Java类初始化顺序问题：静态初始化块，非静态初始化块以及构造函数

在Java类中，初始化方式有三种：静态初始化块，非静态初始化块和构造函数。这就涉及初始化顺序问题，那么有两个问题：

单个类的初始化顺序是怎样的呢？

当有类继承时，Java类初始化顺序是怎样的呢？

实验代码：

class Super {private static int i = getI();{System.out.println("super non-static initalization before");}static {System.out.println("super static initalization before");}public Super() {System.out.println("super constructor initalization");}{System.out.println("super non-static initalization after");}private static int j = getJ();static {System.out.println("super static initalization after");}private static int getI() {System.out.println("super static field before");return 0;}private static int getJ() {System.out.println("super static field after");return 0;}}class Sub extends Super{{System.out.println("sub non-static initalization before");}static {System.out.println("sub static initalization before");}public Sub( ) {System.out.println("sub constructor initalization");}{System.out.println("sub non-static initalization after");}static {System.out.println("sub static initalization after");}}public class ConstructorTest {public static void main(String[] args) {Sub s = new Sub();}}

输出结果：

super static field beforesuper static initalization beforesuper static field aftersuper static initalization aftersub static initalization beforesub static initalization aftersuper non-static initalization beforesuper non-static initalization aftersuper constructor initalizationsub non-static initalization beforesub non-static initalization aftersub constructor initalization

实验可知，java类初始化大致顺序归纳为两条基本原则：

1、先静态，后非静态

2、先父类，后子类

具体原则如下：

1、子类静态优先于父类非静态、构造函数

2、先非静态初始化块，后构造函数；相当于把非静态初始块移到构造函数开始处

3、同类型语句按出现顺序进行初始化；这里静态变量和静态初始化块是同类型语句，所以按语句先后顺序进行初始化