前言

很久以前便决定学习java了，那个时候还不知道J2EE到底是什么。逐渐地走了很远，才发现曾经遇到的、复现的java网站和赛题都只是一知半解。现如今总算是半只脚迈入了javaweb的大门，将成果分享一下，以供后来者参考。
注意：纯servlet项目和jsp项目有一定区别，后续学习若有成果会再次分享。

在此之前

说在前面，想要掌握CURD的基本知识需要一定基础，有关java的集合框架；servlet的部署、调用、request（response）方法；eclipse部署等要有一定了解（相信参考这篇文章学习的你已经掌握了上述知识）。

一，概述

Java2平台包括：标准版（J2SE）、企业版（J2EE）和微缩版（J2ME）三个版。
简而言之，j2EE是一种网站开发标准或者说接口。

二、CURD

备注：表格结构
mysql安装的示例类。这里把city和countrylanguage的外键都删除了，然后为country表新增了一个id列，保留了Name、SurfaceArea、Population这三个字段（String,int,float各一个），其他全部删除。

1介绍

CRUD是常见的页面功能，即我们常说的增删改查
C - Creation 增加
R - Retrieve 查询
U - Update 修改
D - DELETE 删除

备注：实体类一般定义在bean包中，DAO类一般定义在dao包中，Servlet一般定义在servlet包中，便于管理。

2查询并列表显示

准备实体类country

country表有Code、Name、Continent、Region、SurfaceArea、IndepYear、Population、LifeExpectancy、GNP、GNPOld、LocalName、GovernmentForm、HeadOfState、Capital、Code2等列名。
选择name、surfacearea、population三个（string、float、int各一个）作为country类的属性
并且为每一个属性提供public的getter和setter。

package world;
public class country {public int id;public String name;public float surfacearea;public int population;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public float getsurfacearea() {return surfacearea;}public void setsurfacearea(float surfacearea) {this.surfacearea = surfacearea;}public int getpopulation() {return population;}public void setpopulation(int population) {this.population = population;}}

准备一个CountryDAO，提供增加，删除，修改，查询等常规数据库操作方法
注意，查表的结果与字段顺序有关，若表结构更换，以下获取字段的序号都要更换。

package world;import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;import world.country;public class countrydao {public countrydao() {try {Class.forName("com.mysql.jdbc.Driver");} catch (ClassNotFoundException e) {e.printStackTrace();}}public Connection getConnection() throws SQLException {return DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/world?characterEncoding=UTF-8", "root","123456");}public int getTotal() {int total = 0;try (Connection c = getConnection(); Statement s = c.createStatement();) {String sql = "select count(*) from country";ResultSet rs = s.executeQuery(sql);while (rs.next()) {total = rs.getInt(1);}System.out.println("total:" + total);} catch (SQLException e) {e.printStackTrace();}return total;}public void add(country country) {String sql = "insert into country values(null,?,?,?)";try (Connection c = getConnection(); PreparedStatement ps = c.prepareStatement(sql);) {ps.setString(1, country.name);ps.setFloat(2, country.surfacearea);ps.setInt(3, country.population);ps.execute();ResultSet rs = ps.getGeneratedKeys();if (rs.next()) {int id = rs.getInt(1);country.id = id;}} catch (SQLException e) {e.printStackTrace();}}public void update(country country) {String sql = "update country set name= ?, surfacearea = ? , population = ? where id = ?";try (Connection c = getConnection(); PreparedStatement ps = c.prepareStatement(sql);) {ps.setString(1, country.name);ps.setFloat(2, country.surfacearea);ps.setInt(3, country.population);ps.setInt(4, country.id);ps.execute();} catch (SQLException e) {e.printStackTrace();}}public void delete(int id) {try (Connection c = getConnection(); Statement s = c.createStatement();) {String sql = "delete from country where id = " + id;s.execute(sql);} catch (SQLException e) {e.printStackTrace();}}public country get(int id) {//返回值为country类的get方法country country = null;try (Connection c = getConnection(); Statement s = c.createStatement();) {String sql = "select * from country where id = " + id;ResultSet rs = s.executeQuery(sql);if (rs.next()) {country = new country();String name = rs.getString(2);//获取表的第3个列名float surfacearea = rs.getFloat("surfacearea");int population = rs.getInt(4);//获取表的第8个列名country.name = name;country.surfacearea = surfacearea;country.population = population;country.id = id;}} catch (SQLException e) {e.printStackTrace();}return country;}public List<country> list() {return list(0, Short.MAX_VALUE);}public List<country> list(int start, int count) {List<country> countries = new ArrayList<country>();String sql = "select * from country order by id desc limit ?,? ";try (Connection c = getConnection(); PreparedStatement ps = c.prepareStatement(sql);) {ps.setInt(1, start);ps.setInt(2, count);ResultSet rs = ps.executeQuery();while (rs.next()) {country country = new country();int id = rs.getInt(1);String name = rs.getString(2);float surfacearea = rs.getFloat("surfacearea");int population = rs.getInt(4);country.id = id;country.name = name;country.surfacearea = surfacearea;country.population = population;countries.add(country);}} catch (SQLException e) {e.printStackTrace();}return countries;}}

编写 CountryListServlet

做一个Country的维护页面需要一些通用的操作，比如增加，删除，编辑，修改，查询等。
每个不同的操作，都需要一个对应的Servlet，除了做Country之外，还会做到其他的一些表的相关操作，所以好的规范会对将来的维护更有好处。
一般会这样命名，以查询为例 CountryListServlet ： [表][行为]Servlet 这样一种命名规则。
所以对于Country而言就会如此命名：
增加 CountryAddServlet
删除 CountryDeleteServlet
编辑 CountryEditServlet
修改 CountryUpdateServlet
查询 CountryListServlet
在CountryListServlet中，会使用CountryDAO把数据查询出来，然后拼接成一个table用于显示其内容

package world;import java.io.IOException;
import java.util.List;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;
import world.countrydao;public class CountryListServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html; charset=UTF-8");List<country> countries = new countrydao().list();StringBuffer sb = new StringBuffer();sb.append("<table align='center' border='1' cellspacing='0'>\r\n");sb.append("<tr><td>id</td><td>name</td><td>surfacearea</td><td>population</td></tr>\r\n");String trFormat = "<tr><td>%d</td><td>%s</td><td>%f</td><td>%d</td></tr>\r\n";for (country country : countries) {String tr = String.format(trFormat, country.getId(), country.getName(), country.getsurfacearea(), country.getpopulation());sb.append(tr);}sb.append("</table>");response.getWriter().write(sb.toString());}
}

配置web.xml

在web.xml中把路径 listHero映射到HeroListServlet上。

<?xml version="1.0" encoding="UTF-8"?>
<web-app><servlet><servlet-name>HelloServlet</servlet-name><servlet-class>HelloServlet</servlet-class><load-on-startup>10</load-on-startup></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping><servlet><servlet-name>LoginServlet</servlet-name><servlet-class>LoginServlet</servlet-class></servlet><servlet-mapping><servlet-name>LoginServlet</servlet-name><url-pattern>/login</url-pattern></servlet-mapping>   <servlet><servlet-name>RegisterServlet</servlet-name><servlet-class>RegisterServlet</servlet-class></servlet><servlet-mapping><servlet-name>RegisterServlet</servlet-name><url-pattern>/register</url-pattern></servlet-mapping><servlet><servlet-name>CountryListServlet</servlet-name><servlet-class>world.CountryListServlet</servlet-class></servlet><servlet-mapping><servlet-name>CountryListServlet</servlet-name><url-pattern>/listc</url-pattern></servlet-mapping>   </web-app>

为web应用导入mysql-jdbc的jar包

把 mysql 的jar包放在WEB-INF/lib 目录下
放在WEB-INF/lib 下指的是能够web应用中找到对应的class，如果要在eclipse中做调试，还是需要为项目添加该jar才可以。

3增加

准备增加的页面addc.html

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

表示用UTF-8显示中文，同时浏览器也会使用UTF-8编码提交中文
form:
action设置为addHero路径
method设置为post 也是为了提交中文
在web目录下添加

<!DOCTYPE html><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><form action="addc" method="post">国家名称 ： <input type="text" name="name"> <br>国土面积： <input type="text" name="surfacearea"> <br>人口数量： <input type="text" name="population"> <br><input type="submit" value="增加 ">
</form>

编写 CAddServlet

HeroAddServlet 中根据浏览器传过来的参数，创建一个Hero对象。 接着通过HeroDAO把该对象保存到数据库中。
最后使用客户端跳转到listHero查看所有的Hero，就能看到新加入的Hero对象了
request.setCharacterEncoding(“UTF-8”);
表示使用UTF-8的方式获取浏览器传过来的中文

package world;import java.io.IOException;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;
import world.countrydao;public class CAddServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8");country country = new country();country.setName(request.getParameter("name"));country.setsurfacearea(Float.parseFloat(request.getParameter("surfacearea")));country.setpopulation(Integer.parseInt(request.getParameter("population")));new countrydao().add(country);response.sendRedirect("/j2ee/listc");}
}

配置web.xml

    <servlet><servlet-name>HeroAddServlet</servlet-name><servlet-class>servlet.HeroAddServlet</servlet-class></servlet><servlet-mapping><servlet-name>HeroAddServlet</servlet-name><url-pattern>/addHero</url-pattern></servlet-mapping>    

测试

重启tomcat，访问增加页面http://127.0.0.1/addc.html
提交数据，接着客户端跳转到/j2ee/lisc，就可以显示新增加的这条数据了

4删除

修改CountryListServlet（提供delete超链）

修改CountryListServlet，多一个单元格，是一个超链
超链的href属性指向地址 /deleteC?id=242(每条不同的记录id不一样)
可以在左下角的浏览器状态栏里看到

package world;import java.io.IOException;
import java.util.List;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;
import world.countrydao;public class CountryListServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html; charset=UTF-8");List<country> countries = new countrydao().list();StringBuffer sb = new StringBuffer();sb.append("<table align='center' border='1' cellspacing='0'>\r\n");sb.append("<tr><td>id</td><td>name</td><td>surfacearea</td><td>population</td><td>delete</td></tr>\r\n");String trFormat = "<tr><td>%d</td><td>%s</td><td>%f</td><td>%d</td><td><a href='deletec?id=%d'>delete</a></td></tr>\r\n";for (country country : countries) {String tr = String.format(trFormat, country.getId(), country.getName(), country.getsurfacearea(), country.getpopulation(), country.getId());sb.append(tr);}sb.append("</table>");response.getWriter().write(sb.toString());}
}

编写CDeleteServlet

首先获取参数id
然后通过countrydao根据id，删除该对象
然后客户端跳转到 /listc

package world;import java.io.IOException;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;public class CDeleteServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {int id = Integer.parseInt(request.getParameter("id"));new countrydao().delete(id);response.sendRedirect("/j2ee/listc");}
}

配置web.xml

把/deletec指向CDeleteServlet

<servlet><servlet-name>CDeleteServlet</servlet-name><servlet-class>world.CDeleteServlet</servlet-class></servlet><servlet-mapping><servlet-name>CDeleteServlet</servlet-name><url-pattern>/deletec</url-pattern></servlet-mapping>    

5修改

修改CountryListServlet （提供edit超链）

新增加一列 edit，里面放上指向 /edit的超链

package world;import java.io.IOException;
import java.util.List;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;
import world.countrydao;public class CountryListServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html; charset=UTF-8");List<country> countries = new countrydao().list();StringBuffer sb = new StringBuffer();sb.append("<table align='center' border='1' cellspacing='0'>\r\n");sb.append("<tr><td>id</td><td>name</td><td>surfacearea</td><td>population</td><td>edit</td><td>delete</td></tr>\r\n");String trFormat = "<tr><td>%d</td><td>%s</td><td>%f</td><td>%d</td><td><a href='edit?id=%d'>edit</a></td><td><a href='deletec?id=%d'>delete</a></td></tr>\r\n";for (country country : countries) {String tr = String.format(trFormat, country.getId(), country.getName(), country.getsurfacearea(), country.getpopulation(), country.getId(), country.getId());sb.append(tr);}sb.append("</table>");response.getWriter().write(sb.toString());}
}

准备CEditServlet（获取历史数据）

CEditServlet 根据浏览器传过来的id获取一个country对象
然后根据这个对象，准备一个类似add.html的页面，不同之处在于每个输入框都是有值的。
最后还会提供一个type="hidden"的input,用于提交id到路径/update

package world;import java.io.IOException;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;
import world.countrydao;public class CEditServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {int id = Integer.parseInt(request.getParameter("id"));country country = new countrydao().get(id);StringBuffer format = new StringBuffer();response.setContentType("text/html; charset=UTF-8");format.append("<!DOCTYPE html>");format.append("<form action='update' method='post'>");format.append("国家名称： <input type='text' name='name' value='%s' > <br>");format.append("国土面积： <input type='text' name='surfacearea'  value='%f' > <br>");format.append("人口数量： <input type='text' name='population'  value='%d' > <br>");format.append("<input type='hidden' name='id' value='%d'>");format.append("<input type='submit' value='更新'>");format.append("</form>");String html = String.format(format.toString(), country.getName(), country.getsurfacearea(), country.getpopulation(), country.getId());response.getWriter().write(html);}
}

准备CUpdateServlet（修改数据）

package world;import java.io.IOException;import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;import world.country;
import world.countrydao;public class CUpdateServlet extends HttpServlet {protected void service(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8");country country = new country();country.setId(Integer.parseInt(request.getParameter("id")));country.setName(request.getParameter("name"));country.setsurfacearea(Float.parseFloat(request.getParameter("surfacearea")));country.setpopulation(Integer.parseInt(request.getParameter("population")));new countrydao().update(country);response.sendRedirect("/j2ee/listc");}
}

配置web.xml

    <servlet><servlet-name>CEditServlet</servlet-name><servlet-class>world.CEditServlet</servlet-class></servlet><servlet-mapping><servlet-name>CEditServlet</servlet-name><url-pattern>/edit</url-pattern></servlet-mapping><servlet><servlet-name>CUpdateServlet</servlet-name><servlet-class>world.CUpdateServlet</servlet-class></servlet><servlet-mapping><servlet-name>CUpdateServlet</servlet-name><url-pattern>/update</url-pattern></servlet-mapping>

三，总结

以上便是整个CURD案例的全部源代码了，每一步都有详细的介绍，虽然参考了其他的学习案例，但是本测试是根据mysql的案例类专门设计的。测试的过程中，基本把所有的报错都踩了一边，略有感慨，小写一篇文章以分享和记录。