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class=post-block><article class=post-content itemscope itemtype=http://schema.org/Article lang=zh-CN><link href=https://nicksxs.me/2019/12/21/%E8%81%8A%E8%81%8AJava%E4%B8%AD%E7%9A%84%E5%8D%95%E4%BE%8B%E6%A8%A1%E5%BC%8F/ itemprop=mainEntityOfPage><span hidden itemprop=author itemscope itemtype=http://schema.org/Person><meta content=/uploads/avatar.jpg itemprop=image><meta content=Nicksxs itemprop=name></span><span hidden itemprop=publisher itemscope itemtype=http://schema.org/Organization><meta content="Nicksxs's Blog" itemprop=name><meta content="Nicksxs 的技术博客,深入 Java 运行时与生产诊断、低成本大模型推理和 Agent 工程实践。" itemprop=description></span><span hidden itemprop=post itemscope itemtype=http://schema.org/CreativeWork><meta content="聊聊Java中的单例模式 | Nicksxs's Blog" itemprop=name><meta content="这是个 Java 面试的高频问题,我也遇到过,以往都是觉得这类题没意思,网上一搜一大堆,也不愿意记,其实说回来,主要还是没静下心来好好去理解,今天无意中看到一个课程,基本帮我把一些疑惑的点讲清楚了,首先单例是啥意思,这个其实是有范围一说,比如我起了个Spring Boot应用…" itemprop=description></span><header class=post-header><h1 itemprop="name headline" class=post-title>聊聊Java中的单例模式</h1><div class=post-meta-container><div class=post-meta><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-calendar"></i> </span><span class=post-meta-item-text>发表于</span> <time itemprop="dateCreated datePublished" title="创建时间:2019-12-21 01:03:09" datetime=2019-12-21T01:03:09+08:00>2019-12-21</time> </span><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-calendar-check"></i> </span><span class=post-meta-item-text>更新于</span> <time title="修改时间:2019-12-22 08:46:11" datetime=2019-12-22T08:46:11+08:00 itemprop=dateModified>2019-12-22</time> </span><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-folder"></i> </span><span class=post-meta-item-text>分类于</span> <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/ itemprop=url rel=index><span itemprop=name>Java</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/Design-Patterns/ itemprop=url rel=index><span itemprop=name>Design Patterns</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/Singleton/ itemprop=url rel=index><span itemprop=name>Singleton</span></a> </span></span><span class="post-meta-item cloudflare-page-counter" data-increment=true data-path=/2019/12/21/%E8%81%8A%E8%81%8AJava%E4%B8%AD%E7%9A%84%E5%8D%95%E4%BE%8B%E6%A8%A1%E5%BC%8F/ data-title=聊聊Java中的单例模式 title=阅读次数><span class=post-meta-item-icon><i class="far fa-eye"></i> </span><span class=post-meta-item-text>阅读次数:</span> <span class=cloudflare-page-counter-count></span></span></div><div class=post-description>这是个 Java 面试的高频问题,我也遇到过,以往都是觉得这类题没意思,网上一搜一大堆,也不愿意记,其实说回来,主要还是没静下心来好好去理解,今天无意中看到一个课程,基本帮我把一些疑惑的点讲清楚了,首先单例是啥意思,这个其实是有范围一说,比如我起了个Spring Boot应用…</div></div></header><div class=post-body itemprop=articleBody><p>这是个 Java 面试的高频问题,我也遇到过,以往都是觉得这类题没意思,网上一搜一大堆,也不愿意记,其实说回来,主要还是没静下心来好好去理解,今天无意中看到一个课程,基本帮我把一些疑惑的点讲清楚了,首先单例是啥意思,这个其实是有范围一说,比如我起了个<code>Spring Boot</code>应用,在这个应用范围内,我的常规 bean 是单例的,意味着 getBean 的时候其实永远只会拿到那一个对象,那要怎么来写一个单例呢,首先就是传说中的饿汉模式,也是最简单的<h2 id=饿汉模式><a class=headerlink href=#饿汉模式 title=饿汉模式></a>饿汉模式</h2><figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br><span class=line>14</span><br></pre><td class=code><pre><span class=line><span class=keyword>public</span> <span class=keyword>class</span> <span class="title class_">Singleton1</span> {</span><br><span class=line> <span class=comment>// 首先,将构造方法变成私有的</span></span><br><span class=line> <span class=keyword>private</span> <span class="title function_">Singleton1</span><span class=params>()</span> {};</span><br><span class=line> <span class=comment>// 创建私有静态实例,这样第一次使用的时候就会进行创建</span></span><br><span class=line> <span class=keyword>private</span> <span class=keyword>static</span> <span class=type>Singleton</span> <span class=variable>instance</span> <span class=operator>=</span> <span class=keyword>new</span> <span class="title class_">Singleton1</span>();</span><br><span class=line></span><br><span class=line> <span class=comment>// 使用这个对象都是通过这个 getInstance 来获取</span></span><br><span class=line> <span class=keyword>public</span> <span class=keyword>static</span> Singleton1 <span class="title function_">getInstance</span><span class=params>()</span> {</span><br><span class=line> <span class=keyword>return</span> instance;</span><br><span class=line> }</span><br><span class=line> <span class=comment>// 瞎写一个静态方法。这里想说的是,如果我们只是要调用 Singleton.getDate(...),</span></span><br><span class=line> <span class=comment>// 本来是不想要生成 Singleton 实例的,不过没办法,已经生成了</span></span><br><span class=line> <span class=keyword>public</span> <span class=keyword>static</span> Date <span class="title function_">getDate</span><span class=params>(String mode)</span> {<span class=keyword>return</span> <span class=keyword>new</span> <span class="title class_">Date</span>();}</span><br><span class=line>}</span><br></pre></table></figure><p>上面借鉴了一些代码,其实这是最基本,也不会错的方法,但是正如其中<code>getDate</code>方法里说的问题,有时候并没有想那这个对象,但是因为我调用了这个类的静态方法,导致对象已经生成了,可能这也是饿汉模式名字的来由,不管三七二十一给你生成个单例就完事了,不管有没有用,但是这种个人觉得也没啥大问题,如果是面试的话最好说出来它的缺点<h2 id=饱汉模式><a class=headerlink href=#饱汉模式 title=饱汉模式></a>饱汉模式</h2><figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br><span class=line>14</span><br><span class=line>15</span><br><span class=line>16</span><br><span class=line>17</span><br><span class=line>18</span><br><span class=line>19</span><br><span class=line>20</span><br><span class=line>21</span><br></pre><td class=code><pre><span class=line><span class=keyword>public</span> <span class=keyword>class</span> <span class="title class_">Singleton2</span> {</span><br><span class=line> <span class=comment>// 首先,也是先堵死 new Singleton() 这条路,将构造方法变成私有</span></span><br><span class=line> <span class=keyword>private</span> <span class="title function_">Singleton2</span><span class=params>()</span> {}</span><br><span class=line> <span class=comment>// 和饿汉模式相比,这边不需要先实例化出来,注意这里的 volatile,它是必须的</span></span><br><span class=line> <span class=keyword>private</span> <span class=keyword>static</span> <span class=keyword>volatile</span> <span class=type>Singleton2</span> <span class=variable>instance</span> <span class=operator>=</span> <span class=literal>null</span>;</span><br><span class=line></span><br><span class=line> <span class=keyword>private</span> <span class=type>int</span> <span class=variable>m</span> <span class=operator>=</span> <span class=number>9</span>;</span><br><span class=line></span><br><span class=line> <span class=keyword>public</span> <span class=keyword>static</span> Singleton <span class="title function_">getInstance</span><span class=params>()</span> {</span><br><span class=line> <span class=keyword>if</span> (instance == <span class=literal>null</span>) {</span><br><span class=line> <span class=comment>// 加锁</span></span><br><span class=line> <span class=keyword>synchronized</span> (Singleton2.class) {</span><br><span class=line> <span class=comment>// 这一次判断也是必须的,不然会有并发问题</span></span><br><span class=line> <span class=keyword>if</span> (instance == <span class=literal>null</span>) {</span><br><span class=line> instance = <span class=keyword>new</span> <span class="title class_">Singleton2</span>();</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> <span class=keyword>return</span> instance;</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p>这里容易错的有三点,理解了其实就比较好记了<p>第一点,为啥不在 getInstance 上整个代码块加 <code>synchronized</code>,这个其实比较容易理解,就是锁的力度太大,性能太差了,这点其实也要去理解,可以举个夸张的例子,比如我一个电商的服务,如果为了避免一个人的订单出现问题,是不是可以从请求入口就把他锁住,到请求结束释放,那么里面做的事情都有保障,然而这显然不可能,因为我们想要这种竞态条件抢占资源的时间尽量减少,防止其他线程等待。<br>第二点,为啥<code>synchronized</code>之已经检查了 <code>instance == null</code>,还要在里面再检查一次,这个有个术语,叫 <code>double check lock</code>,但是为啥要这么做呢,其实很简单,想象当有两个线程,都过了第一步为空判断,这个时候只有一个线程能拿到这个锁,另一个线程就等待了,如果不再判断一次,那么第一个线程新建完对象释放锁之后,第二个线程又能拿到锁,再去创建一个对象。<br>第三点,为啥要<code>volatile</code>关键字,原先对它的理解是它修饰的变量在 JMM 中能及时将变量值写到主存中,但是它还有个很重要的作用,就是防止指令重排序,<code>instance = new Singleton();</code>这行代码其实在底层是分成三条指令执行的,第一条是在堆上申请了一块内存放这个对象,但是对象的字段啥的都还是默认值,第二条是设置对象的值,比如上面的 m 是 9,然后第三条是将这个对象和虚拟机栈上的指针建立引用关联,那么如果我不用<code>volatile</code>关键字,这三条指令就有可能出现重排,比如变成了 1-3-2 这种顺序,当执行完第二步时,有个线程来访问这个对象了,先判断是不是空,发现不是空的,就拿去直接用了,是不是就出现问题了,所以这个<code>volatile</code>也是不可缺少的<h2 id=嵌套类><a class=headerlink href=#嵌套类 title=嵌套类></a>嵌套类</h2><figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br></pre><td class=code><pre><span class=line><span class=keyword>public</span> <span class=keyword>class</span> <span class="title class_">Singleton3</span> {</span><br><span class=line></span><br><span class=line> <span class=keyword>private</span> <span class="title function_">Singleton3</span><span class=params>()</span> {}</span><br><span class=line> <span class=comment>// 主要是使用了 嵌套类可以访问外部类的静态属性和静态方法 的特性</span></span><br><span class=line> <span class=keyword>private</span> <span class=keyword>static</span> <span class=keyword>class</span> <span class="title class_">Holder</span> {</span><br><span class=line> <span class=keyword>private</span> <span class=keyword>static</span> <span class=type>Singleton3</span> <span class=variable>instance</span> <span class=operator>=</span> <span class=keyword>new</span> <span class="title class_">Singleton3</span>();</span><br><span class=line> }</span><br><span class=line> <span class=keyword>public</span> <span class=keyword>static</span> Singleton3 <span class="title function_">getInstance</span><span class=params>()</span> {</span><br><span class=line> <span class=keyword>return</span> Holder.instance;</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p>这个我个人感觉是饿汉模式的升级版,可以在调用<code>getInstance</code>的时候去实例化对象,也是比较推荐的<h2 id=枚举单例><a class=headerlink href=#枚举单例 title=枚举单例></a>枚举单例</h2><figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br></pre><td class=code><pre><span class=line><span class=keyword>public</span> <span class=keyword>enum</span> <span class="title class_">Singleton</span> {</span><br><span class=line> INSTANCE;</span><br><span class=line> </span><br><span class=line> <span class=keyword>public</span> <span class=keyword>void</span> <span class="title function_">doSomething</span><span class=params>()</span>{</span><br><span class=line> <span class=comment>//todo doSomething</span></span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p>枚举很特殊,它在类加载的时候会初始化里面的所有的实例,而且 JVM 保证了它们不会再被实例化,所以它天生就是单例的。</div><footer class=post-footer><div class="affiliate-creative affiliate-creative--post-end" data-affiliate-id=tencent-cloud-general><a rel="sponsored noopener" aria-label=腾讯云产品优惠 href=https://cloud.tencent.com/act/cps/redirect?redirect=2446&cps_key=2e33a1eedb380997d03618b4a5ba6ca7&from=console target=_blank><img alt=腾讯云服务器、数据库和云产品优惠 decoding=async height=419 loading=lazy src=/uploads/affiliate/tencent-cloud-general.webp width=690> </a><span class=affiliate-disclosure>推广</span></div><div class=reward-container><div>请我一杯咖啡吧!</div><button>赞赏</button><div class=post-reward><div><img alt="Nicksxs 微信" src=https://i.loli.net/2020/01/12/NUlWanT31E8LQGx.png><span>微信</span></div><div><img alt="Nicksxs 支付宝" src=https://ooo.0o0.ooo/2016/08/17/57b421e773057.jpg><span>支付宝</span></div></div></div><div class=post-copyright><ul><li class=post-copyright-author><strong>本文作者: </strong>Nicksxs<li 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