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Unable to redirect to another page in AEM using request dispatcher in sling servlet via POST

I have a scenario where from one page in AEM, I need to call another AEM page in the same application and I need to pass some hidden parameters. I choose to do it via POST and below are the steps which I followed:
From page "A", I did a form submission via POST to the sling servlet and passed some parameters.
2. In the servlet, using request dispatcher I redirected the same request and response to a different page in doPost method using the following code snippet:
request.getRequestDispatcher("/content/company/en/apps/welcomepage.html").forward(request, response);
When I run the code, I am able to call the servlet through form submission but I am not able to redirect to a new page. I see the below error in logs:
18.10.2017 14:41:00.802 ERROR [127.0.0.1 [1508352060795] POST /bin/rap/welcomepage HTTP/1.1] org.apache.sling.servlets.post.impl.operations.ModifyOperation Exception during response processing.
javax.jcr.nodetype.ConstraintViolationException: No matching property definition: appointmentTypeId = platform001d
at org.apache.jackrabbit.oak.jcr.delegate.NodeDelegate.setProperty(NodeDelegate.java:522)
at org.apache.jackrabbit.oak.jcr.session.NodeImpl$35.perform(NodeImpl.java:1375)
at org.apache.jackrabbit.oak.jcr.session.NodeImpl$35.perform(NodeImpl.java:1363)
at org.apache.jackrabbit.oak.jcr.delegate.SessionDelegate.perform(SessionDelegate.java:208)
at org.apache.jackrabbit.oak.jcr.session.ItemImpl.perform(ItemImpl.java:112)
at org.apache.jackrabbit.oak.jcr.session.NodeImpl.internalSetProperty(NodeImpl.java:1363)
If I try the same code in doGet method it works fine. Also if I use response.sendRedirect("/content/company/en/apps/welcomepage.html") it works fine too. But the problem with this is it initiates it as a new request to the page and it looses all the parameters which I get from the form submission. Could someone please let me know like how can I redirect a request to a page in AEM via POST since I need to pass some hidden parameters whic should not be visible in the url ?

Here is the way I understand your question
User visits page "A"
User fills a form, then submits.
Your custom servlet handles the submitted POST request and calls:
request.getRequestDispatcher("/content/company/en/apps/welcomepage.html").forward(request, response);
You get the ConstraintViolationException
Why do you get this exception?
Since you are using forward the POST request is forwarded to /content/company/en/apps/welcomepage.html that node is most likely of type cq:Page, which has constraints on which properties can be added. Think of it as a simple post request trying to store parameters on the cq:Page node.
What can you do?
Since I don't understand your use-case and particularly why you need to preserve the submit params, I cannot recommend a specific solution. However, since you don't want the parameters in the URL, here is a potential solution you can try:
In your servlet handler, see those hidden params to cookies on the response.
use response.sendRedirect("/content/company/en/apps/welcomepage.html")
On any of the components in /content/company/en/apps/welcomepage.html, you can get the request cookies and process them however you like. same way you wanted to process those hidden params.
Now the flow becomes:
User visits page "A"
User fills a form, then submits.
Your custom servlet handles the submitted POST request, adds your special params to cookies on the response, then calls response.sendRedirect("/content/company/en/apps/welcomepage.html")
User's browser receives a 301 response with the cookies, sets the cookies in the browser then requests "/content/company/en/apps/welcomepage.html"
Your components handle the request and get the params from the cookies, then retuns the appropriate response.

what is in background of html submit form?

I'm trying to figure out what really happens when html submit form button is clicked.
I suppose it generates some kind of http request (similar to ajax get or post call) which has data in http body and is sent to address specified in action field.
1) Am I right?
2) I've seen many ways of processing forms with PHP or ASP on server side. Can I process it with Java REST Application using e.g. Jersey? Is submit form capable of hitting REST if I put right URL in action field?
Thank You.

By submitting the form in HTML you basically tell the browser to generate a normal HTTP request, usually POST or GET, for an URL defined in tag with form fields attached according to the specified method either appended to the URL or included in the request data.
There is nothing really special or different from a "normal" HTTP request, in fact you can manually "submit a form" by appending form keys and values to the URL in your browser and navigating to it in case of GET method.
Summarizing:
1) Yes, you are right.
2) From what I've just read (never used REST personally) a REST application is implemented by a servlet mechanism and uses HTTP protocol, so it should be possible to write a REST application for processing HTML forms if the form points to this application's URL.

Establish Connection First, Redirect User Second

I have an idea to make something pretty sweet but I'm not sure if it's possible. Here is an example of a very basic ajax function that I might use to establish a connection a server...
function getFakePage(userId)
{
var ajaxObject, path, params;
ajaxObject = getAjaxObject();
params = "?userId=" + userId
path = getInternalPath() + "someServlet" + params;
ajaxObject.open("GET", path, true);
ajaxObject.send();
// On ready state change stuff here
}
So let's say I have a URL like this...
https://localhost:8443/Instride/user/1/admin
And I wanted to use javascript to redirect the user to this this URL. Normally I would just do this...
window.location = "https://localhost:8443/Instride/user/1/admin";
But my idea is to create a javascript (no js frameworks please) function that could combine the ajax code with the window.location code. Basically what I would like to accomplish is to create a connection with the server via ajax, send a servlet on that server the url I would like the user to be redirected to, and then redirect the user to that URL. So that for however long it takes the user to connect to my server from wherever they are in the world they see a loading icon instead of a blank white page.
So to clarify exactly what I am trying to accomplish; I do not want to put window.location within the success of my ajax function (because that would be encompass two round trips), and I do not want to return a huge chunk of HTML for the requested resource and add it to the page. I want to establish a connection to the server with ajax, send a servlet the URL the user wants to go to, and then somehow override the ajax function to redirect that user. Is this possible?
And I know some of you might think this is stupid but it's not when you're talking about overseas users with slow dial up connections staring at white pages. If it's possible, I'd love to hear some insight. Thank you very much!

First, let me say that the best solution is finding what is causing the slowness and fixing it.
Now as to your question, yes you could do it. You could even shoehorn it onto an existing application. But it wouldn't be pretty. And it comes with it's own set of problems. But here are the steps:
Browser calls ajax cache service requesting "somepage.html"
Browser loads loading icon
Server creates somepage.html and caches it in a temporary cache, (ehcache or other library would be good, probably with file backing for the cache depending on size)
Server responds to ajax request with ID for cached page
Browser now redirects to "somepage.html?cacheId={cacheId}" where the id is from the ajax call.
Server uses a filter to see if any cache can be served up for the page instead of the actual page, thus speeding up the request.
Having said that, it would be better to just have the new page load quickly with a loading icon while it did any of the heavy lifting through ajax.

You can't do an AJAX request and a location change in one. If you want to do only one request you have to choose one of those methods. ie. return some data and replace content on your current page, or load a completely new page.
It doesn't make any sense to want to want to do both. What you could want is stateful URLs; where your URL matches the content displayed, even if that content comes from an AJAX request. In that case an easy solution is the use the # part of the URL which you can change freely (window.location.hash). Some modern browsers support changing the whole URL without causing the page to reload. I've used # with great success myself.

How to get textbox value in the same JSP without submiting it?

Actually i want to use the textbox value and set the session parameter in the same JSP page without submitting it or like using request or response object. This textbox value i want to use in the same JSP page for further use. How can i access the value of a text box in the same page?

You could either utilize the new HTML5 local storage (only supported in the more recent/modern browsers), or you could create a session cookie in JavaScript and store the value in there.
Note that none of those approaches will affect the server side HttpSession in any way. For that you simply can't go around sending a HTTP request containing the desired information, as that's the only way to send information from the client to server side. You could however consider using ajax to send the HTTP request asynchronously and fully transparently in the background.

how do i know whether my html link is clicked or not from Servlet?

My Servlet response type is html and my response contains a hyperlink to another web site.So, now i want to capture the information about whether the user clicked the link or not? and also calculate the total clicks? i am using Tomcat 7 as a server.
Is this possible in setting response header (302 or 404)?...
Please Guide me to get out of this issue?

Yes, you can use a 302: instead of providing the link to the other website, you provide a link to your own servlet, do your accounting and then send back a redirection (301/302) http status with the other web-site URL in the response Location header.
This maybe a bit simplistic though, since the user will leave your original page (is this what you want ?) and search engines may not like this if your web app is public.

I think right now you are redirecting the request(link for another website) at client side.In this approach your server cannot get the information about the click.
What you can do create a servlet and call this servlet on click now this servlet is responsible to redirect the request to another website. Add an static integer counter and increment this when servlet call each time.

Use the method setStatus():-
setStatus(HttpServletResponse.SC_FOUND);
or
setStatus(HttpServletResponse.SC_NOT_FOUND);