Java研学-三层架构实现简单登录操作

一 登录流程

??将服务器资源给有权限的人访问，只有登录的管理员可以访问员工信息进行 CRUD

二 三层架构

??Web 开发中的最佳实践：分层开发模式将整个业务应用划分为：表现层、业务逻辑层、数据访问层。区分层次的目的即为了“高内聚低耦合”的思想。
??表现层（Predentation Layer）：MVC，负责处理与界面交互的相关操作。
??业务层（Business Layer）：Service，负责复杂的业务逻辑计算和判断。
??持久层（Persistent Layer）：DAO，负责将业务逻辑数据进行持久化存储。

三 代码实现

1 数据库表

管理员表

CREATE TABLE `users` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `username` varchar(255) DEFAULT NULL,
  `password` varchar(255) DEFAULT NULL,
  `headImg` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
INSERT INTO user(username, password,headImg) VALUES('yellow', '111111','null');

员工信息表

CREATE TABLE `employee` (
  `id` bigint(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) DEFAULT NULL,
  `salary` double(10,2) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8;
INSERT INTO employee(name, salary) VALUES('kai', '6379');

2 创建实体类

Users

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Users {
    private  Long id;
    private  String username;
    private  String password;
    private  String headImg;
}

Employee

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Employee {
        private Long id;
        private String name;
        private Double salary;
}

3 创建对应的mapper文件

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tj.mapper.UserMapper">
    <!--根据账号查询-->
    <select id="queryByUsername" resultType="Users">
        SELECT * from users WHERE username=#{username}
    </select>
    <!--修改头像-->
    <update id="updateImg">
        update users set headImg =#{headImg} WHERE id=#{id}
    </update>
</mapper>

EmployeeMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tj.mapper.EmployeeMapper">
    <!--增加商品-->
    <insert id="add">
        insert into employee (name,salary)
        values(#{name},#{salary})
    </insert>
    <!--修改-->
    <update id="update">
        update employee set name=#{name},salary=#{salary}
                WHERE id=#{id}
    </update>
    <!--删除-->
    <delete id="delete">
        delete from employee WHERE id=#{id}
    </delete>
    <!--根据id查询-->
    <select id="selectOne" resultType="Employee">
        SELECT * from employee WHERE id=#{id}
    </select>
    <!--查询所有-->
    <select id="list" resultType="Employee">
        SELECT * from employee
    </select>
    <!--查询总条数-->
    <select id="queryCount" resultType="int">
        SELECT count(*) from employee
    </select>
    <!--分页查询-->
    <select id="queryPage" resultType="Employee">
        SELECT * from employee limit #{start},#{pageSize}
    </select>
</mapper>

4 创建resources文件

db.properties

jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql:///play_web
jdbc.user=root
jdbc.password=root

log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.cn.tj=TRACE
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <properties resource="db.properties"></properties>
    <typeAliases>
        <package name="cn.tj.domain"/>
    </typeAliases>
    <environments default="dev">
        <environment id="dev">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.user}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mappers/EmployeeMapper.xml"></mapper>
        <mapper resource="mappers/UserMapper.xml"></mapper>
    </mappers>
</configuration>

5 创建Dao层接口

UserDao

public interface UserDao {
    //根据账号查询
    Users queryByUsername(String username);
    //修改头像
    int updateImg(Users users);
}

EmployeeDao

public interface EmployeeDao {
    /*增加*/
    int add(Employee employee);
    /*修改*/
    int update(Employee employee);
    /*删除*/
    int delete(Long id);
    /*查询所有*/
    List<Employee> list();
    /*根据id查询*/
    Employee selectOne(Long id);
    /*查询总条数*/
    int queryCount(QueryObject qo);
    /*分页查询*/
    List<Employee> queryPage(QueryObject qo);
}

6 创建Dao层实现类

UserDaoImpl

public class UserDaoImpl implements UserDao {
    private SqlSession sqlSession= MybatisUtil.getSqlSession();
    @Override
    public Users queryByUsername(String username) {
        return sqlSession.selectOne("cn.tj.mapper.UserMapper.queryByUsername", username);
    }
    @Override
    public int updateImg(Users users) {
        return sqlSession.update("cn.tj.mapper.UserMapper.updateImg",users);
    }
}

EmployeeDaoImpl

public class EmployeeDaoImpl implements EmployeeDao {
    /*定义成员变量*/
    private SqlSession sqlSession= MybatisUtil.getSqlSession();
    @Override
    public int add(Employee employee) {
        int row = sqlSession.insert("cn.tj.mapper.EmployeeMapper.add", employee);
        return row;
    }
    @Override
    public int update(Employee employee) {
        int row= sqlSession.update("cn.tj.mapper.EmployeeMapper.update",employee);
        return row;
    }
    @Override
    public int delete(Long id) {
        int row= sqlSession.delete("cn.tj.mapper.EmployeeMapper.delete",id);
        return row;
    }
    @Override
    public List<Employee> list() {
        List<Employee> list = sqlSession.selectList("cn.tj.mapper.EmployeeMapper.list");
        return list;
    }
    @Override
    public Employee selectOne(Long id) {
        Employee employee= sqlSession.selectOne("cn.tj.mapper.EmployeeMapper.selectOne", id);
        return employee;
    }
    @Override
    public int queryCount(QueryObject qo) {
        return sqlSession.selectOne("cn.tj.mapper.EmployeeMapper.queryCount",qo);
    }
    @Override
    public List<Employee> queryPage(QueryObject qo) {
        return sqlSession.selectList("cn.tj.mapper.EmployeeMapper.queryPage",qo);
    }
}

7 编写登录 Servlet

UserServlet

@WebServlet("/user")
@MultipartConfig
public class UserServlet extends HttpServlet {
    private UserService userService =new UserServiceImpl();
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        String cmd = req.getParameter("cmd");
        if ("login".equals(cmd)){
            login(req,resp);//登录
        }else if("logout".equals(cmd)){
            logout(req,resp);//退出登录
        }else if("updateImg".equals(cmd)){
            updateImg(req,resp);
        }
    }
    /*修改头像*/
    private void updateImg(HttpServletRequest req, HttpServletResponse resp) {
        try {
            //上传新图片
            Part part = req.getPart("headImg");
            String path=getServletContext().getRealPath("/upload") +"/";
            String headImg = UploadUtil.uploadImg(req, resp, part,path);
            //获取登录账号对象
            HttpSession session = req.getSession();
            Users user= (Users) session.getAttribute("USER_IN_SESSION");
            //修改头像
            user.setHeadImg(headImg);
            //保存新头像名称
            userService.updateImg(user);
            //把session中的对象更新一下
            session.setAttribute("USER_IN_SESSION",user);
            //修改完毕跳转查询商品
            resp.sendRedirect("/employee");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /*退出登录*/
    private void logout(HttpServletRequest req, HttpServletResponse resp) {
        try {
            //获取session
            HttpSession session = req.getSession();
//            session.invalidate();//销毁session
            session.removeAttribute("username");
            //跳转到登录页面
            resp.sendRedirect("login.jsp");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /*用户登录*/
    private void login(HttpServletRequest req, HttpServletResponse resp) {
        try {
            //获取表单的参数
            String username = req.getParameter("username");
            String password = req.getParameter("password");
            //调用业务层登录方法
            Users users = userService.login(username, password);

            //账号密码正确：登录成功,将登录账号显示到商品的列表页面
            HttpSession session = req.getSession();
            session.setAttribute("USER_IN_SESSION",users);
            resp.sendRedirect("/employee");
        } catch (Exception e) {
            try {
                req.setAttribute("msg",e.getMessage());
                req.getRequestDispatcher("/login.jsp").forward(req,resp);
            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }
    }
}

8 编写登录页面

login.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
<h3>用户登录</h3>
<form action="/user?cmd=login" method="post">
    <%--登录错误提示--%>
    <span style="color: red">${msg}</span>
    <p>账号：<input type="text" name="username"></p>
    <p>密码：<input type="password" name="password"></p>
    <p><input type="submit" value="登录"></p>
</form>
</body>
</html>

9 登录后跳转操作员工信息

EmployeeServlet

@WebServlet("/employee")
public class EmployeeServlet extends HttpServlet {
    private EmployeeService employeeService=new ProductServiceImpl();
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //设置编码格式
        req.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");
        // 登录权限判断 操作员工需要先登录
        HttpSession session = req.getSession();
        Users users = (Users) session.getAttribute("USER_IN_SESSION");
        if (users==null){//如果没有登录，跳转登录页面，给出提示
            req.setAttribute("msg","请先登录再操作！");
            req.getRequestDispatcher("/login.jsp").forward(req,resp);
            return;
        }
        //获取请求分发的参数
        String cmd = req.getParameter("cmd");
        if ("delete".equals(cmd)){
            //调用删除
            delete(req,resp);
        }else if("input".equals(cmd)){
            //跳转到增加或修改页面
            input(req,resp);
        } else if("saveOrUpdate".equals(cmd)){
            //保存增加或修改商品
            saveOrUpdate(req,resp);
        }else{
            //查询所有
            list(req,resp);
        }
    }

    /*增加修改保存*/
    private void saveOrUpdate(HttpServletRequest req, HttpServletResponse resp) {
        //封装员工对象
        Employee employee=new Employee();
        /*将参数的获取及封装对象过程抽取成一个方法*/
        req2product(req,employee);
        /*根据请求中是否携带id判断执行增加或者修改操作*/
        String id = req.getParameter("id");
        if (StringUtil.hasLength(id)){
            employee.setId(Long.valueOf(id));
            //执行修改
            employeeService.update(employee);
        }else{
            //调用dao方法执行增加保存操作
            employeeService.add(employee);
        }
        try {
            //跳转到查询
            resp.sendRedirect("/employee");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    /*封装的获取请求参数的方法*/
    private void req2product(HttpServletRequest req, Employee employee) {
        //获取请求中表单参数
        String name = req.getParameter("name");
        //验证输入的表单参数不能为空
        if (StringUtil.hasLength(name)){
            //编辑员工姓名
            employee.setName(name);
        }
        String salary = req.getParameter("salary");
        if (StringUtil.hasLength(salary)){
            employee.setSalary(Double.valueOf(salary));
        }
    }
    /*跳转到编辑页面*/
    private void input(HttpServletRequest req, HttpServletResponse resp) {
        //根据请求中是否携带id判断是增加或者修改操作
        String id = req.getParameter("id");
        if (StringUtil.hasLength(id)){//请求中携带了id执行修改
            //根据id查询员工
            Employee employee = employeeService.selectOne(Long.valueOf(id));
            //将查询的员工存储到作用域
            req.setAttribute("employee",employee);
        }
        try {
            //跳转到WEB-INF下面的页面：input.jsp
            req.getRequestDispatcher("/WEB-INF/views/employee/input.jsp").forward(req,resp);
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    /*分页查询所有*/
    private void list(HttpServletRequest req, HttpServletResponse resp) {
        try {
            /*客户端如果传入了当前页码和每页显示条数则设置给qo对象，如果没有传入则使用默认值*/
            String currentPage = req.getParameter("currentPage");
            String pageSize = req.getParameter("pageSize");
            QueryObject qo=new QueryObject();
            if (StringUtil.hasLength(currentPage)){
                qo.setCurrentPage(Integer.valueOf(currentPage));
            }
            if (StringUtil.hasLength(pageSize)){
                qo.setPageSize(Integer.valueOf(pageSize));
            }
            //调用service分页查询的方法
            PageResult<Employee> pageResult = employeeService.queryByPage(qo);
            //将查询的结果存储到请求作用域
            req.setAttribute("pageResult",pageResult);
            //转发到列表页面
            req.getRequestDispatcher("/WEB-INF/views/employee/list.jsp").forward(req,resp);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /*删除*/
    private void delete(HttpServletRequest req, HttpServletResponse resp) {
        try {
            //获取目标id
            String id = req.getParameter("id");
            //调用删除方法
            int row = employeeService.delete(Long.valueOf(id));
            //删除完毕跳转到查询
            resp.sendRedirect("/employee");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

10 展示与编辑员工页

input.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>员工编辑页面</title>
</head>
<body>
<h3>员工编辑</h3>
<form action="/employee?cmd=saveOrUpdate" method="post">
    <input type="hidden" name="id" value="${employee.id}">
    <p>姓名：<input type="text" name="name" value="${employee.name}"></p>
    <p>工资：<input type="number" name="salary" step="0.01" value="${employee.salary}"></p>
    <p><input type="submit" value="保存"></p>
</form>
</body>
</html>

list.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%--引入核心标签库--%>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>员工信息页面</title>
</head>
<body>
<center>
    <h3>员工列表</h3>
    <p>当前登录账号：${sessionScope.USER_IN_SESSION.username}</p>
    <p>
        <%--点击跳转到上传头像的表单页面--%>
        <a href="/headImg.jsp">
            <%--头像--%>
            <img src="/upload/${sessionScope.USER_IN_SESSION.headImg}" width="100px" height="100px">
            <p>${sessionScope.USER_IN_SESSION.headImg}</p>
        </a>
    </p>
    <p><a href="/employee?cmd=input">增加员工</a></p>
    <p><a href="/user?cmd=logout">退出登录</a></p>
    <%--分页跳转的表单--%>
    <form action="/employee" method="post">

    <table border="1px" cellpadding="0" cellspacing="0"width="800px">
        <thead>
        <tr>
            <th>序号</th>
            <th>姓名</th>
            <th>工资</th>
            <th>操作</th>
        </tr>
        </thead>
       <tbody>
       <%--动态展示员工列表数据--%>
       <c:forEach items="${pageResult.list}" varStatus="vs" var="employee">
           <tr class="trClass">
               <td>${vs.count}</td>
               <td>${employee.name}</td>
               <td>${employee.salary}</td>
               <td>
                   <a href="/employee?cmd=input&id=${employee.id}">修改员工信息</a>&nbsp;&nbsp;
                       <%--                    <a href="/employee?cmd=delete&id=${employee.id}">删除员工信息</a>--%>
                   <a href="#" onclick="deleteTr(${employee.id})">删除员工信息</a>
               </td>
           </tr>
       </c:forEach>
       </tbody>
        <tfoot>
          <tr align="center">
              <td colspan="9">
                  <a href="/employee?currentPage=1&pageSize=${pageResult.pageSize}">首页</a>&nbsp;&nbsp;
                  <a href="/employee?currentPage=${pageResult.prevPage}&pageSize=${pageResult.pageSize}">上一页</a>&nbsp;&nbsp;
                  <a href="/employee?currentPage=${pageResult.nextPage}&pageSize=${pageResult.pageSize}">下一页</a>&nbsp;&nbsp;
                  <a href="/employee?currentPage=${pageResult.totalPage}&pageSize=${pageResult.pageSize}">末页</a>&nbsp;&nbsp;
                  当前是第 ${pageResult.currentPage}/${pageResult.totalPage}页&nbsp;&nbsp;
                  一共${pageResult.totalCount}条数据&nbsp;&nbsp;
                  跳转到第
                  <input type="text" name="currentPage" value="${pageResult.currentPage}"
                         onchange="changePagesize()" style="width: 50px">
                  页&nbsp;&nbsp;
                  每页
                    <select name="pageSize" onchange="changePagesize()">
                        <option value="3" ${pageResult.pageSize==3?'selected':''}>3</option>
                        <option value="6" ${pageResult.pageSize==6?'selected':''}>6</option>
                        <option value="9" ${pageResult.pageSize==9?'selected':''}>9</option>
                    </select>
                  条数据&nbsp;&nbsp;
              </td>
          </tr>
        </tfoot>
    </table>
    </form>
</center>

<%--鼠标移动到当前行，实现背景颜色高亮显示--%>
<script>
    //获取所有的行元素：不包含表头
    var trs = document.getElementsByClassName("trClass");
    //遍历行元素集合
    for(var i=0;i<trs.length;i++){
        //鼠标移入：当前行高亮，背景变成灰色
        trs[i].onmouseover=function (){
            this.style.backgroundColor="gray";
        }
        //鼠标移出：恢复原来背景颜色
        trs[i].onmouseout=function (){
            this.style.backgroundColor="";
        }
    }
</script>

<%--删除确认表--%>
<script>
    function deleteTr(id){
        //确认删除
       var b= window.confirm("确定要删除选中的员工信息吗？");
       if(b){//确定删除，执行后台删除操作
           window.location="/employee?cmd=delete&id="+id;
       }
    }
</script>

<%--分页表单提交函数--%>
<script>
    function changePagesize(){
        document.forms[0].submit();
    }
</script>
</body>
</html>

headImg.jsp – 修改头像

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>头像上传</title>
</head>
<body>
<h3>修改头像</h3>
<form action="/user?cmd=updateImg" method="post" enctype="multipart/form-data">
    <p>头像：<input type="file" name="headImg"></p>
    <p><input type="submit" value="提交"></p>
</form>
</body>
</html>

11 service层接口

UserService

public interface UserService {
    //登录方法
    public Users login(String username, String password);
    //修改头像
    public int updateImg(Users users);
}

EmployeeService

public interface EmployeeService {
    /*增加*/
    int add(Employee employee);
    /*修改*/
    int update(Employee employee);
    /*删除*/
    int delete(Long id);
    /*查询所有*/
    List<Employee> list();
    /*根据id查询*/
    Employee selectOne(Long id);
    /*分页查询*/
    PageResult<Employee> queryByPage(QueryObject qo);
}

12 service实现类

UserServiceImpl

public class UserServiceImpl implements UserService {
    private UserDao userDao=new UserDaoImpl();
    @Override
    public Users login(String username, String password) {
        Users users = userDao.queryByUsername(username);
        //判断账号
        if(users==null){//账号错误
            throw new RuntimeException("账号错误");
        }
        //判断密码
        if(!users.getPassword().equals(password)){//密码错误
            throw new RuntimeException("密码错误");
        }
        return users;
    }
    @Override
    public int updateImg(Users users) {
        int row = userDao.updateImg(users);
        return row;
    }
}

ProductServiceImpl

public class ProductServiceImpl implements EmployeeService {
    private EmployeeDao employeeDao=new EmployeeDaoImpl();
    @Override
    public int add(Employee employee) {
        return employeeDao.add(employee);
    }
    @Override
    public int update(Employee employee) {
        return employeeDao.update(employee);
    }
    @Override
    public int delete(Long id) {
        return employeeDao.delete(id);
    }
    @Override
    public List<Employee> list() {
        return employeeDao.list();
    }
    @Override
    public Employee selectOne(Long id) {
        return employeeDao.selectOne(id);
    }
    /*分页查询*/
    @Override
    public PageResult<Employee> queryByPage(QueryObject qo) {
        //查询总条数
        int totalCount = employeeDao.queryCount(qo);
        if(totalCount==0){
            return new PageResult<>(qo.getCurrentPage(),qo.getPageSize(),
                    0, Collections.emptyList());
        }
        //查询分页商品集合
        List<Employee> list = employeeDao.queryPage(qo);
        //创建分页对象
        PageResult<Employee> pageResult=new PageResult<>(qo.getCurrentPage(),qo.getPageSize(),
                totalCount,list);
        return pageResult;
    }
}

13 分页QO

QueryObject

@Setter
@Getter
public class QueryObject {
    private Integer currentPage=1;
    private Integer pageSize=3;
    public int getStart(){
      return   (currentPage-1)*pageSize;//计算查询的起始索引
    }
}

14 所需的工具类

MybatisUtil

public class MybatisUtil {
    private static  SqlSessionFactory factory=null;
    static {
        try {
            InputStream rs = Resources.getResourceAsStream("mybatis-config.xml");
             factory = new SqlSessionFactoryBuilder().build(rs);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public static SqlSession getSqlSession(){
        return factory.openSession(true);
    }
}

PageResult

@Setter
@Getter
public class PageResult<T> {
    //上一页
    private  Integer prevPage;
    //下一页
    private  Integer nextPage;
    //总条数
    private  Integer totalCount;
    //总页数
    private Integer totalPage;
    //当前页码
    private Integer currentPage;
    //每页显示条数
    private Integer pageSize;
    //查询的集合
    private List<T> list;
    /*初始化分页对象有参构造*/
    public  PageResult(Integer currentPage,Integer pageSize,Integer totalCount,List<T> list){
        this.currentPage=currentPage;
        this.pageSize=pageSize;
        this.totalCount=totalCount;
        this.list=list;
        //总页数=总条数/每页显示条数 当求模运算值不等于0的时候总页数加1
        this.totalPage=(this.totalCount%this.pageSize==0)?(this.totalCount/this.pageSize):(this.totalCount/this.pageSize+1);
        //上一页
        this.prevPage=(this.currentPage==1)?(1):(this.currentPage-1);
        //下一页
        this.nextPage=(this.currentPage==this.totalPage)?(this.totalPage):(this.currentPage+1);
    }
}

StringUtil

public class StringUtil {
    public static boolean hasLength(String s){//判断输入的表单参数是否为空
        return s!=null && s.trim().length()!=0;
    }
}

UploadUtil

public class UploadUtil {
    public static String uploadImg(HttpServletRequest req, HttpServletResponse resp,
        Part part,String path){
        try {
            //上传文件的类型
            String contentType = part.getContentType();
            if (!contentType.startsWith("image")){
                //上传的不是图片，页面给出提示
                req.setAttribute("errorMsg","上传的文件必须是图片！");
                req.getRequestDispatcher("register.jsp").forward(req,resp);
                return null;
            }
            //获取文件的名称：pic.png
            String fileName = part.getSubmittedFileName();
            //获取源文件类型或后缀名
            String type= fileName.substring(fileName.lastIndexOf("."));
            String s = UUID.randomUUID().toString().replace("-","");//使用uuid生成32位随机字符

            //设置新的文件名称
            String newfilename=s+type;
            //设置上传的目标目录地址
//            String path="D:\\java\\upload\\";
            //将上传的文件保存到目标地址
            part.write(path+newfilename);
            return newfilename;
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}