console readPassWord() 的时分为什么显示NUllPointexception

console readPassWord() 的时候为什么显示NUllPointexception ?
RT

Java code

public class scannerAndconsol {    public static void main(String[] args) {        // TODO Auto-generated method stub                Console cons =System.console();        String psw = new String(cons.readPassword());        System.out.println(psw);    }}