无法在NetBeans Java Web应用程序中连接到mysql数据库

Question

I am new to web development to java. I am writing this basic java web application that is going to store customer information into a database. I use the MVC-2 architecture. My Jsp sends a request to a servlet that in turn tries instatiates a bean an inserts that object into a database. When i try to connect to the database (in debugging mode) the connection variable returns empty. So data cannot be inserted.

This is the class that makes connections to the db

/*
 * 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;
    }
}

This is the method that adds a customer to the db

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);
    }

and finally this is the servlet where i call the method to add a customer

   /*
     * 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

First, I'd like to point out that your code is open to SQL injection. Personally, I'm a fan of procedures, so I'll recommend that: you should create a procedure in mysql to insert a new record, and then provide that sproc with the arguments (name, address, etc).

As for the problem at hand: After your statement.executeUpdate line, try closing the connection. Also, extract the Class.forName to your main method. It should only be executed once. Also, remove the .newInstance() off of that once you have done so.

If all that doesn't work, extract the entire connection out to a globally accessible static variable and do not assign to it more than once . See if your program works then. If not, you have a separate problem.