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<code>DevTools</code></p><p>内存泄漏是指不再使用的内存，没有被垃圾回收机制回收。当内存泄漏很大或足够频繁时，用户会有所感知：轻则影响应用性能，表现为迟缓卡顿；重则导致应用奔溃，表现为无法正常使用。为了避免内存泄漏带来的不良影响，需要对垃圾回收机制进行了解，掌握内存泄漏分析方法，完善线上相关监控措施。</p><p>内存泄漏定位和分析一般需要辅助工具，比如 Chrome DevTools。开发者可以通过 DevTools 记录页面活动概况，生成可视化分析结果，从时间轴中直观了解内存泄漏情况；利用 DevTools 获取若干次内存快照，检查内存堆栈变化；以及使用 Chrome 任务管理器，实时监控内存的使用情况。</p><h2><a aria-hidden="true" tabindex="-1" class="anchor anchor__h2 anchorWithHideOnScrollNavbar_3R7-" id="知识点深入"></a>知识点深入<a class="hash-link" href="#知识点深入" title="Direct link to heading">#</a></h2><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="1-排查内存泄漏常见问题"></a>1. 排查内存泄漏常见问题<a class="hash-link" href="#1-排查内存泄漏常见问题" title="Direct link to heading">#</a></h3><p>在 JavaScript 中，当一些不再需要的数据仍然可达时，V8 会认为这些数据仍在被使用，不会释放内存。为了调试内存泄漏，我们需要找到被错误保留的数据，并确保 V8 能够将其清理掉。</p><p>代码量较小时，开发者通常可以基于以下基本原则进行快速自查：</p><ol><li>是否滥用全局变量，没有手动回收。</li><li>是否没有正确销毁定时器、闭包。</li><li>是否没有正确监听事件和销毁事件。</li></ol><p>除此之外，开发者可以借助外部工具进行内存泄漏排查。</p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="2-使用-chrome-devtools-定位内存泄漏"></a>2. 使用 Chrome DevTools 定位内存泄漏<a class="hash-link" href="#2-使用-chrome-devtools-定位内存泄漏" title="Direct link to heading">#</a></h3><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="performance"></a>Performance<a class="hash-link" href="#performance" title="Direct link to heading">#</a></h4><p><img src="https://user-images.githubusercontent.com/17002181/129483290-b6fa5f92-2509-49af-9866-e1b08502fe51.png" alt="image"></p><p>打开准备分析的页面和 DevTools 的 Performance 面板，勾选 Memory 并开始录制，在模拟用户操作一段时间后结束录制，DevTools 会将这段时间内的页面行为活动进行记录和分析。</p><p>通过生成的结果可以直观查看到内存时间线，了解内存随时间的占用变化，如果内存占用曲线成阶梯状一直上升，则可能存在内存泄漏。按需选取时间线中的区域片段，检查对应时间段内的活动类型和时间占用，作为排查和定位内存泄漏的辅助办法。</p><h4><a aria-hidden="true" tabindex="-1" class="anchor anchor__h4 anchorWithHideOnScrollNavbar_3R7-" id="memory"></a>Memory<a class="hash-link" href="#memory" title="Direct link to heading">#</a></h4><p><img src="https://user-images.githubusercontent.com/17002181/130357605-39e4ffb0-f679-4331-91c0-f9958e70e8a9.png" alt="image"></p><p>打开准备分析的页面和 DevTools 的 Memory 面板，按需生成快照。每个快照的内容是快照时刻，进行一次垃圾回收后，应用中所有可达的对象。</p><p>当开发者明确知道与内存泄漏关联的用户交互步骤时，可以生成多次内存快照进行对比，排查出泄漏的对象：在做用户交互操作之前，进行一次正常内存堆栈信息的快照；在做用户交互操作中或操作结束时，进行内存快照。使用 Comparison 视图或使用 filter 按需查看快照之间的差异。</p><p>上面的图中使用 filter 检查快照 2 和快照 3 的差异，通过结果可知在两个快照之间持续被分配 clickCallback 闭包。通过点击文件路径可以定位到内存泄漏的代码。</p><p><img src="https://user-images.githubusercontent.com/17002181/130357708-28497c8f-93f8-4afd-a53f-9506eaa3de9c.png" alt="image"></p><h3><a aria-hidden="true" tabindex="-1" class="anchor anchor__h3 anchorWithHideOnScrollNavbar_3R7-" id="3-nodejs-中的内存泄漏定位"></a>3. Node.js 中的内存泄漏定位<a class="hash-link" href="#3-nodejs-中的内存泄漏定位" title="Direct link to heading">#</a></h3><p>如果需要定位 Node.js 中的内存泄漏，启动 Node.js 时带上 --inspect 参数，以便利用 Chrome DevTools 工具生成 Memory 快照数据。如图所示，启动 Node.js 服务后，打开 Chrome DevTools，会有 Node 标识，点击可以打开 Node 专用 DevTools。</p><p><img src="https://user-images.githubusercontent.com/17002181/130330062-c37a1085-1464-4cc9-8aa0-2f65decead03.png" alt="image"></p><p>除此之外，也可以借助第三方包 heapdump 生成快照文件，导入至 Chrome DevTools 中的 Memory 进行快照对比。</p><p>启动 Node.js 时带上 --expose-gc 参数以便调用 <code>global.gc()</code> 方法触发垃圾回收。借助 <code>process.memoryUsage().heapUsed</code> 检查内存大小，作为内存泄漏的辅助判断。</p><div class="codeBlockContainer_K1bP"><div class="codeBlockContent_hGly js"><pre tabindex="0" class="prism-code language-js codeBlock_23N8 thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_39YC"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> heapdump </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">require</span><span class="token punctuation" style="color:#393A34">(</span><span class="token string" style="color:#e3116c">"heapdump"</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> </span><span class="token function-variable function" style="color:#d73a49">capture</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> global</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">gc</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> heapdump</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">writeSnapshot</span><span class="token punctuation" style="color:#393A34">(</span><span class="token string" style="color:#e3116c">"./HZFE_HEAPSNAPSHOT/"</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">+</span><span class="token plain"> </span><span class="token known-class-name class-name">Date</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">now</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">+</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">".heapsnapshot"</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token console class-name">console</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">log</span><span class="token punctuation" style="color:#393A34">(</span><span class="token string" style="color:#e3116c">"heapUsed:"</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> process</span><span class="token punctuation" style="color:#393A34">.</span><span class="token method function property-access" style="color:#d73a49">memoryUsage</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">.</span><span class="token property-access">heapUsed</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token function" style="color:#d73a49">capture</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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</span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic">/* 可能有内存泄漏的代码片段 start */</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic">// code</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token comment" style="color:#999988;font-style:italic">/* 可能有内存泄漏的代码片段 end */</span><span class="token plain"></span></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block">
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