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how to redirect user for accessing to a page from URL in java?

I am coming to a problem where I have loginFilter where I dont want the user to access a page when they are logged in with the URL. So, all I want to do is redirect the user to the index page. Here is my code below. Thank you.
public class LoginFilter implements Filter {
#Override
public void init(final FilterConfig filterConfig) throws ServletException {
}
#Override
public void doFilter(final ServletRequest request, final ServletResponse response, final FilterChain chain) throws IOException, ServletException {
HttpServletRequest req = (HttpServletRequest)request;
String username = req.getHeader("username");
String password = req.getHeader("password");
if(username == null) {
chain.doFilter(request, response);
return;
}
try {
req.login(username, password);
chain.doFilter(request, response);
} catch (ServletException e) {
((HttpServletResponse)response).setStatus(StatusCodes.UNAUTHORIZED);
}
}
#Override
public void destroy() {
}
}

Change your method as :
HttpServletResponse res = (HttpServletResponse)response;
try {
req.login(username, password);
res.sendRedirect("/index");
} catch (ServletException e) {
// you can use SC_UNAUTHORIZED(401) from HttpServletResponse class as well
res.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
}

Java SSE method in doGet() not working

I wrote a method which return an Array converted as String. When calling this method in the main method and printing it out the array is filled. When I am calling the same method in the doGet method for printing it in my html file, the array is empty and it prints only: []
Normally the doGet method schould work because when the method return not the array but just "hello" the html file print the String.
Here ist the code:
public static String test(senderonpremise s){
String t;
//this should be printed
t = String.valueOf(s.arrivalList);
//startSending();
//this works in doGet()
//return "this works";
// when I return this it works in the main-method but not in DoGet()
return t;
}
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
resp.setContentType("text/event-stream");
resp.setCharacterEncoding("UTF-8");
senderonpremise s = new senderonpremise();
PrintWriter out = resp.getWriter();
String next = "data: " + test(s) + "\n\n";
out.write(next);
out.flush();
}
/**
public static void main(String[] args) {
senderonpremise s = new senderonpremise();
System.out.print(test(s));
}
**/

I recommend you using the JEaSSE library: https://github.com/mariomac/jeasse, which is lightweight and works out of the box with Servlets 3.x
#WebServlet(asyncSupported = true)
public class ExampleServlet1 extends HttpServlet {
EventTarget target;
#Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
target = new ServletEventTarget(req).ok().open();
}
public void onGivenEvent(String info) {
target.send("givenEvent",info);
}
}

NullPointerException with Servlet

I am calling a Servlet using its URL address. This is the URL I am typing
http://localhost:7001/ryan/olympics?action=selectCatalog&id=1
This is the Servlet's URL for sure; if I change the address I get
page not found
This is the code for the Servlet.
public class Servlet extends javax.servlet.http.HttpServlet
implements javax.servlet.Servlet {
private static final long serialVersionUID = 1L;
public Servlet() {
super();
}
public void init(ServletConfig config) throws ServletException {
System.out.println("*** initializing controller servlet.");
super.init(config);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
String action = request.getParameter("action");
if (action.equals("selectCatalog")) {
String categoryId = request.getParameter("id");
ProductModelDAO dao4 = new ProductModelDAOImpl("jpac");
if (categoryId != null && !categoryId.trim().equals("")) {
CategoryDAO dao1 = new CategoryDAOImpl("jpac");
try {
Category category = dao1.getCategoryName(categoryId);
request.setAttribute("category", category);
} catch (Exception e) {
e.printStackTrace();
}
}
try {
#SuppressWarnings("unchecked")
List<Product> products = dao4
.getProductsByCategory(categoryId);
request.setAttribute("products", products);
} catch (Exception e) {
e.printStackTrace();
}
url = "SelectCatalog.jsp";
RequestDispatcher requestDispatcher =
getServletContext().getRequestDispatcher(url);
requestDispatcher.forward(request, response);
I get the NullPointerException pointing to the RequestDispatcher's line. Any help?

Try changing "SelectCatalog.jsp" to "/SelectCatalog.jsp", because, as I understand, You want to use an absolute path.

If you want to use a relative path you have to use:
request.getRequestDispatcher(url);
in place of:
getServletContext().getRequestDispatcher(url);

request.getParameter("action");
code is written in doPost method. Are you invoking this servlet from doPost method of calling servlet? URL parameters will not be used by doPost method.

Null Pointer Exception while communicating between servlets

I have login code in one servlet:LoginGtalkServlet..
XMPPConnection connection;
//#see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String userName = request.getParameter("username");
String password = request.getParameter("password");
System.out.println(userName);
System.out.println(password);
//ProxyInfo proxyInfo = new
ConnectionConfiguration config = new ConnectionConfiguration("talk.google.com",5222,"gmail.com");
connection = new XMPPConnection(config);
config.setSASLAuthenticationEnabled(false);
try {
connection.connect();
} catch (XMPPException e) {
e.printStackTrace();
}
try {
connection.login(userName, password);
} catch (XMPPException e) {
e.printStackTrace();
}
boolean status=connection.isAuthenticated();
if(status==true)
{
System.out.println("Success");
response.sendRedirect("GetRoster");
}
else
{
response.sendRedirect("Loginfailed.html");
}
And my GetRosterServlet has the code for retreving the roster list
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
Roster roster = connection.getRoster();
Collection<RosterEntry> entries = roster.getEntries();
for (RosterEntry r : entries) {
String user = r.getUser();
String name = r.getName();
System.out.println(name + user);
}
roster.addRosterListener(new RosterListener() {
// Ignored events public void entriesAdded(Collection<String>
// addresses) {}
public void entriesDeleted(Collection<String> addresses) {
}
public void entriesUpdated(Collection<String> addresses) {
}
public void presenceChanged(Presence presence) {
System.out.println("Presence changed: " + presence.getFrom()
+ " " + presence);
}
#Override
public void entriesAdded(Collection<String> arg0) {
}
});
}
Now the isssue is my GetRosterServlet is not informed that I haved logged into Gtalk..i.e LoginGtalkServlet is not communicating with GetRosterServlet and hence connection.getRoster() is throwing Null pointer exception...
How do I let the roster servlet know that the user is logged into gtalk and hence get the friends list

Note that I'm not familiar with XMPP/GTalk.
When you log in to it in LoginGtalkServlet, is it possible for you to be handed back some sort of session ID? If so, you could put that into the redirect URL as a query parameter or in a POST body and then GetRosterServlet could extract the session ID from request and use it to Gtalk via that session ID.
(Note: the edit I just made was just to fix some bad grammar. No substantive changes.)

You can save the connection object in the session attribute.
session .setAttribute("connection", connectionObj);
And can get by using session.getAttribute("connection");

Can I implement HttpSessionListener this way?

I'm trying to tracking valid user Ids in my Java servlet, can I implement HttpSessionListener this way ?
public class my_Servlet extends HttpServlet implements HttpSessionListener
{
String User_Id;
static Vector<String> Valid_User_Id_Vector=new Vector<String>();
private static int activeSessions=0;
public void sessionCreated(HttpSessionEvent se)
{
// associate User_Id with session Id;
// add User_Id to Valid_User_Id_Vector
Out(" sessionCreated : "+se.getSession().getId());
activeSessions++;
}
public void sessionDestroyed(HttpSessionEvent se)
{
if (activeSessions>0)
{
// remove User_Id from Valid_User_Id_Vector by identifing it's session Id
Out(" sessionDestroyed : "+se.getSession().getId());
activeSessions--;
}
}
public static int getActiveSessions()
{
return activeSessions;
}
public void init(ServletConfig config) throws ServletException
{
}
public void destroy()
{
}
protected void processRequest(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException
{
User_Id=request.getParameter("User_Id");
}
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";
}
}
How to get the listener notified when a session ends ? I'm trying to bypass "/WEB-INF.web.xml" all together, is it doable ? Or does it make sense ?

This won't bypass /WEB-INF/web.xml. Furthermore, you'll end up with 2 instances of this class, not 1 performing both functions. I suggest you put this Vector in the ServletContext and have 2 separate classes.
In the servlet, you get to it via getServletContext(). In the listener, you'll do something like this:
public void sessionCreated(HttpSessionEvent se) {
Vector ids = (Vector) se.getSession().getServletContext().getAttribute("currentUserIds");
//manipulate ids
}