驱动或连接失败，这是哪里出问题了

Question

驱动或连接失败：com.microsoft.sqlserve.jdbc.SQLServerDriverException in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "java.sql.Connection.createStatement()" because "this.con" is null

代码如下
package bookup;

import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.*;

import javax.swing.*;

public class book extends JFrame implements ActionListener {

JLabel jlbl1,jlbl2,jlbl3,jlbl4,jlbl5,jlbl6;
JTextField jtf1,jtf2,jtf3,jtf4,jtf5,jtf6;
JButton jbtn1,jbtn2,jbtn3,jbtn4,jbtn5,jbtn6;

Connection con=null;
Statement stmt=null;
PreparedStatement pstmt=null;
ResultSet rs=null;


public  book(String title) {
    
    super(title);
    jlbl1 =new JLabel("编号：");
    jlbl2 =new JLabel("书名：");
    jlbl3 =new JLabel("作者：");
    jlbl4 =new JLabel("分类：");
    jlbl5 =new JLabel("出版社：");
    jlbl6 = new JLabel("出版时间：");
    
    
    
    jtf1=new  JTextField();
    jtf1.setEditable(false);
    jtf2=new  JTextField();
    jtf3=new  JTextField();
    jtf4=new  JTextField();
    jtf5=new  JTextField();
    jtf6=new  JTextField();
    
    
    jbtn1=new JButton("查询");
    jbtn2=new JButton("下一条");
    jbtn3=new JButton("增加");
    jbtn4=new JButton("删除");
    jbtn5=new JButton("修改");
    jbtn6=new JButton("清空");
    
    jbtn1.addActionListener(this);
    jbtn2.addActionListener(this);
    jbtn3.addActionListener(this);
    jbtn4.addActionListener(this);
    jbtn5.addActionListener(this);
    jbtn6.addActionListener(this);
    
Container cp=this.getContentPane();
cp.setLayout(new GridLayout(9,2,10,10));
cp.add(jlbl1);
cp.add(jtf1);

cp.add(jlbl2);
cp.add(jtf2);

cp.add(jlbl3);
cp.add(jtf3);

cp.add(jlbl4);
cp.add(jtf4);

cp.add(jlbl5);
cp.add(jtf5);

cp.add(jlbl6);
cp.add(jtf6);


cp.add(jbtn1);
cp.add(jbtn2);
cp.add(jbtn3);
cp.add(jbtn4);
cp.add(jbtn5);
cp.add(jbtn6);


setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setSize(800,400);
setResizable(false);
this.setLocation(500,200);
setVisible(true);
}

public void createCon() 
{
    try 
    {
        Class.forName("com.microsoft.sqlserve.jdbc.SQLServerDriver");
        
String constr="jdbc:sqlserver://localhost:1433;DatabaseName=BookST";
con =DriverManager.getConnection(constr,"sa","123456");

        
    }catch(Exception e) {
        System.out.print("驱动或连接失败："+e.getMessage());
        
        
    }
}
    
    public void getData()
    {
        
        try {
            stmt=con.createStatement();
            String sql="select * form Book Table_1";
            rs=stmt.executeQuery(sql);
            if (rs.next())
                
        {
                jtf1.setText(rs.getNString(1));
                jtf2.setText(rs.getNString(2));
                jtf3.setText(rs.getNString(3));
                jtf4.setText(rs.getNString(4));
                jtf5.setText(rs.getNString(5));
                jtf6.setText(rs.getNString(6));
                    }
            
        
        }catch (SQLException e) {
            System.out.println("数据读取失败");
            
            
        }
        
    
    }
            
        public void releaseZy()
        {
            try {
                rs.close();
                stmt.close();
                con.close();
            }catch(SQLException e){
                System.out.println("释放资源失败");
            }
            
                
                
            }
        {
            
        }
            
            
        

    




    
    
    
     

        




@Override
public void actionPerformed (ActionEvent e) {
    if (e.getSource()==jbtn1)
    {
        createCon();
        getData();
        
        
    }
    
    
    
}

}

Answer 1

java.lang.NullPointerException: Cannot invoke "java.sql.Connection.createStatement()" because "this.con" is null，
报错提醒多么明显，这个con是null

如果不是静态的，每个方法都要重新注册驱动创建连接。