idea连接数据库类
上面的一段是user连接数据库和一些类方法的代码，下面是我模仿着更改的teacher连接数据库的类方法和代码，但是teacher的这个dao类里面就连接不上数据库报错500
package com.demo.dao1;
import com.demo.ben.User;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;
public class UserDao {
PreparedStatement ps = null;
static Connection connection = null;
ResultSet resultSet = null;
public List queryAll() throws Exception {
List user_list = null;
try{
Class.forName("com.mysql.cj.jdbc.Driver");
//先获取connection对象
connection = DriverManager
.getConnection("jdbc:mysql://localhost:3306/gu2","root","");
//获取真正操作数据的对象
ps = connection.prepareCall("select * from `user`");
//执行数据库查询操作
resultSet = ps.executeQuery();
//把数据库结果集转成java的List集合
user_list = new ArrayList();
while(resultSet.next()){
User user = new User();
user.setId((String) resultSet.getObject(1));
user.setName((String) resultSet.getObject(2));
user.setUser_class((String) resultSet.getObject(3));
user.setGender((String) resultSet.getObject(4));
user.setPhone((String) resultSet.getObject(5));
user.setPassword((String) resultSet.getObject(6));
user_list.add(user);
}
}catch (Exception e){
e.printStackTrace();
}
// finally {
// connection.close();
// ps.close();
// resultSet.close();
// }
return user_list;
}
//user register send the information to the database
public static void insert(String Account, String Password, String Name, String Class, String Gender, String Telephone)throws Exception{
PreparedStatement prep = connection.prepareStatement("insert into gu2.user value(?,?,?,?,?,?)");
prep.setString(1,Account);
prep.setString(2,Name);
prep.setString(3,Class);
prep.setString(4,Gender);
prep.setString(5,Telephone);
prep.setString(6,Password);
prep.execute();
}
//user login request the information from database
public static String[] getUser(String Account) throws SQLException {
PreparedStatement prep = connection.prepareStatement("select * from gu2.user where User_id=?");
prep.setString(1,Account);
prep.execute();
ResultSet resultSet = prep.executeQuery();
//get the id from the database
if (resultSet.next()){
String UserAccount = resultSet.getString("User_id");
String UserPassword = resultSet.getString("Password");
String[] a = new String[]{UserAccount,UserPassword};
return a;
}
//do not get the id and pass from the database
String[] a = new String[]{"$Yang Yang$","$Yang Yang$"};
return a;
}
//check the input information should be correct
public static String Rcheck(String Account)throws Exception{
PreparedStatement prep = connection.prepareStatement("select * from gu2.user where User_id=?");
prep.setString(1,Account);
prep.execute();
ResultSet resultSet = prep.executeQuery();
if (resultSet.next()){
String UserAccount = resultSet.getString("User_id");
String exist= "$Yang Yang$";
return exist;
}
return Account;
}
}
package com.demo.dao1;
import com.demo.ben.Teacher;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;
public class TeacherDao {
PreparedStatement ps = null;
static Connection connection = null;
ResultSet resultSet = null;
public List queryAll() throws Exception {
List teacher_list = null;
try{
Class.forName("com.mysql.cj.jdbc.Driver");
//先获取connection对象
connection = DriverManager
.getConnection("jdbc:mysql://localhost:3306/gu2","root","");
//获取真正操作数据的对象
ps = connection.prepareCall("select * from `teacher`");
//执行数据库查询操作
resultSet = ps.executeQuery();
//把数据库结果集转成java的List集合
teacher_list = new ArrayList();
while(resultSet.next()){
Teacher teacher = new Teacher();
teacher.setId((String) resultSet.getObject(1));
teacher.setName((String) resultSet.getObject(2));
teacher.setGender((String) resultSet.getObject(3));
teacher.setPhone((String) resultSet.getObject(4));
teacher.setPassword((String) resultSet.getObject(5));
teacher.setTClass((String) resultSet.getObject(6));
teacher_list.add(teacher);
}
}catch (Exception e){
e.printStackTrace();
}
return teacher_list;
}
//teacher register send the information to the database
public static void insert(String Account, String Password, String Name, String Class, String Gender, String Telephone)throws Exception{
PreparedStatement prep = connection.prepareStatement("insert into gu2.teacher value(?,?,?,?,?,?)");
prep.setString(1,Account);
prep.setString(2,Name);
prep.setString(3,Gender);
prep.setString(4,Telephone);
prep.setString(5,Password);
prep.setString(6,Class);
prep.execute();
}
//user login request the information from database
public static String[] getTeacher(String Account) throws SQLException {
PreparedStatement prep = connection.prepareStatement("select * from gu2.teacher where Teacher_id=?");
prep.setString(1,Account);
prep.execute();
ResultSet resultSet = prep.executeQuery();
//get the id from the database
if (resultSet.next()){
String TeacherAccount = resultSet.getString("Teacher_id");
String TeacherPassword = resultSet.getString("Password");
String[] a = new String[]{TeacherAccount,TeacherPassword};
return a;
}
//do not get the id and pass from the database
String[] a = new String[]{"$Yang Yang$","$Yang Yang$"};
return a;
}
//check the input information should be correct
public static String Rcheck(String Account)throws Exception{
PreparedStatement prep = connection.prepareStatement("select * from gu2.teacher where Teacher_id=?");
prep.setString(1,Account);
prep.execute();
ResultSet resultSet = prep.executeQuery();
if (resultSet.next()){
String UserAccount = resultSet.getString("Teacher_id");
String exist= "$Yang Yang$";
return exist;
}
return Account;
}
}
- 你 58行那里的 connection 没有赋值呀，还是null；
- 你的连接对象，也只是在 queryAll里面才 赋值的
- 写个数据库工具类，获取连接就调这个工具类吧
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
/**
* @author huazie
* @version 2.0.0
* @since 2.0.0
*/
public class DBUtil {
private DBUtil() {
}
static {
try {
Class.forName("com.mysql.cj.jdbc.Driver");
} catch (ClassNotFoundException e) {
// 打印异常
}
}
/**
* 获取数据库连接对象
*
* @param url 数据库URL
* @param name 用户名
* @param pwd 密码
* @return 数据库连接对象
*/
public static Connection getConnection(String url, String name, String pwd) throws SQLException {
return DriverManager.getConnection(url, name, pwd);
}
/**
* <p> 释放连接Connection </p>
*
* @param conn 数据库连接对象
*/
private static void closeConnection(Connection conn) {
try {
if (null != conn) {
conn.close();
}
} catch (SQLException e) {
// 打印异常
}
}
/**
* <p> 释放statement </p>
*
* @param statement Statement对象
*/
private static void closeStatement(Statement statement) {
try {
if (null != statement) {
statement.close();
}
} catch (SQLException e) {
// 打印异常
}
}
/**
* <p> 释放ResultSet结果集 </p>
*
* @param rs 结果集对象
*/
private static void closeResultSet(ResultSet rs) {
try {
if (null != rs) {
rs.close();
}
} catch (SQLException e) {
// 打印异常
}
}
/**
* <p> 释放资源 </p>
*
* @param conn 数据库连接对象
* @param statement 数据库状态对象
* @param rs 数据库结果集对象
*/
public static void close(Connection conn, Statement statement, ResultSet rs) {
closeResultSet(rs);
closeStatement(statement);
closeConnection(conn);
}
/**
* <p> 释放连接 </p>
*
* @param conn 数据库连接对象
*/
public static void close(Connection conn) {
closeConnection(conn);
}
/**
* <p> 释放状态 </p>
*
* @param statement 数据库状态对象
*/
public static void close(Statement statement) {
closeStatement(statement);
}
/**
* <p> 释放结果集 </p>
*
* @param rs 数据库结果集对象
*/
public static void close(ResultSet rs) {
closeResultSet(rs);
}
}
- 你可以看下这个问题的回答https://ask.csdn.net/questions/763669
如果你已经解决了该问题, 非常希望你能够分享一下解决方案, 写成博客, 将相关链接放在评论区, 以帮助更多的人 ^-^