未能自动连线批注

为什么在执行时总是得到userDetailDao异常null:

package com.springweb.service;

import com.springweb.dao.UserDetailDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Configurable;
import org.springframework.dao.DataAccessException;
import org.springframework.security.userdetails.UserDetails;
import org.springframework.security.userdetails.UserDetailsService;
import org.springframework.security.userdetails.UsernameNotFoundException;

public class UserService implements UserDetailsService
{    
    @Autowired
    UserDetailDao userDetailDao;

    public UserDetails loadUserByUsername(String string) throws UsernameNotFoundException, DataAccessException {
        return userDetailDao.queryForUser(string);
    }

}

<security:authentication-provider user-service-ref="userService">
    <security:password-encoder hash="md5" />        
</security:authentication-provider>

<bean name="userService" class="com.springweb.service.UserService">
</bean>

在dispatcher上下文中,我已经定义要扫描包:

<context:component-scan base-package="com.springweb"/>