我试图在我的 servlet 中连接 mysql 数据库。然后我得到一个例外。
但是当我从一个普通的 Java 类测试数据库连接时，连接是好的，我从数据库中获取数据。
我在 tomcat 服务器控制台上遇到的异常是：
java.lang.ClassNotFoundException: com.mysql.jdbc.Driver.
我的代码DatabaseHandler.java是：
import java.sql.*;
public class DatabaseHandler {
private Statement stmt;
private Connection con;
String username = "root";
String password = "thunder";
String dbname = "bidderboy";
public DatabaseHandler()
{
this.createConnection();
}
private void createConnection()
{
//create the connection for first time
try {
Class.forName("com.mysql.jdbc.Driver").newInstance();
con = DriverManager.getConnection("jdbc:mysql://localhost/"+dbname , username , password);
stmt = con.createStatement();
} catch (Exception ex) {
System.out.println(ex.toString());
}
}
public int executeUpdate(String sql)
{
int result = 0;
//before update checks if connection is open
try {
if(con.isClosed()) {
Class.forName("com.mysql.jdbc.Driver").newInstance();
con = DriverManager.getConnection("jdbc:mysql://localhost/"+dbname , username , password);
stmt = con.createStatement();
}
} catch (Exception ex) {
System.out.println(ex.toString());
}
//try to executeUpdate the sql command
try{
result = stmt.executeUpdate(sql);
}
catch(Exception ex){
System.out.println("Couldn't executeUpdate sql command");
}
return result;
}
public ResultSet executeQuery(String sql)
{
ResultSet rs = null;
//before Query checks if connection is open
try {
if(con.isClosed()) {
Class.forName("com.mysql.jdbc.Driver").newInstance();
con = DriverManager.getConnection("jdbc:mysql://localhost:3306/"+dbname , username , password);
stmt = con.createStatement();
}
} catch (Exception ex) {
System.out.println(ex.toString());
}
//try to executeQuery the sql command
try{
rs = stmt.executeQuery(sql);
}
catch(Exception ex){
System.out.println("Couldn't executeQuery sql command");
}
return rs;
}
public void closeConnection()
{
//if connection is open try to close the connection
try {
if(!con.isClosed()) {
con.close();
}
} catch (SQLException ex) {
System.out.println("Failed to close database connection");
}
}
}
我的 servlet 的代码是：
import java.io.IOException;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@SuppressWarnings("serial")
public class UserLogin extends HttpServlet{
public UserLogin()
{
}
@Override
public void init(ServletConfig config) throws ServletException {
super.init(config);
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
this.doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
DatabaseHandler dbh = new DatabaseHandler();
String username="mamun";
String password="1234";
String dbusername="";
String dbpassword="";
String sql = "select * from users where username='"+username+"' and password='"+password+"'";
ResultSet rs = dbh.executeQuery(sql);
try {
while(rs.next())
{
dbusername = rs.getNString(1);
dbpassword = rs.getNString(2);
}
} catch (SQLException e) {}
System.out.println(dbusername+" "+dbpassword);
}
}
我从中获取数据的普通java类的代码是：
import java.sql.ResultSet;
import java.sql.SQLException;
public class DatabaseConnectionTest {
public static void main(String[] args) {
String sql = "Select * from users";
DatabaseHandler dbh = new DatabaseHandler();
ResultSet rs = dbh.executeQuery(sql);
try {
while(rs.next())
{
String n = rs.getNString(1);
String c = rs.getNString(2);
System.out.println("Name: "+n+" Contact: "+c);
}
} catch (SQLException ex) {
System.out.println(ex.toString());
}
}
}
任何人都请解释为什么我得到这个类没有找到异常以及解决方案是什么。提前致谢。