Java 向NetBeans项目添加MySQL驱动程序库
我对JavaEE编程非常陌生，我应该做一些关于它的练习（以便很快完成学业）。 我的问题可能很简单，但我还没有找到答案。 在一个练习中，我应该能够登录到java/jsp/mysql类型的服务 Login.htmJava 向NetBeans项目添加MySQL驱动程序库,java,mysql,netbeans,nullpointerexception,Java,Mysql,Netbeans,Nullpointerexception,我对JavaEE编程非常陌生，我应该做一些关于它的练习（以便很快完成学业）。 我的问题可能很简单，但我还没有找到答案。 在一个练习中，我应该能够登录到java/jsp/mysql类型的服务 Login.htm <html> <head> <title>Login</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <lin
<html>
<head>
<title>Login</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" href="css/style.css" />
</head>
<body>
<h3>Login</h3>
<form name="login" method="post" action="ServletLogin">
<table>
<tr>
<td>Email:</td>
<td><input name="email" type="text" id="email"></td>
</tr>
<tr>
<td>Salasana</td>
<td><input name="password" type="password" id="password"></td>
</tr>
<tr>
<td colspan="2">
<input type="submit" name="Submit" value="Log in;">
</td>
</tr>
</table>
</form>
</body>
</html>
ServletLogin.javaNullPointerExceptionServletLogin.java/*
* Remember to install a MYSQL driver library
* or the file will alert
* NullPointerException.
*/
package Servlets; //the package in which the Servlet is part of.
import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
@WebServlet(name = "ServletLogin", urlPatterns = {"/ServletLogin"})
public class ServletLogin extends HttpServlet {
Connection conn = null;
/**
* Initializes the servlet.
*/
@Override
public void init(ServletConfig config) throws ServletException {
super.init(config);
//Connect to database with database-class
try {
conn = Classes.SQL.openConnection();
}
catch (Exception e) {
System.out.println("Cannot connect to database " + e);
}
}
/**
* Destroys the servlet.
*/
@Override
public void destroy() {
//Closing the database connection
try {
conn.close();
} catch ( SQLException se ) {
System.out.println("Exception " + se);
}
}
//Everything is done in doPost method.
//This Servlet does not use doGet() or ProcessRequest()
/**
* Handles the HTTP <code>POST</code> method.
* @param request servlet request
* @param response servlet response
*/
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out = response.getWriter();
/*Unless there is a session, one is created.
It is used to check if the user has logged in.*/
HttpSession session = request.getSession(true);
//Reading the parameters from login form.
String email = request.getParameter("email");
String password = request.getParameter("password");
out.print(email);
//Created login_ok and set it to false by default.
boolean login_ok = false;
try {
//let's create a Statement with which the sql can be run with.
Statement stmt = conn.createStatement();
//Run SQL and save it to ResultSet rs.
ResultSet rs = stmt.executeQuery("SELECT email, password FROM clients");
//Go trough the results with while loop and next-method, which returns the value true, until we've reached last result.
while(rs.next())
{
//Reading the data
String email2 = rs.getString("email");
String salasana2 = rs.getString("password);
//If user input has been found from database,
//login_ok becomes true and looping ends.
if(email.compareTo(email2) == 0 && password.compareTo(password2) == 0)
{
login_ok = true;
break;
}
}
//If login_ok is true, save info about logging to session and guide the user to Clients.jsp
if( login_ok == true )
{
//session is informed about login
session.setAttribute("login", "ok");
//Debugging printed to console.
System.out.println("Login ok");
//Proceeding to clients
response.sendRedirect("clients.jsp");
//Return stops the servlet run.
return;
}
//Login false -> redirected to login page.
else {
response.sendRedirect("login.htm");
}
}
catch(SQLException se){
out.println("Error: " + se);
}
out.close();
}
}
mysql-connector-java-5.1.23-bin.jarNullPointerExceptionmysql-connector-java-5.1.23-bin.jar似乎也解决不了这个问题。
将MySQL注册到GlassFish，仍然使用NullPointerException
转到“服务”选项卡
选择数据库
右键单击并选择注册MYSQL驱动程序
右键单击项目中的库，然后从添加库中选择MYSQL
您是否在Glassfish中注册了mysql驱动程序？你可以找到如何做到这一点。这很简单，不要像那样迭代SQL结果集。数据库的职责是查找单个记录。学习使用选择。。。其中…
您是否在服务器日志中看到“无法连接到数据库（…）”？你能提供更多关于如何在Java中实现连接的详细信息吗？不允许注册MySQL驱动程序，该选项不可用。
/*
* Remember to install a MYSQL driver library
* or the file will alert
* NullPointerException.
*/
package Servlets; //the package in which the Servlet is part of.
import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
@WebServlet(name = "ServletLogin", urlPatterns = {"/ServletLogin"})
public class ServletLogin extends HttpServlet {
Connection conn = null;
/**
* Initializes the servlet.
*/
@Override
public void init(ServletConfig config) throws ServletException {
super.init(config);
//Connect to database with database-class
try {
conn = Classes.SQL.openConnection();
}
catch (Exception e) {
System.out.println("Cannot connect to database " + e);
}
}
/**
* Destroys the servlet.
*/
@Override
public void destroy() {
//Closing the database connection
try {
conn.close();
} catch ( SQLException se ) {
System.out.println("Exception " + se);
}
}
//Everything is done in doPost method.
//This Servlet does not use doGet() or ProcessRequest()
/**
* Handles the HTTP <code>POST</code> method.
* @param request servlet request
* @param response servlet response
*/
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out = response.getWriter();
/*Unless there is a session, one is created.
It is used to check if the user has logged in.*/
HttpSession session = request.getSession(true);
//Reading the parameters from login form.
String email = request.getParameter("email");
String password = request.getParameter("password");
out.print(email);
//Created login_ok and set it to false by default.
boolean login_ok = false;
try {
//let's create a Statement with which the sql can be run with.
Statement stmt = conn.createStatement();
//Run SQL and save it to ResultSet rs.
ResultSet rs = stmt.executeQuery("SELECT email, password FROM clients");
//Go trough the results with while loop and next-method, which returns the value true, until we've reached last result.
while(rs.next())
{
//Reading the data
String email2 = rs.getString("email");
String salasana2 = rs.getString("password);
//If user input has been found from database,
//login_ok becomes true and looping ends.
if(email.compareTo(email2) == 0 && password.compareTo(password2) == 0)
{
login_ok = true;
break;
}
}
//If login_ok is true, save info about logging to session and guide the user to Clients.jsp
if( login_ok == true )
{
//session is informed about login
session.setAttribute("login", "ok");
//Debugging printed to console.
System.out.println("Login ok");
//Proceeding to clients
response.sendRedirect("clients.jsp");
//Return stops the servlet run.
return;
}
//Login false -> redirected to login page.
else {
response.sendRedirect("login.htm");
}
}
catch(SQLException se){
out.println("Error: " + se);
}
out.close();
}
}