public List<Product> findByIddd(String guess) { Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null; List<Product> products = new ArrayList<>(); try { conn = DBHelper.getConnection(); String sql = "select productid, category, cname, ename, image, " + "listprice, unitcost, descn " + "from products where productid = ? "; pstmt = conn.prepareStatement(sql); pstmt.setString(1, guess); rs = pstmt.executeQuery(); while (rs.next()) { Product p = new Product(); p.setProductid(rs.getString("productid")); p.setCategory(rs.getString("category")); p.setCname(rs.getString("cname")); p.setEname(rs.getString("ename")); p.setImage(rs.getString("image")); p.setListprice(rs.getDouble("listprice")); p.setUnitcost(rs.getDouble("unitcost")); p.setDescn(rs.getString("descn")); products.add(p); } } catch (SQLException e) { e.printStackTrace(); } finally { if (rs != null) { try { rs.close(); } catch (SQLException e) { } } if (pstmt != null) { try { pstmt.close(); } catch (SQLException e) { } } if (conn != null) { try { conn.close(); } catch (SQLException e) { } } } return products; }改成返回输出所有包含guess关键字的数据要怎么改

<%@page contentType="text/html" pageEncoding="UTF-8"%><%@ page language="java" import="java.sql."%><!DOCTYPE html><html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>学生信息查询</title></head><body bgcolor="CCCFFF" align="center">
记录条数学号姓名性别年龄体重 <% Connection conn = null; ResultSet rs = null; Statement stmt = null; String sDBDriver = "com.mysql.jdbc.Driver"; String sConnStr = "jdbc:mysql://localhost:3306/student?useUnicode=true&characterEncoding=utf-8"; String username = "root"; String password = "bl"; int count=0; //2 装载驱动程序 try { Class.forName(sDBDriver); } catch (ClassNotFoundException ex) { System.err.println(ex.getMessage()); } try { //3 建立数据库连接 conn = DriverManager.getConnection(sConnStr, username, password); //4 创建Statement对象 stmt = conn.createStatement(); //5 执行sql语句 String sql = "select from stu;"; rs = stmt.executeQuery(sql); //6 处理结果 //6 处理结果 if (rs != null) { count = 0; while (rs.next()) { count++; int id = rs.getInt("id"); String uname = rs.getString("name"); String sex = rs.getString("sex"); int age = rs.getInt("age"); double weight = rs.getDouble("weight"); %> <%=count%> <%=id%> <%=uname%> <%=sex%> <%=age%> <%=weight%> <% } } } catch (SQLException e1) { out.println(e1); } finally { //7 关闭链接 conn.close(); } %> 你要查询的学生数据表中共有<%=count%> 人</body></html>

<tr><td><%=s.getId()%></td><td><%=s.getName()%></td></tr> <% } %> **优化建议**: 实现服务端分页逻辑，在数据库层面限制返回的数据条数。 java public List<Student> getPaginatedStudents(int offset, ...

Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null;

1. Connection conn = null;: 这是一个空的对象引用，表示一个Connection对象。在Java中，用于建立到数据库的连接。当从数据库操作完成后，需要将其关闭以释放系统资源。 2. PreparedStatement pstmt = null;: ...

// 查询所有商品信息 public List findAllProducts() { // TODO: 实现查询所有商品信息的代码 Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null; List productList = new ArrayList<>(); try { conn = DBUtil.getConnection(); String sql = "SELECT * FROM product"; pstmt = conn.prepareStatement(sql); rs = pstmt.executeQuery(); while (rs.next()) { Product product = new Product(); product.setId(rs.getInt("id")); product.setName(rs.getString("name")); product.setPrice(rs.getDouble("price")); product.setDescription(rs.getString("description")); productList.add(product); } } catch (SQLException e) { e.printStackTrace(); } finally { DBUtil.close(conn, pstmt, rs); } return productList; }，product数据库表有pid，pname，categpry表有cid和cname，第二列为商品id，即pid，第三列为商品名称，即pname，第四列为类别名称，即cname

不过我可以看出这段代码是用于查询所有商品信息，并且将查询结果封装为一个 Product 对象的 List。同时，该表的名称为 product，包含 id、name、price、description 四个字段。另外，你提到了一个 category 表，但并...

找bugif (name.trim().equals(ZYName.trim())) { try { PreparedStatement pstmt2 = conn.prepareStatement(sql2); ResultSet rs2 = pstmt2.executeQuery(); while(rs2.next()){ String A = rs2.getString("A"); if (name.trim().equals(A.trim())) { %> <%=rs2.getString("A")%> <%=rs2.getString("B")%> <%=rs2.getString("C")%> <%=rs2.getString("D")%> <%=rs2.getString("E")%> <%=rs2.getString("F")%> <% } } pstmt2.setString(1,name); count = pstmt2.executeUpdate(); pstmt2.close(); } catch (SQLException e) { e.printStackTrace(); } break; }else{ System.out.print("查无此药"); }

在这段代码中，有一个潜在的 bug，即当 name.trim().equals(ZYName.trim()) 不成立时，只是简单地输出了一条信息查无此药，但并没有终止程序继续执行。这可能会导致程序继续执行下去，执行不必要的 SQL 查询，...

public List<Student> getStudentById(String id) throws SQLException { List<Student> list = new ArrayList<>(); String sql = "SELECT * FROM students WHERE id = ?"; try (Connection conn = DriverManager.getConnection(URL, USER, PASSWORD); PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, id); try (ResultSet rs = pstmt.executeQuery()) { while (rs.next()) { Student s = new Student( rs.getString("id"), rs.getString("name"), rs.getInt("age"), rs.getString("sex") ); list.add(s);增加模糊查询

1. 方法定义：public List<Student> getStudentById(String id) - 返回包含Student对象的List集合 - 通过传入的id参数进行查询 - 声明抛出SQLException 2. SQL语句：SELECT * FROM students WHERE id = ? -...

package com.student.dao; import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import java.util.ArrayList; import java.util.List; import com.student.entity.Student; import com.student.util.DBUtil; public class StudentDAO { // 插入学生 public void insertStudent(Student student) { String sql = "INSERT INTO students (sid, name, age, score) VALUES (?, ?, ?, ?)"; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, student.getSid()); pstmt.setString(2, student.getName()); pstmt.setInt(3, student.getAge()); pstmt.setDouble(4, student.getScore()); pstmt.executeUpdate(); } catch (SQLException e) { System.err.println("插入学生信息时出现错误: " + e.getMessage()); } } // 获取所有学生 public List<Student> getAllStudents() { List<Student> students = new ArrayList<>(); String sql = "SELECT * FROM students"; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql); ResultSet rs = pstmt.executeQuery()) { while (rs.next()) { students.add(extractStudentFromResultSet(rs)); } } catch (SQLException e) { e.printStackTrace(); } return students; } // 删除学生（修改方法名） public void deleteStudent(String sid) { // 这里修改了方法名 String sql = "DELETE FROM students WHERE sid = ?"; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, sid); int rowsAffected = pstmt.executeUpdate(); if (rowsAffected == 0) { System.err.println("未找到学号为 " + sid + " 的学生"); } } catch (SQLException e) { e.printStackTrace(); } } // 更新学生信息 public void updateStudent(Student student) { String sql = "UPDATE students SET name = ?, age = ?, score = ? WHERE sid = ?"; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, student.getName()); pstmt.setInt(2, student.getAge()); pstmt.setDouble(3, student.getScore()); pstmt.setString(4, student.getSid()); int rowsAffected = pstmt.executeUpdate(); if (rowsAffected == 0) { System.err.println("未找到学号为 " + student.getSid() + " 的学生"); } } catch (SQLException e) { e.printStackTrace(); } } // 搜索学生 public List<Student> searchStudents(String keyword) { List<Student> students = new ArrayList<>(); String sql = "SELECT * FROM students WHERE sid LIKE ? OR name LIKE ?"; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql)) { String searchPattern = "%" + keyword + "%"; pstmt.setString(1, searchPattern); pstmt.setString(2, searchPattern); try (ResultSet rs = pstmt.executeQuery()) { while (rs.next()) { students.add(extractStudentFromResultSet(rs)); } } } catch (SQLException e) { e.printStackTrace(); } return students; } // 排序学生 public List<Student> getStudentsOrderBy(String field) { List<Student> students = new ArrayList<>(); String validField = validateSortField(field); String sql = "SELECT * FROM students ORDER BY " + validField; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql); ResultSet rs = pstmt.executeQuery()) { while (rs.next()) { students.add(extractStudentFromResultSet(rs)); } } catch (SQLException e) { e.printStackTrace(); } return students; } // 根据ID获取学生 public Student getStudentById(String sid) { String sql = "SELECT * FROM students WHERE sid = ?"; try (Connection conn = DBUtil.getConnection(); PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, sid); try (ResultSet rs = pstmt.executeQuery()) { if (rs.next()) { return extractStudentFromResultSet(rs); } } } catch (SQLException e) { e.printStackTrace(); } return null; } // 验证排序字段（防止SQL注入） private String validateSortField(String field) { String[] validFields = {"sid", "name", "age", "score"}; for (String valid : validFields) { if (valid.equalsIgnoreCase(field)) { return valid; } } return "sid"; // 默认值 } // 从ResultSet提取学生对象（复用代码） private Student extractStudentFromResultSet(ResultSet rs) throws SQLException { String sid = rs.getString("sid"); String name = rs.getString("name"); int age = rs.getInt("age"); double score = rs.getDouble("score"); return new Student(sid, name, age, score); } } 根据上面的给我一个可运行的完整代码

<think>我们已经在之前的代码中扩展了StudentGUI，现在需要根据提供的StudentDAO类来完善整个项目。注意：在StudentDAO中，我们已经有了一些基本的方法，但为了支持新功能（录入成绩、计算平均分、生成排名），我们...

package book.library.dao; import book.library.beans.BookBeans; import book.library.util.DBCon; import java.sql.; import java.util.ArrayList; import java.util.List; public class BookDAO { private Connection conn; public BookDAO() throws SQLException { this.conn = DBCon.getConnection(); } public void addBook(BookBeans book) throws SQLException { String sql = "INSERT INTO books (title, price, author, publisher, stock, modifiedStock) VALUES (?, ?, ?, ?, ?, ?)"; try (PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, book.getTitle()); pstmt.setDouble(2, book.getPrice()); pstmt.setString(3, book.getAuthor()); pstmt.setString(4, book.getPublisher()); pstmt.setInt(5, book.getStock()); pstmt.executeUpdate(); } } public void updateBook(BookBeans book) throws SQLException { String sql = "UPDATE books SET title = ?, price = ?, author = ?, publisher = ?, stock = ? WHERE id = ?"; try (PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setString(1, book.getTitle()); pstmt.setDouble(2, book.getPrice()); pstmt.setString(3, book.getAuthor()); pstmt.setString(4, book.getPublisher()); pstmt.setInt(5, book.getStock()); pstmt.setInt(6, book.getId()); pstmt.executeUpdate(); } } public void deleteBook(int id) throws SQLException { String sql = "DELETE FROM books WHERE id = ?"; try (PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setInt(1, id); pstmt.executeUpdate(); } } public List<BookBeans> getAllBooks() throws SQLException { List<BookBeans> books = new ArrayList<>(); String sql = "SELECT FROM books"; try (Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery(sql)) { while (rs.next()) { BookBeans book = new BookBeans(); book.setId(rs.getInt("id")); book.setTitle(rs.getString("title")); book.setPrice(rs.getDouble("price")); book.setAuthor(rs.getString("author")); book.setPublisher(rs.getString("publisher")); book.setStock(rs.getInt("stock")); books.add(book); } } return books; } } 这是后台代码，请修改。<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%> <%@ taglib prefix="c" uri="https://jakarta.ee/xml/ns/jakartaee/jstl/core" %> <!DOCTYPE html> <html> <head> <title>图书信息展示</title> </head> <body> 图书信息图书名称图书价格图书作者出版社存放数量修改数量删除 <c:forEach var="book" items="${books}"> ${book.title} ${book.price} ${book.author} ${book.publisher} ${book.stock} 修改 删除 </c:forEach> 添加图书信息 </body> </html>这是前台代码，请修改。

try (PreparedStatement pstmt = conn.prepareStatement(sql)) { pstmt.setInt(1, newStock); pstmt.setInt(2, id); pstmt.executeUpdate(); } } 2. 前端JSP页面修改（添加库存修改表单）： jsp <%@ ...

package com.exam.dao; import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.util.ArrayList; import java.util.Iterator; import java.util.List; import com.exam.bean.User; /* * 用于登录验证的数据库 / public class JDBCTest { //数据库查询语句 public static String findsql = "selectfrom tb_user"; //数据库查询方法 public static List<User> Find() { //初始化查询结果集 ResultSet rs = null; PreparedStatement pstmt = null; Connection conn = null; List<User> list = new ArrayList(); int id = 0; String username, password = null; try { conn = DBUtil.getconn(); pstmt = conn.prepareStatement(findsql); rs = pstmt.executeQuery(); while (rs.next()) { id = rs.getInt(1); username = rs.getString(2); password = rs.getString(3); User user = new User(id, username, password); list.add(user); } } catch (Exception e) { e.printStackTrace(); } finally { DBUtil.closeAll(rs, pstmt, conn); } return list; } // 测试是否能够拿到数据 public static void main(String[] args) { List<User> list = new ArrayList(); list = Find(); User user = new User(); Iterator<User> it = list.iterator(); while (it.hasNext()) { user = it.next(); System.out.println(user.getId()); System.out.println(user.getUsername()); System.out.println(user.getPassword()); } } }填写注释

public static List<User> Find() { ResultSet rs = null; PreparedStatement pstmt = null; Connection conn = null; List<User> list = new ArrayList(); int id = 0; String username, password = null; /...

<%@ page language="java" import="java.util." pageEncoding="utf-8"%> <%@page import="com.util.SQLConn"%> <%@page import="java.sql." %> <%@page import="java.sql.Statement" %> <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %> <%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %> <html> <body> <% // 获取前端数据 String username = request.getParameter("username"); String usernum = request.getParameter("usernum"); String userpwd = request.getParameter("userpwd"); // 执行数据库操作 Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null; int count = 0; try { conn = SQLConn.openDB(); String sql = "INSERT INTO User_ZC(UserID, Name, Code, Power) VALUES (?, ?, ?, ?)"; pstmt = conn.prepareStatement(sql); pstmt.setString(1, usernum); pstmt.setString(2, username); pstmt.setString(3, userpwd); pstmt.setInt(4, 1); count = pstmt.executeUpdate(); } catch (SQLException e) { e.printStackTrace(); } finally { SQLConn.closeDB(conn, pstmt, rs); } // 根据执行结果进行相应的操作 if (count > 0) { request.getRequestDispatcher("ZCS.jsp").forward(request,response); } else { request.getRequestDispatcher("ZCB.jsp").forward(request,response); } %> </body> </html>,优化代码

PreparedStatement pstmt = conn.prepareStatement(INSERT_SQL)) { pstmt.setString(1, usernum); pstmt.setString(2, username); pstmt.setString(3, userpwd); pstmt.setInt(4, 1); count = pstmt....

<%@ page import="util.DbConnet" %> <%@ page import="java.sql.ResultSet" %> <%@ page contentType="text/html;charset=UTF-8" language="java" %> <% //获取用户输入的查询内容 String username = request.getParameter("appliance_type"); if(username==null) username=""; String realname = request.getParameter("appliance_name"); if(realname==null) realname=""; //1.数据总数 String sql = "select count() as total from living_room_appliances " + "where appliance_type like ? and appliance_name like ?;"; Object[] params = new Object[]{ "%"+username+"%","%"+realname+"%" }; ResultSet rs = DbConnet.select(sql, params); rs.next(); int total = rs.getInt("total"); //2.每页显示的行数 int pageSize = 5; //3.总页数 double result = (double)total/pageSize; //向上取整：只要数值带有有效的小数，舍去小数，整数位加一 int pageTotal = (int) Math.ceil(result); //4.当前页码 String pageNoStr = request.getParameter("pageNo"); pageNoStr = pageNoStr==null?"1":pageNoStr; int pageNo = Integer.parseInt(pageNoStr); //获取用户表中的数据，显示出来 sql = "select from living_room_appliances " + "where appliance_type like ? and appliance_name like ? " + "limit ?,?;"; int start = (pageNo - 1) * pageSize;//（当前页码-1）*每页显示的行数 params = new Object[]{ "%"+username+"%","%"+realname+"%",start,pageSize }; rs = DbConnet.select(sql, params); %> <html> <head> <title>用户列表</title> </head> <body> <%--搜索区域 S--%> <form> <label for="username">账号：</label> <input type="text" id="username" name="username" value="<%=username%>"> <label for="username">姓名：</label> <input type="text" id="realname" name="realname" value="<%=realname%>"> <button id="btnSearch" class="primary" type="button">查询</button> <button id="btnReset" type="button">重置</button> </form> <%--搜索区域 E--%> <%--按钮区域 S--%> <button id="btnAdd" class="primary" type="button">新增</button> <%--按钮区域 E--%> <%--表格区域 S--%> 编号账号姓名操作 <% while (rs.next()){ %> <%=rs.getString("appliance_id")%> <%=rs.getString("appliance_type")%> <%=rs.getString("appliance_name")%> <button data-id="<%=rs.getString("id")%>" name="btnEdit" class="primary" type="button">编辑</button> <button data-id="<%=rs.getString("id")%>" name="btnDelete" class="danger" type="button">删除</button> <% } %> <%--表格区域 E--%> <%--页码区域 S--%> 共<%=total%>条数据/每页<%=pageSize%>条 <% if(pageNo>1){%> 首页上一页 <% } %> <%-- 1--%> <% for (int i=1;i<=pageTotal;i++){%> " class="page"> <%=i%> <% } %> <% if(pageTotal>pageNo){%> 下一页尾页 <% } %> <%--页码区域 E--%> <script src="../js/jquery-3.5.1.min.js"></script> <script src="../js/common.js"></script> <script> //绑定搜索按钮的点击事件 $('#btnSearch').on('click', function () { //获取搜索框中的内容：账号、姓名 let username = $('#username').val(); let realname = $('#realname').val(); window.location.href="list.jsp?appliance_type=" + username + "&realname=" + realname; }); //绑定重置按钮的点击事件 $('#btnReset').on('click', function () { window.location.href="list.jsp"; }); //绑定新增按钮的点击事件 $('#btnAdd').on('click', function () { window.location.href="add.jsp"; }); //绑定行内的编辑按钮点击事件 $('button[name=btnEdit]').on('click', function () { let id = $(this).attr('data-id');//从当前点击的按钮身上获取data-id的值 window.location.href = 'edit.jsp?id='+id; }); //绑定行内的删除按钮点击事件 $('button[name=btnDelete]').on('click', function () { if(confirm("确定要删除吗？")) { //获取删除按钮所在行的编号(id) let id = $(this).attr('data-id');//从当前点击的按钮身上获取data-id的值 //无刷新方式提交删除请求 postAction('/user/delete', {id: id}, function (res) { alert(res.msg); if (res.result) window.location.href = res.url; }); } }); </script> </body> </html>

i <= pageTotal; i++) { %> <li class="<%=(pageNo == i ? "active" : "")%> page"> <a href="list.jsp?appliance_type=<%=applianceType%>&appliance_name=<%=applianceName%>&pageNo=<%=i%>"><%=i%></a> </li>...

<!DOCTYPE html> <html> <head> <title>用户登录结果</title> </head> <body>

<% String yonghuming = request.getParameter("username"); String yonghumima = request.getParameter("password"); Connection conn = DBUtil.getConnection(); // System.out.println(conn.isClosed()); String sql = "SELECT name, pass FROM account WHERE name = ? ;"; PreparedStatement pstmt = conn.prepareStatement(sql); pstmt.setString(1, yonghuming); ResultSet rs = pstmt.executeQuery(); // 判断查询的结果集是否为空集 boolean isEmpty = false; if (rs.next() == false) { isEmpty = true; out.println("用户名不存在！"); } else { if (!yonghumima.equals(rs.getString("pass"))) { out.println("密码错误！"); } else { out.println("登录成功！"); } } DBUtil.closeConnection(conn); %> </body> </html> 根据该代码使登录成功直接跳转login.jsp

<think>好的，我现在需要帮用户解决两个问题：一是如何在登录成功后跳转到login.jsp，二是避免主键冲突的错误。首先，用户提到登录成功后跳转到login.jsp，这看起来有点矛盾，因为通常登录成功后应该跳转到欢迎页面...

<% //连接数据库 String driver = "com.mysql.jdbc.Driver"; String url = "jdbc:mysql://localhost:3306/votedb?serverTimezone=UTC&characterEncoding=UTF-8"; String username = "root"; String password = "123456"; Connection conn = null; Statement stmt = null; ResultSet rs = null; try { Class.forName(driver); conn = DriverManager.getConnection(url, username, password); stmt = conn.createStatement(); rs = stmt.executeQuery("SELECT * FROM vote"); while(rs.next()){ String title = rs.getString("title"); %> <input type="radio" name="option" value="<%=title%>"><%=title%>
<% } } catch (Exception e) { e.printStackTrace(); } finally { //关闭连接 try { if (rs != null) { rs.close(); } if (stmt != null) { stmt.close(); } if (conn != null) { conn.close(); } } catch (SQLException e) { e.printStackTrace(); } } %> 提交投票选中后数据库数据变化

PreparedStatement pstmt = null; try { Class.forName(driver); conn = DriverManager.getConnection(url, username, password); pstmt = conn.prepareStatement("INSERT INTO vote_result(vote_id, option_id...

帮我优化下面这段代码：package com.ethan.dao; import java.sql.Connection; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.util.ArrayList; import java.util.List; import com.ethan.model.Student; import com.ethan.util.DBUtil; public class StudentDao { private static Connection conn = DBUtil.getConn(); private static PreparedStatement pstmt = null; private static ResultSet rs = null; public static List<Student> findAllStudent() throws Exception { String sql = "select * from student"; pstmt = conn.prepareStatement(sql); ResultSet rs = pstmt.executeQuery(); List<Student> list = new ArrayList<Student>(); while (rs.next()) { Student student = new Student(); student.setUserId(rs.getInt("userId")); student.setUserName(rs.getString("userName")); student.setSex(rs.getString("sex")); student.setBirthYear(rs.getString("birthYear")); student.setGrade(rs.getString("grade")); student.setCollegeName(rs.getString("collegeName")); list.add(student); } return list; } }

public static List<Student> findAllStudent() throws Exception { String sql = "select * from student"; try (Connection conn = DBUtil.getConn(); PreparedStatement pstmt = conn.prepareStatement(sql);...

public boolean judge(String account1, String password1) throws Exception { boolean f = false; Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null; try { // 更新数据库连接信息 Class.forName("com.mysql.cj.jdbc.Driver"); String url = "jdbc:mysql://localhost:3306/bookmanagement"; String username = "root"; String password = "password"; // 连接数据库 conn = DriverManager.getConnection(url, username, password); String sql = "SELECT * FROM bookmanagement.admin WHERE account=? AND password=?"; pstmt = conn.prepareStatement(sql); pstmt.setString(1, account1); pstmt.setString(2, password1); rs = pstmt.executeQuery(); if (rs.next()) { f = true; } } finally { // 关闭数据库连接和相关资源 if (rs != null) { rs.close(); } if (pstmt != null) { pstmt.close(); } if (conn != null) { conn.close(); } }

它使用 JDBC API 来连接 MySQL 数据库，并使用 PreparedStatement 来执行 SQL 查询语句，查询指定账号和密码是否存在于数据库表中。如果存在，则返回 true，否则返回 false。在最后，程序会关闭连接和相关资源，释放...

请解释这段代码的意思：public int[] select(String username){ Connection conn = null; PreparedStatement pstmt = null; ResultSet rs = null; int[] result= null; try { int grift,level,step; conn = JDBCUtils.getConnection(); String sql = "select grift,lever,step from user where id= ? "; pstmt = conn.prepareStatement(sql); pstmt.setString(1,username); rs = pstmt.executeQuery(); result=new int[3]; while (rs.next()){ grift=rs.getInt("grift"); level=rs.getInt("lever"); step=rs.getInt("step"); result[0]=grift; result[1]=level; result[2]=step; } return result; } catch (SQLException e) { e.printStackTrace(); } finally { JDBCUtils.close(rs, pstmt, conn); } return result; }

2. 获取数据库的连接对象conn，并创建一个PreparedStatement对象pstmt和一个ResultSet对象rs用于执行SQL语句和获取查询结果。 3. 定义SQL语句，这里是查询用户信息的SQL语句：select grift,lever,step from user ...

优化这段代码：package com.ethan.dao; import java.awt.List; import java.sql.Connection; import java.sql.ResultSet; import java.util.ArrayList; import com.ethan.model.; import com.ethan.util.DBUtil; import com.mysql.jdbc.PreparedStatement; public class StudentDao { private static Connection conn = DBUtil.getConn(); private static PreparedStatement pstmt = null; private static ResultSet rs = null; public static ArrayList<Student> findAllStuden() throws Exception { String sql = "select from student"; pstmt = (PreparedStatement) conn.prepareStatement(sql); ResultSet rs = pstmt.executeQuery(); ArrayList<Student> list = new ArrayList<Student>(); while (rs.next()) { Student student = new Student(); student.setUserId(Integer.parseInt(rs.getString("userId"))); student.setUserName(rs.getString("userName")); student.setSex(rs.getString("sex")); student.setBirthYear(rs.getString("birthYear")); student.setGrade(rs.getString("grade")); student.setCollegeName(rs.getString("collegeName")); list.add(student); } return list; } }

public static List<Student> findAllStudent() throws Exception { String sql = "SELECT * FROM student"; try (Connection conn = DBUtil.getConn(); PreparedStatement pstmt = conn.prepareStatement(sql);...

<%@ page language="java" import="java.util., java.sql." pageEncoding="utf-8"%> <%@page import="com.util.SQLConn"%> <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %> <%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>登陆跳转servlet</title> </head> <body> <% String usernum = request.getParameter("usernum"); String userpwd = request.getParameter("userpwd"); Connection conn = SQLConn.openDB(); // 使用PreparedStatment进行预编译 PreparedStatement pstmt = conn.prepareStatement("select UserId, Code,Login from User_ZC where UserId=? and Code=? and Login=0"); pstmt.setString(1, usernum); pstmt.setString(2, userpwd); ResultSet rs = pstmt.executeQuery(); boolean v = rs.next(); if(v){ final String UPDATE_SQL = "update User_ZC set Login=? where UserId=?"; int count = 0; try { Connection connn = SQLConn.openDB(); PreparedStatement pstmte = conn.prepareStatement(UPDATE_SQL); { pstmt.setInt(1, 1); pstmt.setString(2, usernum); count = pstmt.executeUpdate(); } } catch (SQLException e) { e.printStackTrace(); } if (count > 0){ request.getRequestDispatcher("ZCS.jsp").forward(request, response); }else{System.out.print("登陆失败");} } else{ request.getRequestDispatcher("ZCB.jsp").forward(request, response); } %> </body> </html>找bug

在第24行，应该使用已经预编译好的 PreparedStatement 对象 pstmte 而不是重新使用 conn.prepareStatement() 方法创建一个新的 PreparedStatement 对象。正确的代码应该是： pstmte.setInt(1, 1); pstmte....

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