JAVA中静态块、静态变量加载顺序详解

源博客：https://www.cnblogs.com/leiqiannian/p/7922824.html

public class test {                         //1.第一步，准备加载类
<span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public</span> </span><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">static</span> </span><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">void</span> </span><span class="hljs-title"><span class="hljs-function"><span class="hljs-title">main</span>(<span class="hljs-params">String[] args</span>) </span>{
    <span class="hljs-keyword"><span class="hljs-keyword">new</span> test();                         <span class="hljs-comment"><span class="hljs-comment">//4.第四步，new一个类，但在new之前要处理匿名代码块        </span>
}

<span class="hljs-keyword"><span class="hljs-keyword">static</span> <span class="hljs-keyword"><span class="hljs-keyword">int</span> num = <span class="hljs-number"><span class="hljs-number">4</span>;                    <span class="hljs-comment"><span class="hljs-comment">//2.第二步，静态变量和静态代码块的加载顺序由编写先后决定 </span>

{
    num += <span class="hljs-number"><span class="hljs-number">3</span>;
    System.<span class="hljs-keyword"><span class="hljs-keyword">out</span>.println(<span class="hljs-string"><span class="hljs-string">"b"</span>);           <span class="hljs-comment"><span class="hljs-comment">//5.第五步，按照顺序加载匿名代码块，代码块中有打印</span>
}

<span class="hljs-keyword"><span class="hljs-keyword">int</span> a = <span class="hljs-number"><span class="hljs-number">5</span>;                             <span class="hljs-comment"><span class="hljs-comment">//6.第六步，按照顺序加载变量</span>

{ <span class="hljs-comment"><span class="hljs-comment">// 成员变量第三个</span>
    System.<span class="hljs-keyword"><span class="hljs-keyword">out</span>.println(<span class="hljs-string"><span class="hljs-string">"c"</span>);           <span class="hljs-comment"><span class="hljs-comment">//7.第七步，按照顺序打印c</span>
}

test() { <span class="hljs-comment"><span class="hljs-comment">// 类的构造函数，第四个加载</span>
    System.<span class="hljs-keyword"><span class="hljs-keyword">out</span>.println(<span class="hljs-string"><span class="hljs-string">"d"</span>);           <span class="hljs-comment"><span class="hljs-comment">//8.第八步，最后加载构造函数，完成对象的建立</span>
}

<span class="hljs-keyword"><span class="hljs-keyword">static</span> {                              <span class="hljs-comment"><span class="hljs-comment">// 3.第三步，静态块，然后执行静态代码块，因为有输出，故打印a</span>
    System.<span class="hljs-keyword"><span class="hljs-keyword">out</span>.println(<span class="hljs-string"><span class="hljs-string">"a"</span>);
}

<span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">static</span> </span><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">run</span>(<span class="hljs-params"></span>)                    </span><span class="hljs-comment"><span class="hljs-function"><span class="hljs-comment">// 静态方法，调用的时候才加载// 注意看，e没有加载</span>
</span>{
    System.<span class="hljs-keyword"><span class="hljs-keyword">out</span>.println(<span class="hljs-string"><span class="hljs-string">"e"</span>);
}

}

一般顺序：静态块（静态变量）——>成员变量——>构造方法——>静态方法 
1、静态代码块（只加载一次） 2、构造方法（创建一个实例就加载一次）3、静态方法需要调用才会执行，所以最后结果没有e 

 public class Print {
 <span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public</span> </span><span class="hljs-title"><span class="hljs-function"><span class="hljs-title">Print</span>(<span class="hljs-params">String s</span>)</span>{
     System.<span class="hljs-keyword"><span class="hljs-keyword">out</span>.print(s + <span class="hljs-string"><span class="hljs-string">" "</span>);
 }

}

 public class Parent{

     public static Print obj1 = new Print("1"); public Print obj2 = new Print("2"); public static Print obj3 = new Print("3"); static{ new Print("4"); } public static Print obj4 = new Print("5"); public Print obj5 = new Print("6"); public Parent(){ new Print("7"); } }

 public class Child extends Parent{ static{ new Print("a"); } public static Print obj1 = new Print("b"); public Print obj2 = new Print("c"); public Child (){ new Print("d"); } public static Print obj3 = new Print("e"); public Print obj4 = new Print("f"); public static void main(String [] args){ Parent obj1 = new Child (); Parent obj2 = new Child (); } }

执行main方法，程序输出顺序为： 1 3 4 5 a b e 2 6 7 c f d 2 6 7 c f d 

输出结果表明，程序的执行顺序为： 
如果类还没有被加载： 
1、先执行父类的静态代码块和静态变量初始化，并且静态代码块和静态变量的执行顺序只跟代码中出现的顺序有关。 
2、执行子类的静态代码块和静态变量初始化。 
3、执行父类的实例变量初始化 
4、执行父类的构造函数 
5、执行子类的实例变量初始化 
6、执行子类的构造函数 

如果类已经被加载： 
则静态代码块和静态变量就不用重复执行，再创建类对象时，只执行与实例相关的变量初始化和构造方法。