Problem in Login using Servlets

index.jsp
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>JSP Page</title>
</head>
<body>
<center><h1>Fast Learn</h1></center>
<br/><br/>
<form method="POST" action="validate.do">
<center>
<table border="0" cellspacing="5" cellpadding="5">
<tr>
<td>Username</td>
<td><input type="text" name="username" size="30"></td>
</tr>
<tr>
<td>Password</td>
<td><input type="password" name="password" size="30"></td>
</tr>
<tr>
<td colspan="2"><input type="submit" value="Sign in"></td>
</tr>
</table>
</center>
</form>
</body>
</html>

validate
import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class validate extends HttpServlet {

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
List errorMsg=new LinkedList();
errorMsg.clear();
try {
String username=request.getParameter("username").trim();
String password=request.getParameter("password").trim();
if(username.isEmpty()&&password.isEmpty()){
errorMsg.add("Please enter username and password.");
}
else if(username.isEmpty()){
errorMsg.add("Please enter username.");
}
else if(password.isEmpty()){
errorMsg.add("Please enter password.");
}
if(!errorMsg.isEmpty()){
RequestDispatcher view=request.getRequestDispatcher("login.view");
view.forward(request, response);
}
HttpSession session = request.getSession(true);
session.setAttribute("username", username);
RequestDispatcher view=request.getRequestDispatcher("home.view");
view.forward(request, response);
} finally {
errorMsg.add("Please Re-Login.");
RequestDispatcher view=request.getRequestDispatcher("login.view");
view.forward(request, response);
out.close();
}
}

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
public String getServletInfo() {
return "Short description";
}
}

login
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class login extends HttpServlet {

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
List errorMsg=new LinkedList();
errorMsg=(List)request.getAttribute("errorMsg");

try {
out.println("<html>");
out.println("<head>");
out.println("<title>Login</title>");
out.println("</head>");
out.println("<body>");
out.println("<center><h1>Fast Learn</h1><center>");
out.println("<br/><br/>");
out.println("<form method=\"POST\" action=\"validate.do\">");
out.println("<center>");
out.println("<table border=\"0\" cellspacing=\"5\" cellpadding=\"5\">");
out.println("<tr>");
out.println("<td>Username</td>");
out.println("<td><input type=\"text\" name=\"username\" size=\"30\"></td>");
out.println("</tr>");
out.println("<tr>");
out.println("<td>Password</td>");
out.println("<td><input type=\"password\" name=\"password\" size=\"30\"></td>");
out.println("</tr>");
if(errorMsg!=null)
{
out.println("<tr><td>");
out.println("<font color=\"red\"");
out.println("<p>");
Iterator items=errorMsg.iterator();
while(items.hasNext()){
out.println(items.next());
}
out.println("</p>");
out.println("</font>");
out.println("</td></tr>");
}
out.println("<tr>");
out.println("<td colspan=\"2\"><input type=\"submit\" value=\"Sign in\"></td>");
out.println("</tr>");
out.println("</table>");
out.println("</center>");
out.println("</form>");
out.println("</body>");
out.println("</html>");
} finally {
out.close();
}
}

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}

public String getServletInfo() {
return "Short description";
}
}

home

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 javax.servlet.http.HttpSession;

public class home extends HttpServlet {

protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
HttpSession session=request.getSession();
try {
out.println("<html>");
out.println("<head>");
out.println("<title>Servlet home</title>");
out.println("</head>");
out.println("<body>");
out.println("<h4>Welcome "+session.getAttribute("username")+"</h4>");
out.println("<br/><br/><br/>");
out.println("<a href=\"logout.jsp\"><b>Logout</b></a>");
out.println("</body>");
out.println("</html>");
} finally {
out.close();
}
}

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}

public String getServletInfo() {
return "Short description";
}
}

Help me to correct the following errors:

WARNING: StandardWrapperValve[validate]: PWC1406: Servlet.service() for servlet validate threw exception
java.lang.IllegalStateException: PWC1227: Cannot forward after response has been committed
at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:370)
at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:350)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:300)
at Controller.validate.processRequest(validate.java:76)
at Controller.validate.doPost(validate.java:109)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:754)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98)
at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:330)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:174)
at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:828)
at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:725)
at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1019)
at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
at java.lang.Thread.run(Thread.java:722)

Miz,
First, welcome to coderanch..
Next, Please use code tags while posting to forum. UnFormatted codes are very difficult to read.

A couple of points on the code,
1. Having both doGet and doPost do same thing is not a good practice. They both are present for different purposes.
2. In validate servlet, why view.forward() is present in both try as well as finally. In try it had already forwarded the response, so in finally a exception is thrown (did you mistakenly wrote finally block, instead of catch block here?).

I removed view.forward() in validate. Still I get this error.

WARNING: StandardWrapperValve[validate]: PWC1406: Servlet.service() for servlet validate threw exception
java.lang.IllegalStateException: PWC1227: Cannot forward after response has been committed
at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:370)
at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:350)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:300)
at Controller.validate.processRequest(validate.java:72)
at Controller.validate.doPost(validate.java:109)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:754)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98)
at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:330)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:174)
at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:828)
at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:725)
at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1019)
at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
at java.lang.Thread.run(Thread.java:722)

Validate

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package Controller;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 *
 * @author Zubair
 */
public class validate 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();
        List errorMsg=new LinkedList();
        errorMsg.clear();
        try {
            /*
             * TODO output your page here. You may use following sample code.
             
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet validate</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Servlet validate at " + request.getContextPath() + "</h1>");
            out.println("</body>");
            out.println("</html>");
            * 
            */
            String username=request.getParameter("username").trim();
            String password=request.getParameter("password").trim();
            if(username.isEmpty()&&password.isEmpty()){
                errorMsg.add("Please enter username and password.");
            }
            else if(username.isEmpty()){
                errorMsg.add("Please enter username.");
            }
            else if(password.isEmpty()){
                errorMsg.add("Please enter password.");
            }
            if(!errorMsg.isEmpty()){
                RequestDispatcher view=request.getRequestDispatcher("login.view");
                view.forward(request, response);
            }
            HttpSession session = request.getSession(true);
            session.setAttribute("username", username);
            RequestDispatcher view=request.getRequestDispatcher("home.view");
            view.forward(request, response);
        } finally {
            //errorMsg.add("Please Re-Login.");
            //RequestDispatcher view=request.getRequestDispatcher("login.view");
            //view.forward(request, response);
            out.close();
        }
    }

// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}

can you put this part of code.. in else condition and check once.

Add return; statement after each view.forward(), that should resolve the issue.

Kindly see the following code and corresponding errors which I get. The main idea is to display the errorMsg in the login page.

index.jsp

validate

import java.io.IOException;
import java.io.PrintWriter;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 *
 * @author Zubair
 */
public class validate 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();
        List errorMsg=new LinkedList();
        errorMsg.clear();
        try {
            /*
             * TODO output your page here. You may use following sample code.
             
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet validate</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Servlet validate at " + request.getContextPath() + "</h1>");
            out.println("</body>");
            out.println("</html>");
            * 
            */
            String username=request.getParameter("username").trim();
            String password=request.getParameter("password").trim();
            if(username.isEmpty()&&password.isEmpty()){
                errorMsg.add("Please enter username and password.");
            }
            else if(username.isEmpty()){
                errorMsg.add("Please enter username.");
            }
            else if(password.isEmpty()){
                errorMsg.add("Please enter password.");
            }
            request.setAttribute("errorMsg", errorMsg);
            if(!errorMsg.isEmpty()){
                RequestDispatcher view=request.getRequestDispatcher("login.view");
                view.forward(request, response);
                return;
            }
            HttpSession session = request.getSession(true);
            session.setAttribute("username", username);
            RequestDispatcher view=request.getRequestDispatcher("home.view");
            view.forward(request, response);
            return;
        } finally {
            //errorMsg.add("Please Re-Login.");
            //RequestDispatcher view=request.getRequestDispatcher("login.view");
            //view.forward(request, response);
            out.close();
        }
    }

// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}

login

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 *
 * @author Zubair
 */
public class login 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();
        List errorMsg=new LinkedList();
        errorMsg=(List)request.getAttribute("errorMsg");

try {
            /*
             * TODO output your page here. You may use following sample code.
             */
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Login</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<center><h1>Fast Learn</h1><center>");
            out.println("<br/><br/>");
            out.println("<form method=\"POST\" action=\"validate.do\">");
            out.println("<center>");
            out.println("<table border=\"0\" cellspacing=\"5\" cellpadding=\"5\">");
            out.println("<tr>");
            out.println("<td>Username</td>");
            out.println("<td><input type=\"text\" name=\"username\" size=\"30\"></td>");
            out.println("</tr>");
            out.println("<tr>");
            out.println("<td>Password</td>");
            out.println("<td><input type=\"password\" name=\"password\" size=\"30\"></td>");
            out.println("</tr>");
            if(!errorMsg.isEmpty())
            {
                out.println("<tr><td>");
                out.println("<font color=\"red\"");
                out.println("<p>");
                Iterator items=errorMsg.iterator();
                while(items.hasNext()){
                    out.println(items.next());
                    System.out.println(items.next());
                }
                out.println("</p>");
                out.println("</font>");
                out.println("</td></tr>");
            }
            out.println("<tr>");
            out.println("<td colspan=\"2\"><input type=\"submit\" value=\"Sign in\"></td>");
            out.println("</tr>");
            out.println("</table>");
            out.println("</center>");
            out.println("</form>");
            out.println("</body>");
            out.println("</html>");
        } finally {            
            out.close();
        }
    }

// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}

home

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 javax.servlet.http.HttpSession;

/**
 *
 * @author Zubair
 */
public class home 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();
        HttpSession session=request.getSession();
        try {
            /*
             * TODO output your page here. You may use following sample code.
             */
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet home</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<h4>Welcome "+session.getAttribute("username")+"</h4>");
            out.println("<br/><br/><br/>");
            out.println("<a href=\"logout.jsp\"><b>Logout</b></a>");
            out.println("</body>");
            out.println("</html>");
        } finally {            
            out.close();
        }
    }

// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

/**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}

Error in server log

WARNING: StandardWrapperValve[validate]: PWC1406: Servlet.service() for servlet validate threw exception
java.util.NoSuchElementException
at java.util.LinkedList$ListItr.next(LinkedList.java:888)
at View.login.processRequest(login.java:70)
at View.login.doPost(login.java:117)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:754)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539)
at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:787)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:649)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:483)
at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:454)
at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:350)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:300)
at Controller.validate.processRequest(validate.java:68)
at Controller.validate.doPost(validate.java:112)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:754)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:847)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1539)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:281)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:655)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:595)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:98)
at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:91)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:162)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:330)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:231)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:174)
at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:828)
at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:725)
at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1019)
at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
at java.lang.Thread.run(Thread.java:722)

Irfan Zubair wrote:login

while(items.hasNext()){ out.println(items.next()); System.out.println(items.next()); }

When you call items.next(), it moves forward in the iterator. If it can't (because there are no more elements), you get an exception.
It seems like you want to print the next element twice. You can't call next() twice for that; you need to call it once, cache its return value, and use that twice:

Thanks all for helping me.