spring+hibernate 调用 DAO 出现java.lang.NullPointerException异常，先谢谢了

applicationcontex.xml配置如下：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">

<beans>

<bean name="/login" class="com.test.struts.action.LoginAction" singleton="false"></bean>

<bean id="DataSource" class="org.apache.commons.dbcp.BasicDataSource">
<property name="driverClassName">
<value>com.mysql.jdbc.Driver</value>
</property>
<property name="url">
<value>jdbc:mysql://localhost:3306/SAMPLEDB</value>
</property>
<property name="username">
<value>root</value>
</property>
<property name="password">
<value>123</value>
</property>
</bean>
<bean id="SessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<property name="dataSource">
<ref bean="DataSource"></ref>
</property>
<property name="hibernateProperties">
<props>
<prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
</props>
</property>
<property name="mappingResources">
<list>
<value>com/test/Hibernate/User.hbm.xml</value>
</list>
</property>
</bean>
<bean id="UserDAO" class="com.test.Hibernate.UserDAO">
<property name="sessionFactory">
<ref bean="SessionFactory" />
</property>
</bean>
</beans>


DAO.java如下：
package com.test.Hibernate;

import java.util.List;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.hibernate.LockMode;
import org.hibernate.Session;
import org.hibernate.criterion.Example;
import org.springframework.context.ApplicationContext;
import org.springframework.orm.hibernate3.support.HibernateDaoSupport;

/**
* Data access object (DAO) for domain model class User.
* @see com.test.Hibernate.User
* @author MyEclipse - Hibernate Tools
*/
public class UserDAO extends HibernateDaoSupport {

private static final Log log = LogFactory.getLog(UserDAO.class);

protected void initDao() {
//do nothing
}

private static String hql = "from User u where u.username=? ";

public boolean isValidUser(String username, String password) {



List userList = this.getHibernateTemplate().find(hql, username);

if (userList.size() > 0) {

return true;

}

return false;

}

private Session openSession() {
// TODO Auto-generated method stub
return null;
}


}
调用如下：
private UserDAO userDAO;

public UserDAO getUserDAO() {

return userDAO;
}

public void setUserDAO(UserDAO userDAO) {
this.userDAO = userDAO;
}

public ActionForward execute(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response)
{
DynaActionForm loginForm = (DynaActionForm) form;
String username=loginForm.getString("username");
String password=loginForm.getString("password");
if (userDAO.isValidUser(username,password)) {
return mapping.findForward("indexGo");
} else {
return mapping.getInputForward();
}



}