作者 ： 不良使
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连接数据库无非就是和数据库建立连接，和前面的python连接数据库差不多。
首先在连接Java数据库的时候需要数据库的jar包，这点不同于python连接数据库
mysql的jar包
下面让我们来看看数据库是如何让连接的，是如何完成数据库的一系列操作，增删改查等…
Java连接数据库。
1、注册驱动
Class.forName("com.mysql.jdbc.Driver");2、获取连接
conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/shop", "root", "123456");注意：获取连接很重要，数据的一点点错误都会导致你连接不上数据库。
1、localhost代表本机不用变
2、3306是端口号，即port,一般默认都是3306。
3、root一般指管理员，不用变，直接用即可。
4、"123456"指的是密码，一般大家常用的密码就123、111111、root或者123456几种。
3、获取数据库操作对象
stmt = conn.createStatement();4、//执行sql
String sql = "select id,name,num,price from testmodel_car";rs = stmt.executeQuery(sql);while (rs.next()) {String id = rs.getString("id");String name = rs.getString("name");String num = rs.getString("num");String price = rs.getString("price");System.out.println(id + "," + name + "," + num + "," + price);}} catch (Exception e) {e.printStackTrace();} finally {注意：
1、java"select id,name,num,price from testmodel_car";，从表 testmodel_car中查询 id,name,num,price四列数据
2、rs = stmt.executeQuery(sql);，获取的对象编译一下sql语句
3、String id = rs.getString("id");，取出id那一列的值赋值给id(String id 中的id可以随便换，只要不和其他变量冲突都是可以的)
5、释放资源
if (rs != null) {try {rs.close();} catch (Exception e) {e.printStackTrace();}}if (stmt != null) {try {stmt.close();} catch (Exception e) {e.printStackTrace();}}if (conn != null) {try {conn.close();} catch (Exception e) {e.printStackTrace();}}不释放资源也行，在上面抛出异常也可以解决
6、完整代码
import com.mysql.jdbc.Driver;import java.sql.Connection;import java.sql.DriverManager;import java.sql.ResultSet;import java.sql.Statement;public class Test2 {public static void main(String[] args) {Connection conn = null;Statement stmt = null;ResultSet rs = null;try {//注册驱动Class.forName("com.mysql.jdbc.Driver");//获取连接conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/shop", "root", "123456");//获取数据库操作对象stmt = conn.createStatement();//执行sqlString sql = "select id,name,num,price from testmodel_car";rs = stmt.executeQuery(sql);while (rs.next()) {String id = rs.getString("id");String name = rs.getString("name");String num = rs.getString("num");String price = rs.getString("price");System.out.println(id + "," + name + "," + num + "," + price);}} catch (Exception e) {e.printStackTrace();} finally {//释放资源if (rs != null) {try {rs.close();} catch (Exception e) {e.printStackTrace();}}if (stmt != null) {try {stmt.close();} catch (Exception e) {e.printStackTrace();}}if (conn != null) {try {conn.close();} catch (Exception e) {e.printStackTrace();}}}}}连接数据库——增加操作
add.jsp
<html lang="en"><head><meta charset="UTF-8"><link href="h/favicon2.ico" rel="icon" type="image/x-icon"/><title>添加信息</title><style>body {background-color:#FDF5E6;}</style><style>body {width: 100%;height: 100%;background: url('./https://blog.csdn.net/qq_46906413/article/details/img/olypic.jpg');background-repeat: no-repeat;background-size: 100% 100%;background-attachment: fixed;}</style><style>input{width: 20%;padding: 12px 20px;margin: 8px 0;box-sizing: border-box;border: 1px solid #555;outline: none;}input:focus {background-color: lightblue;}</style><style>#boxshadow {position: relative;-moz-box-shadow: 1px 2px 4px rgba(0, 0, 0,0.5);-webkit-box-shadow: 1px 2px 4px rgba(0, 0, 0, .5);box-shadow: 1px 2px 4px rgba(0, 0, 0, .5);padding: 10px;background:#F0F8FF;border-radius:8px;}/* Make the image fit the box */#boxshadow img {width: 50%;border: 0px solid #8a4419;border-style: inset;}#boxshadow::after {content: '';position: absolute;z-index: -1; /* hide shadow behind image */-webkit-box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);-moz-box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);width: 70%;left: 15%; /* one half of the remaining 30% */height: 100px;bottom: 0;}</style><style>.button {background-color:#337AB7; /* Green */border: none;color: white;padding: 16px 32px;text-align: center;text-decoration: none;display: inline-block;font-size: 16px;margin: 4px 2px;-webkit-transition-duration: 0.4s; /* Safari */transition-duration: 0.4s;cursor: pointer;}.button1 {background-color: white;color: black;border: 2px solid #337AB7;}.button1:hover {background-color: #337AB7;color: white;box-shadow: 0 12px 16px 0 rgba(0,0,0,0.24),0 17px 50px 0 rgba(0,0,0,0.19);}</style></head><body><div style="padding:2px;width:1080px;height:470px;margin-top:10%;margin-left:20%;opacity: 0.8" id="boxshadow"><div><img src="https://blog.csdn.net/qq_46906413/article/details/img/olypic.jpg" style="float:left;margin-top:5%;"></div><div style="float:right;"><form action="add" method="post"><p style="margin-right:200px;">国家名称   <input type="text"name="Country"style="border-radius:8px;width:150px;height:30px;"></p><p>  金牌数     <input type="text" name="Glod_num" style="border-radius:8px;width:150px;height:30px;margin-top:1%"></p><p>  银牌数     <input type="text" name="Silver_num" style="border-radius:8px;width:150px;height:30px;margin-top:1%"></p><p>  铜牌数     <input type="text" name="Copper_num" style="border-radius:8px;width:150px;height:30px;margin-top:1%"></p><p>     id           <input type="text" name="id" style="border-radius:8px;width:150px;height:30px;margin-top:1%"></p><input type="submit" value="保存"/><br/></form></form></div></div></body></html>Add.java
package com.bjpowernode.javaweb.servlet;import com.bjpowernode.javaweb.utils.DButil;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.*;@WebServlet(name = "add", value = "/add")public class Add extends HttpServlet {@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html;charset=UTF-8");request.setCharacterEncoding("UTF-8");//获取信息，前端页面注册的信息String country=request.getParameter("Country");//String logo=request.getParameter("logo");String Glod_num=request.getParameter("Glod_num");String Silver_num=request.getParameter("Silver_num");String Copper_num=request.getParameter("Copper_num");String id =request.getParameter("id");//String Sum=request.getParameter("Sum");//连接数据库，执行insert插入语句Connection conn = null;PreparedStatement ps = null;int count=0;try {//注册驱动try {Class.forName("com.mysql.jdbc.Driver");} catch (ClassNotFoundException e) {e.printStackTrace();}//获取连接conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mofa", "root", "123456");// TODO 2、开启事务 默认开启我们将它关闭（）自动提交机制关闭String sql = "insert ignore into medals_table(Country,Glod_num,Silver_num,Copper_num,id) values (?,?,?,?,?);";ps = conn.prepareStatement(sql);ps.setString(1, country);//ps.setString(2, logo);ps.setString(2, Glod_num);ps.setString(3, Silver_num);ps.setString(4, Copper_num);ps.setString(5, id);//ps.setString(7, Sum);//返回一个值，指的是影响了数据库中多少条记录主要是判断是否大于0就是是否印象数据库count = ps.executeUpdate();}catch (SQLException e){e.printStackTrace();}finally {DButil.close(conn,ps,null);}if(count==1){//增加成功 任然跳到列表页面request.getRequestDispatcher("Take_01").forward(request,response);}else{//删除失败request.getRequestDispatcher("error.jsp").forward(request,response);}}}
增加功能就是通过前端jsp文件text填写数据，然后通过submit提交数据，传递给后端servlet，也就是上面的Add，注意Java文件不要小写，我就没少挨骂
步骤：
1、前端jsp文件text写入通过submit提交
2、通过form表单提交给后端的servel(Add)
3、String country=request.getParameter("Country");通过getParameter取到前端的值然后赋值给country，为后面刷数据做准备
4、后端servlet访问数据库，步骤访问上面第一个标题《Java连接数据库》
5、成功将前端传来的数据刷如数据库
连接数据库——删除操作
package com.bjpowernode.javaweb.servlet;import com.bjpowernode.javaweb.utils.DButil;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.io.PrintWriter;import java.sql.*;@WebServlet(name = "delete", value = "/delete")public class Delete extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {//根据部门编号删除部门//获取部门编号String country = request.getParameter("name");//连接数据库Connection conn = null;PreparedStatement ps = null;ResultSet rs = null;int count=0;try {//注册驱动Class.forName("com.mysql.jdbc.Driver");//获取连接conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mofa", "root", "123456");// TODO 2、开启事务 默认开启我们将它关闭（）自动提交机制关闭conn.setAutoCommit(false);String sql = "delete from medals_table where Country=?";ps = conn.prepareStatement(sql);ps.setString(1,country);//返回一个值，指的是影响了数据库中多少条记录主要是判断是否大于0就是是否印象数据库count= ps.executeUpdate();//事务提交conn.commit();} catch (SQLException e) {if(conn!=null){try {conn.rollback();} catch (SQLException ex) {ex.printStackTrace();}}e.printStackTrace();} catch (ClassNotFoundException e) {e.printStackTrace();}finally {DButil.close(conn,ps,null);}if(count==1){PrintWriter out = response.getWriter();out.println("");out.println("alert('Dear, the deletion is unrecoverable. Are you sure you deleted it？');");out.println("location.href='Take_01'");//跳转到用户列表页面out.println("");////删除成功 任然跳到列表页面//request.getRequestDispatcher("Take_01").forward(request,response);}else{//删除失败request.getRequestDispatcher("error.jsp").forward(request,response);}}}String country = request.getParameter("name");，老规矩，前端取值进行操作
连接数据库——修改操作
edit.jsp
<html><head><title><html><head><title>新增</title></head><body><h1>新增国家奖牌信息</h1><hr color="red" width="100%"><form action="edit" method="post">国家名称 :<input type='text' name='Country' value='"+country+"'/><br/><%--logo :<input type='text' name='logo' value='"+logo+"'/><br/>--%>金牌数 :<input type='text' name='Glod_num' value='"+Glod+"'/><br/>银牌数 :<input type='text' name='Silver_num' value='"+Silver+"'/><br/>铜牌数 :<input type='text' name='Copper_num' value='"+Copper+"'/><br/>id: <input type='text' name='Sum' value='"+sum+"'/><br/>总数 : <input type='text' name='id' value='"+id+"'/><br/><input type="submit" value="保存"/><br/></form></body></html>Edit.java
package com.bjpowernode.javaweb.servlet;import com.bjpowernode.javaweb.utils.DButil;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.io.PrintWriter;import java.sql.*;@WebServlet(name = "edit",value = "/edit")public class Edit extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {String contextPath = request.getContextPath();response.setContentType("text/html;charset=UTF-8");request.setCharacterEncoding("UTF-8");PrintWriter out =response.getWriter();//jsp文档out.println("");out.println("");out.println("");out.println("");out.println("添加信息 ");out.println("");out.println("body {");out.println("background-color:#FDF5E6;");out.println("}");out.println("");out.println("");out.println("body {");out.println("width: 100%;");out.println("height: 100%;");out.println("background: url('./https://blog.csdn.net/qq_46906413/article/details/img/olypic.jpg');");out.println("background-repeat: no-repeat;");out.println("background-size: 100% 100%;");out.println("background-attachment: fixed;");out.println("}");out.println("");out.println("");out.println("input{");out.println("width: 20%;");out.println("padding: 12px 20px;");out.println("margin: 8px 0;");out.println("box-sizing: border-box;");out.println("border: 1px solid #555;");out.println("outline: none;");out.println("}");out.println("");out.println("input:focus {");out.println("background-color: lightblue;");out.println("}");out.println("");out.println("");out.println("#boxshadow {");out.println("position: relative;");out.println("-moz-box-shadow: 1px 2px 4px rgba(0, 0, 0,0.5);");out.println("-webkit-box-shadow: 1px 2px 4px rgba(0, 0, 0, .5);");out.println("box-shadow: 1px 2px 4px rgba(0, 0, 0, .5);");out.println("padding: 10px;");out.println("background:#F0F8FF;");out.println("border-radius:8px;");out.println("}");out.println("");out.println("/* Make the image fit the box */");out.println("#boxshadow img {");out.println("width: 50%;");out.println("border: 0px solid #8a4419;");out.println("border-style: inset;");out.println("}");out.println("");out.println("#boxshadow::after {");out.println("content: '';");out.println("position: absolute;");out.println("z-index: -1; /* hide shadow behind image */");out.println("-webkit-box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);");out.println("-moz-box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);");out.println("box-shadow: 0 15px 20px rgba(0, 0, 0, 0.3);");out.println("width: 70%;");out.println("left: 15%; /* one half of the remaining 30% */");out.println("height: 100px;");out.println("bottom: 0;");out.println("}");out.println("");out.println("");out.println(" .button {");out.println("background-color:#337AB7; /* Green */");out.println("border: none;");out.println("color: white;");out.println("padding: 16px 32px;");out.println("text-align: center;");out.println("text-decoration: none;");out.println("display: inline-block;");out.println("font-size: 16px;");out.println("margin: 4px 2px;");out.println("-webkit-transition-duration: 0.4s; /* Safari */");out.println("transition-duration: 0.4s;");out.println("cursor: pointer;");out.println("}");out.println("");out.println(" .button1 {");out.println("background-color: white;");out.println("color: black;");out.println("border: 2px solid #337AB7;");out.println("}");out.println(" .button1:hover {");out.println("background-color: #337AB7;");out.println("color: white;");out.println("box-shadow: 0 12px 16px 0 rgba(0,0,0,0.24),0 17px 50px 0 rgba(0,0,0,0.19);");out.println("}");out.println("");out.println("");out.println("");out.println("");out.println("");out.println("");out.println("");out.println("
");out.println("");out.println("");out.println("<form action='"+contextPath+"/update' method='post'>");//获取国家名称String country=request.getParameter("name");//连接数据库Connection conn=null;PreparedStatement ps=null;ResultSet rs=null;try {try {Class.forName("com.mysql.jdbc.Driver");} catch (ClassNotFoundException e) {e.printStackTrace();}//获取连接conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mofa", "root", "123456");String sql = "select * from medals_table where Country=?";//编译sql语句ps = conn.prepareStatement(sql);//赋值第一个问好赋值 country 国家名称ps.setString(1, country);//查询返回结果集这个结果集只有一条记录，国家名称是主键，故只有一条数据rs = ps.executeQuery();if(rs.next()){String logo=rs.getString("logo");String Glod = rs.getString("Glod_num");String Silver = rs.getString("Silver_num");String Copper = rs.getString("Copper_num");String sum=rs.getString("Sum");String id=rs.getString("id");System.out.println("你好，你好，你好，齐天大圣！！！");//输出动态网页out.println(""
);out.println("国家名称   <input type='text'name='Country'value='"+country+"' style='border-radius:8px;width:150px;height:30px;'>");out.println("");//out.println("");
//out.println("   logo      ");//out.println("");out.println(""
);out.println("  金牌数     <input type='text' name='Glod_num' value='"+Glod+"' style='border-radius:8px;width:150px;height:30px;margin-top:1%'>");out.println("");out.println("");out.println(""
);out.println("  银牌数     <input type='text' name='Silver_num' value='"+Silver+"' style='border-radius:8px;width:150px;height:30px;margin-top:1%'>");out.println("");out.println(""
);out.println("  铜牌数     <input type='text' name='Copper_num' value='"+Copper+"' style='border-radius:8px;width:150px;height:30px;margin-top:1%'>");out.println("");out.println(""
);out.println("     id           <input type='text' name='id' value='"+id+"' style='border-radius:8px;width:150px;height:30px;margin-top:1%'>");out.println("");}} catch (SQLException e) {e.printStackTrace();}finally {DButil.close(conn,ps,rs);}out.println("
");out.println("");out.println(" ");out.println("");out.println("");out.println("");out.println("");}}Update.java
package com.bjpowernode.javaweb.servlet;import com.bjpowernode.javaweb.utils.DButil;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.*;@WebServlet(name = "update",value = "/update")public class Update extends HttpServlet {@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html;charset=UTF-8");request.setCharacterEncoding("UTF-8");//解决中文乱码问题String country=request.getParameter("Country");String logo=request.getParameter("logo");String Glod_num=request.getParameter("Glod_num");String Silver_num=request.getParameter("Silver_num");String Copper_num=request.getParameter("Copper_num");String id=request.getParameter("id");//连接数据库执行更新操作Connection conn = null;PreparedStatement ps = null;ResultSet rs = null;int count=0;try {try {Class.forName("com.mysql.jdbc.Driver");} catch (ClassNotFoundException e) {e.printStackTrace();}//获取连接conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mofa", "root", "123456");String sql = "update medals_table set Glod_num=?,Silver_num=?,Copper_num=?,id=? where Country=?";//编译sql语句ps = conn.prepareStatement(sql);//ps.setString(1, logo);ps.setString(1, Glod_num);ps.setString(2, Silver_num);ps.setString(3, Copper_num);ps.setString(4, id);ps.setString(5, country);//System.out.println(logo);count=ps.executeUpdate();System.out.println(count);} catch (SQLException e) {e.printStackTrace();} finally {DButil.close(conn, ps, rs);}if(count==1){//更新成功request.getRequestDispatcher("Take_01").forward(request,response);}else{//更新失败request.getRequestDispatcher("error.jsp").forward(request,response);}}}1、前端传输数据给后端edit,edit通过getParameter取到前端值完成后面的操作。
2、为了让修改的时候可以显示修改前的值，我们在Edit.java中写入前端代码。
3、Edit.jav中代码通过form表单把数据传输给Update.java。
4、Update.java通过getParameter取到由Edit.java中传来的值。
5、把取到的值存进数据库，完成刷入操作。
注意：我这里有个bug，就是不能修改国家名称，因为手贱的我为了让数据库看着好看就把国家名设置为主键了，，，，，，，
连接数据库——查询操作
Select.java
package com.bjpowernode.javaweb.servlet;import com.bipowernode.pojo.Olypic;import com.bjpowernode.javaweb.utils.DButil;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.io.PrintWriter;import java.sql.*;import java.util.ArrayList;import java.util.List;@WebServlet(name = "select",value = "/select")public class Select extends HttpServlet {@Overrideprotected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType("text/html;charset=UTF-8");request.setCharacterEncoding("UTF-8");String name = request.getParameter("select_name");System.out.println(name);//连接数据库Connection conn = null;PreparedStatement ps = null;ResultSet rs = null;List<Olypic> olypicList=new ArrayList<>();try {Class.forName("com.mysql.jdbc.Driver");//获取连接conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mofa", "root", "123456");String sql = "select * from medals_table where Country=?";//编译sql语句ps = conn.prepareStatement(sql);//赋值第一个问好赋值 country 国家名称ps.setString(1, name);//查询返回结果集这个结果集只有一条记录，国家名称是主键，故只有一条数据rs = ps.executeQuery();if (rs.next()) {String country = rs.getString("Country");String Glod = rs.getString("Glod_num");String Silver = rs.getString("Silver_num");String Copper = rs.getString("Copper_num");String id=rs.getString("id");String logo=rs.getString("logo");System.out.println("这是数据");System.out.println(country);System.out.println("Glod");// String Sum=rs.getString("Sum");int G = Integer.valueOf(Glod).intValue();int S = Integer.valueOf(Silver).intValue();int C = Integer.valueOf(Copper).intValue();int A=G+S+C;String Sum=String.valueOf(A);//将数据封装程Java对象Olypic olpic=new Olypic();olpic.setCountry(country);olpic.setGlod_num(Glod);olpic.setSilver_num(Silver);olpic.setCopper_num(Copper);olpic.setId(id);olpic.setSum(Sum);olpic.setLogo(logo);// TODO 将数据放入上面定义好的容器olypicList.add(olpic);}} catch (SQLException e) {e.printStackTrace();} catch (ClassNotFoundException e) {e.printStackTrace();} finally {DButil.close(conn, ps, rs);}request.setAttribute("olypicList",olypicList);request.getRequestDispatcher("./take_03.jsp").forward(request,response);}}可以从前端或者后端传值，完成搜索操作。但是一般是要通过前端搜索框输入值，通过form表单传走，后端通过getParameter取值完成一些列操作。
现在的项目都是通过框架，但是 了解传输和运行机制还是十分重要的。
最后介绍一下牛客。Leetcode有的刷题牛客都有，除此之外牛客里面还有招聘（社招和校招）、一些上岸大厂的大佬的面试经验。
牛客是可以伴随一生的编程软件（完全免费），从学校到社会工作，时时刻刻你都可以用到，感兴趣的可以去注册试试可以伴随一生的刷题app
完整的JavaWeb项目
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