面试被问设计模式？不要怕看这里：单例模式

短命女 2022-05-27 01:54 102阅读 0赞

设计模式是老生常谈的问题，有人工作多年却对设计模式一窍不通，但是更多的人是懂一点点，但是不求甚解。其实这样不好，暂且不说在工作中的应用，即便是在面试时，被面试官问到设计模式时一脸懵逼，是非常尴尬的事情。本文不废话，不谈大篇理论教学，只针对面试，给出设计模式的关键点，从应试的角度，让大家认识和理解设计模式。

首先搞清楚一点，设计模式不是高深技术，不是奇淫技巧。设计模式只是一种设计思想，针对不同的业务场景，用不同的方式去设计代码结构，其最最本质的目的是为了解耦，延伸一点的话，还有为了可扩展性和健壮性，但是这都是建立在解耦的基础之上。

我们都知道大名鼎鼎的GoF的21种设计模式，看过head first的这本书的人应该不少。针对这21种设计模式，面试官问出的问题可能千变万化，让人莫名的忧(dan)伤(teng)，但是不要怕，只要你搞清楚了每种设计模式的关键点和精髓，就可以举一反三，迎刃而解了。

今天我们来看看最简单的单例模式。理论我就不介绍了，没听说过的可以去查一下。即便是最简单的单例，也有关键点。举个不太恰当地例子，看过修仙修道之类小说的同学都知道，一般阵法高手做阵都有一个或多个“阵眼”，同理，我们每种设计模式也有“阵眼”，那么单例的“阵眼”在哪里？各位莫慌，我们先来看看代码。根据单例模式的理论：保证系统中只有一个实例，于是我撸了以下代码

好了，如果我是面试官，你是候选人，我要你撸个单例给我，你撸以上代码问我资词不资词，我肯定是不资词的。为什么？真的不是因为我想搞个大新闻，而是这段单例的代码一般情况下还是可以勉强运行，但是，遇到多线程的并发条件下就大清药丸了。因为这段代码是线程不安全的，有可能给new出多个单例实例，都多个了，还是屁的“单例”啊。

好，废话不多说，继续撸代码，你可能会说：”不是说线程不安全吗？小爷我加上线程安全判断呗，度娘在手天下我有，来了您呐~~~“

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      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">=</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">new</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">Singleton</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">return</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

好了，这回该消停了吧，锁也加了，线程也安全了。But，你还是太连清。。。这样依然有问题。问题在哪里？就在关键点2，检测单例是否被构造。虽然这里判断了一次，但是由于某些情况下，可能有延迟加载或者缓存的原因，只有关键点2这一次判断，仍然不能保证系统是否只创建了一个单例，也可能出现多个实例的情况，那么怎么办呢？

<table style="margin-left:0px;font-size:13px;border:none;margin-top:0px;margin-right:0px;margin-bottom:0px;padding:0px;border-spacing:0px;width:auto;background:none;table-layout:auto;"> 
 <tbody style="margin:0px;padding:0px;"> 
  <tr style="background:none;border-style:none;margin:0px;padding:0px;vertical-align:top;"> 
   <td style="font-size:13px;text-align:center;background-image:none;border-style:none;margin:0px;padding:0px;vertical-align:top;font-family:Monaco, MonacoRegular, 'Courier New', monospace;background-color:rgb(238,238,238);"> 
    <div style="border:none;margin:0px;padding-right:0px;padding-left:0px;background:none;text-align:left;white-space:nowrap;padding-top:5px;padding-bottom:3px;line-height:15px;"> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       1 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       2 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       3 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       4 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       5 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       6 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       7 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       8 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       9 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       10 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       11 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       12 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       13 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       14 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       15 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       16 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       17 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       18 
     </div> 
     <div style="border-top:none;border-right:1px solid rgb(222,222,222);border-bottom:none;border-left:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;text-align:right;height:inherit;min-width:1.2em;color:rgb(170,170,170);">
       19 
     </div> 
    </div></td> 
   <td style="font-size:13px;text-align:center;width:622px;background:none;border-style:none;margin:0px;padding:0px;vertical-align:top;font-family:Monaco, MonacoRegular, 'Courier New', monospace;"> 
    <div style="margin:0px;padding-right:0px;padding-left:0px;text-align:left;color:rgb(0,0,0);white-space:pre;border:none;padding-top:5px;padding-bottom:3px;background:none;line-height:15px;"> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">public</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;color:rgb(128,0,128);">class</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">Singleton</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">{ </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">private</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">Singleton</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">{ </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(153,153,153);font-style:italic;">//关键点0：构造函数是私有的</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">private</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">static</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">Singleton </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">=</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;color:rgb(128,0,128);">null</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(153,153,153);font-style:italic;">//关键点1：声明单例对象是静态的</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">private</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">static</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;color:rgb(128,0,128);">object</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">obj</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">=</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">new</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;color:rgb(128,0,128);">object</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">public</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(128,0,128);">static</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">Singleton </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">GetInstance</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(153,153,153);font-style:italic;">//通过静态方法来构造对象</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">{ </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">if</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">==</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;color:rgb(128,0,128);">null</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(153,153,153);font-style:italic;">//关键点2：判断单例对象是否已经被构造</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">{ </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">lock</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">obj</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(153,153,153);font-style:italic;">//关键点3：加线程锁</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">{ </span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">if</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">==</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;color:rgb(128,0,128);">null</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(153,153,153);font-style:italic;">//关键点4：二次判断单例是否已经被构造</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">{ </span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">=</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">new</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:#008080;">Singleton</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">(</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">)</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;font-weight:bold;">return</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);"> </span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,45,122);">single</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;&nbsp;&nbsp;</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(0,111,224);">&nbsp;&nbsp;</span> 
     </div> 
     <div style="border:none;margin:0px;padding:0px 5px;font-family:inherit;background:none;height:inherit;"> 
      <span style="border:0px;margin:0px;padding:0px;font-family:inherit;height:inherit;color:rgb(51,51,51);">}</span> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

所以，在判断单例实例是否被构造时，需要检测两次，在线程锁之前判断一次，在线程锁之后判断一次，再去构造实例，这样就万无一失了。是不是很简(Tu)单(Xue)呢？

最后，我们来归纳一下。下次面试别人再问你单例模式，你可以这样说：

单例是为了保证系统中只有一个实例，其关键点有5

一.私有构造函数

二.声明静态单例对象

三.构造单例对象之前要加锁（lock一个静态的object对象）

四.需要两次检测单例实例是否已经被构造，分别在锁之前和锁之后

如果要你撸代码，你就撸最后这一段，完美~~~面试官要是个女的准想和你生猴子。。。

哦，别高兴太早了，面试官要是个男的，也可能会问你下列问题

0.为何要检测两次？

如上面所述，有可能延迟加载或者缓存原因，造成构造多个实例，违反了单例的初衷。

1.构造函数能否公有化？

不行，单例类的构造函数必须私有化，单例类不能被实例化，单例实例只能静态调用

2.lock住的对象为什么要是object对象，可以是int吗？

不行，锁住的必须是个引用类型。如果锁值类型，每个不同的线程在声明的时候值类型变量的地址都不一样，那么上个线程锁住的东西下个线程进来会认为根本没锁，相当于每次都锁了不同的门，没有任何卵用。而引用类型的变量地址是相同的，每个线程进来判断锁多想是否被锁的时候都是判断同一个地址，相当于是锁在通一扇门，起到了锁的作用。

好了，这下估计男的都想跟你生猴子了。。。