报错：java.lang.NoClassDefFoundError: Could not initialize class XXXX

用servet做的登陆注册的程序

，点击之后出现

下面是配置xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>UserLoginServlet</servlet-name>
    <servlet-class>com.bie.servlet.UserLoginServlet</servlet-class>
  </servlet>
  <servlet>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>UserRegisterServlet</servlet-name>
    <servlet-class>com.bie.servlet.UserRegisterServlet</servlet-class>
  </servlet>




  <servlet-mapping>
    <servlet-name>UserLoginServlet</servlet-name>
    <url-pattern>/userlogin</url-pattern>
  </servlet-mapping>
  <servlet-mapping>
    <servlet-name>UserRegisterServlet</servlet-name>
    <url-pattern>/userregister</url-pattern>
  </servlet-mapping>

</web-app>

好像是缺失类的问题，不过不知道怎么操作