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Finding why an Angular 5 service subscription is returning a null observable

I have the following service subscription in my component:
this.userService.loginUser(this.user).subscribe(users => {
if (users == null) {
console.log('users is null');
this.isValid = !this.isValid;
} else {
this.userService.subscribers.next(users);
localStorage.setItem('user', JSON.stringify(users));
console.log(`User, ${this.user.username}, successfully logged in!`);
console.log(localStorage.getItem('user'));
this.router.navigate(['dashboard']);
}
});
The service method looks like this:
public loginUser(user: User) {
console.log(`Attempting to login user: ${user.username}`);
const json = JSON.stringify(user);
this.currentUser = user;
return this.http.post<User>(API_URL + 'login', json, HTTP_OPTIONS);
}
The post request goes to this java servlet method:
#Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("login attempt");
if (req.getInputStream() != null) {
ObjectMapper mapper = new ObjectMapper();
User user = mapper.readValue(req.getInputStream(), User.class);
user = userService.loginUser(user);
if (user != null) {
user.setPassword("");
HttpSession session = req.getSession();
session.setAttribute("user", user);
}
}
}
On the java end, a username and password match is found and the session attribute is correctly set. However, on the Angular end, 'users' is null, as evidenced by the console.log('users is null') in the first code block. So my question is, why is this null? Can someone please help me understand the mechanics of this? I'm having a very hard time grasping how certain aspects of Angular even work...

So the problem was that in the servlet I didn't have a response. At the end I needed
PrintWriter out = resp.getWriter();
out.write(mapper.writeValueAsString(user));

session attribute is being set to an object in Java, but it is null in the JSP. Why is the JSP using a new session?

I ran the code in the debugger and confirmed that the object was being created in the Java code and was null in the JSP. Why is the JSP using a new session?
In debugger, it goes into Java code and sets the captcha in a session with an id. When I run the JSP, it gets the session with a different id, fails, then goes into doGet() and sets the current id session with a new captcha object. Storing captcha in a session, but that session isn't being used when the JSP runs.
Here are some code snippets
Java:
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
ColoredEdgesWordRenderer wordRenderer = new ColoredEdgesWordRenderer(COLORS, FONTS);
Captcha captcha = new Captcha.Builder(_width, _height).
addText().addNoise().
addBackground(new BrightGradiatedBackgroundProducer()).
build();
CaptchaServletUtil.writeImage(resp, captcha.getImage());
req.getSession().setAttribute("simpleCaptcha", captcha); // object is getting set
}
JSP:
session=request.getSession(false);
if (session==null)
session=request.getSession(true);
boolean isCaptchaTrue = false;
String strCaptcha = request.getParameter("captcha");
String captchaType = request.getParameter("captchaType");
if (strCaptcha != null && captchaType != null) {
if(session.getAttribute("simpleCaptcha") instanceof Captcha){
Captcha captcha = (Captcha) session.getAttribute("simpleCaptcha");
isCaptchaTrue = captcha.isCorrect(strCaptcha);
}else if(session.getAttribute("simpleCaptcha") instanceof AudioCaptcha){
AudioCaptcha captcha = (AudioCaptcha) session.getAttribute("simpleCaptcha");
isCaptchaTrue = captcha.isCorrect(strCaptcha);
}
}

I found my problem. At the top of the JSP the session was being invalidated. I removed session.invalidate() and it works now.

How to check in servlet that is an user new with session?

I am very new to java servlet programming. I have been writing a simple program for practicing java session. There are two .jsp file. first one called index.jsp, and another one is selection.jsp. And there is a servlet called controller. At first the index.jsp will be called, and user will be submit a input. That will be redirect in servlet controller. In that servlet will check whether it is new request or not. If new then it redirect to other page, else will do some other work.
I am checking whether it is new request or not by session.isNew() method. But it always says it is not new session. But, if I disable the browser cookies option then it is working fine. Now what is my observation is that when in the first I request the index.jsp to the container it assign a session along with that request. So when it comes to servlet it treat as a old session. I got this idea from Head first book Servlet and JSP.
Here is my servlet code -
public class Controller extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String user;
HttpSession session = request.getSession(false);
if (session == null) {
user = request.getParameter("user");
if (user == null) {
response.sendRedirect("index.jsp");
}
session.setAttribute("username", user);
SelectItem selectItem = new SelectItem();
selectItem.setUser(user);
response.sendRedirect("selection.jsp");
session.setAttribute("selectItem", selectItem);
} else {
String selectionItem = request.getParameter("selection");
SelectItem selectItem = (SelectItem) session.getAttribute("selectItem");
if (selectItem != null) {
selectItem.add(selectionItem);
session.setAttribute("selectItem", selectItem);
}
response.sendRedirect("selection.jsp");
}
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}
So, how to determine whether it is a new session or old one? Thank you.

HttpSession.isNew API:
Returns true if the client does not yet know about the session or if the client chooses not to join the session. For example, if the server used only cookie-based sessions, and the client had disabled the use of cookies, then a session would be new on each request.
So, you're getting true because the client has cookies disabled. The "new session" check in done in the else block of this check:
HttpSession session = request.getSession(false);
if (session == null) {
// create new session
session = request.getSession();
} else {
// existing session so don't create
}
In your code, you don't appear to be creating a new session when a new session is detected. Perhaps that's where you're stumbling.
Note: learning the basic Servlet API is a good thing. However, for my professional work I use frameworks which simplify my programming, like Spring Boot and Spring Security.

Declare a session variable golbaly to access from DoGet and DoPost in a sevlet

I have a servlet where I need to declare a session which can be acceptable form doGet and doPost both how I should do this?
I have done
#WebServlet(name = "LoginLogout", urlPatterns = {"/LoginLogout.do"})public class LoginLogout extends HttpServlet {//For Session
HttpSession session = request.getSession(true);
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
}
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String status = request.getParameter("status");
System.out.println(status);
}
#Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
try {
String loginId = request.getParameter("login_id");
String password = request.getParameter("password");
System.out.println(loginId);
//Inserting value to the Pogo named "newLoginPogo"
loginData newLoginPogo = new loginData();
newLoginPogo.setLoginId(loginId);
newLoginPogo.setPassword(password);
//Creating a obj of ModelLogin to send the loginId and Password via a method which is in ModelLogin class
ModelLogin loginBis = new ModelLogin();
loginData userData = loginBis.checkUser(newLoginPogo);
String userExist = userData.getUserExist();
System.out.println(userExist);
if ("yes".equals(userExist)) {
System.out.println("In while loop of Servlet");
String firstName = userData.getFirstName();
String userId = userData.getUserId();
boolean IsSu = userData.getIsSu();
//conveting boolean to string
String superuser = new Boolean(IsSu).toString();
//Creating a session
session.setAttribute("firstName", firstName);
session.setAttribute(userId, "userId");
session.setAttribute(superuser, "IsSu");
//==============================================================================================================
//If user does exist show the Success Message and forward Dashboard
//==============================================================================================================
//Session for success message
String succmsg = "Login Successful";
session.setAttribute("succmsg", succmsg);
getServletConfig().getServletContext().getRequestDispatcher("/WEB-INF/ViewPages/dashboard/dashboard.jsp").forward(request, response);
} //==============================================================================================================
//If user does not exist show the Error Message
//==============================================================================================================
else if ("no".equals(userExist)) {
//Session for success message
System.out.println("inside NO");
String emsg = "Login Error";
session.setAttribute("errmsg", emsg);
getServletConfig().getServletContext().getRequestDispatcher("/index.jsp").forward(request, response);
} else {
}
/*
//===============================================================================================================
//code for Logout
//===============================================================================================================
String status = request.getParameter("status");
if ("logout".equals(status)) {
//clearing the session
session.invalidate();
//forwarding to index page
getServletConfig().getServletContext().getRequestDispatcher("/index.jsp").forward(request, response);
}
*/
} finally {
}
}}
But it says
Can Not find Symbol
in this line HttpSession session = request.getSession(true);

You don't need to have session variable in servlet as field. In general - this is kind of common mistake. There will be only one onstance of servlet serving lots of requests, and unless you declare it as single-threaded - the requests would be handled concurrently.
HttpSession will be pre-exist for you in doGet and doPost via request object. Servlet container will guarantee this. So simply obtain reference to the session in doGet/doPost and do whatever you want.

What you desire is one of the roles of HTTP session.
You can look at it as a conversation between the client and the server.
As long as the "conversation" (HTTP session) is open and alive, you can set variables on the HTTP session, and access them from different requests that will sent on the same session.
Look at this as some sort of "shared memory" that exists during the "conversation time".
You can find many examples on how to do that over the internet.
Here is an example for session tracking.

What extra do I need to do while using HttpSession with GAE?

I read , to set a session attribute I need to put :
<sessions-enabled>true</sessions-enabled>
inside appengine-web.xml and also implement java.io.Serializable. (though I don't understand the reason for this !)
Following is one of the servlet that uses HttpSession :
#Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String userName = request.getParameter("username");
String password = request.getParameter("password");
ArrayList list = new ArrayList();
if(userName.compareTo("user") != 0 ) {
list.add("Wrong Username");
} else if(password.compareTo("password") != 0) {
list.add("Wrong Password");
}
if(list.isEmpty()) {
HttpSession session = request.getSession();
if(session.isNew()) {
session.setAttribute("UserRole", "PW :Admin");
session.setMaxInactiveInterval(900);
RequestDispatcher rd = request.getRequestDispatcher("abc/cpanel/PcPanel.jsp");
request.setAttribute("SessionStatus", "JC"); // Just Created
rd.forward(request, response);
}
} else {
response.sendRedirect("abc/cpanel/PcPanel_Login.jsp");
}
}
After validating the username and password and entering the if block,request should be forwarded to PcPanel Login.jsp but it doesn't happen.Intead a blank page appears with the address of this servlet. But if I remove/comment all the session junk,it works fine. Why does it happen ? Am I missing something or I making a mistake somewhere ?

Don't use isNew(). isNew() is only true when session is first established = when user first lands on any of your pages.
See the answer to this question: Session is NOT working - GAE/J