<span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> Java与C++有一个不同之处在于，Java不但有构造函数，还有一个”初始化块“(Initialization Block)的概念。下面探究一下它的执行顺序与可能的用途。</span></p> <h3><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">执行顺序</span></h3> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 首先定义A, B, C三个类用作测试，其中B继承了A，C又继承了B，并分别给它们加上静态初始化块、非静态初始化块和构造函数，里面都是一句简单的输出。</span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 主类Main里面也如法炮制。</span></p> <div class="cnblogs_code" onclick="cnblogs_code_show('569f199d-483b-44d7-8c22-9728510ac299')"><img id="code_img_closed_569f199d-483b-44d7-8c22-9728510ac299" class="code_img_closed" src="http://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif" alt=""><img id="code_img_opened_569f199d-483b-44d7-8c22-9728510ac299" class="code_img_opened" style="display: none;" onclick="cnblogs_code_hide('569f199d-483b-44d7-8c22-9728510ac299',event)" src="http://images.cnblogs.com/OutliningIndicators/ExpandedBlockStart.gif" alt=""> <div id="cnblogs_code_open_569f199d-483b-44d7-8c22-9728510ac299" class="cnblogs_code_hide"> <pre><span style="color: #008080;"> 1</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> A { </span><span style="color: #008080;"> 2</span> <span style="color: #0000ff;">static</span><span style="color: #000000;"> { </span><span style="color: #008080;"> 3</span> System.out.println("Static init A."<span style="color: #000000;">); </span><span style="color: #008080;"> 4</span> <span style="color: #000000;"> } </span><span style="color: #008080;"> 5</span> <span style="color: #008080;"> 6</span> <span style="color: #000000;"> { </span><span style="color: #008080;"> 7</span> System.out.println("Instance init A."<span style="color: #000000;">); </span><span style="color: #008080;"> 8</span> <span style="color: #000000;"> } </span><span style="color: #008080;"> 9</span> <span style="color: #008080;">10</span> <span style="color: #000000;"> A() { </span><span style="color: #008080;">11</span> System.out.println("Constructor A."<span style="color: #000000;">); </span><span style="color: #008080;">12</span> <span style="color: #000000;"> } </span><span style="color: #008080;">13</span> <span style="color: #000000;">} </span><span style="color: #008080;">14</span> <span style="color: #008080;">15</span> <span style="color: #0000ff;">class</span> B <span style="color: #0000ff;">extends</span><span style="color: #000000;"> A { </span><span style="color: #008080;">16</span> <span style="color: #0000ff;">static</span><span style="color: #000000;"> { </span><span style="color: #008080;">17</span> System.out.println("Static init B."<span style="color: #000000;">); </span><span style="color: #008080;">18</span> <span style="color: #000000;"> } </span><span style="color: #008080;">19</span> <span style="color: #008080;">20</span> <span style="color: #000000;"> { </span><span style="color: #008080;">21</span> System.out.println("Instance init B."<span style="color: #000000;">); </span><span style="color: #008080;">22</span> <span style="color: #000000;"> } </span><span style="color: #008080;">23</span> <span style="color: #008080;">24</span> <span style="color: #000000;"> B() { </span><span style="color: #008080;">25</span> System.out.println("Constructor B."<span style="color: #000000;">); </span><span style="color: #008080;">26</span> <span style="color: #000000;"> } </span><span style="color: #008080;">27</span> <span style="color: #000000;">} </span><span style="color: #008080;">28</span> <span style="color: #008080;">29</span> <span style="color: #0000ff;">class</span> C <span style="color: #0000ff;">extends</span><span style="color: #000000;"> B { </span><span style="color: #008080;">30</span> <span style="color: #008080;">31</span> <span style="color: #0000ff;">static</span><span style="color: #000000;"> { </span><span style="color: #008080;">32</span> System.out.println("Static init C."<span style="color: #000000;">); </span><span style="color: #008080;">33</span> <span style="color: #000000;"> } </span><span style="color: #008080;">34</span> <span style="color: #008080;">35</span> <span style="color: #000000;"> { </span><span style="color: #008080;">36</span> System.out.println("Instance init C."<span style="color: #000000;">); </span><span style="color: #008080;">37</span> <span style="color: #000000;"> } </span><span style="color: #008080;">38</span> <span style="color: #008080;">39</span> <span style="color: #000000;"> C() { </span><span style="color: #008080;">40</span> System.out.println("Constructor C."<span style="color: #000000;">); </span><span style="color: #008080;">41</span> <span style="color: #000000;"> } </span><span style="color: #008080;">42</span> <span style="color: #000000;">} </span><span style="color: #008080;">43</span> <span style="color: #008080;">44</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Main { </span><span style="color: #008080;">45</span> <span style="color: #008080;">46</span> <span style="color: #0000ff;">static</span><span style="color: #000000;"> { </span><span style="color: #008080;">47</span> System.out.println("Static init Main."<span style="color: #000000;">); </span><span style="color: #008080;">48</span> <span style="color: #000000;"> } </span><span style="color: #008080;">49</span> <span style="color: #008080;">50</span> <span style="color: #000000;"> { </span><span style="color: #008080;">51</span> System.out.println("Instance init Main."<span style="color: #000000;">); </span><span style="color: #008080;">52</span> <span style="color: #000000;"> } </span><span style="color: #008080;">53</span> <span style="color: #008080;">54</span> <span style="color: #0000ff;">public</span><span style="color: #000000;"> Main() { </span><span style="color: #008080;">55</span> System.out.println("Constructor Main."<span style="color: #000000;">); </span><span style="color: #008080;">56</span> <span style="color: #000000;"> } </span><span style="color: #008080;">57</span> <span style="color: #008080;">58</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) { </span><span style="color: #008080;">59</span> C c = <span style="color: #0000ff;">new</span><span style="color: #000000;"> C(); </span><span style="color: #008080;">60</span> <span style="color: #008000;">//</span><span style="color: #008000;">B b = new B();</span> <span style="color: #008080;">61</span> <span style="color: #000000;"> } </span><span style="color: #008080;">62</span> }</pre> </div> <span class="cnblogs_code_collapse">测试代码</span></div> <p> </p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 当然这里不使用内部类，因为内部类不能使用静态的定义；而用静态内部类就失去了一般性。</span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 那么可以看到，当程序进入了main函数，并创建了一个类C的对象之后，输出是这样子的：</span></p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="复制代码"><img src="//common.cnblogs.com/images/copycode.gif" alt="复制代码"></a></span></div> <pre>Static init Main. Static init A. Static init B. Static init C. Instance init A. Constructor A. Instance init B. Constructor B. Instance init C. Constructor C.</pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="复制代码"><img src="//common.cnblogs.com/images/copycode.gif" alt="复制代码"></a></span></div></div> <p> </p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 观察上面的输出，可以观察到两个有趣的现象：</span></p> <ol> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">Main类是肯定没有被实例化过的，但是由于执行main入口函数用到了Main类，于是static初始化块也被执行了；</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">所有的静态初始化块都优先执行，其次才是非静态的初始化块和构造函数，它们的执行顺序是：</span></li> <ol> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">父类的静态初始化块</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">子类的静态初始化块</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">父类的初始化块</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">父类的构造函数</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">子类的初始化块</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">子类的构造函数</span></li> </ol></ol> <p><span style="font-family: 'Microsoft YaHei';"><span style="font-size: 16px; line-height: 24px;"> 那么如果有多个实例化对象，又会不会发生变化呢？于是在第一个C类的对象后面，再实例化一个B类的对象，再观察输出：</span></span></p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="复制代码"><img src="//common.cnblogs.com/images/copycode.gif" alt="复制代码"></a></span></div> <pre>Static init Main. Static init A. Static init B. Static init C. Instance init A. Constructor A. Instance init B. Constructor B. Instance init C. Constructor C. Instance init A. Constructor A. Instance init B. Constructor B.</pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="复制代码"><img src="//common.cnblogs.com/images/copycode.gif" alt="复制代码"></a></span></div></div> <p> </p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 可以发现这输出跟前面的基本长得一样对吧？只是在后面多了4行，那是新的B类对象实例化时产生的信息，同样也是父类A的初始化块和构造函数先执行，再轮到子类B的初始化块和构造函数执行；同时还发现，静态初始化块的输出只出现了一次，也就是说每个类的静态初始化块都只在第一次实例化该类对象时执行一次。</span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 无论如何，初始化块和构造函数总在一起执行是件有趣的事情，让我们反编译一下看看吧！</span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 查看生成目录发现已经生成了4个.class文件，分别是A.class, B.class, C.class, Main.class，先看看Main.class的结构（这里重新注释了new B）：</span></p> <div class="cnblogs_code"> <pre><span style="color: #008080;">1</span> javap -c Main</pre> </div> <div class="cnblogs_code" onclick="cnblogs_code_show('03b903ac-5e54-4294-8c77-60be2b898fac')"><img id="code_img_closed_03b903ac-5e54-4294-8c77-60be2b898fac" class="code_img_closed" src="http://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif" alt=""><img id="code_img_opened_03b903ac-5e54-4294-8c77-60be2b898fac" class="code_img_opened" style="display: none;" onclick="cnblogs_code_hide('03b903ac-5e54-4294-8c77-60be2b898fac',event)" src="http://images.cnblogs.com/OutliningIndicators/ExpandedBlockStart.gif" alt=""> <div id="cnblogs_code_open_03b903ac-5e54-4294-8c77-60be2b898fac" class="cnblogs_code_hide"> <pre><span style="color: #008080;"> 1</span> Compiled from <span style="color: #800000;">"</span><span style="color: #800000;">Main.java</span><span style="color: #800000;">"</span> <span style="color: #008080;"> 2</span> <span style="color: #000000;">public class Main { </span><span style="color: #008080;"> 3</span> <span style="color: #000000;"> public Main(); </span><span style="color: #008080;"> 4</span> <span style="color: #000000;"> Code: </span><span style="color: #008080;"> 5</span> <span style="color: #800080;">0</span><span style="color: #000000;">: aload_0 </span><span style="color: #008080;"> 6</span> <span style="color: #800080;">1</span>: invokespecial #<span style="color: #800080;">1</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Method java/lang/Object."<init>":()V</span> <span style="color: #008080;"> 7</span> <span style="color: #800080;">4</span>: getstatic #<span style="color: #800080;">2</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Field java/lang/System.out:Ljava/io/PrintStream;</span> <span style="color: #008080;"> 8</span> <span style="color: #800080;">7</span>: ldc #<span style="color: #800080;">3</span> <span style="color: #008000;">//</span><span style="color: #008000;"> String Instance init Main.</span> <span style="color: #008080;"> 9</span> <span style="color: #800080;">9</span>: invokevirtual #<span style="color: #800080;">4</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Method java/io/PrintStream.println:(Ljava/lang/String;)V</span> <span style="color: #008080;">10</span> <span style="color: #800080;">12</span>: getstatic #<span style="color: #800080;">2</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Field java/lang/System.out:Ljava/io/PrintStream;</span> <span style="color: #008080;">11</span> <span style="color: #800080;">15</span>: ldc #<span style="color: #800080;">5</span> <span style="color: #008000;">//</span><span style="color: #008000;"> String Constructor Main.</span> <span style="color: #008080;">12</span> <span style="color: #800080;">17</span>: invokevirtual #<span style="color: #800080;">4</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Method java/io/PrintStream.println:(Ljava/lang/String;)V</span> <span style="color: #008080;">13</span> <span style="color: #800080;">20</span><span style="color: #000000;">: return </span><span style="color: #008080;">14</span> <span style="color: #008080;">15</span> <span style="color: #000000;"> public static void main(java.lang.String[]); </span><span style="color: #008080;">16</span> <span style="color: #000000;"> Code: </span><span style="color: #008080;">17</span> <span style="color: #800080;">0</span>: new #<span style="color: #800080;">6</span> <span style="color: #008000;">//</span><span style="color: #008000;"> class C</span> <span style="color: #008080;">18</span> <span style="color: #800080;">3</span><span style="color: #000000;">: dup </span><span style="color: #008080;">19</span> <span style="color: #800080;">4</span>: invokespecial #<span style="color: #800080;">7</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Method C."<init>":()V</span> <span style="color: #008080;">20</span> <span style="color: #800080;">7</span><span style="color: #000000;">: astore_1 </span><span style="color: #008080;">21</span> <span style="color: #800080;">8</span><span style="color: #000000;">: return </span><span style="color: #008080;">22</span> <span style="color: #008080;">23</span> <span style="color: #000000;"> static {}; </span><span style="color: #008080;">24</span> <span style="color: #000000;"> Code: </span><span style="color: #008080;">25</span> <span style="color: #800080;">0</span>: getstatic #<span style="color: #800080;">2</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Field java/lang/System.out:Ljava/io/PrintStream;</span> <span style="color: #008080;">26</span> <span style="color: #800080;">3</span>: ldc #<span style="color: #800080;">8</span> <span style="color: #008000;">//</span><span style="color: #008000;"> String Static init Main.</span> <span style="color: #008080;">27</span> <span style="color: #800080;">5</span>: invokevirtual #<span style="color: #800080;">4</span> <span style="color: #008000;">//</span><span style="color: #008000;"> Method java/io/PrintStream.println:(Ljava/lang/String;)V</span> <span style="color: #008080;">28</span> <span style="color: #800080;">8</span><span style="color: #000000;">: return </span><span style="color: #008080;">29</span> }</pre> </div> <span class="cnblogs_code_collapse">Main.class的反编译结果</span></div> <p> </p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 可以看到整个Main类被分成三个部分，static {}部分很显然，就是我们的static初始化块，在里面调用了println并输出了String“Static init Main.”；而main入口函数也很清晰，首先新实例化了一个类C的对象，然后调用了类C的构造函数，最后返回；而上面public Main();的部分就很有意思了，这是类Main的构造函数，但我们看到里面调用了两次println，分别输出了String“Instance init Main.”和String“Constructor Main.”。难道初始化块和构造函数被合并到一起了？</span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 我们再看看C类的反编译结果吧：</span></p> <div class="cnblogs_code"> <pre><span style="color: #008080;">1</span> javap -c C</pre> </div> <div class="cnblogs_code" onclick="cnblogs_code_show('d5b28da1-f393-4afc-87af-f6175e29e926')"><img id="code_img_closed_d5b28da1-f393-4afc-87af-f6175e29e926" class="code_img_closed" src="http://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif" alt=""><img id="code_img_opened_d5b28da1-f393-4afc-87af-f6175e29e926" class="code_img_opened" style="display: none;" onclick="cnblogs_code_hide('d5b28da1-f393-4afc-87af-f6175e29e926',event)" src="http://images.cnblogs.com/OutliningIndicators/ExpandedBlockStart.gif" alt=""> <div id="cnblogs_code_open_d5b28da1-f393-4afc-87af-f6175e29e926" class="cnblogs_code_hide"> <pre>Compiled from "Main.java" class C extends B { C(); Code: 0: aload_0 1: invokespecial #1 // Method B."<init>":()V 4: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 7: ldc #3 // String Instance init C. 9: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 12: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 15: ldc #5 // String Constructor C. 17: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 20: return static {}; Code: 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #6 // String Static init C. 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return }</pre> </div> <span class="cnblogs_code_collapse">C.class的反编译结果</span></div> <p> </p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 静态初始化块仍然单独分出一部分，输出了我们的调试语句。而另一部分，仍然还是类C的构造函数C();，可以看到它先调用了父类B的构造函数，接着输出了我们初始化块中的语句，然后才输出我们写在构造函数中的语句，最后返回。多次试验也都是如此。于是我们能够推断：初始化块的代码是被加入到子类构造函数的前面，父类初始化的后面了。</span></p> <p> </p> <h3><span style="font-family: 'Microsoft YaHei';"><span style="font-size: 16px; line-height: 24px;">可能的用途：</span></span></h3> <p><span style="font-family: 'Microsoft YaHei';"><span style="font-size: 16px; line-height: 24px;"> 既然执行顺序和大概原理都摸清了，那么就要探讨一下初始化块的可能的用途。</span></span></p> <h5><span style="font-family: 'Microsoft YaHei';"><span style="font-size: 16px; line-height: 24px;"> 静态初始化块</span></span></h5> <p><span style="font-family: 'Microsoft YaHei';"><span style="font-size: 16px; line-height: 24px;"> 1. 用于初始化静态成员变量<br></span></span></p> <p><span style="font-family: 'Microsoft YaHei';"><span style="font-size: 16px; line-height: 24px;"> 比如给类C增加一个静态成员变量sub，我们在static块里面给它赋值为5：</span></span></p> <div class="cnblogs_code"><div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="复制代码"><img src="//common.cnblogs.com/images/copycode.gif" alt="复制代码"></a></span></div> <pre><span style="color: #008080;"> 1</span> <span style="color: #0000ff;">class</span> C <span style="color: #0000ff;">extends</span><span style="color: #000000;"> B { </span><span style="color: #008080;"> 2</span> <span style="color: #008080;"> 3</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">int</span><span style="color: #000000;"> a; </span><span style="color: #008080;"> 4</span> <span style="color: #008080;"> 5</span> <span style="color: #0000ff;">static</span><span style="color: #000000;"> { </span><span style="color: #008080;"> 6</span> a = 5<span style="color: #000000;">; </span><span style="color: #008080;"> 7</span> System.out.println("Static init C."<span style="color: #000000;">); </span><span style="color: #008080;"> 8</span> <span style="color: #000000;"> } </span><span style="color: #008080;"> 9</span> <span style="color: #008080;">10</span> <span style="color: #000000;">...... </span><span style="color: #008080;">11</span> <span style="color: #008080;">12</span> }</pre> <div class="cnblogs_code_toolbar"><span class="cnblogs_code_copy"><a href="javascript:void(0);" onclick="copyCnblogsCode(this)" title="复制代码"><img src="//common.cnblogs.com/images/copycode.gif" alt="复制代码"></a></span></div></div> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> main函数里输出这个静态变量C.sub：</span></p> <div class="cnblogs_code"> <pre><span style="color: #008080;">1</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) { </span><span style="color: #008080;">2</span> System.out.println("Value of C.sub: " +<span style="color: #000000;"> C.sub); </span><span style="color: #008080;">3</span> }</pre> </div> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 则输出结果：</span></p> <div class="cnblogs_code"> <pre>Static init Main. Static init A. Static init B. Static init C. Value of C.sub: 5</pre> </div> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 符合类被第一次加载时执行静态初始化块的结论，且C.sub被正确赋值为5并输出了出来。</span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 但是乍一看似乎没有什么用，因为静态成员变量在定义时就可以顺便赋值了。因此在赋值方面有点鸡肋。</span></p> <p> </p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 2. 执行初始化代码<br></span></p> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 比如可以记录第一次访问类的日志，或方便单例模式的初始化等。对于单例模式，可以先用static块初始化一些可能还被其他类访问的基础参数，等到真正需要加载大量资源的时候(getInstance)再构造单体，在构造函数中加载资源。</span></p> <p> </p> <h5><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 非静态初始化块</span></h5> <p><span style="font-family: 'Microsoft YaHei'; font-size: 16px;"> 这个就没什么好说的了，基本跟构造函数一个功能，但比构造函数先执行。最常见的用法应该还是<strong>代码复用</strong>，即多个重载构造函数都有若干段相同的代码，那么可以把这些重复的代码拉出来放到初始化块中，但仍然要注意它的执行顺序，对顺序有严格要求的初始化代码就不适合使用了。</span></p> <p> </p> <h3><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">总结：</span> </h3> <ol> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">静态初始化块的优先级最高，也就是最先执行，并且仅在<strong>类第一次被加载时</strong>执行；</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">非静态初始化块和构造函数后执行，并且在<strong>每次生成对象时</strong>执行一次；</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">非静态初<strong>始化块的代码会在类构造函数之前执行</strong>。因此若要使用，应当养成把初始化块写在构造函数之前的习惯，便于调试；</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">静态初始化块既可以用于<strong>初始化静态成员变量</strong>，也可以<strong>执行初始化代码</strong>；</span></li> <li><span style="font-family: 'Microsoft YaHei'; font-size: 16px;">非静态初始化块可以针对多个重载构造函数进行<strong>代码复用</strong>。</span>参考 http://www.cnblogs.com/BlackStorm/p/5699965.html
java的初始化块、静态初始化块、构造函数的执行顺序
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