奇怪的组数length属性

Java中的数组其实也是一个对象，但是确实是一个特殊的对象，实在是太特殊了，继承自Object, 多出一个属性length,改写了clone方法。

 

我debug了数组对象的运行时的Class对象，只有一个name属性，用[L开头，其他属性全是null。

调用getDeclaredFields,getDeclaredMethods都没有数据。这就奇怪了，明明可以用的length属性在哪的呢

 

stackoverflow上有简单的解释：

Arrays are special objects in java, they have a simple attribute named length which is final.

There is no "class definition" of an array (you can't find it in any .class file), they're a part of the language itself.

10.7. Array Members

The members of an array type are all of the following:

• The public final field length, which contains the number of components of the array. lengthmay be positive or zero.

• The public method clone, which overrides the method of the same name in class Objectand throws no checked exceptions. The return type of the clone method of an array type T[]is T[].

  A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared.

• All the members inherited from class Object; the only method of Object that is not inherited is its clone method.

 

看了上面英文我的理解是数组是java底层的类，可能是用c语言写的，所以在java语言Class类定义上，都找不到length属性和clone方法的声明。这里强调一点数组的clone是浅复制，也就是多维数组的时候，只会克隆第一维，后面维的都是指向相同地址的指针。

 

It's "special" basically, with its own bytecode instruction: arraylength. So this method:

public static void main(String[] args){
　　int x = args.length;
}

is compiled into bytecode like this:

public static void main(java.lang.String[]);

Code:
　 0:   aload_0
   1:   arraylength
   2:   istore_1
   3:   return

通过javap更好的说明了数组的length属性，其实是一个单独的二进制指令：arraylength