单一职责原则

（1）类图

 

（2）源代码

//DBUtil.java
package Test;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class DBUtil {
    public static String db_url = "jdbc:mysql://localhost:3308/db3?serverTimezone=GMT%2B8&useSSL=false";
    public static String db_user = "root";
    public static String db_pass = "123456";
    public static Connection getConn () {
        Connection conn = null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            conn = DriverManager.getConnection(db_url, db_user, db_pass);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return conn;
    }//end getConn
    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();
            }
        }
    }
    public static void main(String[] args) throws SQLException {
        Connection conn = getConn();
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        String sql ="select * from user";
        pstmt = conn.prepareStatement(sql);
        rs = pstmt.executeQuery();
        if(rs.next()){
            System.out.println("连接成功");
        }else{
            System.out.println("连接失败");
        }
    }
}
//LoginForm.java
package Test;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class LoginForm extends JFrame {
    
    private static final long serialVersionUID = 1L;
    private  UserDao dao = new UserDao();
    //设置按钮组件
    
    private  JButton jb=new JButton("登录");    //添加一个登录按钮
    private JButton button=new JButton("重置");    //添加一个确定按钮
    //设置文本框组件
    private JTextField username = new JTextField();//用户名框
    private JPasswordField password = new JPasswordField();//密码框：为加密的***    
    JLabel user_name=new JLabel("账号：");//设置左侧用户名文字
    JLabel pass_word=new JLabel("密码：");//设置左侧密码文字  
    public void init()
    {
    /* 组件绝对位置  */
    user_name.setBounds(50, 70, 300, 25);
    pass_word.setBounds(50, 130, 200, 25);        
    username.setBounds(110, 70, 300, 25);//设置用户名框的宽，高，x值，y值
    password.setBounds(110, 130, 300, 25);//设置密码框的宽，高，x值，y值        
    button.setBounds(315, 225, 90, 20);//设置确定按钮的宽，高，x值，y值
    jb.setBounds(95, 225, 90, 20);//设置确定按钮的宽，高，x值，y值      
    /* 组件透明化*/
    user_name.setOpaque(false);
    pass_word.setOpaque(false);     
     //监听事件
    jb.addActionListener(new ActionListener(){        //为确定按钮添加监听事件                
        @SuppressWarnings("deprecation")
        public void actionPerformed(ActionEvent arg0) {
            validate(username.getText().trim(),password.getText().trim());
        }
    });          
    //重置按钮
    button.addActionListener(new ActionListener(){        //为重置按钮添加监听事件
        //同时清空name、password的数据
        public void actionPerformed(ActionEvent arg0) {
            // TODO 自动生成方法存根
            username.setText("");
            password.setText("");
        }
    });        
    }   
    public void display()
     {
    JFrame f =new JFrame();
    f.setTitle("登录页面");
        //窗口退出行为
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //设置窗口大小不可变
        f.setResizable(false);
        //设置窗口打开居中
        f.setLocationRelativeTo(null);
        //窗口大小
        f.setSize(500, 300);   
        init(); 
        //添加组件
        Container contentPanel= new Container();//添加一个contentPanel容器
         contentPanel.setLayout(null);//设置添加的contentPanel容器为流布局管理器
         contentPanel.add(user_name);
         contentPanel.add(pass_word);
         contentPanel.add(username);
         contentPanel.add(password);
         contentPanel.add(jb);
         contentPanel.add(button);        
            f.add(contentPanel); 
             //展示窗口
            f.setVisible(true);
    }
 public  void  validate(String username,String password)
     {
        
           if(username.trim().length()==0||password.trim().length()==0){
       JOptionPane.showMessageDialog(null, "用户名,密码不允许为空");               
        return;
         }
         
       if(dao.findUser(username, password))
        {
              
        JOptionPane.showMessageDialog(null, "登录成功！");                      
          }else {
              JOptionPane.showMessageDialog(null, "用户名或密码错误");            
          }    
       }        
}
//MainClass.java
package Test;
public class MainClass { 
     public static void main(String[] args)
     {
            LoginForm loginForm=new LoginForm() ;    //调用
            loginForm.display();              
     }
}
//UserDao.java
package Test;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class UserDao {
    public boolean findUser(String username, String password) {
        //准备SQL语句
        String sql = "select * from user where username ='" + username + "'";
        Connection conn= DBUtil.getConn();
        //创建语句传输对象
        Statement state = null;
        ResultSet rs= null;
        int flag=0;
        String c_password=null;
        try {
            state = conn.createStatement();
            rs = state.executeQuery(sql);
            while(rs.next()) {
                ++flag;
                c_password=rs.getString("password");
            }    if (flag == 0) {
                return false;
            }
            if (!password.equals(c_password)) {   //判断密码
                return false;
            }
    }catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }finally {
         DBUtil.close(rs, state, conn);
    }
    return true;
        }
}