无法在netbeans java web应用程序中连接到mysql数据库

Question

我是java web开发的新手。我正在编写这个基本的javaweb应用程序，它将把客户信息存储到数据库中。我使用MVC-2体系结构。我的Jsp向servlet发送一个请求，servlet反过来尝试安装bean并将该对象插入数据库。当我尝试连接到数据库时（在调试模式下），连接变量返回空。因此无法插入数据。

这是连接到数据库的类

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package customer;
import java.io.Serializable;
import java.sql.Connection;
import java.sql.DriverManager;
/**
 *
 * @author 
 */
public class DatabaseOperations implements Serializable 
{
    private static Connection connection;
    public DatabaseOperations()
    {
        try
        {
            String username = "root";
            String password = "root";
            String url = "jdbc:mysql://localhost/test";
            Class.forName ("com.mysql.jdbc.Driver").newInstance();
            connection = DriverManager.getConnection (url, username, password);
            System.out.println("Database connection established");
        }
        catch(Exception e)
        {
        }
    }
    public static Connection getConnection()
    {
        return connection;
    }
}

这是将客户添加到数据库的方法

public void addCustomer(CustomerBean customer) throws SQLException {
        DatabaseOperations db = new DatabaseOperations();
        connection = DatabaseOperations.getConnection();
        statement = connection.createStatement();
        String query = "insert into customer (name, address, phone, email) "
                + "values (" + customer.name + ","
                + customer.address + ","
                + customer.phone + ","
                + customer.email + "," + ")";
        statement.executeUpdate(query);
    }

最后，这是servlet，我在其中调用添加客户的方法

   /*
     * To change this template, choose Tools | Templates
     * and open the template in the editor.
     */
    package customer;
    import java.io.IOException;
    import java.io.PrintWriter;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import customer.CustomerBean;
    import javax.servlet.RequestDispatcher;
    /**
     *
     * @author 
     */
    public class CustomerServlet extends HttpServlet {
        /**
         * Processes requests for both HTTP
         * <code>GET</code> and
         * <code>POST</code> methods.
         *
         * @param request servlet request
         * @param response servlet response
         * @throws ServletException if a servlet-specific error occurs
         * @throws IOException if an I/O error occurs
         */
        protected void processRequest(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            response.setContentType("text/html;charset=UTF-8");
            PrintWriter out = response.getWriter();
            CustomerBean customer = new CustomerBean();
            try {
                out.println("tests");
                customer.setName(request.getParameter("name"));
                customer.setEmail(request.getParameter("email"));
                customer.setAddress(request.getParameter("address"));
                customer.setPhone(request.getParameter("phone"));
/************** ADD CUSTOMER TO DB HERE***********************/
                customer.addCustomer(customer);
                request.setAttribute("cust", customer);
                request.getRequestDispatcher("/index.jsp").forward(request, response);
            } 
            catch (Exception e) 
            {            
                e.printStackTrace();
            }
        }

Answer 1

首先，我想指出您的代码对SQL注入是开放的。就我个人而言，我是过程的粉丝，所以我建议：您应该在mysql中创建一个过程来插入一个新记录，然后为该存储过程提供参数（名称、地址等）。

至于手头的问题：在statement.executeUpdate线路之后，尝试关闭连接。此外，将Class.forName提取到您的主方法中。它应该只执行一次。此外，一旦你这样做了，就把.newInstance()从上面去掉

如果所有这些都不起作用，请将整个连接提取到全局可访问的静态变量，并且不要多次分配给它。然后看看你的程序是否有效。如果没有，你就有一个单独的问题。