连接数据库的增删改查

Posted 2021-01-25 zhaoxinhui

      首先要完成一个java web连接数据库的增删改查，要有前台的.jsp文件，还要有后台的文件，前台的文件负责产生网页，来直接使进行增删改查，而后台的分为util层，这一层主要负责完成与数据库的连接，代码如下。

package util;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
//连接数据库
public class DButil {
    public static String db_url = "jdbc:mysql://localhost:3306/haohaoxuexi";
    public static String db_user = "root";
    public static String db_pass = "zxh521+.";
    public static Connection getConn () {
        Connection conn = null;
        
        try {
            Class.forName("com.mysql.jdbc.Driver");//加载驱动
            conn = DriverManager.getConnection(db_url, db_user, db_pass);
        } catch (Exception e) {
            e.printStackTrace();
        }
        
        return conn;
    }
    public static void main(String[] args) {
         try
         {
         //第三步：获取连接类实例con，用con创建Statement对象类实例 sql_statement
         Connection con = getConn();
         Statement sql_statement = con.createStatement();
         String query = "select * from tiantianxiangshang";
         ResultSet result = sql_statement.executeQuery(query);
         //显示数据中network表中的内容：
         System.out.println("数据库haohaoxuexi表tiantianxiangshang中的数据如下:");
         System.out.println("------------------------");
         System.out.println("name" + "        " + "teacher" + "        " + "classroom");
         System.out.println("------------------------");
         //对获得的查询结果进行处理，对Result类的对象进行操作
         while (result.next())
         {
         String name = result.getString("name");
         String teacher = result.getString("teacher");
         String classroom = result.getString("classroom");
         //取得数据库中的数据
         System.out.println(" " +name + "       " +teacher+ "      " + classroom);
         
         }
         } catch (SQLException ex) {
             System.err.println("SQLException: " + ex.getMessage());
             }
    }
    /**
     * 关闭连接
     * @param state
     * @param conn
     */
    public static void close (Statement state, Connection conn) {
        if (state != null) {
            try {
                state.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    
    public static void close (ResultSet rs, Statement state, Connection conn) {
        if (rs != null) {
            try {
                rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        
        if (state != null) {
            try {
                state.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
    
}

这个类主要完成数据库的连接，并且把数据库里的内容显示出来。要实现数据库的连接首先要完成驱动，然后完成与conn的连接。如果把数据库比喻成一个小岛，那么数据库的连接就相当于在连接小岛时搭建一座桥，而Connection con = getConn();就是实现搭建桥的过程。

在后台的层中有一层为entity层，这一层建立实体类，里面建立的是数据库中的变量，然后建立set、get方法，并且有无参和有参的构造函数。代码如下。

package entity;

public class course {
    private int id;
    private String name;
    private String teacher;
    private String classroom;
    
    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 String getTeacher() {
        return teacher;
    }
    public void setTeacher(String teacher) {
        this.teacher = teacher;
    }
    public String getClassroom() {
        return classroom;
    }
    public void setClassroom(String classroom) {
        this.classroom = classroom;
    }
    
    public course() {}
    
    public course(int id, String name, String teacher, String classroom) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
        this.classroom = classroom;
    }
    
    public course(String name, String teacher, String classroom) {
        this.name = name;
        this.teacher = teacher;
        this.classroom = classroom;
    }
}

建立dao层，dao层是工具层，这一层主要是进行与数据库之间的操作，搭建后台与数据库之间的桥梁。代码如下。

package dao;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;

import entity.course;
import util.DButil;
//与数据库建立联系
public class coursedao {
public boolean add(course course) {//建立course类的对象
    String sql="insert into tiantianxiangshang(name,teacher,classroom)values(‘"+course.getName()+"‘,‘"+course.getTeacher()+"‘,‘"+course.getClassroom()+"‘)";
            //添加语句，调用course类里面的方法，完成添加
Connection conn=DButil.getConn();//建立桥
Statement state=null;//建立车
Boolean f=false;
int a=0;

try {
    state = conn.createStatement();//桥给车一个可以运输货物的功能
    
    a=state.executeUpdate(sql);//把运输货物的多少赋值给a
} catch (Exception e) {
    e.printStackTrace();
} finally {
    DButil.close(state, conn);//在运输结束后，毁掉桥和车
}

if (a > 0) {
    f = true;
}
return f;
}
public boolean delete (int id) {
    boolean f = false;
    String sql = "delete from course where id=‘" + id + "‘";
    Connection conn = DButil.getConn();
    Statement state = null;
    int a = 0;
    
    try {
        state = conn.createStatement();
        a = state.executeUpdate(sql);
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DButil.close(state, conn);
    }
    
    if (a > 0) {
        f = true;
    }
    return f;
}

public boolean update(course course) {
    String sql = "update course set name=‘" + course.getName() + "‘, teacher=‘" + course.getTeacher() + "‘, classroom=‘" + course.getClassroom()
        + "‘ where id=‘" + course.getId() + "‘";
    Connection conn = DButil.getConn();
    Statement state = null;
    boolean f = false;
    int a = 0;

    try {
        state = conn.createStatement();
        a = state.executeUpdate(sql);
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DButil.close(state, conn);
    }
    
    if (a > 0) {
        f = true;
    }
    return f;
}

public boolean name(String name) {
    Boolean flag=false;
    String sql="select from tiantianxiangshang where name=‘"+name+"‘";
    Connection conn=DButil.getConn();//建立桥
    Statement state=null;//建立车
    ResultSet rs = null;
    
    try {
        state = conn.createStatement();
        rs = state.executeQuery(sql);
        while (rs.next()) {
            flag = true;
        }
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DButil.close(rs, state, conn);
    }
    return flag;
}
public course getCourseById(int id) {
    String sql = "select * from course where id =‘" + id + "‘";
    Connection conn = DButil.getConn();
    Statement state = null;
    ResultSet rs = null;
    course course = null;
    
    try {
        state = conn.createStatement();
        rs = state.executeQuery(sql);
        while (rs.next()) {
            String name = rs.getString("name");
            String teacher = rs.getString("teacher");
            String classroom = rs.getString("classroom");
            course = new course(id, name, teacher, classroom);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        DButil.close(rs, state, conn);
    }
    
    return course;
}

public course getCourseByName(String name) {
    String sql = "select * from course where name =‘" + name + "‘";
    Connection conn = DButil.getConn();
    Statement state = null;
    ResultSet rs = null;
    course course = null;
    
    try {
        state = conn.createStatement();
        rs = state.executeQuery(sql);
        while (rs.next()) {
            int id = rs.getInt("id");
            String teacher = rs.getString("teacher");
            String classroom = rs.getString("classroom");
            course = new course(id, name, teacher, classroom);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        DButil.close(rs, state, conn);
    }
    
    return course;
}

public List<course> search(String name, String teacher, String classroom) {
    String sql = "select * from course where ";
    if (name != "") {
        sql += "name like ‘%" + name + "%‘";
    }
    if (teacher != "") {
        sql += "teacher like ‘%" + teacher + "%‘";
    }
    if (classroom != "") {
        sql += "classroom like ‘%" + classroom + "%‘";
    }
    List<course> list = new ArrayList<>();
    Connection conn = DButil.getConn();
    Statement state = null;
    ResultSet rs = null;

    try {
        state = conn.createStatement();
        rs = state.executeQuery(sql);
        Course bean = null;
        while (rs.next()) {
            int id = rs.getInt("id");
            String name2 = rs.getString("name");
            String teacher2 = rs.getString("teacher");
            String classroom2 = rs.getString("classroom");
            bean = new course(id, name2, teacher2, classroom2);
            list.add(bean);
        }
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DButil.close(rs, state, conn);
    }
    
    return list;
}

public List<course> list() {
    String sql = "select * from course";
    List<course> list = new ArrayList<>();
    Connection conn = DButil.getConn();
    Statement state = null;
    ResultSet rs = null;

    try {
        state = conn.createStatement();
        rs = state.executeQuery(sql);
        course bean = null;
        while (rs.next()) {
            int id = rs.getInt("id");
            String name2 = rs.getString("name");
            String teacher2 = rs.getString("teacher");
            String classroom2 = rs.getString("classroom");
            bean = new course(id, name2, teacher2, classroom2);
            list.add(bean);
        }
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DButil.close(rs, state, conn);
    }
    
    return list;
}
}

这一层主要是完成与数据库之间的操作，比如在写添加这个方法时，建立实体类的对象，然后写添加语句，之后是搭建与小岛之间的桥梁，然后建立一个可也在桥梁上运行货物的车，但是先给它赋一个空指针，使用try catch语句，在try语句中给车一个可以拉货物的功能，然后把车拉了的货物赋给一个常量。最后判断如果这个常量的值大于0，那么就返回true,否则就返回false。里面还有可以查找课程名称的方法name，这个方法在service类中可以用来查找输入的名字是否已经存在。还有其他的几个方法。

另一个层是service层，这个层是服务层，用来沟通dao层和sevlet层之间的桥梁。代码如下。

package service;

import java.awt.List;

import dao.coursedao;
import entity.course;

/*服务层
 * 实现sevelet与dao层之间的联系
 */

public class courseservice {
    coursedao cdao=new coursedao();
public boolean add(course course) {
    Boolean f=false;
    if(!cdao.name(course.getName())) {
        cdao.add(course);
        f=true;
    }
    return f;
}
public void del(int id) {
    cdao.delete(id);
}

public void update(course course) {
    cdao.update(course);
}

public course getCourseById(int id) {
    return cdao.getCourseById(id);
}

public course getCourseByName(String name) {
    return cdao.getCourseByName(name);
}


public List<course> search(String name, String teacher, String classroom) {
    return cdao.search(name, teacher, classroom);
}

public List<course> list() {
    return cdao.list();
}
}
package service;

import java.awt.List;

import dao.coursedao;
import entity.course;

/*服务层
 * 实现sevelet与dao层之间的联系
 */

public class courseservice {
    coursedao cdao=new coursedao();
public boolean add(course course) {
    Boolean f=false;
    if(!cdao.name(course.getName())) {
        cdao.add(course);
        f=true;
    }
    return f;
}
public void del(int id) {
    cdao.delete(id);
}

public void update(course course) {
    cdao.update(course);
}

public course getCourseById(int id) {
    return cdao.getCourseById(id);
}

public course getCourseByName(String name) {
    return cdao.getCourseByName(name);
}


public List<course> search(String name, String teacher, String classroom) {
    return cdao.search(name, teacher, classroom);
}

public List<course> list() {
    return cdao.list();
}
}

首先建立dao层类的对象，以添加为例，如果输入的名字不存在，则成功输入，否则返回false。

另外一层是servlet层，这一层是完成与后台之间的操作。代码如下。

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

public class courseservlet extends HttpServlet {
    
    private static final long serialVersionUID = 1L;

    courseservice service = new courseservice();
    
    /**
     * 方法选择
     */
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        String method = req.getParameter("method");
        if ("add".equals(method)) {
            add(req, resp);
        } else if ("del".equals(method)) {
            del(req, resp);
        } else if ("update".equals(method)) {
            update(req, resp);
        } else if ("search".equals(method)) {
            search(req, resp);
        } else if ("getcoursebyid".equals(method)) {
            getCourseById(req, resp);
        } else if ("getcoursebyname".equals(method)) {
            getCourseByName(req, resp);
        } else if ("list".equals(method)) {
            list(req, resp);
        }
    }

    /**
     * 添加
     * @param req
     * @param resp
     * @throws IOException 
     * @throws ServletException 
     */
    private void add(HttpServletRequest req, HttpServletResponse resp) throws IOException, ServletException {
        req.setCharacterEncoding("utf-8");
        String name = req.getParameter("name");
        String teacher = req.getParameter("teacher");
        String classroom = req.getParameter("classroom");
        Course course = new Course(name, teacher, classroom);
        
        //添加后消息显示
        if(service.add(course)) {
            req.setAttribute("message", "添加成功");//传递参数给jsp
            req.getRequestDispatcher("add.jsp").forward(req,resp);//与前台add.jsp文件取得联系
        } else {
            req.setAttribute("message", "课程名称重复，请重新录入");
            req.getRequestDispatcher("add.jsp").forward(req,resp);
        }
    }
}

这一层里面我只写了添加的方法。getParameter方法用来接受在文本框中输入的内容。将.jsp中的name、teacher、classroom重新赋值给字符串类型的name,teacher,classroom。然后建立course类的对象，然后把之间字符串类型的name,teacher,classroom封装在一个包里，方便直接使用。使用try  catch语句，在try语句中判断，如果在service类中的add方法可以正确运行，那么就使用setAttribute方法，传递参数到.jsp文件，然后用getRequestDispatcher方法，实现servlet与.jsp文件的连接。

后台的文件就是这些。下面是前台，需要运行然后出现网页，直接进行增改查的.jsp文件。

显示主页的为index.jsp文件。代码如下。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>首页</title>
<style>
    .a{
        font-size: 26px;
        margin-top: 20px;
    }
</style>
</head>
<body>
<div align="center">//给文本框设置格式
        <h1 style="color: red;">课程信息管理系统</h1>
        <div class="a">
            <a href="add.jsp">课程信息录入</a>
        </div>
        <div class="a">
            <a href="courseservlet?method=list">课程信息修改</a>
        </div>
        <div class="a">
            <a href="del.jsp">课程信息删除</a>
        </div>
        <div class="a">
            <a href="search.jsp">课程信息查询</a>
        </div>
    </div>
</body>
</html>

在这个里面主要完成给文本框设置格式，然后可以进行增删改查的选择，在选择之后直接进去相关的网页来进行操作。

添加的.jsp代码如下。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>//设置字体的格式
    .a{
        margin-top: 20px;
    }
    .b{
        font-size: 20px;
        width: 160px;
        color: white;
        background-color: greenyellow;
    }
</style>
</head>
<body>
<%
         Object message = request.getAttribute("message");//接收从sevelet里面传来的参数
         if(message!=null && !"".equals(message)){
     
    %>
         <script type="text/javascript">
              alert("<%=request.getAttribute("message")%>");
         </script>
    <%} %>
    <div align="center">//给文本框设置格式
        <h1 style="color: red;">课程信息录入</h1>
        <a href="index.jsp">返回主页</a>
        <form action="courseservlet?method=add" method="post" onsubmit="return check()">
            <div class="a">
                课程名称<input type="text" id="name" name="name"/>
            </div>
            <div class="a">
                任课教师<input type="text" id="teacher" name="teacher" />
            </div>
            <div class="a">
                上课地点<input type="text" id="classroom" name="classroom" />
            </div>
            <div class="a">
                <button type="submit" class="b">保&nbsp;&nbsp;&nbsp;存</button>
            </div>
        </form>
    </div>
    <script type="text/javascript">
        function check() {
            var name = document.getElementById("name");;
            var teacher = document.getElementById("teacher");
            var classroom = document.getElementById("classroom");
            
            //非空
            if(name.value == ‘‘) {
                alert(‘课程名称为空‘);
                name.focus();
                return false;
            }
            if(teacher.value == ‘‘) {
                alert(‘教师为空‘);
                teacher.focus();
                return false;
            }
            if(classroom.value == ‘‘) {
                alert(‘上课地点为空‘);
                classroom.focus();
                return false;
            }
            
            //教师
            if(teacher.value != ‘wangjianmin‘ && teacher.value != ‘wanghui‘ && teacher.value != ‘liudan‘ && teacher.value != ‘liulijia‘ && teacher.value != ‘yangziguang‘){
                alert(‘教师名称错误‘);
                return false;
            }
            
            //教室
            if(!/^jijiao/.test(classroom.value) && !/^yijiao/.test(classroom.value) && !/^erjiao/.test(classroom.value) && !/^sanjiao/.test(classroom.value)) {
                alert(‘上课地点错误‘);
                return false;
            }
        }
    </script>
</body>
</html>

在servlet类中层使用setAttribute方法来向.jsp文件中传递参数，那么在.jsp文件中就要使用getAttribute方法来接收从servlet文件中传递来的参数。同样需要对文本框设置格式，然后在添加时判断输入的课程名称、任课老师、上课地点是否为空，如果为空就使用alert来弹出一个框提示错误。然后判断输入的老师是否为这五个老师中的一个，否则提示错误。然后运用四则公式判断输入的上课地点是否为以基教、一教、二教、三教开头，否则提示错误。

然后是删，删分为两步，第一步是输入课程名称，然后点击删除，出现这一科课程的全部信息，然后提示是否真的删除。代码如下。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>
    .a{
        margin-top: 20px;
    }
    .b{
        font-size: 20px;
        width: 160px;
        color: white;
        background-color: greenyellow;
    }
</style>
</head>
<body>
    <%
         Object message = request.getAttribute("message");
         if(message!=null && !"".equals(message)){
     
    %>
         <script type="text/javascript">
              alert("<%=request.getAttribute("message")%>");
         </script>
    <%} %>
    <div align="center">
        <h1 style="color: red;">课程信息删除</h1>
        <a href="index.jsp">返回主页</a>
        <form action="CourseServlet?method=getcoursebyname" method="post" onsubmit="return check()">
            <div class="a">
                课程名称<input type="text" id="name" name="name"/>
            </div>
            <div class="a">
                <button type="submit" class="b">查&nbsp;&nbsp;&nbsp;找</button>
            </div>
        </form>
    </div>
    <script type="text/javascript">
        function check() {
            var name = document.getElementById("name");;
            
            //非空
            if(name.value == ‘‘) {
                alert(‘课程名称为空‘);
                name.focus();
                return false;
            }
        }
    </script>
</body>
</html>
<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>
    .a{
        margin-top: 20px;
    }
    .b{
        font-size: 20px;
        width: 160px;
        color: white;
        background-color: greenyellow;
    }
    .tb, td {
        border: 1px solid black;
        font-size: 22px;
    }
</style>
</head>
<body>
    <div align="center">
        <h1 style="color: red;">课程信息删除</h1>
        <a href="index.jsp">返回主页</a>
        <table class="tb">
            <tr>
                <td>课程名称</td>
                <td>${course.name}</td>
            </tr>
            <tr>
                <td>任课教师</td>
                <td>${course.teacher}</td>
            </tr>
            <tr>
                <td>上课地点</td>
                <td>${course.classroom}</td>
            </tr>
        </table>
        <div class="a">
            <a onclick="return check()" href="CourseServlet?method=del&id=${course.id}">删&nbsp;&nbsp;&nbsp;除</a>
        </div>
    </div>
    <script type="text/javascript">
        function check() {
            if (confirm("真的要删除吗？")){
                return true;
            }else{
                return false;
            }
        }
    </script>
</body>
</html>

然后是修改的.jsp文件。代码如下。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>
    .a{
        margin-top: 20px;
    }
    .b{
        font-size: 20px;
        width: 160px;
        color: white;
        background-color: greenyellow;
    }
</style>
</head>
<body>
    <%
         Object message = request.getAttribute("message");
         if(message!=null && !"".equals(message)){
     
    %>
         <script type="text/javascript">
              alert("<%=request.getAttribute("message")%>");
         </script>
    <%} %>
    <div align="center">
        <h1 style="color: red;">课程信息修改</h1>
        <a href="index.jsp">返回主页</a>
        <form action="CourseServlet?method=update" method="post" onsubmit="return check()">
            <div class="a">
                课程名称<input type="text" id="name" name="name" value="${course.name}"/>
            </div>
            <div class="a">
                任课教师<input type="text" id="teacher" name="teacher" value="${course.teacher}"/>
            </div>
            <div class="a">
                上课地点<input type="text" id="classroom" name="classroom" value="${course.classroom}"/>
            </div>
            <input type="hidden" id="id" name="id" value="${course.id}"/>
            <div class="a">
                <button type="submit" class="b">修&nbsp;&nbsp;&nbsp;改</button>
            </div>
        </form>
    </div>
    <script type="text/javascript">
        function check() {
            var name = document.getElementById("name");;
            var teacher = document.getElementById("teacher");
            var classroom = document.getElementById("classroom");
            
            //非空
            if(name.value == ‘‘) {
                alert(‘课程名称为空‘);
                name.focus();
                return false;
            }
            if(teacher.value == ‘‘) {
                alert(‘教师为空‘);
                teacher.focus();
                return false;
            }
            if(classroom.value == ‘‘) {
                alert(‘上课地点为空‘);
                classroom.focus();
                return false;
            }
            
            //教师
            if(teacher.value != ‘王建民‘ && teacher.value != ‘王辉‘ && teacher.value != ‘刘丹‘ && teacher.value != ‘刘立嘉‘ && teacher.value != ‘杨子光‘){
                alert(‘教师名称错误‘);
                return false;
            }
            
            //教室
            if(!/^基教/.test(classroom.value) && !/^一教/.test(classroom.value) && !/^二教/.test(classroom.value) && !/^三教/.test(classroom.value)) {
                alert(‘上课地点错误‘);
                return false;
            }
        }
    </script>
</body>
</html>

在进行修改的时候也会进行判断，是否是这五个老师中的一个，并且判断是否是以基教、一教、二教、三教开头的。

然后是查询，查询要求实现模糊查询。代码如下。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>
    .a{
        margin-top: 20px;
    }
    .b{
        font-size: 20px;
        width: 160px;
        color: white;
        background-color: greenyellow;
    }
</style>
</head>
<body>
    <div align="center">
        <h1 style="color: red;">课程信息查询</h1>
        <a href="index.jsp">返回主页</a>
        <form action="CourseServlet?method=search" method="post" onsubmit="return check()">
            <div class="a">
                课程名称<input type="text" id="name" name="name"/>
            </div>
            <div class="a">
                任课教师<input type="text" id="teacher" name="teacher" />
            </div>
            <div class="a">
                上课地点<input type="text" id="classroom" name="classroom" />
            </div>
            <div class="a">
                <button type="submit" class="b">查&nbsp;&nbsp;&nbsp;询</button>
            </div>
        </form>
    </div>
    <script type="text/javascript">
        function check() {
            var name = document.getElementById("name");;
            var teacher = document.getElementById("teacher");
            var classroom = document.getElementById("classroom");
            
            //非空
            if(name.value == ‘‘ && teacher.value == ‘‘ && classroom.value == ‘‘) {
                alert(‘请填写一个条件‘);
                return false;
            }
        }
    </script>
</body>
</html>

要填写一个条件，否则提示错误。

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<style>
    .a{
        margin-top: 20px;
    }
    .b{
        font-size: 20px;
        width: 160px;
        color: white;
        background-color: greenyellow;
    }
    .tb, td {
        border: 1px solid black;
        font-size: 22px;
    }
</style>
</head>
<body>
    <div align="center">
        <h1 style="color: red;">课程信息列表</h1>
        <a href="index.jsp">返回主页</a>
        <table class="tb">
            <tr>
                <td>id</td>
                <td>课程名称</td>
                <td>任课教师</td>
                <td>上课地点</td>
            </tr>
            <!-- forEach遍历出adminBeans -->
            <c:forEach items="${courses}" var="item" varStatus="status">
                <tr>
                    <td>${item.id}</td>
                    <td><a>${item.name}</a></td>
                    <td>${item.teacher}</td>
                    <td>${item.classroom}</td>
                </tr>
            </c:forEach>
        </table>
    </div>
</body>
</html>

然后输入一个条件之后就会遍历出相关的所有信息。