public User selectByLoginnameAndPassword(@Param("loginname")String loginname,@Param("password") String password);

 User login(String loginname,String password);

@Autowired

private UserDao userDao;

public User login(String loginname, String password) {

        return userDao.selectByLoginnameAndPassword(loginname, password);

	}

<mapper namespace="com.yongxing.manager.dao.UserDao">
<select id="selectByLoginnameAndPassword" resultType="com.yongxing.manager.domain.User" >

  select * from user_inf where loginname=#{loginname} and password=#{password}

</select>

@Controller

public class UserController {

	@Autowired

	private UserService userServcie;

	@RequestMapping("/checklogin")

	@ResponseBody

   public User checklogin( String loginname,String paasword,HttpSession session){

		System.out.println(loginname);

		User user =userServcie.login(loginname, paasword);

		if(user!=null) {

			session.setAttribute("user_session", user);

			return user;

		}else {

			return null;

		}

}

<form>

 <input name="loginname" type="text"  id="loginname"/ >

 <input name="password" type="password" id="password"/>

 <input value="登录" style="width: 100%;" type="button" onclick="requestJson()" >

</form>

<script type="text/javascript">

function requestJson(){

	    var name =$("#loginname").val();//#是id选择器

            var pwd =$("#password").val();

            if(name==""){

                alert("用户名不能为空！");

                return false;

            }

            else if(pwd==" "){

                alert("密码不能为空！");

                return false;

            }

            else

            {

            $.ajax({

                type:'post',

                url:'checklogin',

                dataType:"json",//注意使用的是打他dataType，而不是Content-Type

                async: true,

                data:{loginname:name,paasword:pwd},

                success:function(data){

                   if(data==null){

                       alert("没有这个用户！")

                   }

                   else{

                       alert("登录成功")

                       window.location.href ="main";

                   }

                }

            });

           }

       }

   </script>