Jsp 学习笔记

代码可参考: Demo地址

1 入门

1.1 环境搭建

创建moven项目目录结构如下
在这里插入图片描述

1.2 依赖配置

<!-- 依赖 -->
<dependencies>
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>3.1.0</version>
        <scope>provided</scope>
    </dependency>

    <dependency>
        <groupId>javax.servlet.jsp</groupId>
        <artifactId>jsp-api</artifactId>
        <version>2.2</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

<!-- 插件 -->
<plugins>
   <plugin>
       <groupId>org.apache.tomcat.maven</groupId>
       <artifactId>tomcat7-maven-plugin</artifactId>
       <version>2.2</version>
   </plugin>
</plugins>

1.3 创建页面

在webapp下创建 hello.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
	<h1>hello jsp</h1>
    <%
        System.out.println("hello, jso~");
    %>
</body>
</html>

1.4 测试

启动tomcat
访问 http://localhost:8080/Jsp-Demo/hello.jsp后
控制台输出

hello, jso~

2 原理

hello.jsp是Servlet
访问hello.jsp会生成如下的类
在这里插入图片描述
继承关系

class HttpJspBase extends HttpServlet implements HttpJspPage

public final class hello_jsp extends HttpJspBase implements JspSourceDependent {
    private static final JspFactory _jspxFactory = JspFactory.getDefaultFactory();
    private static Map<String, Long> _jspx_dependants;
    private ExpressionFactory _el_expressionfactory;
    private InstanceManager _jsp_instancemanager;

    public hello_jsp() {
    }

    public Map<String, Long> getDependants() {
        return _jspx_dependants;
    }

    public void _jspInit() {
        this._el_expressionfactory = _jspxFactory.getJspApplicationContext(this.getServletConfig().getServletContext()).getExpressionFactory();
        this._jsp_instancemanager = InstanceManagerFactory.getInstanceManager(this.getServletConfig());
    }

    public void _jspDestroy() {
    }

    public void _jspService(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        JspWriter out = null;
        JspWriter _jspx_out = null;
        PageContext _jspx_page_context = null;

        try {
            response.setContentType("text/html;charset=UTF-8");
            PageContext pageContext = _jspxFactory.getPageContext(this, request, response, (String)null, true, 8192, true);
            _jspx_page_context = pageContext;
            pageContext.getServletContext();
            pageContext.getServletConfig();
            pageContext.getSession();
            out = pageContext.getOut();
            out.write("\n");
            out.write("<html>\n");
            out.write("<head>\n");
            out.write("    <title>Title</title>\n");
            out.write("</head>\n");
            out.write("<body>\n");
            out.write("    <h1>hello jsp</h1>\n");
            out.write("    ");
            System.out.println("hello, jso~");
            out.write("\n");
            out.write("</body>\n");
            out.write("</html>\n");
        } catch (Throwable var13) {
            if (!(var13 instanceof SkipPageException)) {
                out = (JspWriter)_jspx_out;
                if (_jspx_out != null && ((JspWriter)_jspx_out).getBufferSize() != 0) {
                    try {
                        out.clearBuffer();
                    } catch (IOException var12) {
                    }
                }

                if (_jspx_page_context == null) {
                    throw new ServletException(var13);
                }

                _jspx_page_context.handlePageException(var13);
            }
        } finally {
            _jspxFactory.releasePageContext(_jspx_page_context);
        }

    }
}

在_jspService()方法中，会将html标签写入到JspWritter中返回给浏览器

3 jsp实践

3.1 jsp类型

3.1.1 <%…%>

内容会被放到_jspService()中

3.1.2 <%=…%>

内容会放到out.println()中，作为out.print()函数

3.1.3 <%!..%>

内容会被放到_jspService()方法之外，被类直接包含

下面是示例

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>hello jsp</h1>
    <%
        System.out.println("hello, jso~");
        int i = 5;
    %>

    <%="hello"%>
    <%=i%>

    <%!
        void show() {}
        String name = "zhangsan";
    %>
</body>
</html>

public final class hello_jsp extends HttpJspBase implements JspSourceDependent {
	//<%!void show() {} String name = "zhangsan"; %>
    String name = "zhangsan";
    void show() {}
    public void _jspService(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        	...
            System.out.println("hello, jso~"); //<%System.out.println("hello, jso~");%>
            int i = 5; //<% int i = 5%>
            out.write("\n");
            out.write("\n");
            out.write("    ");
            out.print("hello"); //<%="hello"%>
            out.write("\n");
            out.write("    ");
            out.print(i);	// <%=i%>
            ...
        }
    }
}

3.2 示例

<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.List" %>
<%@ page import="com.jsp.Brand" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%
    List<Brand> brandList = new ArrayList();
    brandList.add(new Brand(1, "华为", "华为", 200, "5G厂商", 1));
    brandList.add(new Brand(2, "比亚迪", "比亚迪", 300, "国产电车", 1));
    brandList.add(new Brand(3, "茅台", "茅台", 400, "白酒酱香", 1));
%>
<html>
<head>
    <title>Title</title>
</head>
<body>
<input type="button" value="新郑"><br>
<hr>
<table border="1" cellspacing="0" width="800">
    <tr>
        <th>序号</th>
        <th>品牌名称</th>
        <th>企业名称</th>
        <th>排序</th>
        <th>品牌介绍</th>
        <th>状态</th>
        <th>操作</th>
    </tr>

    <%
        for (int i = 0; i < brandList.size(); i++) {
            Brand brand = brandList.get(i);
    %>

    <tr align="center">
        <td><%=brand.getId()%></td>
        <td><%=brand.getBrandName()%></td>
        <td><%=brand.getCompanyName()%></td>
        <td><%=brand.getOrdered()%></td>
        <td><%=brand.getDescription()%></td>
        <td><%=brand.getStatus() == 1 ? "启用" : "禁用"%></td>
        <td><a href="#">修改</a> <a href="#">删除</a> </td>
    </tr>

    <%
        }
    %>
</table>

</body>
</html>

在浏览器地址栏输入

http://localhost:8080/Jsp-Demo/brand.jsp

效果展示
在这里插入图片描述

遇到报错

Only a type can be imported. * resolves to a package

解决方式: 在Avaliable Elements 这里选择对应的项目后，右键，选择 Put into Output Root
在这里插入图片描述

4 EL表达式

EL(全称Expression Language) 表达式语言，用于简化jsp页面的Java代码
主要作用是获取数据，然后展示在页面上
语法$ {expression}，如${brands}就是获取域中存储的key为brands的数据

4.1 案例

<%@ page contentType="text/html;charset=UTF-8"  isELIgnored="false" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
    ${brands}
</body>
</html>

@WebServlet("/demo1")
public class ServletDemo1 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("ServletDemo1 doGet");
        List<Brand> brandList = new ArrayList();
        brandList.add(new Brand(1, "华为", "华为", 200, "5G厂商", 1));
        brandList.add(new Brand(2, "比亚迪", "比亚迪", 300, "国产电车", 1));
        brandList.add(new Brand(3, "茅台", "茅台", 400, "白酒酱香", 1));

        req.setAttribute("brands", brandList);
        req.setAttribute("status", 111);

        System.out.println(req.getAttribute("brands"));

        req.getRequestDispatcher("/el-demo.jsp").forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

访问http://localhost:8080/Jsp-Demo/demo1
后将会输出brands中的内容

如果el表达式获取不到值,则可能是WEB-INF包下的web.xml中的版本过低
修改成

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns="http://xmlns.jcp.org/xml/ns/javaee"
        xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
        version="3.1">
</web-app>

另外jsp中添加 isELIgnored=“false”

<%@ page contentType="text/html;charset=UTF-8"  isELIgnored="false" language="java" %>

4.2 域对象

page: 当前页面有效
request: 当前请求有效
session: 当前会话有效
applicaton: 当前应用有效

6 JSTL标签

jsp标准标签库(jsp Standarded Tab Library), 使用标签替代jsp页面上的代码
使用:
导入依赖

<dependency>
   <groupId>jstl</groupId>
   <artifactId>jstl</artifactId>
   <version>1.2</version>
</dependency>
<dependency>
    <groupId>taglibs</groupId>
    <artifactId>standard</artifactId>
    <version>1.1.2</version>
</dependency>

使用标签

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

使用示例:
jstp.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<html>
<head>
    <title>Title</title>
</head>
<body>
    <%-- <c:if>标签 --%>
    <c:if test="${status == 1}">
        开启
    </c:if>

    <c:if test="${status == 0}">
        关闭
    </c:if>
    <br>

    <%-- <c:forEach>标签 --%>
    <c:forEach items="${brands}" var="brand">
        <tr align="center">
            <td>${brand.id}</td>
            <td>${brand.brandName}</td>
            <td>${brand.companyName}</td>
            <td>${brand.description}</td>
        </tr>
        <br>
    </c:forEach>
    <br>

    <%-- <c:forEach>标签 --%>
    <c:forEach begin="0" end="10" step="1" var="i">
        ${i}
    </c:forEach>

</body>
</html>

ServletDemo2.java

@WebServlet("/demo2")
public class ServletDemo2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //将数据存储到request域中
        req.setAttribute("status", 1);

        List<Brand> brandList = new ArrayList<>();
        brandList.add(new Brand(1, "华为", "华为", 200, "5G厂商", 1));
        brandList.add(new Brand(2, "比亚迪", "比亚迪", 300, "国产电车", 1));
        brandList.add(new Brand(3, "茅台", "茅台", 400, "白酒酱香", 1));

        req.setAttribute("brands", brandList);

        //转发到jstl-if.jsp中
        req.getRequestDispatcher("/jstl.jsp").forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

使用浏览器访问

http://localhost:8080/Jsp-Demo/demo2

浏览器中输出内容

开启
1 华为 华为 5G厂商
2 比亚迪 比亚迪 国产电车
3 茅台 茅台 白酒酱香

0 1 2 3 4 5 6 7 8 9 10

7 示例

7.1 环境准备

1 创建web工程
2 创建数据库，添加数据

-- 创建tb_brand表
create table tb_brand(
	id int primary key auto_increment,
	brand_name varchar(20),
	company_name varchar(20),
	ordered int,
	description varchar(100),
	status int
);

-- 添加数据
insert into tb_brand (brand_name, company_name, ordered, description, status)
values('比亚迪', '比亚迪汽车有限公司', 100, '高科技专业电动汽车', 0),
	  ('宁德时代', '宁德时代科技公司', 200, '高科技锂电池', 1),
	  ('小米', '小米科技有限公司', 300, '让每个人享受科技带来的美好生活', 1)

7.2 查询所有

1 配置web-app的版本，否则jsp中获取不到el表达式的值
WEB-INFO/web.xml

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">
</web-app>

2 实体类 Brand

package com.test.pojo;

public class Brand {
    private Integer id;
    private String brandName;
    private String companyName;
    private Integer ordered;
    private String description;
    private Integer status;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getBrandName() {
        return brandName;
    }

    public void setBrandName(String brandName) {
        this.brandName = brandName;
    }

    public String getCompanyName() {
        return companyName;
    }

    public void setCompanyName(String companyName) {
        this.companyName = companyName;
    }

    public Integer getOrdered() {
        return ordered;
    }

    public void setOrdered(Integer ordered) {
        this.ordered = ordered;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public Integer getStatus() {
        return status;
    }

    public void setStatus(Integer status) {
        this.status = status;
    }

    @Override
    public String toString() {
        return "Brand{" +
                "id=" + id +
                ", brandName='" + brandName + '\'' +
                ", companyName='" + companyName + '\'' +
                ", ordered=" + ordered +
                ", description='" + description + '\'' +
                ", status=" + status +
                '}';
    }
}

3 定义公共的查询工具类

package com.test.utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

public class SqlSessionFactoryUtils {
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try {
            String rescource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(rescource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSessionFactory getSqlSessionFactory() {
        return sqlSessionFactory;
    }
}

4 定义查询的Mapper类和xml
BrandMapper.java

public interface BrandMapper {
    List<Brand> selectAll();
}

BrandMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.test.mapper.BrandMapper">
    <resultMap id="brandResultMap" type="brand">
        <result column="brand_name" property="brandName"></result>
        <result column="company_name" property="companyName"></result>
    </resultMap>

    <select id="selectAll" resultType="com.test.pojo.Brand" resultMap="brandResultMap">
        select * from tb_brand;
    </select>
</mapper>

5 定义查询的方法

public class BrandService {
    SqlSessionFactory factory  = SqlSessionFactoryUtils.getSqlSessionFactory();

    public List<Brand> selectAll() {
        SqlSession sqlSession = factory.openSession();
        BrandMapper mapper = sqlSession.getMapper(BrandMapper.class);

        List<Brand> brands = mapper.selectAll();
        System.out.println("brands = " + brands);
        sqlSession.close();
        return brands;
    }
}

6 定义服务接口Servlet

@WebServlet("/selectAllServlet")
public class SelectAllServlet extends HttpServlet {
    private BrandService service = new BrandService();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        List<Brand> brands = service.selectAll();
        req.setAttribute("brands", brands);
        req.getRequestDispatcher("/brand.jsp").forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doGet(req, resp);
    }
}

7 jsp页面

<%--
  Created by IntelliJ IDEA.
  User: yl
  Date: 2022/11/30
  Time: 5:11 PM
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
  <hr>
  <table border="1" cellspacing="0" width="80%">
    <tr>
        <th>序号</th>
        <th>品牌名称</th>
        <th>企业名称</th>
        <th>排序</th>
        <th>品牌介绍</th>
        <th>状态</th>
        <th>操作</th>
    </tr>
      <c:forEach items="${brands}" var="brand" varStatus="status">
          <tr align="center">
              <td>${status.count}</td>
              <td>${brand.brandName}</td>
              <td>${brand.companyName}</td>
              <td>${brand.ordered}</td>
              <td>${brand.description}</td>
              <c:if test="${brand.status == 1}">
                  <td>启用</td>
              </c:if>

               <c:if test="${brand.status != 1}">
                  <td>禁用</td>
              </c:if>
              <td>
                  <a href="/Mvc-Demo/selectByIdServlet?id=${brand.id}">修改

                  </a> <a href="#"> 删除</a>
              </td>
          </tr>
      </c:forEach>
  </table>
</body>
</html>

访问http://localhost:8080/Mvc-Demo/selectAllServlet地址后，
浏览器页面效果
在这里插入图片描述

7.3 添加

具体代码可参考顶部Demo中
1 Mapper方法

@Insert("insert into tb_brand values(null, #{brandName}, #{companyName}," +
        " #{ordered}, #{description}, #{status})")
void add(Brand brand);

2 Service中的方法

public void add(Brand brand) {
   SqlSession sqlSession = factory.openSession();
    BrandMapper mapper = sqlSession.getMapper(BrandMapper.class);

    mapper.add(brand);
    sqlSession.commit();
    sqlSession.close();
}

3 添加页面jsp

<%--
  Created by IntelliJ IDEA.
  User: yl
  Date: 2022/11/30
  Time: 9:16 PM
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>添加品牌</title>
</head>
<body>
    <h3>添加品牌</h3>
    <form action="/Mvc-Demo/addServlet" method="post">
        品牌名称: <input name="brandName"><br> <br>
        企业名称: <input name="companyName"><br> <br>
        品牌排序: <input name="ordered"><br> <br>
        描述信息: <textarea rows="5" cols="20" name="description"></textarea> <br> <br>
        状态:
        <input type="radio" name="status" value="0">禁用
        <input type="radio" name="status" value="1">启用 <br><br>

        <input type="submit" value="提交">
    </form>
</body>
</html>

4 Servlet接口

package com.test.web;

import com.test.pojo.Brand;
import com.test.service.BrandService;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;

@WebServlet("/selectAllServlet")
public class SelectAllServlet extends HttpServlet {
    private BrandService service = new BrandService();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        List<Brand> brands = service.selectAll();
        req.setAttribute("brands", brands);
        req.getRequestDispatcher("/brand.jsp").forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doGet(req, resp);
    }
}

5 列表页面新增

<input type="button" value="新增" id="add"><br>

<script>
    document.getElementById("add").onclick = function () {
        location.href = "/Mvc-Demo/addBrand.jsp"
    }
</script>

请求地址: http://localhost:8080/Mvc-Demo/addBrand.jsp
效果展示
在这里插入图片描述
提交后，商品列表中多了一项

7.4 查询并展示单条数据

1 BrandMapper中添加查询语句

@Select("select * from tb_brand where id = #{id}")
@ResultMap("brandResultMap")
Brand selectById(int id);

2 BrandService添加查询Mapper逻辑

public Brand selectById(int id) {
    SqlSession sqlSession = factory.openSession();
    BrandMapper mapper = sqlSession.getMapper(BrandMapper.class);

    Brand brand = mapper.selectById(id);
    sqlSession.commit();
    sqlSession.close();

    return brand;
}

3 添加查询的Servlet

@WebServlet("/selectByIdServlet")
public class SelectByIdServlet extends HttpServlet {
    private BrandService service = new BrandService();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String id = req.getParameter("id");
        Brand brand = service.selectById(Integer.parseInt(id));
        req.setAttribute("brand", brand);

        req.getRequestDispatcher("/update.jsp").forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doGet(req, resp);
    }
}

4 添加update.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<html>
<head>
    <title>修改品牌信息</title>
</head>
<body>
    <h3>修改品牌信息</h3>
    <form action="/Mvc-Demo/updateServlet" method="post">
        品牌名称: <input name="brandName" value="${brand.brandName}"><br> <br>
        企业名称: <input name="companyName" value="${brand.companyName}"><br> <br>
        品牌排序: <input name="ordered" value="${brand.ordered}"><br> <br>
        描述信息: <textarea rows="5" cols="20" name="description"> ${brand.description} </textarea> <br> <br>
        状态:
        <c:if test="${brand.status == 0}">
            <input type="radio" name="status" value="0" checked>禁用
            <input type="radio" name="status" value="1">启用 <br><br>
        </c:if>

        <c:if test="${brand.status == 1}">
            <input type="radio" name="status" value="0">禁用
            <input type="radio" name="status" value="1" checked>启用 <br><br>
        </c:if>


        <input type="submit" value="提交">
    </form>
</body>
</html>

5 访问http://localhost:8080/Mvc-Demo/selectAllServlet 后，选择任意一项品牌，然后点修改
展示效果:
在这里插入图片描述

7.5 修改

1 添加update sql 语句方法
BrandMapper.java

@Update("update tb_brand set brand_name = #{brandName}, company_name = #{companyName}, ordered = #{ordered}" +
        ", description = #{description}, status = #{status} where id = #{id}")
void update(Brand brand);

2 添加业务方法

public void update(Brand brand) {
    SqlSession sqlSession = factory.openSession();
    BrandMapper mapper = sqlSession.getMapper(BrandMapper.class);

    System.out.println("update brand = " + brand);
    mapper.update(brand);
    sqlSession.commit();
    sqlSession.close();
}

3 updateServlet

WebServlet("/updateServlet")
public class UpdateServlet extends HttpServlet {
	private BrandService service = new BrandService();
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
	   req.setCharacterEncoding("utf-8");
	
	   String id = req.getParameter("id");
	   String brandName = req.getParameter("brandName");
	   String companyName = req.getParameter("companyName");
	   String ordered = req.getParameter("ordered");
	   String description = req.getParameter("description");
	   String status = req.getParameter("status");
	
	   Brand brand = new Brand();
	   brand.setBrandName(brandName);
	   brand.setCompanyName(companyName);
	   brand.setOrdered(Integer.parseInt(ordered));
	   brand.setDescription(description);
	   brand.setStatus(Integer.parseInt(status));
	   brand.setId(Integer.parseInt(id));
	
	   service.update(brand);
	
	   req.getRequestDispatcher("/selectAllServlet").forward(req, resp);
	}
	
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
	    this.doGet(req, resp);
	}
}

4 隐藏id选项，id不可编辑

<input type="hidden" name="id" value="${brand.id}"><br>

在update.jsp页面中，修改内容，提交后，可看到内容被修改了