输入密码时，在keypress函数里检测输入的按键，如果是图形字符，显示星号，并讲键入的字符存为全局变量，当输入del或backspace时，清除全局变量和星号；当输入为enter或点击确认时，开始比较所键入的全局变量值与密码，当然，用户名要对应密码-Enter a password, key input detection in the keypress function, if it is a graphic character showing the asterisks. And save the characters you typed as global variables, when the input del or backspace to remove global variables and asterisk　when the input to enter or click on the confirmation, began to compare the type of global variables values and password, of course, the user name to the corresponding password