile"><a class="link-muted" href="/2025/06/08/java%E5%A4%9A%E7%BA%BF%E7%A8%8B/">java多线程笔记</a></p><div class="content"><!-- toc -->
<ul>
<li><a href="#%E5%88%9B%E5%BB%BA%E6%96%B0%E7%BA%BF%E7%A8%8B">创建新线程</a></li>
<li><a href="#%E7%BA%BF%E7%A8%8B%E7%9A%84%E7%8A%B6%E6%80%81">线程的状态</a></li>
<li><a href="#%E4%B8%AD%E6%96%AD%E7%BA%BF%E7%A8%8B">中断线程</a></li>
<li><a href="#%E5%AE%88%E6%8A%A4%E7%BA%BF%E7%A8%8B">守护线程</a></li>
<li><a href="#%E7%BA%BF%E7%A8%8B%E5%90%8C%E6%AD%A5">线程同步</a><ul>
<li><a href="#%E5%90%8C%E6%AD%A5%E6%96%B9%E6%B3%95">同步方法</a></li>
<li><a href="#%E6%AD%BB%E9%94%81">死锁</a></li>
<li><a href="#wait%E5%92%8Cnotify">wait()和notify()</a></li>
<li><a href="#reentrantlock">ReentrantLock()</a></li>
<li><a href="#condition">Condition</a></li>
<li><a href="#readwritelock">ReadWriteLock</a></li>
<li><a href="#stampedlock">StampedLock</a></li>
<li><a href="#semphore">Semphore</a></li>
<li><a href="#javautilconcurrent%E5%8C%85%E6%8F%90%E4%BE%9B%E7%9A%84%E7%BA%BF%E7%A8%8B%E5%AE%89%E5%85%A8%E9%9B%86%E5%90%88">java.util.concurrent包提供的线程安全集合</a></li>
<li><a href="#javautilconcurrentatomic%E5%8C%85%E6%8F%90%E4%BE%9B%E5%8E%9F%E5%AD%90%E6%93%8D%E4%BD%9C">java.util.concurrent.atomic包提供原子操作</a></li>
</ul>
</li>
<li><a href="#%E4%BD%BF%E7%94%A8%E7%BA%BF%E7%A8%8B%E6%B1%A0">使用线程池</a></li>
<li><a href="#future">Future</a></li>
<li><a href="#completablefuture">CompletableFuture</a></li>
<li><a href="#forkjoin">ForkJoin</a></li>
<li><a href="#threadlocal">ThreadLocal</a></li>
<li><a href="#%E8%99%9A%E6%8B%9F%E7%BA%BF%E7%A8%8B">虚拟线程</a></li>
</ul>
<!-- tocstop -->
<p>Java语言内置了多线程支持：一个Java程序实际上是一个JVM进程，JVM进程用一个主线程来执行<code>main()</code>方法，在<code>main()</code>方法内部，我们又可以启动多个线程。此外，JVM还有负责垃圾回收的其他工作线程等。</p>
<h2><span id="创建新线程">创建新线程</span></h2><p><strong>方法1：</strong><br>Java用<code>Thread</code>对象表示一个线程，通过调用<code>start()</code>启动一个新线程；<br>一个线程对象只能调用一次<code>start()</code>方法；<br>线程的执行代码写在<code>run()</code>方法中；<br> <figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"> <span class="comment">// 多线程</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> {</span><br><span class="line"> <span class="type">Thread</span> <span class="variable">t</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MyThread</span>();</span><br><span class="line"> t.start(); <span class="comment">// 启动新线程</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">MyThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="meta">@Override</span></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> System.out.println(<span class="string">"start new thread!"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> { </span><br><span class="line"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> { </span><br><span class="line"><span class="type">Thread</span> <span class="variable">t1</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Thread</span>(<span class="keyword">new</span> <span class="title class_">MyRunnable</span>()); </span><br><span class="line"> <span class="type">Thread</span> <span class="variable">t2</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Thread</span>(() -> { </span><br><span class="line">System.out.println(<span class="string">"start new thread with lambda."</span>); </span><br><span class="line">}); </span><br><span class="line"> t1.start(); </span><br><span class="line"> t2.start(); </span><br><span class="line"> } </span><br><span class="line"> </span><br><span class="line"> </span><br><span class="line">} </span><br><span class="line"> <span class="keyword">class</span> <span class="title class_">MyRunnable</span> <span class="keyword">implements</span> <span class="title class_">Runnable</span> { </span><br><span class="line"> <span class="meta">@Override</span> </span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> { </span><br><span class="line"> System.out.println(<span class="string">"start new thread."</span>); </span><br><span class="line"> } </span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p><strong>线程优先级</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Thread.setPriority(<span class="type">int</span> n) <span class="comment">// 1~10, 默认值5</span></span><br></pre></td></tr></table></figure>
<p>多线程经常需要读写共享数据，并且需要同步。</p>
<h2><span id="线程的状态">线程的状态</span></h2><figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"> ┌─────────────┐</span><br><span class="line"> │ New │</span><br><span class="line"> └─────────────┘</span><br><span class="line"> │</span><br><span class="line"> ▼</span><br><span class="line">┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐</span><br><span class="line"> ┌─────────────┐ ┌─────────────┐</span><br><span class="line">││ Runnable │ │ Blocked ││</span><br><span class="line"> └─────────────┘ └─────────────┘</span><br><span class="line">│┌─────────────┐ ┌─────────────┐│</span><br><span class="line"> │ Waiting │ │Timed Waiting│</span><br><span class="line">│└─────────────┘ └─────────────┘│</span><br><span class="line"> ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─</span><br><span class="line"> │</span><br><span class="line"> ▼</span><br><span class="line"> ┌─────────────┐</span><br><span class="line"> │ Terminated │</span><br><span class="line"> └─────────────┘</span><br></pre></td></tr></table></figure>
<p> 线程状态转移图<br><strong>线程状态</strong>包含：</p>
<ul>
<li>New：新创建的线程，尚未执行；</li>
<li>Runnable：运行中的线程，正在执行<code>run()</code>方法的Java代码；</li>
<li>Blocked：运行中的线程，因为某些操作被阻塞而挂起；</li>
<li>Waiting：运行中的线程，因为某些操作在等待中；</li>
<li>Timed Waiting：运行中的线程，因为执行<code>sleep()</code>方法正在计时等待；</li>
<li>Terminated：线程已终止，因为<code>run()</code>方法执行完毕。</li>
</ul>
<p><strong>状态切换</strong></p>
<ul>
<li>通过对另一个线程对象调用<code>join()</code>方法可以等待其执行结束；</li>
<li>可以指定等待时间，超过等待时间线程仍然没有结束就不再等待；</li>
<li>对已经运行结束的线程调用<code>join()</code>方法会立刻返回。</li>
</ul>
<h2><span id="中断线程">中断线程</span></h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 中断线程</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException {</span><br><span class="line"> <span class="type">Thread</span> <span class="variable">t</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MyThread</span>();</span><br><span class="line"> t.start();</span><br><span class="line"> Thread.sleep(<span class="number">1</span>); <span class="comment">// 暂停1毫秒</span></span><br><span class="line"> t.interrupt(); <span class="comment">// 中断t线程</span></span><br><span class="line"> t.join(); <span class="comment">// 等待t线程结束</span></span><br><span class="line"> System.out.println(<span class="string">"end"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">MyThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> <span class="type">int</span> <span class="variable">n</span> <span class="operator">=</span> <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">while</span> (! isInterrupted()) { <span class="comment">// 必要</span></span><br><span class="line"> n ++;</span><br><span class="line"> System.out.println(n + <span class="string">" hello!"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">// 捕捉Exception</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">MyThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> <span class="type">Thread</span> <span class="variable">hello</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">HelloThread</span>();</span><br><span class="line"> hello.start(); <span class="comment">// 启动hello线程</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> hello.join(); <span class="comment">// 等待hello线程结束</span></span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> System.out.println(<span class="string">"interrupted!"</span>);</span><br><span class="line"> }</span><br><span class="line"> hello.interrupt();</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">HelloThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> <span class="type">int</span> <span class="variable">n</span> <span class="operator">=</span> <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">while</span> (!isInterrupted()) {</span><br><span class="line"> n++;</span><br><span class="line"> System.out.println(n + <span class="string">" hello!"</span>);</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> Thread.sleep(<span class="number">100</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p><code>interrupt()</code>方法仅仅向<code>t</code>线程发出了“中断请求”，至于<code>t</code>线程是否能立刻响应，要看具体代码。而<code>t</code>线程的<code>while</code>循环会检测<code>isInterrupted()</code>，所以上述代码能正确响应<code>interrupt()</code>请求，使得自身立刻结束运行<code>run()</code>方法。<br>目标线程检测到<code>isInterrupted()</code>为<code>true</code>或者捕获了<code>InterruptedException</code>都应该立刻结束自身线程；</p>
<p><strong>设置标志位</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 中断线程</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException {</span><br><span class="line"> <span class="type">HelloThread</span> <span class="variable">t</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">HelloThread</span>();</span><br><span class="line"> t.start();</span><br><span class="line"> Thread.sleep(<span class="number">1</span>);</span><br><span class="line"> t.running = <span class="literal">false</span>; <span class="comment">// 标志位置为false</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">HelloThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">volatile</span> <span class="type">boolean</span> <span class="variable">running</span> <span class="operator">=</span> <span class="literal">true</span>;</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> <span class="type">int</span> <span class="variable">n</span> <span class="operator">=</span> <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">while</span> (running) {</span><br><span class="line"> n ++;</span><br><span class="line"> System.out.println(n + <span class="string">" hello!"</span>);</span><br><span class="line"> }</span><br><span class="line"> System.out.println(<span class="string">"end!"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>注意到<code>HelloThread</code>的标志位<code>boolean running</code>是一个线程间共享的变量。线程间共享变量需要使用<code>volatile</code>关键字标记，确保每个线程都能读取到更新后的变量值。<br>[[为什么要对线程间共享的变量用关键字volatile声明？]]<br><code>volatile</code>关键字的目的是告诉虚拟机：</p>
<ul>
<li>每次访问变量时，总是获取主内存的最新值；</li>
<li>每次修改变量后，立刻回写到主内存。<br><em><code>volatile</code>关键字解决的是可见性问题：当一个线程修改了某个共享变量的值，其他线程能够立刻看到修改后的值。</em></li>
</ul>
<h2><span id="守护线程">守护线程</span></h2><p>守护线程（Daemon Thread）是指为其他线程服务的线程。在JVM中，所有非守护线程都执行完毕后，无论有没有守护线程，虚拟机都会自动退出。<br><strong>创建守护线程</strong></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">Thread</span> <span class="variable">t</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">MyThread</span>();</span><br><span class="line">t.setDaemon(<span class="literal">true</span>); <span class="comment">// 标记</span></span><br><span class="line">t.start();</span><br></pre></td></tr></table></figure>
<p>守护线程不能持有需要关闭的资源（如打开文件等）。</p>
<h2><span id="线程同步">线程同步</span></h2><p>使用<code>synchronized</code>解决了多线程同步访问共享变量的正确性问题。但是，它的缺点是带来了性能下降。因为<code>synchronized</code>代码块无法并发执行。</p>
<p>我们来概括一下如何使用<code>synchronized</code>：</p>
<ol>
<li>找出修改共享变量的线程代码块；</li>
<li>选择一个共享实例作为锁；</li>
<li>使用<code>synchronized(lockObject) { ... }</code>。</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 多线程</span></span><br><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> Exception {</span><br><span class="line"> <span class="type">var</span> <span class="variable">add</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">AddThread</span>();</span><br><span class="line"> <span class="type">var</span> <span class="variable">dec</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">DecThread</span>();</span><br><span class="line"> add.start();</span><br><span class="line"> dec.start();</span><br><span class="line"> add.join();</span><br><span class="line"> dec.join();</span><br><span class="line"> System.out.println(Counter.count);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Counter</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">final</span> <span class="type">Object</span> <span class="variable">lock</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">Object</span>();</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="type">int</span> <span class="variable">count</span> <span class="operator">=</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">AddThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="type">int</span> i=<span class="number">0</span>; i<<span class="number">10000</span>; i++) {</span><br><span class="line"> <span class="keyword">synchronized</span>(Counter.lock) {</span><br><span class="line"> Counter.count += <span class="number">1</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">DecThread</span> <span class="keyword">extends</span> <span class="title class_">Thread</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="type">int</span> i=<span class="number">0</span>; i<<span class="number">10000</span>; i++) {</span><br><span class="line"> <span class="keyword">synchronized</span>(Counter.lock) {</span><br><span class="line"> Counter.count -= <span class="number">1</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>在使用<code>synchronized</code>的时候，不必担心抛出异常。因为无论是否有异常，都会在<code>synchronized</code>结束处正确释放锁：</p>
<p><strong>不需要synchronized</strong></p>
<p>JVM规范定义了几种原子操作：</p>
<ul>
<li>基本类型（<code>long</code>和<code>double</code>除外）赋值，例如：<code>int n = m</code>；</li>
<li>引用类型赋值，例如：<code>List<String> list = anotherList</code>。</li>
</ul>
<p>单条原子操作不需要同步<br>不可变对象无需同步</p>
<h3><span id="同步方法">同步方法</span></h3><p>如果一个类被设计为允许多线程正确访问，我们就说这个类就是“线程安全”的（thread-safe）<br>Java标准库的<code>java.lang.StringBuffer</code>也是线程安全的。<br>还有一些不变类，例如<code>String</code>，<code>Integer</code>，<code>LocalDate</code>，它们的所有成员变量都是<code>final</code>，多线程同时访问时只能读不能写，这些不变类也是线程安全的。<br>类似<code>Math</code>这些只提供静态方法，没有成员变量的类，也是线程安全的。无状态的方法天然线程安全，因为<strong>没有共享数据可被竞争</strong>。</p>
<p>[[synchronized static 方法的锁机制]]</p>
<h3><span id="死锁">死锁</span></h3><p>JVM允许同一个线程重复获取同一个锁，这种能被同一个线程反复获取的锁，就叫做<strong>可重入锁</strong>。<br>所以，获取锁的时候，不但要判断是否是第一次获取，还要记录这是第几次获取。每获取一次锁，记录+1，每退出<code>synchronized</code>块，记录-1，减到0的时候，才会真正释放锁。</p>
<p>两个线程各自持有不同的锁，然后各自试图获取对方手里的锁，造成了双方无限等待下去，这就是死锁。<br>死锁发生后，没有任何机制能解除死锁，只能强制结束JVM进程。<br>**解决方式：**线程获取锁的顺序要一致。</p>
<h3><span id="wait和notify">wait()和notify()</span></h3><p><strong>wait()</strong><br><strong>JVM 源码中的定义</strong>：<br><code>wait()</code> 的本地方法实现在 HotSpot JVM 的 C++ 代码中（如 <code>objectMonitor.cpp</code>），核心逻辑围绕 <strong>对象监视器（ObjectMonitor）</strong> 展开。</p>
<ul>
<li><strong>关键步骤</strong>：<ol>
<li><strong>检查锁状态</strong>：确认当前线程持有对象的锁（通过 <code>ObjectMonitor::owner</code> 字段）。</li>
<li><strong>释放锁</strong>：通过 <code>ObjectMonitor::exit()</code> 释放锁，允许其他线程进入同步块。</li>
<li><strong>线程挂起</strong>：调用操作系统原语（如 <code>pthread_cond_wait</code> 或 <code>WaitForSingleObject</code>）将线程挂起，进入等待队列。</li>
<li><strong>被唤醒后重新竞争锁</strong>：当其他线程调用 <code>notify()</code>/<code>notifyAll()</code> 时，线程被移入竞争队列，重新尝试获取锁（通过 <code>ObjectMonitor::enter()</code>）<br>因此，只能在锁对象上调用<code>wait()</code>方法。</li>
</ol>
</li>
</ul>
<p><strong>notify()</strong></p>
<ul>
<li><strong>唤醒一个等待线程</strong>：<br> 当调用 <code>object.notify()</code> 时，会随机唤醒一个正在该对象上通过 <code>object.wait()</code> 阻塞的线程（如果有多个线程在等待）。</li>
<li><strong>不释放锁</strong>：<br> <code>notify()</code> 本身不会释放锁，被唤醒的线程需要等到当前线程退出 <code>synchronized</code> 块后，才能重新竞争锁并继续执行。</li>
</ul>
<table>
<thead>
<tr>
<th>方法</th>
<th>行为</th>
<th>适用场景</th>
</tr>
</thead>
<tbody><tr>
<td><code>notify()</code></td>
<td>唤醒<strong>一个</strong>等待线程</td>
<td>仅需唤醒一个线程（如单生产者-单消费者）</td>
</tr>
<tr>
<td><code>notifyAll()</code></td>
<td>唤醒<strong>所有</strong>等待线程</td>
<td>需唤醒所有线程（多线程竞争同一资源时）</td>
</tr>
</tbody></table>
<ol>
<li><strong><code>wait()</code> 必须放在 <code>while</code> 循环中</strong>，确保被唤醒后条件仍然成立。</li>
<li><strong><code>notifyAll()</code> 比 <code>notify()</code> 更安全</strong>，避免某些线程永远不被唤醒。</li>
</ol>
<h3><span id="reentrantlock">ReentrantLock()</span></h3><blockquote>
<p><code>synchronized</code>关键字用于加锁，但这种锁一是很重，二是获取时必须一直等待，没有额外的尝试机制。</p>
</blockquote>
<p><code>java.util.concurrent.locks</code>包提供的<code>ReentrantLock</code>用于替代<code>synchronized</code>加锁</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Counter</span> {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="type">Lock</span> <span class="variable">lock</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">ReentrantLock</span>();</span><br><span class="line"> <span class="keyword">private</span> <span class="type">int</span> count;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">add</span><span class="params">(<span class="type">int</span> n)</span> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> count += n;</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p><code>ReentrantLock</code>获取锁更安全；</p>
<p><code>ReentrantLock</code>可以尝试获取锁：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">if</span> (lock.tryLock(<span class="number">1</span>, TimeUnit.SECONDS)) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> ...</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h3><span id="condition">Condition</span></h3><p><code>Condition</code>提供的<code>await()</code>、<code>signal()</code>、<code>signalAll()</code>原理和<code>synchronized</code>锁对象的<code>wait()</code>、<code>notify()</code>、<code>notifyAll()</code>是一致的，并且其行为也是一样的：</p>
<ul>
<li><code>await()</code>会释放当前锁，进入等待状态；</li>
<li><code>signal()</code>会唤醒某个等待线程；</li>
<li><code>signalAll()</code>会唤醒所有等待线程；</li>
<li>唤醒线程从<code>await()</code>返回后需要重新获得锁。</li>
</ul>
<p>此外，和<code>tryLock()</code>类似，<code>await()</code>可以在等待指定时间后，如果还没有被其他线程通过<code>signal()</code>或<code>signalAll()</code>唤醒，可以自己醒来：</p>
<h3><span id="readwritelock">ReadWriteLock</span></h3><p>使用<code>ReadWriteLock</code>可以解决这个问题，它保证：</p>
<ul>
<li>只允许一个线程写入（其他线程既不能写入也不能读取）；</li>
<li>没有写入时，多个线程允许同时读（提高性能）。</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Counter</span> {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="type">ReadWriteLock</span> <span class="variable">rwlock</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">ReentrantReadWriteLock</span>();</span><br><span class="line"> <span class="comment">// 注意: 一对读锁和写锁必须从同一个rwlock获取:</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="type">Lock</span> <span class="variable">rlock</span> <span class="operator">=</span> rwlock.readLock();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="type">Lock</span> <span class="variable">wlock</span> <span class="operator">=</span> rwlock.writeLock();</span><br><span class="line"> <span class="keyword">private</span> <span class="type">int</span>[] counts = <span class="keyword">new</span> <span class="title class_">int</span>[<span class="number">10</span>];</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">inc</span><span class="params">(<span class="type">int</span> index)</span> {</span><br><span class="line"> wlock.lock(); <span class="comment">// 加写锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> counts[index] += <span class="number">1</span>;</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> wlock.unlock(); <span class="comment">// 释放写锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="type">int</span>[] get() {</span><br><span class="line"> rlock.lock(); <span class="comment">// 加读锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">return</span> Arrays.copyOf(counts, counts.length);</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> rlock.unlock(); <span class="comment">// 释放读锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h3><span id="stampedlock">StampedLock</span></h3><p>[[乐观锁与悲观锁]]</p>
<blockquote>
<p>乐观锁的意思就是乐观地估计读的过程中大概率不会有写入，因此被称为乐观锁。反过来，悲观锁则是读的过程中拒绝有写入，也就是写入必须等待。</p>
</blockquote>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Point</span> {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="type">StampedLock</span> <span class="variable">stampedLock</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">StampedLock</span>();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="type">double</span> x;</span><br><span class="line"> <span class="keyword">private</span> <span class="type">double</span> y;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">move</span><span class="params">(<span class="type">double</span> deltaX, <span class="type">double</span> deltaY)</span> {</span><br><span class="line"> <span class="type">long</span> <span class="variable">stamp</span> <span class="operator">=</span> stampedLock.writeLock(); <span class="comment">// 获取写锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> x += deltaX;</span><br><span class="line"> y += deltaY;</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> stampedLock.unlockWrite(stamp); <span class="comment">// 释放写锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="type">double</span> <span class="title function_">distanceFromOrigin</span><span class="params">()</span> {</span><br><span class="line"> <span class="type">long</span> <span class="variable">stamp</span> <span class="operator">=</span> stampedLock.tryOptimisticRead(); <span class="comment">// 获得一个乐观读锁</span></span><br><span class="line"> <span class="comment">// 注意下面两行代码不是原子操作</span></span><br><span class="line"> <span class="comment">// 假设x,y = (100,200)</span></span><br><span class="line"> <span class="type">double</span> <span class="variable">currentX</span> <span class="operator">=</span> x;</span><br><span class="line"> <span class="comment">// 此处已读取到x=100，但x,y可能被写线程修改为(300,400)</span></span><br><span class="line"> <span class="type">double</span> <span class="variable">currentY</span> <span class="operator">=</span> y;</span><br><span class="line"> <span class="comment">// 此处已读取到y，如果没有写入，读取是正确的(100,200)</span></span><br><span class="line"> <span class="comment">// 如果有写入，读取是错误的(100,400)</span></span><br><span class="line"> <span class="keyword">if</span> (!stampedLock.validate(stamp)) { <span class="comment">// 检查乐观读锁后是否有其他写锁发生</span></span><br><span class="line"> stamp = stampedLock.readLock(); <span class="comment">// 获取一个悲观读锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> currentX = x;</span><br><span class="line"> currentY = y;</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> stampedLock.unlockRead(stamp); <span class="comment">// 释放悲观读锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> Math.sqrt(currentX * currentX + currentY * currentY);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h3><span id="semphore">Semphore</span></h3><blockquote>
<p>如果要对某一受限资源进行限流访问，可以使用<code>Semaphore</code>，保证同一时间最多N个线程访问受限资源。</p>
</blockquote>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Point</span> {</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="type">StampedLock</span> <span class="variable">stampedLock</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">StampedLock</span>();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="type">double</span> x;</span><br><span class="line"> <span class="keyword">private</span> <span class="type">double</span> y;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title function_">move</span><span class="params">(<span class="type">double</span> deltaX, <span class="type">double</span> deltaY)</span> {</span><br><span class="line"> <span class="type">long</span> <span class="variable">stamp</span> <span class="operator">=</span> stampedLock.writeLock(); <span class="comment">// 获取写锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> x += deltaX;</span><br><span class="line"> y += deltaY;</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> stampedLock.unlockWrite(stamp); <span class="comment">// 释放写锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="type">double</span> <span class="title function_">distanceFromOrigin</span><span class="params">()</span> {</span><br><span class="line"> <span class="type">long</span> <span class="variable">stamp</span> <span class="operator">=</span> stampedLock.tryOptimisticRead(); <span class="comment">// 获得一个乐观读锁</span></span><br><span class="line"> <span class="comment">// 注意下面两行代码不是原子操作</span></span><br><span class="line"> <span class="comment">// 假设x,y = (100,200)</span></span><br><span class="line"> <span class="type">double</span> <span class="variable">currentX</span> <span class="operator">=</span> x;</span><br><span class="line"> <span class="comment">// 此处已读取到x=100，但x,y可能被写线程修改为(300,400)</span></span><br><span class="line"> <span class="type">double</span> <span class="variable">currentY</span> <span class="operator">=</span> y;</span><br><span class="line"> <span class="comment">// 此处已读取到y，如果没有写入，读取是正确的(100,200)</span></span><br><span class="line"> <span class="comment">// 如果有写入，读取是错误的(100,400)</span></span><br><span class="line"> <span class="keyword">if</span> (!stampedLock.validate(stamp)) { <span class="comment">// 检查乐观读锁后是否有其他写锁发生</span></span><br><span class="line"> stamp = stampedLock.readLock(); <span class="comment">// 获取一个悲观读锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> currentX = x;</span><br><span class="line"> currentY = y;</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> stampedLock.unlockRead(stamp); <span class="comment">// 释放悲观读锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> Math.sqrt(currentX * currentX + currentY * currentY);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br></pre></td></tr></table></figure>
<h3><span id="javautilconcurrent包提供的线程安全集合">java.util.concurrent包提供的线程安全集合</span></h3><p>针对<code>List</code>、<code>Map</code>、<code>Set</code>、<code>Deque</code>等，<code>java.util.concurrent</code>包也提供了对应的并发集合类。我们归纳一下：</p>
<table>
<thead>
<tr>
<th>interface</th>
<th>non-thread-safe</th>
<th>thread-safe</th>
</tr>
</thead>
<tbody><tr>
<td>List</td>
<td>ArrayList</td>
<td>CopyOnWriteArrayList</td>
</tr>
<tr>
<td>Map</td>
<td>HashMap</td>
<td>ConcurrentHashMap</td>
</tr>
<tr>
<td>Set</td>
<td>HashSet / TreeSet</td>
<td>CopyOnWriteArraySet</td>
</tr>
<tr>
<td>Queue</td>
<td>ArrayDeque / LinkedList</td>
<td>ArrayBlockingQueue / LinkedBlockingQueue</td>
</tr>
<tr>
<td>Deque</td>
<td>ArrayDeque / LinkedList</td>
<td>LinkedBlockingDeque</td>
</tr>
</tbody></table>
<p>还提供了新旧集合转换器</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">Map</span> <span class="variable">unsafeMap</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">HashMap</span>();</span><br><span class="line"><span class="type">Map</span> <span class="variable">threadSafeMap</span> <span class="operator">=</span> Collections.synchronizedMap(unsafeMap);</span><br></pre></td></tr></table></figure>
<h3><span id="javautilconcurrentatomic包提供原子操作">java.util.concurrent.atomic包提供原子操作</span></h3><p>我们以<code>AtomicInteger</code>为例，它提供的主要操作有：</p>
<ul>
<li>增加值并返回新值：<code>int addAndGet(int delta)</code></li>
<li>加1后返回新值：<code>int incrementAndGet()</code></li>
<li>获取当前值：<code>int get()</code></li>
<li>用CAS方式设置：<code>int compareAndSet(int expect, int update)</code></li>
</ul>
<p>Atomic类是通过无锁（lock-free）的方式实现的线程安全（thread-safe）访问。它的主要原理是利用了CAS：Compare and Set。</p>
<p>[[CAS]]是指，在这个操作中，如果<code>AtomicInteger</code>的当前值是<code>prev</code>，那么就更新为<code>next</code>，返回<code>true</code>。如果<code>AtomicInteger</code>的当前值不是<code>prev</code>，就什么也不干，返回<code>false</code>。通过CAS操作并配合<code>do ... while</code>循环，即使其他线程修改了<code>AtomicInteger</code>的值，最终的结果也是正确的。</p>
<h2><span id="使用线程池">使用线程池</span></h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 创建固定大小的线程池:</span></span><br><span class="line"><span class="type">ExecutorService</span> <span class="variable">executor</span> <span class="operator">=</span> Executors.newFixedThreadPool(<span class="number">3</span>);</span><br><span class="line"><span class="comment">// 提交任务:</span></span><br><span class="line">executor.submit(task1);</span><br><span class="line">executor.submit(task2);</span><br><span class="line">executor.submit(task3);</span><br><span class="line">executor.submit(task4);</span><br><span class="line">executor.submit(task5);</span><br><span class="line"><span class="comment">// 关闭线程池；</span></span><br><span class="line">es.shutdown();</span><br></pre></td></tr></table></figure>
<p>因为<code>ExecutorService</code>只是接口，Java标准库提供的几个常用实现类有：</p>
<ul>
<li>FixedThreadPool：线程数固定的线程池；</li>
<li>CachedThreadPool：线程数根据任务动态调整的线程池；</li>
<li>SingleThreadExecutor：仅单线程执行的线程池。</li>
<li>ScheduledThreadPool：(任务定期触发)</li>
</ul>
<h2><span id="future">Future</span></h2><p><code>Runnable</code>接口有个问题，它的方法没有返回值。如果任务需要一个返回结果，那么只能保存到变量。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">class</span> <span class="title class_">Task</span> <span class="keyword">implements</span> <span class="title class_">Callable</span><String> {</span><br><span class="line"> <span class="keyword">public</span> String <span class="title function_">call</span><span class="params">()</span> <span class="keyword">throws</span> Exception {</span><br><span class="line"> <span class="keyword">return</span> longTimeCalculation(); </span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>获取结果：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">ExecutorService</span> <span class="variable">executor</span> <span class="operator">=</span> Executors.newFixedThreadPool(<span class="number">4</span>); </span><br><span class="line"><span class="comment">// 定义任务:</span></span><br><span class="line">Callable<String> task = <span class="keyword">new</span> <span class="title class_">Task</span>();</span><br><span class="line"><span class="comment">// 提交任务并获得Future:</span></span><br><span class="line">Future<String> future = executor.submit(task);</span><br><span class="line"><span class="comment">// 从Future获取异步执行返回的结果:</span></span><br><span class="line"><span class="type">String</span> <span class="variable">result</span> <span class="operator">=</span> future.get(); <span class="comment">// 可能阻塞</span></span><br><span class="line"></span><br></pre></td></tr></table></figure>
<p>当我们提交一个<code>Callable</code>任务后，我们会同时获得一个<code>Future</code>对象，然后，我们在主线程某个时刻调用<code>Future</code>对象的<code>get()</code>方法，就可以获得异步执行的结果。在调用<code>get()</code>时，如果异步任务已经完成，我们就直接获得结果。如果异步任务还没有完成，那么<code>get()</code>会阻塞，直到任务完成后才返回结果。</p>
<p>一个<code>Future<V></code>接口表示一个未来可能会返回的结果，它定义的方法有：</p>
<ul>
<li><code>get()</code>：获取结果（可能会等待）</li>
<li><code>get(long timeout, TimeUnit unit)</code>：获取结果，但只等待指定的时间；</li>
<li><code>cancel(boolean mayInterruptIfRunning)</code>：取消当前任务；</li>
<li><code>isDone()</code>：判断任务是否已完成。</li>
</ul>
<h2><span id="completablefuture">CompletableFuture</span></h2><p>从Java 8开始引入了<code>CompletableFuture</code>，它针对<code>Future</code>做了改进，可以传入回调对象，当异步任务完成或者发生异常时，自动调用回调对象的回调方法。</p>
<p><code>Future</code> 和 <code>CompletableFuture</code> 都支持异步计算，但 <code>CompletableFuture</code> 提供了更灵活、更强大的 <strong>链式调用（串行执行）</strong> 能力，而 <code>Future</code> 在这方面的能力非常有限</p>
<ul>
<li><strong><code>Future</code> 无法直接依赖另一个 <code>Future</code> 的结果</strong>，必须手动等待前一个 <code>Future</code> 完成，再启动下一个任务。</li>
<li><strong>代码复杂</strong>：需要嵌套 <code>get()</code> 调用，容易导致阻塞或回调地狱。</li>
</ul>
<p><code>CompletableFuture</code> 提供了 <code>thenApply()</code>, <code>thenCompose()</code>, <code>thenAccept()</code> 等方法，可以 <strong>直接在前一个任务完成后触发下一个任务</strong>，无需手动等待。</p>
<h2><span id="forkjoin">ForkJoin</span></h2><p>Fork/Join是一种基于“<strong>分治</strong>”的算法：通过分解任务，并行执行，最后合并结果得到最终结果。</p>
<p><code>ForkJoinPool</code>线程池可以把一个大任务分拆成小任务并行执行，任务类必须继承自<code>RecursiveTask</code>或<code>RecursiveAction</code>。</p>
<p>使用Fork/Join模式可以进行并行计算以提高效率。</p>
<h2><span id="threadlocal">ThreadLocal</span></h2><p><code>Thread.currentThread()</code>获取当前线程。</p>
<p>如何在一个线程中传递状态？</p>
<blockquote>
<p>这种在一个线程中，横跨若干方法调用，需要传递的对象，我们通常称之为上下文（Context），它是一种状态，可以是用户身份、任务信息等。<br>给每个方法增加一个context参数非常麻烦，而且有些时候，如果调用链有无法修改源码的第三方库，<code>User</code>对象就传不进去了。</p>
</blockquote>
<p>Java标准库提供了一个特殊的<code>ThreadLocal</code>，它可以在一个线程中传递同一个对象。</p>
<p><code>ThreadLocal</code>表示线程的“局部变量”，它确保每个线程的<code>ThreadLocal</code>变量都是各自独立的；</p>
<p><code>ThreadLocal</code>适合在一个线程的处理流程中保持上下文（避免了同一参数在所有方法中传递）；</p>
<blockquote>
<p>使用<code>ThreadLocal</code>要用<code>try ... finally</code>结构，并在<code>finally</code>中清除。why?</p>
</blockquote>
<p><strong>ThreadLocal 使用注意事项</strong></p>
<ol>
<li><p><strong>存储机制</strong></p>
<ul>
<li><code>ThreadLocal.set()</code> 将数据存储到<strong>当前线程的 <code>ThreadLocalMap</code></strong>中，而非 <code>ThreadLocal</code> 本身。</li>
<li>线程死亡时，<code>ThreadLocalMap</code> 自动销毁，无需手动 <code>remove()</code>。</li>
</ul>
</li>
<li><p><strong>线程池场景</strong></p>
<ul>
<li><strong>线程会被复用</strong>（如 Tomcat、Spring 线程池），导致 <code>ThreadLocal</code> 数据残留，引发内存泄漏或数据错乱。</li>
<li><strong>必须清理</strong>：在 <code>finally</code> 中调用 <code>ThreadLocal.remove()</code>，或使用 <code>AutoCloseable</code> 自动清除。</li>
</ul>
</li>
<li><p><strong>最佳实践</strong></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">try {</span><br><span class="line"> threadLocal.set(value);</span><br><span class="line"> // 业务逻辑...</span><br><span class="line">} finally {</span><br><span class="line"> threadLocal.remove(); // 强制清除，避免线程池复用污染</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p> <strong>或</strong>（推荐）</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">try (AutoCloseable cleanup = threadLocal.withInitial(value)) {</span><br><span class="line"> // 业务逻辑...</span><br><span class="line">} // 自动调用 remove()</span><br></pre></td></tr></table></figure></li>
</ol>
<p><strong>核心原则</strong>：只要用线程池，就必须清理 <code>ThreadLocal</code>！</p>
<h2><span id="虚拟线程">虚拟线程</span></h2><p>Java 19引入的虚拟线程是为了解决<strong>IO密集型任</strong>务的吞吐量，它可以高效通过少数线程去调度大量虚拟线程；</p>
<p>虚拟线程在执行到IO操作或Blocking操作时，会<strong>自动切换到其他虚拟线程执行</strong>，从而避免当前线程等待，能最大化线程的执行效率；</p>
<p>虚拟线程使用普通线程相同的接口，最大的好处是无需修改任何代码，就可以将现有的IO操作异步化获得更大的吞吐能力。</p>
<p>计算密集型任务不应使用虚拟线程，只能通过增加CPU核心解决，或者利用分布式计算资源。</p>
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