Where is array's length property defined?

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. length may be positive or zero.

• The public method clone, which overrides the method of the same name in class Object and 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.

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Resources:

• JLS - Arrays

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

So it's not accessed as if it were a normal field. Indeed, if you try to get it as if it were a normal field, like this, it fails:

// Fails...
Field field = args.getClass().getField("length");
System.out.println(field.get(args));

So unfortunately, the JLS description of each array type having a public final field length is somewhat misleading :(

It's defined in the Java language specification:

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. length may be positive or zero.

Since there is a limitless number of array types (for every class there is a corresponding array type, and then there are multidimensional arrays), they cannot be implemented in a class file; the JVM has to do it on the fly.