引入Dao - 技术栈

1.crm和数据库的结合

我们先前实现的crm项目的数据都是自定义的而非数据库获取因此现在我们应该实现crm和数据库的集成

ListServlet.java
- doPost方法中在处理异常的选项中并没有发现throws方式而只有try-catch方式这是因为子类throws的异常必须和父类throws异常一致或者是父类throws异常的子类型异常显然子类throws异常超出了父类throws异常的范围
- ResultSet属于资源需要关闭以释放资源所以我们需要通过try-with-resources对其自动关闭
- 我们在写sql语句的时候尽量写明字段名以代替*
- 转发操作可以保证attribute的共享具体是为了在ListServlet和list.jsp之间进行共享重定向则不然并且我们的转发操作一定不要忽略forward方法

java 复制代码

package com.axihh.servlet;

import com.axihh.bean.Customer;

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.sql.*;
import java.util.ArrayList;
import java.util.List;

@WebServlet("/list")
public class ListServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 通过变量保存url、用户名、密码
        String url = "jdbc:mysql://localhost:3306/crm?serverTimezone=UTC";// 由于是新版的驱动包 所以我们需要指定时区
        String username = "root";
        String password = "root";
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            try (Connection conn = DriverManager.getConnection(url, username, password);// 连接数据库(注意获取Connection对象的时候会抛出异常 处理异常的方式却只有一种 即try-catch 并没有throws方式 很奇怪 这是因为我们的doPost方法是继承自HttpServlet的doPost 而父类的该方法限制了子类throws的对象只能是ServletException/IOException或者他们的子类型异常 而当前获取连接操作所抛出的异常超出了这个范围 所以他不允许你通过throws手段对异常进行处理 只能通过try-catch进行处理)
                 PreparedStatement pstmt = conn.prepareStatement("SELECT id, name, age, height FROM customer");// 创建语句(注意创建语句需要同时传入要执行的sql语句 同时他属于资源 所以也要放置在try-with-resources的()中 sql语句中的*尽量写清楚具体的字段名)
                 ResultSet rs = pstmt.executeQuery())// 执行sql语句 将sql语句的返回值用ResultSet进行保存)
            {
                // 定义一个集合 用于存放用户bean
                List<Customer> customers = new ArrayList<>();
                // 循环遍历数据库中的用户 并且将用户存入集合中
                while (rs.next()) {
                    // 创建一个bean实例 用于存放当前用户的字段
                    Customer customer = new Customer();
                    Integer id = rs.getInt("id");
                    String name = rs.getString("name");
                    Integer age = rs.getInt("age");
                    Double height = rs.getDouble("height");
                    customer.setId(id);
                    customer.setName(name);
                    customer.setAge(age);
                    customer.setHeight(height);
                    customers.add(customer);
                }
                // 将集合存放到attribute中
                request.setAttribute("customers", customers);
                // 转发到list.jsp中
                request.getRequestDispatcher("page/list.jsp").forward(request, response);
            }
        }catch (Exception e) {
            e.printStackTrace();
        }

}

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

list.jsp

html 复制代码

<%--
  Created by IntelliJ IDEA.
  User: 19938
  Date: 2024/5/25
  Time: 9:51
  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>
    <style>
        th, td {
            border: 1px solid red;
        }
    </style>
</head>
<body>
<a href="page/add.html">添加</a>
<table>
    <thead>
        <tr>
            <th>id</th>
            <th>姓名</th>
            <th>年龄</th>
            <th>身高</th>
        </tr>
    </thead>
    <tbody>
        <%-- 我们在获取存储在attribute中的customers时 我们应该通过el表达式去获取 具体的格式就是${} 而本质上就是在调用getAttribute --%>
        <c:forEach items="${customers}" var="customer">
            <tr>
                <%-- 我们以下的el表达式的本质是调用getter获取属性 --%>
                <td>${customer.id}</td>
                <td>${customer.name}</td>
                <td>${customer.age}</td>
                <td>${customer.height}</td>
            </tr>
        </c:forEach>
    </tbody>
</table>

</body>
</html>

add.html

html 复制代码

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="/crm5/save" method="post">
    <div>姓名 <input type="text" name="name"></div>
    <div>年龄 <input type="text" name="age"></div>
    <div>身高 <input type="text" name="height"></div>
    <div><button>保存</button></div>
</form>
</body>
</html>

SaveServlet

java 复制代码

package com.axihh.servlet;

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.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

@WebServlet("/save")
public class SaveServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 变量保存url username password
        String url = "jdbc:mysql://localhost:3306/crm?serverTimezone=UTC";
        String username = "root";
        String password = "root";
        try{
            // 注册驱动
            Class.forName("com.mysql.cj.jdbc.Driver");
            try(Connection conn = DriverManager.getConnection(url, username, password);
                PreparedStatement pstmt = conn.prepareStatement("INSERT INTO customer(name, age, height) VALUES (?, ?, ?)")){
                // 为pstmt设置参数
                pstmt.setString(1, request.getParameter("name"));
                pstmt.setInt(2, Integer.parseInt(request.getParameter("age")));
                pstmt.setDouble(3, Double.parseDouble(request.getParameter("height")));
                // 执行sql语句
                int i = pstmt.executeUpdate();
                // 重定向到指定的页面
                response.sendRedirect("/crm5/list");
            }
        }catch(Exception e) {
            e.printStackTrace();
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

值得注意的是有些时候我们在启动Tomcat以后会出现资源无法找到的现象这时候有可能是某个文件未及时编译打包到out/artifacts目录中(实际上部署到Tomcat上的项目是存放在out/artifacts中的其中web-inf目录用于存放我们的src文件以及关联的函数库而其他的一些html、jsp文件则存放在web-inf同目录下) 我们可以将out/artifacts目录下的contextPath文件删除然后重启Tomcat以重新生成contextPath文件或者说有更快速的手段即idea->build->build artifacts->指定的contextPath文件->build/clean

2.项目缺陷

对于上述这个项目实现至此仍然存在着许多的问题

数据库访问代码大量重复

解决方法：将数据库访问代码抽取出来放置在Dao类(即Database Access Object 该类用于专门负责数据库的访问)中里面内置save、list等提供调用的方法
需要注意的是我们在进行保存操作的时候如果某个字段以unique修饰说明不能重复我们要对重复添加做出反应即转发到失败页面为此我们可以定义一个error.jsp展示失败信息反之则执行重定向操作
当然我们也可以将常量单独抽取到一个Constant类这边我们就直接将其放置在dao中
CustomerDao

java 复制代码

package com.axihh.dao;

import com.axihh.bean.Customer;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;

public class CustomerDao {
    // 定义私有成员
    private static final String URL = "jdbc:mysql://localhost:3306/crm?serverTimezone=UTC";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "root";
    public boolean save(Customer customer){
        try {
            // 注册驱动
            Class.forName("com.mysql.cj.jdbc.Driver");
            try(// 连接数据库
                Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
                // 创建语句
                PreparedStatement pstmt = conn.prepareStatement("INSERT INTO customer(name, age, height) VALUES (?, ?, ?)")) {
                // 设置参数
                pstmt.setString(1, customer.getName());
                pstmt.setInt(2, customer.getAge());
                pstmt.setDouble(3, customer.getHeight());
                // 执行sql语句
                int a = pstmt.executeUpdate();
                // 如果a的值大于0的话 那么说明dml语句执行成功
                return a > 0;
            }
        }catch(Exception e) {
            // 打印报错信息
            e.printStackTrace();
            // 返回false 表示执行失败
            return false;
        }
    }
    public List<Customer> list() {
        try {
            // 注册驱动
            Class.forName("com.mysql.cj.jdbc.Driver");
            try(// 建立连接
                Connection conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
                // 创建语句
                PreparedStatement pstmt = conn.prepareStatement("SELECT id, name, age, height FROM customer" );
                // 执行sql语句
                ResultSet rs = pstmt.executeQuery()) {
                // 接着将数据库中的用户信息存入到customer bean类中 并且将customer bean类储存到customer集合中
                List<Customer> customers = new ArrayList<>();
                while(rs.next()) {
                    Customer customer = new Customer();
                    customer.setId(rs.getInt("id"));
                    customer.setName(rs.getString("name"));
                    customer.setAge(rs.getInt("age"));
                    customer.setHeight(rs.getDouble("height"));
                    customers.add(customer);
                }
                // 返回用户信息集合
                return customers;
            }
        } catch(Exception e) {
            // 打印报错信息
            e.printStackTrace();
            // 返回空值
            return null;
        }
    }
}

ListServlet

java 复制代码

package com.axihh.servlet;

import com.axihh.bean.Customer;
import com.axihh.dao.CustomerDao;

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.awt.*;
import java.io.IOException;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;

@WebServlet("/list")
public class ListServlet extends HttpServlet {
    // 定义一个客户数据库访问类对象
    private final CustomerDao cd = new CustomerDao();
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<Customer> customers = cd.list();
        // 将customers储存到attribute中
        request.setAttribute("customers", customers);
        // 转发
        request.getRequestDispatcher("/page/list.jsp").forward(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

SaveServlet

java 复制代码

package com.axihh.servlet;

import com.axihh.bean.Customer;
import com.axihh.dao.CustomerDao;

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.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

@WebServlet("/save")
public class SaveServlet extends HttpServlet {
    private final CustomerDao cd = new CustomerDao();
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 定义一个Customer bean对象
        Customer customer = new Customer();
        customer.setName(request.getParameter("name"));
        customer.setAge(Integer.parseInt(request.getParameter("age")));
        customer.setHeight(Double.parseDouble(request.getParameter("height")));
        if(cd.save(customer)) {
            // 表明添加成功 执行重定向操作
            response.sendRedirect("/crm5/list");
        }else {
            // 表明添加失败或者出现异常 需要执行转发操作
            request.setAttribute("error", "添加失败 请重新添加");
            request.getRequestDispatcher("/page/error.jsp").forward(request, response);
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

list.jsp

html 复制代码

<%--
  Created by IntelliJ IDEA.
  User: 19938
  Date: 2024/5/25
  Time: 9:51
  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>
    <style>
        th, td {
            border: 1px solid red;
        }
    </style>
</head>
<body>
<a href="page/add.html">添加</a>
<table>
    <thead>
        <tr>
            <th>id</th>
            <th>姓名</th>
            <th>年龄</th>
            <th>身高</th>
        </tr>
    </thead>
    <tbody>
        <%-- 我们在获取存储在attribute中的customers时 我们应该通过el表达式去获取 具体的格式就是${} 而本质上就是在调用getAttribute --%>
        <c:forEach items="${customers}" var="customer">
            <tr>
                <%-- 我们以下的el表达式的本质是调用getter获取属性 --%>
                <td>${customer.id}</td>
                <td>${customer.name}</td>
                <td>${customer.age}</td>
                <td>${customer.height}</td>
            </tr>
        </c:forEach>
    </tbody>
</table>

</body>
</html>

error.jsp

html 复制代码

<%--
  Created by IntelliJ IDEA.
  User: 19938
  Date: 2024/5/25
  Time: 17:05
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<h1>${error}</h1>
</body>
</html>

add.html

html 复制代码

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="/crm5/save" method="post">
    <div>姓名 <input type="text" name="name"></div>
    <div>年龄 <input type="text" name="age"></div>
    <div>身高 <input type="text" name="height"></div>
    <div><button>保存</button></div>
</form>
</body>
</html>

每一次请求都对应着一个Servlet
存在繁杂的异常处理代码