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<meta name="keywords" content="Java,String,StringBuilder,StringBuffer"><meta property="og:url" content="https://tobebetterjavaer.com/string/builder-buffer.html"><meta property="og:site_name" content="Java程序员进阶之路"><meta property="og:title" content="聊聊 Java String、StringBuilder、StringBuffer 三兄弟"><meta property="og:description" content="在本文中，我们将详细探讨 Java 中 String、StringBuilder 和 StringBuffer 的差异和使用场景。学习如何根据实际需求选择正确的字符串处理类，了解它们在性能、线程安全和内存开销方面的优缺点。本文还将解析它们的内部实现，帮助你更好地理解 Java 字符串处理的原理和最佳实践。"><meta property="og:type" content="article"><meta property="og:locale" content="zh-CN"><meta property="og:updated_time" content="2023-04-24T02:39:04.000Z"><meta property="article:tag" content="数组&字符串"><meta property="article:modified_time" content="2023-04-24T02:39:04.000Z"><script type="application/ld+json">{"@context":"https://schema.org","@type":"Article","headline":"聊聊 Java String、StringBuilder、StringBuffer 三兄弟","image":[""],"dateModified":"2023-04-24T02:39:04.000Z","author":[]}</script><meta name="robots" content="all"><meta name="author" content="沉默王二"><meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate"><meta http-equiv="Pragma" content="no-cache"><meta http-equiv="Expires" content="0"><meta name="apple-mobile-web-app-capable" content="yes"><script>
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class="title">二、Java基础</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.1 Java概述及环境配置</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.2 Java语法基础</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">2.3 数组&字符串</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><a href="/array/array.html" class="nav-link sidebar-link sidebar-page" aria-label="掌握Java数组"><!---->掌握Java数组<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/array/double-array.html" class="nav-link sidebar-link sidebar-page" aria-label="掌握Java二维数组"><!---->掌握Java二维数组<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/array/print.html" class="nav-link sidebar-link sidebar-page" aria-label="打印Java数组"><!---->打印Java数组<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/string-source.html" class="nav-link sidebar-link sidebar-page" aria-label="解读String类源码"><!---->解读String类源码<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/immutable.html" class="nav-link sidebar-link sidebar-page" aria-label="String为什么不可变"><!---->String为什么不可变<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/constant-pool.html" class="nav-link sidebar-link sidebar-page" aria-label="深入理解Java字符串常量池"><!---->深入理解Java字符串常量池<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/string/intern.html" class="nav-link sidebar-link sidebar-page" aria-label="详解 String.intern 方法"><!---->详解 String.intern 方法<!----></a><ul 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clickable"><!----><span class="title">2.4 面向对象编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.5 集合框架（容器）</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.6 Java IO</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.7 异常处理</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.8 常用工具类</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.9 Java新特性</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 网络编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.11 NIO</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.12 Java重要知识点</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.13 并发编程</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.14 JVM</span><span class="arrow end"></span></button><!----></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">三、Java进阶</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">四、数据库</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">五、计算机基础</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">六、求职面试</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">七、学习建议</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">八、知识库搭建</span><span class="arrow end"></span></button><!----></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">九、联系作者</span><span class="arrow end"></span></button><!----></section></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->聊聊 Java String、StringBuilder、StringBuffer 三兄弟</h1><div class="page-info"><span class="page-author-info" aria-label="作者🖊" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="page-author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><!----><span class="page-date-info" aria-label="写作日期📅" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="calendar icon"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></svg><span><!----></span><meta property="datePublished" content="2023-02-24T04:41:13.000Z"></span><span class="page-category-info" aria-label="分类🌈" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="category icon"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H593.927c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.31-40.31zM730.22 920.502H623.926c-40.925 0-74.22-33.388-74.22-74.425V623.992c0-41.038 33.387-74.424 74.425-74.424h222.085c41.038 0 74.424 33.226 74.424 74.067v114.233c0 10.244-8.304 18.548-18.547 18.548s-18.548-8.304-18.548-18.548V623.635c0-20.388-16.746-36.974-37.33-36.974H624.13c-20.585 0-37.331 16.747-37.331 37.33v222.086c0 20.585 16.654 37.331 37.126 37.331H730.22c10.243 0 18.547 8.304 18.547 18.547 0 10.244-8.304 18.547-18.547 18.547z"></path></svg><span class="page-category-item category0 clickable" role="navigation">Java核心</span><meta property="articleSection" content="Java核心"></span><span class="page-tag-info" aria-label="标签🏷" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon tag-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="tag icon"><path d="M939.902 458.563L910.17 144.567c-1.507-16.272-14.465-29.13-30.737-30.737L565.438 84.098h-.402c-3.215 0-5.726 1.005-7.634 2.913l-470.39 470.39a10.004 10.004 0 000 14.164l365.423 365.424c1.909 1.908 4.42 2.913 7.132 2.913s5.223-1.005 7.132-2.913l470.39-470.39c2.01-2.11 3.014-5.023 2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></svg><span class="page-tag-item tag5 clickable" role="navigation">数组&字符串</span><meta property="keywords" content="数组&字符串"></span><span class="page-word-info" aria-label="字数🔠" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon word-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="word icon"><path d="M518.217 432.64V73.143A73.143 73.143 0 01603.43 1.097a512 512 0 01419.474 419.474 73.143 73.143 0 01-72.046 85.212H591.36a73.143 73.143 0 01-73.143-73.143z"></path><path d="M493.714 566.857h340.297a73.143 73.143 0 0173.143 85.577A457.143 457.143 0 11371.566 117.76a73.143 73.143 0 0185.577 73.143v339.383a36.571 36.571 0 0036.571 36.571z"></path></svg><span>约 2086 字</span><meta property="wordCount" content="2086"></span><span class="page-reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewBox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 7 分钟</span><meta property="timeRequired" content="PT7M"></span></div><hr></div><!----><!----><div class="theme-hope-content"><h1 id="_4-8-string、stringbuilder、stringbuffer" tabindex="-1"><a class="header-anchor" href="#_4-8-string、stringbuilder、stringbuffer" aria-hidden="true">#</a> 4.8 String、StringBuilder、StringBuffer</h1><p>“哥，<a href="https://tobebetterjavaer.com/string/intern.html" target="_blank" rel="noopener noreferrer">上一篇深入理解 String.intern()<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> 讲到了 StringBuilder，这一节我们就来聊聊吧！”三妹很期待。</p><p>“好啊，它们之间的关系还真的是挺和谐的。”看着三妹好奇的样子，我感到学技术就应该是这个样子才对。</p><p>由于字符串是不可变的，所以当遇到<a href="https://tobebetterjavaer.com/string/join.html" target="_blank" rel="noopener noreferrer">字符串拼接<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>（尤其是使用<code>+</code>号操作符）的时候，就需要考量性能的问题，你不能毫无顾虑地生产太多 String 对象，对珍贵的内存造成不必要的压力。</p><p>于是 Java 就设计了一个专门用来解决此问题的 StringBuffer 类。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">StringBuffer</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractStringBuilder</span> <span class="token keyword">implements</span> <span class="token class-name">Serializable</span><span class="token punctuation">,</span> <span class="token class-name">CharSequence</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token class-name">StringBuffer</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">synchronized</span> <span class="token class-name">StringBuffer</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">synchronized</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 其他方法</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>不过，由于 StringBuffer 操作字符串的方法加了 <a href="https://tobebetterjavaer.com/thread/synchronized-1.html" target="_blank" rel="noopener noreferrer"><code>synchronized</code> 关键字<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>进行了同步，主要是考虑到多线程环境下的安全问题，所以执行效率会比较低。</p><p>于是 Java 就给 StringBuffer “生了个兄弟”，名叫 StringBuilder，说，“孩子，你别管线程安全了，你就在单线程环境下使用，这样效率会高得多，如果要在多线程环境下修改字符串，你到时候可以使用 <a href="https://tobebetterjavaer.com/thread/ThreadLocal.html" target="_blank" rel="noopener noreferrer"><code>ThreadLocal</code><span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> 来避免多线程冲突。”</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">StringBuilder</span> <span class="token keyword">extends</span> <span class="token class-name">AbstractStringBuilder</span>
<span class="token keyword">implements</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span><span class="token punctuation">,</span> <span class="token class-name">CharSequence</span>
<span class="token punctuation">{</span>
<span class="token comment">// ...</span>
<span class="token keyword">public</span> <span class="token class-name">StringBuilder</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// Create a copy, don't share the array</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>除了类名不同，方法没有加 synchronized，基本上完全一样。</p><p>实际开发中，StringBuilder 的使用频率也是远高于 StringBuffer，甚至可以这么说，StringBuilder 完全取代了 StringBuffer。</p><p><a href="https://tobebetterjavaer.com/overview/what-is-java.html" target="_blank" rel="noopener noreferrer">之前我们也曾聊过<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>，Java 是一门解释型的编程语言，所以当编译器遇到 <code>+</code> 号这个操作符的时候，会将 <code>new String("二哥") + new String("三妹")</code> 这行代码编译为以下代码：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">new</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"二哥"</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span><span class="token string">"三妹"</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>这个过程是我们看不见的，但这正是 Java 的“智能”之处，它可以在编译的时候偷偷地帮我们做很多优化，这样既可以提高我们的开发效率（<code>+</code> 号写起来比创建 StringBuilder 对象便捷得多），也不会影响 JVM 的执行效率。</p><p>当然了，如果我们使用 <a href="https://tobebetterjavaer.com/jvm/bytecode.html" target="_blank" rel="noopener noreferrer">javap<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> 反编译 <code>new String("二哥") + new String("三妹")</code> 的字节码的时候，也是能看出 StringBuilder 的影子的。</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>0: new #2 // class java/lang/StringBuilder
3: dup
4: invokespecial #3 // Method java/lang/StringBuilder."<init>":()V
7: new #4 // class java/lang/String
10: dup
11: ldc #5 // String 二哥
13: invokespecial #6 // Method java/lang/String."<init>":(Ljava/lang/String;)V
16: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: new #4 // class java/lang/String
22: dup
23: ldc #8 // String 三妹
25: invokespecial #6 // Method java/lang/String."<init>":(Ljava/lang/String;)V
28: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
31: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: areturn
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>可以看到 Java 编译器将字符串拼接操作（<code>+</code>）转换为了 StringBuilder 对象的 append 方法，然后再调用 StringBuilder 对象的 toString 方法返回拼接后的字符串。</p><p>来看一下 StringBuilder 的 toString 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>value 是一个 char 类型的数组：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token doc-comment comment">/**
* The value is used for character storage.
*/</span>
<span class="token keyword">char</span><span class="token punctuation">[</span><span class="token punctuation">]</span> value<span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>在 StringBuilder 对象创建时，会为 value 分配一定的内存空间（初始容量 16），用于存储字符串。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token doc-comment comment">/**
* Constructs a string builder with no characters in it and an
* initial capacity of 16 characters.
*/</span>
<span class="token keyword">public</span> <span class="token class-name">StringBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">(</span><span class="token number">16</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>随着字符串的拼接，value 数组的长度会不断增加，因此在 StringBuilder 对象的实现中，value 数组的长度是可以<a href="https://tobebetterjavaer.com/collection/arraylist.html" target="_blank" rel="noopener noreferrer">动态扩展的，就像ArrayList那样<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>。</p><p>继续来看 StringBuilder 的 toString 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>value 用于存储 StringBuilder 对象中包含的字符序列。count 是一个 int 类型的变量，表示字符序列的长度。toString() 方法会调用 <code>new String(value, 0, count)</code>，使用 value 数组中从 0 开始的前 count 个元素创建一个新的字符串对象，并将其返回。</p><p>再来看一下 append 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">StringBuilder</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">append</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>实际上是调用了 AbstractStringBuilder 中的 append(String str) 方法。在 AbstractStringBuilder 中，append(String str) 方法会检查当前字符序列中的字符是否够用，如果不够用则会进行扩容，并将指定字符串追加到字符序列的末尾。</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token doc-comment comment">/**
* Appends the specified string to this character sequence.
* <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>p</span><span class="token punctuation">></span></span>
* The characters of the <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java"><span class="token class-name">String</span></span></span><span class="token punctuation">}</span> argument are appended, in order,
* increasing the length of this sequence by the length of the argument.
* If <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java">str</span></span><span class="token punctuation">}</span> is <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java"><span class="token keyword">null</span></span></span><span class="token punctuation">}</span>, then the four characters <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java"><span class="token string">"null"</span></span></span><span class="token punctuation">}</span>
* are appended.
* <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>p</span><span class="token punctuation">></span></span>
* Let <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>i</span><span class="token punctuation">></span></span>n<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>i</span><span class="token punctuation">></span></span> be the length of this character sequence just prior to
* execution of the <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java">append</span></span><span class="token punctuation">}</span> method. Then the character at index
* <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>i</span><span class="token punctuation">></span></span>k<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>i</span><span class="token punctuation">></span></span> in this character sequence is equal to the character at index
* <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>i</span><span class="token punctuation">></span></span>k<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>i</span><span class="token punctuation">></span></span> in the argument <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java">str</span></span><span class="token punctuation">}</span>, if <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>i</span><span class="token punctuation">></span></span>k<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>i</span><span class="token punctuation">></span></span> is less than
* <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>i</span><span class="token punctuation">></span></span>n<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>i</span><span class="token punctuation">></span></span>; otherwise, it is equal to the character at index
* <span class="token tag"><span class="token tag"><span class="token punctuation"><</span>i</span><span class="token punctuation">></span></span>k-n<span class="token tag"><span class="token tag"><span class="token punctuation"></</span>i</span><span class="token punctuation">></span></span> in the argument <span class="token punctuation">{</span><span class="token keyword">@code</span> <span class="token code-section"><span class="token code language-java">str</span></span><span class="token punctuation">}</span>.
*
* <span class="token keyword">@param</span> <span class="token parameter">str</span> a string.
* <span class="token keyword">@return</span> a reference to this object.
*/</span>
<span class="token keyword">public</span> <span class="token class-name">AbstractStringBuilder</span> <span class="token function">append</span><span class="token punctuation">(</span><span class="token class-name">String</span> str<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>str <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
<span class="token keyword">return</span> <span class="token function">appendNull</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> len <span class="token operator">=</span> str<span class="token punctuation">.</span><span class="token function">length</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span>count <span class="token operator">+</span> len<span class="token punctuation">)</span><span class="token punctuation">;</span>
str<span class="token punctuation">.</span><span class="token function">getChars</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> len<span class="token punctuation">,</span> value<span class="token punctuation">,</span> count<span class="token punctuation">)</span><span class="token punctuation">;</span>
count <span class="token operator">+=</span> len<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>append(String str)</code> 方法将指定字符串追加到当前字符序列中。如果指定字符串为 null，则追加字符串 "null"；否则会检查指定字符串的长度，然后根据当前字符序列中的字符数和指定字符串的长度来判断是否需要扩容。</p><p>如果需要扩容，则会调用 <code>ensureCapacityInternal(int minimumCapacity) </code>方法进行扩容。扩容之后，将指定字符串的字符拷贝到字符序列中。</p><p>来看一下 ensureCapacityInternal 方法：</p><div class="language-java line-numbers-mode" data-ext="java"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">ensureCapacityInternal</span><span class="token punctuation">(</span><span class="token keyword">int</span> minimumCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// overflow-conscious code</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>minimumCapacity <span class="token operator">-</span> value<span class="token punctuation">.</span>length <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span>
<span class="token function">expandCapacity</span><span class="token punctuation">(</span>minimumCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">expandCapacity</span><span class="token punctuation">(</span><span class="token keyword">int</span> minimumCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> newCapacity <span class="token operator">=</span> value<span class="token punctuation">.</span>length <span class="token operator">*</span> <span class="token number">2</span> <span class="token operator">+</span> <span class="token number">2</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator">-</span> minimumCapacity <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span>
newCapacity <span class="token operator">=</span> minimumCapacity<span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>newCapacity <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>minimumCapacity <span class="token operator"><</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// overflow</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">OutOfMemoryError</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
newCapacity <span class="token operator">=</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span><span class="token constant">MAX_VALUE</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
value <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> newCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><code>ensureCapacityInternal(int minimumCapacity)</code> 方法用于确保当前字符序列的容量至少等于指定的最小容量 minimumCapacity。如果当前容量小于指定的容量，就会为字符序列分配一个新的内部数组。新容量的计算方式如下：</p><ul><li>如果指定的最小容量大于当前容量，则新容量为两倍的旧容量加上 2；</li><li>如果指定的最小容量小于等于当前容量，则不会进行扩容，直接返回当前对象。</li></ul><p>在进行扩容之前，<code>ensureCapacityInternal(int minimumCapacity)</code> 方法会先检查当前字符序列的容量是否足够，如果不足就会调用 <code>expandCapacity(int minimumCapacity)</code> 方法进行扩容。<code>expandCapacity(int minimumCapacity)</code> 方法首先计算出新容量，然后使用 <code>Arrays.copyOf(char[] original, int newLength)</code> 方法将原字符数组扩容到新容量的大小。</p><p>关于扩容，后面在讲<a href="https://tobebetterjavaer.com/collection/arraylist.html" target="_blank" rel="noopener noreferrer">ArrayList<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>的时候会再次说明，今天就先聊到这吧。</p><p>“我想，关于 String、StringBuilder、StringBuilder 之间的差别，你都搞清楚了吧？”我问。</p><p>“哥，你真棒！区别我是搞清楚了，你后面讲的源码扩容还没消化，我一会去加个餐，再细看一下。”三妹说。</p><p>“可以的，实际上，你现在只需要知道 StringBuilder 的用法就可以了。”喝了一口右手边的可口可乐（无糖）后，我感觉好爽快啊。</p><hr><p>GitHub 上标星 7600+ 的开源知识库《<a href="https://github.com/itwanger/toBeBetterJavaer" target="_blank" rel="noopener noreferrer">二哥的 Java 进阶之路<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>》第一版 PDF 终于来了！包括Java基础语法、数组&字符串、OOP、集合框架、Java IO、异常处理、Java 新特性、网络编程、NIO、并发编程、JVM等等，共计 32 万余字，可以说是通俗易懂、风趣幽默……详情戳：<a href="https://tobebetterjavaer.com/overview/" target="_blank" rel="noopener noreferrer">太赞了，GitHub 上标星 7600+ 的 Java 教程<span><svg 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