向service里面注入dao。为什么注入不进？有空的帮个忙看看。
就是在下面service里面，发觉userDao为null.为什么不能注入呢？兄弟们帮帮忙看看。
public   class   LoginServiceImpl   implements   LoginService   {

private   UserDAO   userDao;

public   void   setUserDao(UserDAO   userDao)   {
this.userDao   =   userDao;
}

public   User   findUserById(String   userId)   {
User   user   =   userDao.findUserById(userId);
return   user;
}
}

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

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

<bean   id= "loginAction "   class= "freeunite.action.LoginAction "   />
<bean   id= "logoutAction "   class= "freeunite.action.LogoutAction "   />

<bean   id= "userDao "   class= "freeunite.dao.UserDAOImpl "   singleton= "false ">
<property   name= "sessionFactory ">
<ref   bean= "sessionFactory "   />
</property>
</bean>

web.xml:
<?xml   version= "1.0 "   encoding= "UTF-8 "?>
<web-app   xmlns= "http://java.sun.com/xml/ns/j2ee "
xmlns:xsi= "http://www.w3.org/2001/XMLSchema-instance "   version= "2.4 "
xsi:schemaLocation= "http://java.sun.com/xml/ns/j2ee     http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd ">

<filter>
<filter-name> encodingFilter </filter-name>
<filter-class>
org.springframework.web.filter.CharacterEncodingFilter
</filter-class>
<init-param>
<param-name> encoding </param-name>
<param-value> UTF-8 </param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name> encodingFilter </filter-name>
<url-pattern> /* </url-pattern>
</filter-mapping>

<filter>
<filter-name> hibernateFilter </filter-name>
<filter-class>
org.springframework.orm.hibernate3.support.OpenSessionInViewFilter
</filter-class>
</filter>
<filter-mapping>
<filter-name> hibernateFilter </filter-name>
<url-pattern> /* </url-pattern>
</filter-mapping>

<listener>
<listener-class>
org.springframework.web.context.ContextLoaderListener
</listener-class>
</listener>
<listener>
<listener-class>
org.springframework.web.util.IntrospectorCleanupListener
</listener-class>
</listener>

<listener>