为什么这段代码会抛出“空指针异常”？

Question

public class register extends HttpServlet {
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String uname=request.getParameter("Uname");
        String uemail=request.getParameter("Uemail");
        String upass=request.getParameter("Upass");
        
        System.out.println(uname+" "+uemail+" "+upass);
        
        
        Connection connection=ConnectionProvider.getConn();
        try {
            java.sql.PreparedStatement pq=connection.prepareStatement("insert into login values(?,?,?)");
            pq.setString(1,uname);
            pq.setString(2,uemail);
            pq.setString(3,upass);
            int i=pq.executeUpdate();
            if(i>0) {
                response.getWriter().println("Data Inserted");
            }
            else {
                response.getWriter().println("No Data Inserted");
            }
    
        } 
        catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        
    }

}

我正在编写此代码以将值输入到数据库中，但是当我填写表单并单击“提交”时，它将在此行显示“NullPointerException”： java.sql.PreparedStatement pq=connection.prepareStatement("插入登录值(?,?,?)"); 此代码中的错误的实际原因是什么？