Exceptions in Web App

Hi i'm practicing with a simple web application that just verifies a username and a password. I got two servlets and I'm using netbeans 5.5 and the bundled Tomcat 5.17. The Web Application works normal with any problems so far...but my problem is why the Tomcat Log shows me some exceptions i still don't understand ...i hope you people can help me out with this.

import javax.servlet.http.*;

import javax.servlet.*;

import java.io.*;

import java.sql.*;

publicclass Loginextends HttpServlet{

private Statement s;

private ConectionDB a;

publicvoid init()throws ServletException{

ServletContext sc = getServletConfig().getServletContext();

String driver = sc.getInitParameter("driver");

String url = sc.getInitParameter("url");

String user = sc.getInitParameter("username_DB");

String pass = sc.getInitParameter("password_DB");

try{

a =new ConectionDB();

s = a.openDBConnection(driver, url, user, pass);

}

catch (SQLException e){

e.printStackTrace();

}

catch (ClassNotFoundException e){

e.printStackTrace();

}

publicvoid doPost(HttpServletRequest request, HttpServletResponse response){

try{

response.setContentType("text/html");

PrintWriter out = response.getWriter();

String username = request.getParameter("username");

String password = request.getParameter("password");

ResultSet res = s.executeQuery("select username,password from users where username = " +

"'" + username +"'" +

" and password = " +

"'" + password +"'");

if (res.next()){

out.println("<HTML><BODY>");

out.println("Hello "+ username +". Welcome to our magazines! ");

out.println("

");

out.println("<img src='00.jpg' width='655' height='1000'>");

out.println("

");

}

else{

out.println("Id/Password combination is not valid");

}

out.println("</BODY></HTML>");

}

catch (Exception e){

e.printStackTrace();

}

publicvoid destroy(){

try{

s.close();

a.closeConnection();

}catch(SQLException e){

e.printStackTrace();

}

import java.sql.*;

import javax.servlet.ServletContext;

import javax.servlet.http.HttpServlet;

publicclass ConectionDBextends HttpServlet{

private Connection c;

public Statement openDBConnection(String driver, String url, String user, String pass)

throws SQLException, ClassNotFoundException{

Class.forName(driver);

c = DriverManager.getConnection(url,user,pass);

Statement s = c.createStatement();

return s;

}

publicvoid closeConnection()throws SQLException{

c.close();

}

This web-app works without any problems but i can't figure out what why i got those exception...what do they mean?

The Tomcat Log

22/01/2007 04:36:28 PM org.apache.catalina.core.ApplicationContext log

GRAVE: StandardWrapper.Throwable

java.lang.NullPointerException

at ConectionDB.openDBConnection(ConectionDB.java:12)

at Login.init(Login.java:18)

at javax.servlet.GenericServlet.init(GenericServlet.java:211)

at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1105)

at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:757)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:130)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)

at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)

at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)

at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)

at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)

at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)

at java.lang.Thread.run(Thread.java:619)

22/01/2007 04:36:28 PM org.apache.catalina.core.StandardWrapperValve invoke

GRAVE: Excepci髇 de reserva de espacio para servlet Login