public class Login extends HttpServlet {
private static final long serialVersionUID = 1L;
private static final Logger log = LogManager.getLogger();

/**
 * @see HttpServlet#HttpServlet()
 */
public Login() {
    super();
}

@SuppressWarnings("unchecked")
public void init(){


    Map<String, User> DB = null;

    if (this.getServletContext().getAttribute("DB") == null){

        DB = new HashMap<String, User>();

        User user1 = new User("user1", "pass1");
        user1.setFullName("The User 1");
        User user2 = new User("user2", "pass2");
        user2.setFullName("The User 2");

        DB.put("user1", user1);
        DB.put("user2", user2);

        log.debug("The following users have been added to the database: " + user1.getUsername() + " " + user2.getUsername());

    } else {

        DB = (HashMap<String, User>) this.getServletContext().getAttribute("DB");

    }

    if (DB.isEmpty()){
        log.info("No Users");
    }

    log.info("Usuarios en DB {}", DB.keySet());
}

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

    if (request.getSession(true).getAttribute("user") != null){

        request.getRequestDispatcher("/WEB-INF/views/session.jsp").forward(request, response);

    } else {
        doPost(request,response);
    }
}

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    List<String> errors = new ArrayList<String>();


    if (request.getParameter("inputUsername") != null && request.getParameter("inputPassword") != null ){

        String username = request.getParameter("inputUsername");
        String password = request.getParameter("inputPassword");

        if (username != null || password != null) {
            log.info("Autentificando a {}", username);
        }
        //This is the line where I get the second NullPointerException
        User user = login(username, password);

        if (user != null){

            request.getSession(true).setAttribute("user", user.getUsername());

            Cookie fullName = new Cookie("fullName", user.getFullName());
            response.addCookie(fullName);

        } else{
            log.error("Invalid username or password.");
            errors.add("Invalid username or password.");
            request.setAttribute("errors", errors);
            request.getRequestDispatcher("/WEB-INF/views/login.jsp").forward(request, response);
        }
    } else {

        log.warn("You must login first");
        errors.add("You must login first.");

        request.getRequestDispatcher("/WEB-INF/views/login.jsp").forward(request, response);
    }
}

private User login(String username, String password){


    if (username == null || password == null){

        return null;
    }

    HashMap<String, User> DB = null;
    DB = (HashMap<String, User>) getServletContext().getAttribute("DB");

    User user = (User) DB.get(username);


    if (user == null){

        return null;
    }

    if (!user.getPassword().equals(password.trim())){

        return null;
    }

    return user;

}

User user = login(username, password);

User user = (User) DB.get(username);