求解spring+hibernate+webwork框架搭建出现空指针异常原因

各位高人，小妹最近遇见点框架问题，还望到这边一游的高人指教。在spring+hibernate+webwork框架搭建出现空指针异常。

具体代码如下：

xwork.xml
<!DOCTYPE xwork PUBLIC "-//OpenSymphony Group//XWork 1.0//EN"
"http://www.opensymphony.com/xwork/xwork-1.0.dtd">
<xwork>
<include file="webwork-default.xml"></include>
<package name="default" extends="webwork-default">
<action name="login" class="action.UserAction" method="login" >
<result name="success">/ok.jsp</result>
<result name="error">/err.jsp</result>
</action>
</package>
</xwork>
applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/tx
http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop-2.0.xsd">
<bean id="dataSource"
class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName"
value="com.mysql.jdbc.Driver">
</property>
<property name="url" value="jdbc:mysql://localhost:3306/test"></property>
<property name="username" value="root"></property>
<property name="password" value="root"></property>
</bean>
<bean id="sessionFactory"
class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<property name="dataSource">
<ref bean="dataSource" />
</property>
<property name="hibernateProperties">
<props>
<prop key="hibernate.dialect">
org.hibernate.dialect.MySQLDialect
</prop>
</props>
</property>
<property name="mappingResources">
<list>
<value>vo/Users.hbm.xml</value>
<value>vo/Result.hbm.xml</value></list>
</property></bean>

<bean id="BaseDao" class="dao.BaseDao" >
<property name="sessionFactory">
<ref bean="seesionfactory" />
</property>
</bean>
<bean id="UserDao" class="dao.impl.UserDao"
parent="BaseDao">
</bean>
<bean id="userservice" class="service.impl.UserService">
<property name="userdao">
<ref bean="UserDao" />
</property>
</bean>
<bean id="UserAction" class="action.UserAction" >

<property name="userservice">
<ref bean="userservice" />
</property>
</bean>
</beans>

异常：
java.lang.NullPointerException
action.UserAction.login(UserAction.java:33)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
com.opensymphony.xwork.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:364)
com.opensymphony.xwork.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:216)
com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:190)
com.opensymphony.xwork.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:168)

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<filter>
<filter-name>webwork</filter-name>
<filter-class>
com.opensymphony.webwork.dispatcher.FilterDispatcher
</filter-class>
</filter>
<filter-mapping>
<filter-name>webwork</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<listener>
<listener-class>
org.springframework.web.context.ContextLoaderListener
</listener-class>
</listener>
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>
/WEB-INF/applicationContext-*.xml,classpath:applicationContext*.xml
</param-value>
</context-param>
<taglib>
<taglib-uri>/webwork</taglib-uri>
<taglib-location>
/WEB-INF/lib/webwork-2.2.4.jar
</taglib-location>
</taglib>

</web-app>

谢谢各位慷慨指点，这个难题已经困扰我很久了