类的加载连接初始化顺序 （二）

1.数组对象的使用，不会导致对应类的初始化。

在创建 ClassName[]数组时，并不会创建对应对象，也不会初始化对应类，会创建一个编译时类型(其数组类型是JVM在运行时期动态生成的，不是由类加载器加载的，即没有对应的ClassLoader。调用getClassLoader会返回对用元素的ClassLoader) [+L+类全名 如下：

public class MyTest02 {

	public static void main(String[] args) {
		Sub[] subs = new Sub[1];
		Sub[][] subss = new Sub[1][1];
		String[] strs = new String[1];
		int[] is = new int[1];
		char[] chars = new char[1];
		byte[] bs = new byte[1];
		boolean[] bo = new boolean[1];
		System.out.println("Sub[]'s class is       "+subs.getClass());
		System.out.println("Sub[][]'s class is     "+subss.getClass());
		System.out.println("Sub[]'s father's class "+subs.getClass().getSuperclass());
		System.out.println("String[]'s class is    "+strs.getClass());
		System.out.println("int[]'s class is       "+is.getClass());
		System.out.println("char[]'s class is      "+chars.getClass());
		System.out.println("byte[]'s class is      "+bs.getClass());
		System.out.println("boolean[]'s class is   "+bo.getClass());
	}

}
class Sub{
	static {
		System.out.println("Sub static block");
	}
}
//~out:
Sub[]'s class is       class [Ljvm.Sub;
Sub[][]'s class is     class [[Ljvm.Sub;
Sub[]'s father's class class java.lang.Object
String[]'s class is    class [Ljava.lang.String;
int[]'s class is       class [I
char[]'s class is      class [C
byte[]'s class is      class [B
boolean[]'s class is   class [Z

　特别的 这些数组的父类为Object，二维数组则多加一个[符号。

 

2.初始化中各字段初始化的先后顺序

关于类初始化的复制顺序 如下代码的输出结果应该很好判断

public class MyTest03 {

	public static void main(String[] args) {
		;
		System.out.println("Singleton.value1 = "+Singleton.getInstance().value1);
		System.out.println("Singleton.value2 = "+Singleton.getInstance().value2);

	}
	

}
class Singleton{
	public static int value1;
	public static int value2 = 0;
	private static Singleton instance = new Singleton();
	Singleton(){
		value1++;
		value2++;
	}
	public static Singleton getInstance() {
		return instance;
	}
	
}//~out:

Singleton.value1 = 1
Singleton.value2 = 1

　但我们调整一下static域字段的定义顺序：

public class MyTest03 {
	public static void main(String[] args) {
		System.out.println("Singleton.value1 = "+Singleton.getInstance().value1);
		System.out.println("Singleton.value2 = "+Singleton.getInstance().value2);
	}
	

}
class Singleton{
	private static Singleton instance = new Singleton();
        public static int value1;
	public static int value2 = 0;
	Singleton(){
		value1++;
		value2++;
	}
	public static Singleton getInstance() {
		return instance;
	}
	
}//~out:
Singleton.value1 = 1
Singleton.value2 = 0

会发现value2在自增后又被初始化为零了,而value1没有，static域的对象初始化的顺序是依照源文件声明的顺序初始化的，

在连接阶段的准备阶段会为静态域赋初值(int为0)

初始化阶段依照声明顺序，先初始化instance引用对象，调用Singleton构造函数 此前value1，value2的值在连接阶段被初始化为默认值0.

调用后各自增了1 然后再初始化value1，value2的值，因为value1未被赋初值，故不进行操作，value2的值为1。

==================================可不看=======================================================public class MyTest0 public static void main(String[] args) {  Singleton.getInstance(); }

	

}
class Singleton{
	
	private static Singleton instance = new Singleton();
	public Sub sub1;
	public Sub sub2 = new Sub("sub2");
        {
        　　sub1 = new Sub("123");
　　　　　}
	public Sub sub3 = new Sub("sub3");
	Singleton(){
		sub1 = new Sub("sub1");
		System.out.println("creating");
	}
	public static Singleton getInstance() {
		return instance;
	}
	
}
class Sub{
	Sub(String str){
		System.out.println(str);
	}
}//~out：
sub2
123
sub3
sub1
creating

　类成员对象的初始化也依照定义顺序

==============================================================================================

3.接口的初始化

接口中所有字段均为 public static final

public class MyTest04 {
	MyTest04(String str){
		System.out.println(str);
	}
	public static void main(String[] args) {
		System.out.println(Father04.str);
		System.out.println(Child04.str);

	}

}
interface Father04{
	String str = "Father04";
}
interface Child04 extends Father04{
	String str = "Child04";
}//~out:
Father04
Child04

编译后就算把接口的字节码删除依然不会报错，

static final的字段由于是编译时常量 被写入了MyTest04的常量池中，所以不会首次主动使用Father04。

JVM规范允许类加载器在预料到某个类将要被使用时就预先加载它，如果在预先加载时遇到了.class文件缺失或者存在错误，类加载器必须在程序首次主动使用时才报告错误(LinkageError错误)

public class MyTest04 {
	public static void main(String[] args) {
		Child04.sub.toString();

	}

}
interface Father04{
	String str = "Father04";
	Sub sub = new Sub("father");
}
interface Child04 extends Father04{
	String str = "Child04";

	Sub sub = new Sub("child");
}
class Sub{
	Sub(String str){
		System.out.println(str);
	}
}
//~out:child

初始化子接口时，并不会初始化其父接口