spring依赖注入问题~求教~

Question

spring依赖注入问题：
配置文件如下：
<?xml version="1.0" encoding="UTF-8"?>
<beans
xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd">


<bean name="transactionManager"
class="org.springframework.orm.hibernate3.HibernateTransactionManager">
<property name="sessionFactory">
<ref bean="sessionFactory" />
</property>
</bean>

<bean id="sessionFactory"
class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<property name="configLocation"
value="classpath:hibernate.cfg.xml">
</property>
</bean>

<bean id="abstractTxDefinition"
class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"
lazy-init="true">
<property name="transactionManager">
<ref bean="transactionManager" />
</property>
<property name="transactionAttributes">
<props>
<prop key="*">
PROPAGATION_REQUIRED,
-AccessException,-CustomerSericeException
</prop>
</props>
</property>
</bean>

<!-- Dao -->
<bean id="userDao" class="kingsoft.dao.impl.UserDaoImpl"></bean>

<!-- Service --> <bean id="userService" parent="abstractTxDefinition">
<property name="target">
<bean class="kingsoft.service.ipml.UserServiceImpl">
<property name="userDao">
<ref bean="userDao"/>
</property>
</bean>
</property>
</bean>

</beans>
在Action中的部分代码，仅是为了测试获得注入的userService没有：
public String execute(){
    User user = userService.findUserByUsernameAndPassword(username, password);
if(user != null){
    return "success";
}
    return "failed";
}
但是这里报空指针错误~希望朋友们指教