Java解决线程安全问题_java线程安全问题 - wpsshop博客

查看

编辑修改

首页
UNITY
NODEJS
PYTHON
AI
GIT
PHP
GO
CEF3
JAVA
HTML
CSS

不正经

这个屌丝很懒，什么也没留下！

热门标签

jquery
HTML
CSS
PHP
ASP
PYTHON
GO
AI
C
C++
C#
PHOTOSHOP
UNITY
iOS
android
vue
xml
爬虫
SEO
LINUX
WINDOWS
JAVA
MFC
CEF3
CAD
NODEJS
GIT
Pyppeteer
article

热门文章

1手把手系列！使用 Zilliz Cloud 和 AWS Bedrock 搭建 RAG 应用
2kafka-消费者组-点对点测试
3华为OD机试 - 生成哈夫曼树（Java & JS & Python & C & C++）_生成哈夫曼树华为od
4计算机专业近几年的参考文献,近几年计算机专业发参考文献计算机专业发专著类参考文献哪里找...
5人工智能轨道交通行业周刊-第29期（2023.1.2-1.8）_昆明南站小红车机器人
6清览云题库--数据可视化
7消息队列:RabbitMQ_消息队列rabbitmq
8小华最多能得到多少克黄金【华为OD机试】(JAVA&Python&C++&JS题解)_小华按照地图去寻宝
9Makefile入门_makefile学习
10html标签“”中文本对齐实现，让人蛋疼的知识_html textarea跟前两行数据对齐</a></li></ul></div></div></div><link rel="stylesheet" href="https://cdn.wpsshop.cn/public/blog/css/mid_detail.css?v=890" type="text/css"><link rel="stylesheet" href="https://cdn.wpsshop.cn/public/blog/css/mid_clk.css?v=890" type="text/css"><script type="text/javascript" src="https://cdn.wpsshop.cn/public/blog/js/mid_.js?v=890"></script><link rel="stylesheet" href="https://cdn.wpsshop.cn/public/study/js/j/vs.css?v=890" type="text/css"><script type="text/javascript" src="https://cdn.wpsshop.cn/public/study/js/j/highlight.pack.js?v=890" ></script><link rel="stylesheet" href="https://cdn.wpsshop.cn/public/blog/css/index_contenthljs.css?v=890" type="text/css"><script type="text/javascript" src="https://cdn.wpsshop.cn/public/wpsshopindex/js/detail01tj.js?v=531890" ></script><link rel="stylesheet" href="https://cdn.wpsshop.cn/public/wpsshopindex/css/mid_detail_last.css?v=3890" type="text/css"><script type="text/javascript" src="https://cdn.wpsshop.cn/public/wpsshopindex/js/mid_detail_last.js?v=33890"></script><style type="text/css"></style><script type="text/javascript"></script><div class="interactive_info" id="body-data-0098" attrv="50082"></div><div class="article_box" style="overflow: hidden;"><div class="article_channel"><div class="channel_top_line"><div class="cn_t_l"></div><div class="cn_t_r"></div></div><div class="article_channel_info">当前位置: <a href="/article/list/1">article</a> > 正文 </div></div><div class="article_title"><h1>Java解决线程安全问题_java线程安全问题</h1></div><div class="article_info"><div class="article_c_info"> 作者：不正经 | 2024-05-31 03:18:42</div><div class="article_click" style="width: 140px;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div class="article_intro">java线程安全问题</div><div class="article_content article_contentheight"> <link rel="stylesheet" href="https://csdnimg.cn/release/blogv2/dist/mdeditor/css/editerView/kdoc_html_views-1a98987dfd.css"> <link rel="stylesheet" href="https://csdnimg.cn/release/blogv2/dist/mdeditor/css/editerView/ck_htmledit_views-044f2cf1dc.css"> <div id="content_views" class="markdown_views prism-atom-one-light"> <svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> <path stroke-linecap="round" d="M5,0 0,2.5 5,5z" id="raphael-marker-block" style="-webkit-tap-highlight-color: rgba(0, 0, 0, 0);"></path> </svg> <p></p> <div class="toc"> <h4><a name="t0"></a>文章目录</h4> <ul><li><ul><li><a href="#_1" rel="nofollow" target="_self">背景</a></li><li><a href="#1__4" rel="nofollow" target="_self">1. 线程安全问题</a></li><li><ul><li><a href="#11__6" rel="nofollow" target="_self">1.1 什么是线程安全？</a></li><li><a href="#12__22" rel="nofollow" target="_self">1.2 产生的原因</a></li><li><a href="#13__29" rel="nofollow" target="_self">1.3 实例（买票超卖问题）</a></li><li><a href="#14__113" rel="nofollow" target="_self">1.4 如何确定是否存在线程安全问题？</a></li></ul> </li><li><a href="#2__119" rel="nofollow" target="_self">2. 如何解决线程安全问题？</a></li><li><ul><li><a href="#21_Immutable_125" rel="nofollow" target="_self">2.1 不可变（Immutable）</a></li><li><a href="#22__148" rel="nofollow" target="_self">2.2 变量私有化</a></li><li><ul><li><a href="#221__152" rel="nofollow" target="_self">2.2.1 栈封闭（主要为局部变量）</a></li><li><a href="#222_Thread_Local_Storage_186" rel="nofollow" target="_self">2.2.2 线程本地存储（Thread Local Storage）</a></li></ul> </li><li><a href="#23__274" rel="nofollow" target="_self">2.3 互斥同步</a></li><li><a href="#24__332" rel="nofollow" target="_self">2.4 非阻塞同步</a></li><li><ul><li><a href="#241_CAS_346" rel="nofollow" target="_self">2.4.1 CAS</a></li><li><a href="#242_Atomic_399" rel="nofollow" target="_self">2.4.2 Atomic（原子操作）</a></li></ul> </li></ul> </li><li><a href="#3__414" rel="nofollow" target="_self">3. 总结和分析</a></li><li><a href="#_424" rel="nofollow" target="_self">参考</a></li></ul> </li></ul> </div> <p></p> <h3><a name="t1"></a><a id="_1"></a>背景</h3> <p>原文地址：<a href="https://duktig.cn/archives/36/" rel="nofollow">https://duktig.cn/archives/36/</a></p> <h3><a name="t2"></a><a id="1__4"></a>1. <a href="https://so.csdn.net/so/search?q=%E7%BA%BF%E7%A8%8B%E5%AE%89%E5%85%A8&spm=1001.2101.3001.7020" target="_blank" class="hl hl-1" data-report-view="{"spm":"1001.2101.3001.7020","dest":"https://so.csdn.net/so/search?q=%E7%BA%BF%E7%A8%8B%E5%AE%89%E5%85%A8&spm=1001.2101.3001.7020","extra":"{\"searchword\":\"线程安全\"}"}" data-report-click="{"spm":"1001.2101.3001.7020","dest":"https://so.csdn.net/so/search?q=%E7%BA%BF%E7%A8%8B%E5%AE%89%E5%85%A8&spm=1001.2101.3001.7020","extra":"{\"searchword\":\"线程安全\"}"}" data-tit="线程安全" data-pretit="线程安全">线程安全</a>问题</h3> <h4><a name="t3"></a><a id="11__6"></a>1.1 什么是线程安全？</h4> <blockquote> <p>线程安全：多个线程不管以何种方式访问某个类，并且在主调代码中不需要进行同步，都能表现正确的行为。</p> </blockquote> <p>“线程安全”不是指线程的安全，而是指内存的安全。为什么如此说呢？这和操作系统有关。</p> <p>目前主流操作系统都是多任务的，即多个进程同时运行。为了保证安全，每个进程只能访问分配给自己的内存空间，而不能访问别的进程的，这是由操作系统保障的。但是<strong>进程中的多个线程共享进程的堆内存</strong>，这就是造成问题的潜在原因。</p> <p>假设某个线程把数据处理到一半，觉得很累，就去休息了一会，回来准备接着处理，却发现数据已经被修改了，不是自己离开时的样子了。可能被其它线程修改了。</p> <p><em>比如把你住的小区看作一个进程，小区里的道路/绿化等就属于公共区域。你拿1万块钱往地上一扔，就回家睡觉去了。睡醒后你打算去把它捡回来，发现钱已经不见了。可能被别人拿走了。因为公共区域人来人往，你放的东西在没有看管措施时，一定是不安全的。内存中的情况亦然如此。</em></p> <blockquote> <p>所以线程安全指的是，在堆内存中的数据由于可以被任何线程访问到，在没有限制的情况下存在被意外修改的风险。</p> </blockquote> <p>即堆内存空间在没有保护机制的情况下，对多线程来说是不安全的地方，因为你放进去的数据，可能被别的线程“破坏”。</p> <h4><a name="t4"></a><a id="12__22"></a>1.2 产生的原因</h4> <ol><li>多个线程执行的不确定性引起执行结果的不稳定。</li><li>多个线程对数据的共享，会造成操作的不完整性，会破坏数据。</li></ol> <p>当多条语句在操作同一个线程共享数据时，一个线程对多条语句只执行了一部分，还没有执行完，另一个线程参与进来执行，导致共享数据的错误。</p> <h4><a name="t5"></a><a id="13__29"></a>1.3 实例（买票超卖问题）</h4> <pre data-index="0" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token comment">/** * 例子：创建三个窗口卖票，总票数为100张.使用继承Thread类的方式 * * 存在线程的安全问题，待解决。 */</span> <span class="token keyword">class</span> <span class="token class-name">Window</span> <span class="token keyword">extends</span> <span class="token class-name">Thread</span><span class="token punctuation">{</span> <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">int</span> ticket <span class="token operator">=</span> <span class="token number">100</span><span class="token punctuation">;</span> <span class="token annotation punctuation">@Override</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">while</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">{</span> <span class="token keyword">try</span> <span class="token punctuation">{</span> Thread<span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">InterruptedException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span> e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">if</span><span class="token punctuation">(</span>ticket <span class="token operator">></span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{</span> System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"：卖票，票号为："</span> <span class="token operator">+</span> ticket<span class="token punctuation">)</span><span class="token punctuation">;</span> ticket<span class="token operator">--</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span> <span class="token keyword">break</span><span class="token punctuation">;</span> <span class="token punctuation">}</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">class</span> <span class="token class-name">WindowTest</span> <span class="token punctuation">{</span> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span> Window t1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Window</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> Window t2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Window</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> Window t3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Window</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> t1<span class="token punctuation">.</span><span class="token function">setName</span><span class="token punctuation">(</span><span class="token string">"窗口1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> t2<span class="token punctuation">.</span><span class="token function">setName</span><span class="token punctuation">(</span><span class="token string">"窗口2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> t3<span class="token punctuation">.</span><span class="token function">setName</span><span class="token punctuation">(</span><span class="token string">"窗口3"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> t1<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> t2<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> t3<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li><li style="color: rgb(153, 153, 153);">9</li><li style="color: rgb(153, 153, 153);">10</li><li style="color: rgb(153, 153, 153);">11</li><li style="color: rgb(153, 153, 153);">12</li><li style="color: rgb(153, 153, 153);">13</li><li style="color: rgb(153, 153, 153);">14</li><li style="color: rgb(153, 153, 153);">15</li><li style="color: rgb(153, 153, 153);">16</li><li style="color: rgb(153, 153, 153);">17</li><li style="color: rgb(153, 153, 153);">18</li><li style="color: rgb(153, 153, 153);">19</li><li style="color: rgb(153, 153, 153);">20</li><li style="color: rgb(153, 153, 153);">21</li><li style="color: rgb(153, 153, 153);">22</li><li style="color: rgb(153, 153, 153);">23</li><li style="color: rgb(153, 153, 153);">24</li><li style="color: rgb(153, 153, 153);">25</li><li style="color: rgb(153, 153, 153);">26</li><li style="color: rgb(153, 153, 153);">27</li><li style="color: rgb(153, 153, 153);">28</li><li style="color: rgb(153, 153, 153);">29</li><li style="color: rgb(153, 153, 153);">30</li><li style="color: rgb(153, 153, 153);">31</li><li style="color: rgb(153, 153, 153);">32</li><li style="color: rgb(153, 153, 153);">33</li><li style="color: rgb(153, 153, 153);">34</li><li style="color: rgb(153, 153, 153);">35</li><li style="color: rgb(153, 153, 153);">36</li><li style="color: rgb(153, 153, 153);">37</li><li style="color: rgb(153, 153, 153);">38</li><li style="color: rgb(153, 153, 153);">39</li><li style="color: rgb(153, 153, 153);">40</li><li style="color: rgb(153, 153, 153);">41</li><li style="color: rgb(153, 153, 153);">42</li><li style="color: rgb(153, 153, 153);">43</li><li style="color: rgb(153, 153, 153);">44</li></ul></pre> <p><strong>结果</strong></p> <p>出现了共享数据错误</p> <pre data-index="1" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;">窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">100</span> 窗口<span class="token number">1</span>：卖票，票号为：<span class="token number">100</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">98</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">97</span> 窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">97</span> 窗口<span class="token number">1</span>：卖票，票号为：<span class="token number">95</span> 窗口<span class="token number">1</span>：卖票，票号为：<span class="token number">94</span> 窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">93</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">92</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">91</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> 窗口<span class="token number">1</span>：卖票，票号为：<span class="token number">10</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">10</span> 窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">10</span> 窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">7</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">7</span> 窗口<span class="token number">1</span>：卖票，票号为：<span class="token number">5</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">4</span> 窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">4</span> 窗口<span class="token number">1</span>：卖票，票号为：<span class="token number">2</span> 窗口<span class="token number">2</span>：卖票，票号为：<span class="token number">1</span> 窗口<span class="token number">3</span>：卖票，票号为：<span class="token number">1</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li><li style="color: rgb(153, 153, 153);">9</li><li style="color: rgb(153, 153, 153);">10</li><li style="color: rgb(153, 153, 153);">11</li><li style="color: rgb(153, 153, 153);">12</li><li style="color: rgb(153, 153, 153);">13</li><li style="color: rgb(153, 153, 153);">14</li><li style="color: rgb(153, 153, 153);">15</li><li style="color: rgb(153, 153, 153);">16</li><li style="color: rgb(153, 153, 153);">17</li><li style="color: rgb(153, 153, 153);">18</li><li style="color: rgb(153, 153, 153);">19</li><li style="color: rgb(153, 153, 153);">20</li><li style="color: rgb(153, 153, 153);">21</li><li style="color: rgb(153, 153, 153);">22</li></ul></pre> <p><strong>分析</strong></p> <ol><li>问题：三条线程同时共享ticket的票，卖票过程中，出现了重票、错票 -->出现了线程的安全问题，导致数据错误。</li><li>原因：当某个线程操作车票的过程中，尚未操作完成时，其他线程参与进来，也操作车票。</li><li>解决：当一个线程a在操作ticket的时候，其他线程不能参与进来。直到线程a操作完ticket时，其他线程才可以开始操作ticket。这种情况即使线程a出现了阻塞，也不能被改变。</li></ol> <h4><a name="t6"></a><a id="14__113"></a>1.4 如何确定是否存在线程安全问题？</h4> <ul><li>明确哪些代码是多线程运行的代码</li><li>明确多个线程是否有共享数据</li><li>明确多线程运行代码中是否有多条语句操作共享数据</li></ul> <h3><a name="t7"></a><a id="2__119"></a>2. 如何解决线程安全问题？</h3> <p>如何解决线程安全问题？解决的过程其实就是一个取舍的过程，不同的方案有不同的侧重点。</p> <h4><a name="t8"></a><a id="21_Immutable_125"></a>2.1 不可变（Immutable）</h4> <p>不可变的对象一定是线程安全的，不需要采取任何的线程安全措施。只要一个不可变的对象被正确的构建出来，在多线程的状态下也不允许修改，那么一定不会发生不一致的状态。</p> <p>形象比喻：<strong>只能看，不能摸</strong>。自然不会存在线程安全问题。</p> <p>在多线程的环境下，应尽量是对象成为不可变的状态，来满足线程安全，这基本也是最小的开销方式来保证线程安全。</p> <blockquote> <p>不可变类的意思是创建该类的实例后，该实例的实例变量是不可改变的。</p> </blockquote> <p>不可变的类型：</p> <ul><li><code>String</code></li><li>枚举类型</li><li>基本数据类型的包装类以及<code>BigInteger</code> 和 <code>BigDecimal</code> 等大数据类型</li><li>final关键字修饰的基本数据类型</li></ul> <p>注：<strong>final修饰的引用数据类型不可变，但其成员变量的类可能会发生改变</strong>。</p> <h4><a name="t9"></a><a id="22__148"></a>2.2 变量私有化</h4> <p>要保证线程安全，并不是一定就要进行同步。如果一个方法本来就不涉及共享数据，那它自然就无须任何同步措施去保证正确性。</p> <h5><a id="221__152"></a>2.2.1 栈封闭（主要为局部变量）</h5> <p><strong>多个线程访问同一个方法的局部变量时，不会出现线程安全问题，因为局部变量存储在虚拟机栈中，属于线程私有的</strong>。</p> <p>形象比喻：<strong>私有的东西就不该让别人知道</strong>。</p> <p><strong>如果一些数据只有某个线程会使用，其它线程不能操作也不需要操作，这些数据就可以放入线程的栈内存中</strong>。较为常见的就是局部变量。</p> <pre data-index="2" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token keyword">double</span> <span class="token function">avgScore</span><span class="token punctuation">(</span><span class="token keyword">double</span><span class="token punctuation">[</span><span class="token punctuation">]</span> scores<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">double</span> sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">double</span> score <span class="token operator">:</span> scores<span class="token punctuation">)</span> <span class="token punctuation">{</span> sum <span class="token operator">+=</span> score<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token keyword">int</span> count <span class="token operator">=</span> scores<span class="token punctuation">.</span>length<span class="token punctuation">;</span> <span class="token keyword">double</span> avg <span class="token operator">=</span> sum <span class="token operator">/</span> count<span class="token punctuation">;</span> <span class="token keyword">return</span> avg<span class="token punctuation">;</span> <span class="token punctuation">}</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li><li style="color: rgb(153, 153, 153);">9</li></ul></pre> <p>这里的变量<code>sum</code>，<code>count</code>，<code>avg</code>都是局部变量，它们都会被分配在线程栈内存中。</p> <p>假如现在A线程来执行这个方法，这些变量会在A的栈内存分配。与此同时，B线程也来执行这个方法，这些变量也会在B的栈内存中分配。</p> <p>也就是说这些局部变量会在每个线程的栈内存中都分配一份。由于线程的栈内存只能自己访问，所以栈内存中的变量只属于自己，其它线程根本就不知道，也就不存在线程安全问题。</p> <p><strong>问题</strong>：</p> <p>不可能所有的变量都只声明成局部变量，而只供某个线程使用，去解决线程安全问题。如果想要声明成员变量，还想要保证线程安全怎么办？</p> <p>可以使用<code>ThreadLocal</code>，使每个线程都私有化一份这个变量的本地副本，互相不受影响。</p> <h5><a id="222_Thread_Local_Storage_186"></a>2.2.2 线程本地存储（Thread Local Storage）</h5> <p>如果一段代码中所需要的数据必须与其他代码共享，那就看看这些共享数据的代码是否能保证在同一个线程中执行。如果能保证，我们就可以把共享数据的可见范围限制在同一个线程之内，这样，无须同步也能保证线程之间不出现数据争用的问题。</p> <p>可以使用<code>java.lang.ThreadLocal</code>类来实现线程本地存储功能。</p> <p>通俗来说：要让公共区域堆内存中的数据对于每个线程都是安全的，那就每个线程都拷贝它一份，每个线程只处理自己的这一份拷贝而不去影响别的线程的，这不就安全了嘛。</p> <p>形象比喻：<strong>大家不要抢，人人有份</strong>。</p> <p>符合这种特点的应用并不少见，大部分使用消费队列的架构模式（如“生产者-消费者”模式）都会将产品的消费过程尽量在一个线程中消费完。其中最重要的一个应用实例就是经典 Web 交互模型中的“一个请求对应一个服务器线程”（Thread-per-Request）的处理方式，这种处理方式的广泛应用使得很多 Web 服务端应用都可以使用线程本地存储来解决线程安全问题。</p> <p>为了理解 <code>ThreadLocal</code>，先看以下代码：</p> <pre data-index="3" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ThreadLocalExample1</span> <span class="token punctuation">{</span> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span> ThreadLocal threadLocal1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ThreadLocal</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> ThreadLocal threadLocal2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ThreadLocal</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> Thread thread1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">></span> <span class="token punctuation">{</span> threadLocal1<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> threadLocal2<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> Thread thread2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">-</span><span class="token operator">></span> <span class="token punctuation">{</span> threadLocal1<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> threadLocal2<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span> thread1<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> thread2<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li><li style="color: rgb(153, 153, 153);">9</li><li style="color: rgb(153, 153, 153);">10</li><li style="color: rgb(153, 153, 153);">11</li><li style="color: rgb(153, 153, 153);">12</li><li style="color: rgb(153, 153, 153);">13</li><li style="color: rgb(153, 153, 153);">14</li><li style="color: rgb(153, 153, 153);">15</li><li style="color: rgb(153, 153, 153);">16</li></ul></pre> <p>它所对应的底层结构图为:</p> <p><a href="https://cos.duktig.cn/2021/07/05/1625453067.png" rel="nofollow"><img src="https://img-blog.csdnimg.cn/img_convert/e3e222725afa145d01a9036ae83dece0.png" alt="ThreadLocal实例内存结构"></a></p> <p><a href="https://cos.duktig.cn/2021/07/05/1625453067.png" rel="nofollow">ThreadLocal实例内存结构</a></p> <p>每个 Thread 都有一个 <code>ThreadLocal.ThreadLocalMap</code> 对象。</p> <pre data-index="4" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token comment">/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */</span> ThreadLocal<span class="token punctuation">.</span>ThreadLocalMap threadLocals <span class="token operator">=</span> null<span class="token punctuation">;</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li></ul></pre> <p>当调用一个 <code>ThreadLocal</code> 的 <code>set(T value)</code> 方法时，先得到当前线程的 <code>ThreadLocalMap</code> 对象，然后将 <code>ThreadLocal->value</code> 键值对插入到该 Map 中。</p> <pre data-index="5" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">set</span><span class="token punctuation">(</span>T value<span class="token punctuation">)</span> <span class="token punctuation">{</span> Thread t <span class="token operator">=</span> Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> ThreadLocalMap map <span class="token operator">=</span> <span class="token function">getMap</span><span class="token punctuation">(</span>t<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>map <span class="token operator">!=</span> null<span class="token punctuation">)</span> map<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">else</span> <span class="token function">createMap</span><span class="token punctuation">(</span>t<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li></ul></pre> <p>get() 方法类似。</p> <pre data-index="6" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token keyword">public</span> T <span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> Thread t <span class="token operator">=</span> Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> ThreadLocalMap map <span class="token operator">=</span> <span class="token function">getMap</span><span class="token punctuation">(</span>t<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>map <span class="token operator">!=</span> null<span class="token punctuation">)</span> <span class="token punctuation">{</span> ThreadLocalMap<span class="token punctuation">.</span>Entry e <span class="token operator">=</span> map<span class="token punctuation">.</span><span class="token function">getEntry</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token operator">!=</span> null<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token annotation punctuation">@SuppressWarnings</span><span class="token punctuation">(</span><span class="token string">"unchecked"</span><span class="token punctuation">)</span> T result <span class="token operator">=</span> <span class="token punctuation">(</span>T<span class="token punctuation">)</span>e<span class="token punctuation">.</span>value<span class="token punctuation">;</span> <span class="token keyword">return</span> result<span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token keyword">return</span> <span class="token function">setInitialValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li><li style="color: rgb(153, 153, 153);">9</li><li style="color: rgb(153, 153, 153);">10</li><li style="color: rgb(153, 153, 153);">11</li><li style="color: rgb(153, 153, 153);">12</li><li style="color: rgb(153, 153, 153);">13</li></ul></pre> <p><code>ThreadLocal</code> 从理论上讲并不是用来解决多线程并发问题的，因为根本不存在多线程竞争。<code>ThreadLocal</code>就是，把一个数据复制N份，每个线程认领一份，各玩各的，互不影响。</p> <p>在一些场景 (尤其是使用线程池) 下，由于 <code>ThreadLocal.ThreadLocalMap</code> 的底层数据结构导致 <code>ThreadLocal</code> 有内存泄漏的情况，应该<strong>尽可能在每次使用 <code>ThreadLocal</code> 后手动调用 <code>remove()</code>，以避免出现 <code>ThreadLocal</code> 经典的内存泄漏甚至是造成自身业务混乱的风险</strong>。</p> <h4><a name="t10"></a><a id="23__274"></a>2.3 互斥同步</h4> <p>前面给出的一些方案，有点“理想化”了，现实中的情况其实是非常混乱嘈杂的，没有规则的。</p> <p>形象比喻：<strong>没有规则，那就先入为主</strong>。</p> <p>例子：</p> <p>比如在中午高峰期你去饭店吃饭，进门后发现只剩一个空桌子了，你心想先去点餐吧，回来就坐这里吧。当你点完餐回来后，发现已经被别人捷足先登了。</p> <p>因为桌子是属于公共区域的物品，任何人都可以坐，那就只能谁先抢到谁坐。虽然你在人群中曾多看了它一眼，但它并不会记住你容颜。</p> <p>解决方法就不用我说了吧，让一个人在那儿看着座位，其它人去点餐。这样当别人再来的时候，你就可以理直气壮的说，“不好意思，这个座位，我，已经占了”。</p> <p>相信聪明的你已经猜到了我要说的东西了，没错，就是<strong>互斥锁</strong> 。</p> <p>如果公共区域（堆内存）的数据，要被多个线程操作时，为了确保数据的安全（或一致）性，需要在数据旁边放一把锁，要想操作数据，先获取锁再说吧。</p> <p>假设一个线程来到数据跟前一看，发现锁是空闲的，没有人持有。于是它就拿到了这把锁，然后开始操作数据。</p> <p>这时，又来了一个线程，发现锁被别人持有着，按照规定，它不能操作数据，因为它无法得到这把锁。当然，它可以选择等待，或放弃，转而去干别的。</p> <p>因为第一个线程持有锁，可以大胆干事而不用担心其他线程的影响。</p> <p><strong>对于互斥同步锁，可以使用<code>synchronized</code> 和 <code>ReentrantLock</code></strong>。</p> <pre data-index="7" class="set-code-show prettyprint"><code class="prism language-java has-numbering" onclick="mdcp.signin(event)" style="position: unset;"><span class="token keyword">class</span> <span class="token class-name">ClassAssistant</span> <span class="token punctuation">{</span> <span class="token keyword">double</span> totalScore <span class="token operator">=</span> <span class="token number">60</span><span class="token punctuation">;</span> <span class="token keyword">final</span> Lock lock <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Lock</span><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">addScore</span><span class="token punctuation">(</span><span class="token keyword">double</span> score<span class="token punctuation">)</span> <span class="token punctuation">{</span> lock<span class="token punctuation">.</span><span class="token function">obtain</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> totalScore <span class="token operator">+=</span> score<span class="token punctuation">;</span> lock<span class="token punctuation">.</span><span class="token function">release</span><span class="token punctuation">(</span><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">subScore</span><span class="token punctuation">(</span><span class="token keyword">double</span> score<span class="token punctuation">)</span> <span class="token punctuation">{</span> lock<span class="token punctuation">.</span><span class="token function">obtain</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> totalScore <span class="token operator">-=</span> score<span class="token punctuation">;</span> lock<span class="token punctuation">.</span><span class="token function">release</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <div class="hljs-button signin active" data-title="登录复制" data-report-click="{"spm":"1001.2101.3001.4334"}"></div></code><ul class="pre-numbering" style=""><li style="color: rgb(153, 153, 153);">1</li><li style="color: rgb(153, 153, 153);">2</li><li style="color: rgb(153, 153, 153);">3</li><li style="color: rgb(153, 153, 153);">4</li><li style="color: rgb(153, 153, 153);">5</li><li style="color: rgb(153, 153, 153);">6</li><li style="color: rgb(153, 153, 153);">7</li><li style="color: rgb(153, 153, 153);">8</li><li style="color: rgb(153, 153, 153);">9</li><li style="color: rgb(153, 153, 153);">10</li><li style="color: rgb(153, 153, 153);">11</li><li style="color: rgb(153, 153, 153);">12</li><li style="color: rgb(153, 153, 153);">13</li><li style="color: rgb(153, 153, 153);">14</li><li style="color: rgb(153, 153, 153);">15</li><li style="color: rgb(153, 153, 153);">16</li><li style="color: rgb(153, 153, 153);">17</li></ul></pre> <p>假定一个班级的初始分数是60分，这个班级抽出10名学生来同时参加10个不同的答题节目，每个学生答对一次为班级加上5分，答错一次减去5分。因为10个学生一起进行，所以这一定是一个并发情形。</p> <p>因此加分和减分这两个方法被并发的调用，它们共同操作总分数。为了保证数据的一致性，需要在每次操作前先获取锁，操作完成后再释放锁。</p> <p><strong>问题</strong>：</p> <p>互斥阻塞会有<strong>线程阻塞和唤醒</strong>所带来的性能问题。</p> <h4><a name="t11"></a><a id="24__332"></a>2.4 非阻塞同步</h4> <p>互斥同步属于一种<strong>悲观的并发策略</strong>，总是认为只要不去做正确的同步措施，那就肯定会出现问题。无论共享数据是否真的会出现竞争，它都要进行加锁（这里讨论的是概念模型，实际上虚拟机会优化掉很大一部分不必要的加锁）、用户态核心态转换、维护锁计数器和检查是否有被阻塞的线程需要唤醒等操作。</p> <p>随着硬件指令集的发展，我们可以使用基于<strong>冲突检测的乐观并发策略</strong>：先进行操作，如果没有其它线程争用共享数据，那操作就成功了，否则采取补偿措施（不断地重试，直到成功为止）。这种乐观的并发策略的许多实现都不需要将线程阻塞，因此这种同步操作称为非阻塞同步。</p> <p>形象比喻：<strong>相信世界充满爱，即使被伤害</strong>。</p> <p>由于无锁操作中没有锁的存在，因此不可能出现死锁的情况，也就是说<strong>乐观锁天生免疫死锁</strong> 。</p> <p>乐观锁多用于“读多写少“的环境，避免频繁加锁影响性能；而悲观锁多用于”写多读少“的环境，避免频繁失败和重试影响性能。</p> <h5><a id="241_CAS_346"></a>2.4.1 CAS</h5> <p>例子解释：</p> <p>例子，假如你往地上仍1万块钱，是不是一定会丢呢？这要看情况了，如果是在人来人往的都市，可以说肯定会丢的。如果你跑到无人区扔地上，可以说肯定不会丢。</p> <p>可以看到，都是把东西无保护的放到公共区域里，结果却相差很大。这说明安全问题还和公共区域的环境状况有关系。</p> <p>比如我把数据放到公共区域的堆内存中，但是始终都只会有1个线程，也就是单线程模型，那这数据肯定是安全的。</p> <p>再者说，2个线程操作同一个数据和200个线程操作同一个数据，这个数据的安全概率是完全不一样的。肯定线程越多数据不安全的概率越大，线程越少数据不安全的概率越小。取个极限情况，那就是只有1个线程，那不安全概率就是0，也就是安全的。</p> <p>因为锁的获取和释放是要花费一定代价的，如果在线程数目特别少的时候，可能可能就不会有别的线程来操作数据，此时你还要获取锁和释放锁，可以说是一种浪费。</p> <p>针对这种“地广人稀”的情况，专门提出了一种方法，叫CAS。就是在并发很小的情况下，数据被意外修改的概率很低，但是又存在这种可能性，此时就用CAS。</p> <p>CAS的全称是：比较并交换（Compare And Swap）。在CAS中，有这样三个值：</p> <ul><li>V：要更新的变量(var)——内存地址</li><li>E：预期值(expected)——旧值</li><li>N：新值(new)</li></ul> <p>比较并交换的过程如下：</p> <p>判断V是否等于E，如果等于，将V的值设置为N；如果不等，说明已经有其它线程更新了V，则当前线程放弃更新，什么都不做。</p> <p>我们以一个简单的例子来解释这个过程：</p> <ol><li>如果有一个多个线程共享的变量<code>i</code>原本等于5，我现在在线程A中，想把它设置为新的值6;</li><li>我们使用CAS来做这个事情；</li><li>首先我们用i去与5对比，发现它等于5，说明没有被其它线程改过，那我就把它设置为新的值6，此次CAS成功，<code>i</code>的值被设置成了6；</li><li>如果不等于5，说明<code>i</code>被其它线程改过了（比如现在<code>i</code>的值为2），那么我就什么也不做，此次CAS失败，<code>i</code>的值仍然为2。</li></ol> <p>在这个例子中，<code>i</code>就是V，5就是E，6就是N。</p> <p>那有没有可能我在判断了<code>i</code>为5之后，正准备更新它的新值的时候，被其它线程更改了<code>i</code>的值呢？</p> <p>不会的。因为CAS是一种<a href="https://so.csdn.net/so/search?q=%E5%8E%9F%E5%AD%90%E6%93%8D%E4%BD%9C&spm=1001.2101.3001.7020" target="_blank" class="hl hl-1" data-report-view="{"spm":"1001.2101.3001.7020","dest":"https://so.csdn.net/so/search?q=%E5%8E%9F%E5%AD%90%E6%93%8D%E4%BD%9C&spm=1001.2101.3001.7020","extra":"{\"searchword\":\"原子操作\"}"}" data-report-click="{"spm":"1001.2101.3001.7020","dest":"https://so.csdn.net/so/search?q=%E5%8E%9F%E5%AD%90%E6%93%8D%E4%BD%9C&spm=1001.2101.3001.7020","extra":"{\"searchword\":\"原子操作\"}"}" data-tit="原子操作" data-pretit="原子操作">原子操作</a>，它是一种系统原语，是一条CPU的原子指令，从CPU层面保证它的原子性。</p> <p>CAS是一种原子操作，在Java中，有一个<code>Unsafe</code>类，它在<code>sun.misc</code>包中。它里面是一些<code>native</code>方法，其中就有几个关于CAS的，他们都是<code>public native</code>的。</p> <p><strong>当多个线程同时使用CAS操作一个变量时，只有一个会胜出，并成功更新，其余均会失败，但失败的线程并不会被挂起，仅是被告知失败，并且允许再次尝试，当然也允许失败的线程放弃操作。</strong></p> <p><strong>ABA问题？(狸猫换太子)</strong></p> <p>因为CAS需要在操作值的时候，检查值有没有发生变化，如果没有发生变化则更新，但是如果一个值原来是A，变成了B，又变成了A，那么使用CAS进行检查时会发现它的值没有发生变化，但是实际上却变化了，那 CAS 操作就会误认为它从来没有被改变过。</p> <p>ABA问题的解决思路就是使用版本号。在变量前面追加上版本号，每次变量更新的时候把版本号加1，那么A→B→A就会变成1A→2B→3A。</p> <p>Java 1.5开始，JDK的Atomic包里提供了一个类AtomicStampedReference来解决ABA问题。这个类的compareAndSet方法的作用是首先检查当前引用是否等于预期引用，并且检查当前标志是否等于预期标志，如果全部相等，则以原子方式将该引用和该标志的值设置为给定的更新值。</p> <h5><a id="242_Atomic_399"></a>2.4.2 Atomic（原子操作）</h5> <p>Unsafe类支持CAS的方法。那Java具体是如何使用这几个方法来实现原子操作的呢？</p> <p>JDK提供了一些用于原子操作的类，在<code>java.util.concurrent.atomic</code>包下面。在JDK 8中，有如下17个类：</p> <p><a href="https://cos.duktig.cn/2021/07/05/1625455891.png" rel="nofollow"><img src="https://img-blog.csdnimg.cn/img_convert/78fa390c885023601e2a3adbe40060c2.png" alt="java.util.concurrent.atomic"></a></p> <p>从名字就可以看得出来这些类大概的用途：</p> <ul><li>原子更新基本类型</li><li>原子更新数组</li><li>原子更新引用</li><li>原子更新字段（属性）</li></ul> <h3><a name="t12"></a><a id="3__414"></a>3. 总结和分析</h3> <p><strong>“栈封闭”</strong>：找个只有自己知道的地方藏起来，当然安全了。</p> <p><strong>“<code>ThreadLocal</code>”</strong>：每人复制1份，各玩各的，互不影响，当然也安全了。</p> <p><strong>“不可变”</strong>：更狠了，直接规定，只能读取，禁止修改，当然也安全了。</p> <p><strong>互斥同步</strong>和<strong>非阻塞同步</strong>，分别对应<strong>悲观锁</strong>和<strong>乐观锁</strong>的策略。</p> <h3><a name="t13"></a><a id="_424"></a>参考</h3> <ul><li><a href="https://github.com/CyC2018/CS-Notes">CS-Notes——Java并发</a></li><li><a href="https://zhuanlan.zhihu.com/p/67905621" rel="nofollow">如果你这样回答“什么是线程安全”，面试官都会对你刮目相看</a></li><li><a href="https://www.cnblogs.com/yumiaoxia/p/9010721.html" rel="nofollow">Java面向对象进阶篇（包装类，不可变类）</a></li></ul> </div> <link href="https://csdnimg.cn/release/blogv2/dist/mdeditor/css/editerView/markdown_views-f23dff6052.css" rel="stylesheet"> <link href="https://csdnimg.cn/release/blogv2/dist/mdeditor/css/style-c216769e99.css" rel="stylesheet"> <div><div><span>声明：</span>本文内容由网友自发贡献，不代表【wpsshop博客】立场，版权归原作者所有，本站不承担相应法律责任。如您发现有侵权的内容，请联系我们。转载请注明出处：<a href="https://www.wpsshop.cn/w/不正经/article/detail/650082">https://www.wpsshop.cn/w/不正经/article/detail/650082</a></div></div></div><div class="article_recommend"><div class="list_top">推荐阅读</div><div class="ListItems"><ul class="NewsList"><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="java.lang.ArrayIndexOutOfBoundsException" href="/w/凡人多烦事01/article/detail/630199" target="_blank"><em>java</em>.<em>lang</em>.ArrayIndexOutOfBoundsException...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="java.lang.ArrayIndexOutOfBoundsException" href="/w/凡人多烦事01/article/detail/630199" target="_blank"><em>java</em>.<em>lang</em>.ArrayIndexOutOfBoundsException is a runtime except... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="报错：java.lang.ArrayIndexOutOfBoundsException——数组越界_" href="/w/知新_RL/article/detail/630204" target="_blank">报错：<em>java</em>.<em>lang</em>.<em>ArrayIndexOutOfBoundsException</em>——<em>数组</em>越界_...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="报错：java.lang.ArrayIndexOutOfBoundsException——数组越界_" href="/w/知新_RL/article/detail/630204" target="_blank">题目：给定一个排序<em>数组</em>，你需要在原地删除重复出现的元素，使得每个元素只出现一次，返回移除后<em>数组</em>的新长度。不要使用额外的数... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="【JAVA】双向链表详解_java 双向链表" href="/w/花生_TL007/article/detail/630205" target="_blank">【<em>JAVA</em>】<em>双向</em><em>链表</em>详解_<em>java</em> <em>双向</em><em>链表</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="【JAVA】双向链表详解_java 双向链表" href="/w/花生_TL007/article/detail/630205" target="_blank">LinkedList的底层是<em>双向</em><em>链表</em>结构(<em>链表</em>后面介绍)，由于<em>链表</em>没有将元素存储在连续的空间中，元素存储在单独的节点中，... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="Java——正则表达式" href="/w/我家小花儿/article/detail/630241" target="_blank"><em>Java</em>——<em>正则表达式</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="Java——正则表达式" href="/w/我家小花儿/article/detail/630241" target="_blank">①Pattern类为<em>正则表达式</em>的编译表现形式②Matcher类为Pattern对Character sequence(文... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="【Java】实现冒泡排序讲解_java实现冒泡排序" href="/w/你好赵伟/article/detail/630410" target="_blank">【<em>Java</em>】<em>实现</em><em>冒泡排序</em>讲解_<em>java</em><em>实现</em><em>冒泡排序</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="【Java】实现冒泡排序讲解_java实现冒泡排序" href="/w/你好赵伟/article/detail/630410" target="_blank"><em>Java</em><em>实现</em><em>冒泡排序</em>_<em>java</em><em>实现</em><em>冒泡排序</em><em>java</em><em>实现</em><em>冒泡排序</em> 目录 <em>冒泡排序</em> <em>冒泡排序</em>思路示... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="JAVA 几种排序算法（附）动画演示_java 冒泡算法动画" href="/w/羊村懒王/article/detail/630415" target="_blank"><em>JAVA</em> 几种<em>排序</em><em>算法</em>（附）<em>动画</em>演示_<em>java</em> <em>冒泡</em><em>算法</em><em>动画</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="JAVA 几种排序算法（附）动画演示_java 冒泡算法动画" href="/w/羊村懒王/article/detail/630415" target="_blank">几种<em>排序</em><em>算法</em>的属性对比一、<em>冒泡</em><em>排序</em> 特点：效率低，实现简单原理：将待<em>排序</em>列中最大的数往后<em>冒泡</em>，成为新的序列，重复以上... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="2024OD机试卷-螺旋数字矩阵 (java\python\c++)_疫情期间,小明隔离在家,百无聊" href="/w/从前慢现在也慢/article/detail/630446" target="_blank"><em>2024OD</em>机试卷-螺旋<em>数字</em><em>矩阵</em> (<em>java</em>\<em>python</em>\<em>c++</em>)_<em>疫情</em>期间<em>,</em><em>小明</em>隔离在家<em>,</em>百无聊...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="2024OD机试卷-螺旋数字矩阵 (java\python\c++)_疫情期间,小明隔离在家,百无聊" href="/w/从前慢现在也慢/article/detail/630446" target="_blank"><em>疫情</em>期间，<em>小明</em>隔离在家，<em>百无聊赖</em>，在纸上<em>写</em><em>数字</em>玩。他发明了一种<em>写</em>法：给出<em>数字</em>个数 n （0 < n ≤ 999）和行数 ... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="华为OD机试C、D卷 - 螺旋数字矩阵（Java & JS & Python & C & C++）_" href="/w/盐析白兔/article/detail/630457" target="_blank">华为<em>OD</em>机试C<em>、</em>D卷 - 螺旋<em>数字</em><em>矩阵</em>（<em>Java</em> & JS & <em>Python</em> & C & C++）_...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="华为OD机试C、D卷 - 螺旋数字矩阵（Java & JS & Python & C & C++）_" href="/w/盐析白兔/article/detail/630457" target="_blank">## 题目描述疫情期间，小明隔离在家，百无聊赖，在纸上写<em>数字</em>玩。他发明了一种写法：给出<em>数字</em>个数 n （0 < n ≤ 9... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="Java kafka producer 的常用参数的意义说明及默认值_acks=0" href="/w/凡人多烦事01/article/detail/630490" target="_blank"><em>Java</em> <em>kafka</em> <em>producer</em> 的常用<em>参数</em>的意义<em>说明</em>及<em>默认值</em>_<em>acks</em>=0...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="Java kafka producer 的常用参数的意义说明及默认值_acks=0" href="/w/凡人多烦事01/article/detail/630490" target="_blank">生产端核心<em>参数</em>1. <em>acks</em><em>参数</em><em>说明</em>：这是一个非常重要的<em>参数</em>，表示指定分区中成功写入消息的副本数量，这是Kafka生产端... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="华为OD机试 -螺旋矩阵 II（java& c++& python & javascript & g" href="/w/AllinToyou/article/detail/630492" target="_blank">华为OD机试 -<em>螺旋</em><em>矩阵</em> II（<em>java</em>& <em>c</em>++& <em>python</em> & <em>java</em>s<em>c</em>ript & g...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="华为OD机试 -螺旋矩阵 II（java& c++& python & javascript & g" href="/w/AllinToyou/article/detail/630492" target="_blank">华为OD机试 -<em>螺旋</em><em>矩阵</em> II（<em>java</em>& <em>c</em>++& <em>python</em> & <em>java</em>s<em>c</em>ript & <em>golang</em> & <em>c</em>#... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="华为OD机试 -螺旋矩阵（java& c++& python & javascript & gola" href="/w/羊村懒王/article/detail/630496" target="_blank">华为OD机试 -<em>螺旋</em><em>矩阵</em>（<em>java</em>& c++& <em>python</em> & <em>java</em>script & gola...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="华为OD机试 -螺旋矩阵（java& c++& python & javascript & gola" href="/w/羊村懒王/article/detail/630496" target="_blank">华为OD机试 -<em>螺旋</em><em>矩阵</em>（<em>java</em>& c++& <em>python</em> & <em>java</em>script & <em>golang</em> & c# & ... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="Java web应用性能分析之【性能点概叙】_javaweb如何统计性能" href="/w/不正经/article/detail/630541" target="_blank"><em>Java</em> <em>web</em>应用<em>性能</em>分析之【<em>性能</em>点概叙】<em>_</em><em>java<em>web</em></em><em>如何</em><em>统计</em><em>性能</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="Java web应用性能分析之【性能点概叙】_javaweb如何统计性能" href="/w/不正经/article/detail/630541" target="_blank">在<em>性能</em>问题排查过程中，我们依赖的就是监控，实时监控和历史监控，要么通过监控工具，查某段时间内服务器资源消耗情况，结合业务... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="java - JVM CPU100%，问题排查_java接口压力测试cpu100%" href="/w/Monodyee/article/detail/630544" target="_blank"><em>java</em> - JVM <em>CPU100%</em>，<em>问题</em><em>排查</em>_<em>java</em><em>接口</em>压力测试<em>cpu100%</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="java - JVM CPU100%，问题排查_java接口压力测试cpu100%" href="/w/Monodyee/article/detail/630544" target="_blank">前段时间我们新上了一个新的应用，因为流量一直不大，集群QPS大概只有5左右，写<em>接口</em>的rt在30ms左右。因为最近接入了新... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="java cpu过高排查_利用 Arthas 精准定位 Java 应用 CPU 负载过高问题" href="/w/我家自动化/article/detail/630561" target="_blank"><em>java</em> <em>cpu</em>过高<em>排查</em>_利用 <em>Arthas</em> 精准定位 <em>Java</em> 应用 <em>CPU</em> 负载过高<em>问题</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="java cpu过高排查_利用 Arthas 精准定位 Java 应用 CPU 负载过高问题" href="/w/我家自动化/article/detail/630561" target="_blank">转自：有赞coder作者：张云翔最近我们线上有个应用服务器有点上头，<em>CPU</em>总能跑到99%，我寻思着它流量也不大啊，为啥能... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="「秘籍」JAVA求职面试宝典，轻松吊打面试官，拿下大厂offer_招聘软件怎么打招呼模板 java" href="/w/羊村懒王/article/detail/630621" target="_blank">「秘籍」<em>JAVA</em><em>求职</em><em>面试</em>宝典，轻松吊打<em>面试</em>官，拿下大厂offer_<em>招聘</em>软件<em>怎么</em>打招呼模板 <em>java</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="「秘籍」JAVA求职面试宝典，轻松吊打面试官，拿下大厂offer_招聘软件怎么打招呼模板 java" href="/w/羊村懒王/article/detail/630621" target="_blank">又到了一年<em>求职</em>季，有粉丝找小编咨询小编收集了些<em>面试</em>相关应对技巧分享给大家<em>面试</em>好比就像是场约会你是不是常常担心自己没有表现... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="Java 程序员必备基础知识温故而知新_java程序员必备知识" href="/w/AllinToyou/article/detail/630670" target="_blank"><em>Java</em> <em>程序员</em> 必备基础<em>知识</em> 温故而知新_<em>java</em><em>程序员</em>必备<em>知识</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="Java 程序员必备基础知识温故而知新_java程序员必备知识" href="/w/AllinToyou/article/detail/630670" target="_blank">前言正文自我介绍数据结构和算法<em>Java</em>篇<em>Java</em> EE<em>知识</em>点储备计算机网络操作系统数据库相关XML常识性<em>知识</em>总结前言准备... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="java 保留2位小数_string.format %.2丢精度" href="/w/繁依Fanyi0/article/detail/630691" target="_blank"><em>java</em> 保留<em>2</em>位小数_<em>string</em>.<em>format</em> <em>%.</em><em>2</em>丢精度...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="java 保留2位小数_string.format %.2丢精度" href="/w/繁依Fanyi0/article/detail/630691" target="_blank"><em>java</em> 保留<em>2</em>位小数，计算同比环比_<em>string</em>.<em>format</em> <em>%.</em><em>2</em>丢精度<em>string</em>.<em>format</em> <em>%.</em><em>2</em>丢精度... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="Java • 保留2位小数（1、String.format(“%.2f“,参数)，参数的数据类型是d" href="/w/羊村懒王/article/detail/630694" target="_blank"><em>Java</em> • 保留<em>2</em>位小数（<em>1</em>、<em>String</em>.<em>format</em>(“<em>%.</em><em>2</em>f“,<em>参数</em>)，<em>参数</em>的<em>数据类型</em>是d...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="Java • 保留2位小数（1、String.format(“%.2f“,参数)，参数的数据类型是d" href="/w/羊村懒王/article/detail/630694" target="_blank"><em>Java</em> • 保留<em>2</em>位小数（<em>1</em>、<em>String</em>.<em>format</em>("<em>%.</em><em>2</em>f",<em>参数</em>)，<em>参数</em>的<em>数据类型</em>是<em>double</em>；<em>2</em>、例如... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="Java 学习（一）Java语言基础_java学习" href="/w/知新_RL/article/detail/630704" target="_blank"><em>Java</em> <em>学习</em>（一）<em>Java</em><em>语言</em>基础_<em>java</em><em>学习</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="Java 学习（一）Java语言基础_java学习" href="/w/知新_RL/article/detail/630704" target="_blank">初学<em>Java</em>的编程小白的自学法宝，<em>Java</em><em>语言</em>基础带您走近<em>Java</em>世界。_<em>java</em><em>学习</em><em>java</em><em>学习</em> ... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li><li><div class="NewTitle"><a title="article" class="cat" href="/article/list/1" target="_blank">article<i></i></a><a title="java 输出保留小数点后2位_运行结果输出为小数点后二位web" href="/w/不正经/article/detail/630709" target="_blank"><em>java</em> <em>输出</em>保留<em>小数点</em><em>后</em><em>2</em>位_运行结果<em>输出</em>为<em>小数点</em><em>后</em>二位<em>web</em>...</a></div><div class="NewsInfo"><div class="NewsDesc" style=""><a title="java 输出保留小数点后2位_运行结果输出为小数点后二位web" href="/w/不正经/article/detail/630709" target="_blank">在函数中返回值以double的类型返回，最<em>后</em>打印的时候利用：System.out.printf("%.<em>2</em>f ",rs);... </a><div class="article_click rice1" style="width: 240px;float: left;"><p class="operation-b-img operation-b-img-active"><i class="img-up txclick" attc="upclick" attn="0"></i><span class="num"> 赞</span></p><p class="operation-b-img operation-b-img-active"><i class="img-down txclick" attc="downclick" attn="0"></i><span class="num">踩</span></p></div></div><div style="clear: both;"></div></div></li></ul><div class="list_tools_top">相关标签</div><div class="list_tools_box"><ul><li><a title="java" rel="nofollow" href="/s?w=java" target="_self">java</a></li><li><a title="python" rel="nofollow" href="/s?w=python" target="_self">python</a></li><li><a title="jvm" rel="nofollow" href="/s?w=jvm" target="_self">jvm</a></li><li><a title="多线程" rel="nofollow" href="/s?w=多线程" target="_self">多线程</a></li><li><a title="编程语言" rel="nofollow" href="/s?w=编程语言" target="_self">编程语言</a></li><li><a title="程序报错" rel="nofollow" href="/s?w=程序报错" target="_self">程序报错</a></li><li><a title="链表" rel="nofollow" href="/s?w=链表" target="_self">链表</a></li><li><a title="数据结构" rel="nofollow" href="/s?w=数据结构" target="_self">数据结构</a></li><li><a title="正则表达式" rel="nofollow" href="/s?w=正则表达式" target="_self">正则表达式</a></li><li><a title="intellij-idea" rel="nofollow" href="/s?w=intellij-idea" target="_self">intellij-idea</a></li><li><a title="程序人生" rel="nofollow" href="/s?w=程序人生" target="_self">程序人生</a></li><li><a title="c++" rel="nofollow" href="/s?w=c++" target="_self">c++</a></li><li><a title="矩阵" rel="nofollow" href="/s?w=矩阵" target="_self">矩阵</a></li><li><a title="od" rel="nofollow" href="/s?w=od" target="_self">od</a></li><li><a title="od机试" rel="nofollow" href="/s?w=od机试" target="_self">od机试</a></li><li><a title="开发语言" rel="nofollow" href="/s?w=开发语言" target="_self">开发语言</a></li><li><a title="华为od" rel="nofollow" href="/s?w=华为od" target="_self">华为od</a></li><li><a title="c语言" rel="nofollow" href="/s?w=c语言" target="_self">c语言</a></li><li><a title="javascript" rel="nofollow" href="/s?w=javascript" target="_self">javascript</a></li><li><a title="kafka" rel="nofollow" href="/s?w=kafka" target="_self">kafka</a></li><li><a title="分布式" rel="nofollow" href="/s?w=分布式" target="_self">分布式</a></li><li><a title="大数据" rel="nofollow" href="/s?w=大数据" target="_self">大数据</a></li><div style="clear: both;"></div></ul></div><div class="list_tools_top"></div></div></div></div><style type="text/css"></style><link rel="stylesheet" href="https://cdn.wpsshop.cn/public/blog/css/phone.css?v=17352890" type="text/css"><script type="text/javascript"></script></div></div></div></div></div></div><style type="text/css"> /* footer css */ .index006-cover-page-foot { background-color: #F7F7F7; } </style><div class="cppui-row-1_100 cppui-row-view-1 row_1_100 ui-draggable" style="display: block;"><div class="cppui-column-1_100-0-1 column-view-com cppui-column-1_100-0-view-1 ui-sortable"><div class="drag-item-show-view-1 footermsg_view_pic ui-draggable" style="display: block;"><div class="index006-cover-page-foot drag-item-hide-view-1"><p class="copyright">Copyright © 2003-2013 www.wpsshop.cn 版权所有，并保留所有权利。 </p><div style="background-color:#F7F7F7; text-align:center; height:20px;padding-top:5px;"><a target="_blank" href="#" style="display:inline-block;text-decoration:none;height:20px;line-height:20px;"><img src="https://cdnimages.wpsshop.cn/57public/skin/index/default/ui/images/beian_ghs.png" style="float:left;"> <p style="float:left;height:20px;line-height:20px;margin: 0px 0px 0px 5px; color:#939393;">闽ICP备14008679号</p></a> <a href="https://www.wpsshop.cn/xml/w/%E4%B8%8D%E6%AD%A3%E7%BB%8F/article/detail/650082.xml" target="_blank" style="display:inline-block;text-decoration:none;height:20px;line-height:20px;"><img src="https://cdn.wpsshop.cn/public/blog/images/site.png" style="float:left;"></a> <a href="/xml/w/g007/article/detail/new.xml" target="_blank" style="display:inline-block;text-decoration:none;height:20px;line-height:20px;"><img src="https://cdn.wpsshop.cn/public/blog/images/site.png" style="float:left;"></a> <a href="/site.xml" target="_blank" style="display:inline-block;text-decoration:none;height:20px;line-height:20px;"><img src="https://cdn.wpsshop.cn/public/blog/images/site.png" style="float:left;"></a></div></div></div></div></div><script type="text/javascript">var domain = document.domain;var hr=encodeURIComponent(window.location.href+'=='+document.referrer); document.write('<script src="https://ad.wpsshop.cn/admin.appflux?s=ad_flu_ind&fluxuserauto=yes&hr='+hr+'&p57weburl='+domain+'&p57usercode1='+domain+'&u=Vy1RJQhqAXUEMQNnAm0HaVVu&i='+p57ref("id")+'&r='+escape(document.referrer)+'" language="JavaScript"><\/script>');function p57ref(id){var reg=new RegExp("(^|&)"+id+"=([^&]*)(&|$)");var ref=window.location.search.substr(1).match(reg);if(ref!=null)return unescape(ref[2]);return null;} </script></body></html>