Nginx and Tomcat - subdomain pointing to subdirectory

I have the following configuration for my application example.com:

1. Nginx as a reverse proxy to redirect traffic from port 80 to:
2. Tomcat 7 where I deploy my JSF app, port 8080
3. My xhtml files are mostly in the ROOT folder: /usr/local/tomcat7/webapps/ROOT
4. But I have some files in a separate folder "forum": /usr/local/tomcat7/webapps/ROOT/forum

This is my default nginx configuration:

server {
    server_name  www.example.com;
    return 301 $scheme://example.com$request_uri;
}
server {
  listen          80;
  server_name     example.com.br;
  root            /usr/local/tomcat7/webapps/ROOT;
  location / {
        proxy_set_header X-Forwarded-Host $host;
        proxy_set_header X-Forwarded-Server $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_pass http://127.0.0.1:8080/;
  }
}

So, to access the forum page I have to go to example.com/forum/forum.xhtml (forum.xhtml is the index)

Now, I'm trying to set a subdomain, forum.example.com, in nginx, that will point to the forum folder, keeping the URL. That is, I don't want ever to see the sufix "/forum/" in the URL. I want that the forum folder behaves as the root. So if the user tries forum.example.com/anotherPageFromROOT.xhtml he will not be able to.

This is what I have so far:

server {
 listen          80;
 server_name     forum.example.com;

 location / {
       proxy_set_header X-Forwarded-Host $host;
       proxy_set_header X-Forwarded-Server $host;
       proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
       proxy_pass http://127.0.0.1:8080/;
       rewrite ^/$ /forum/forum.xhtml break;    
 }
}

Problems:

1. If I click in a link (POST request) to the file "topic.xhtml" also inside the "forum" folder the URL will be forum.example.com/forum/topic.xhtml instead of forum.example.com/topic.xhtml as I want.
2. The user is able to access forum.example.com/anotherPageFromROOT.xhtml normally

Could some one please help me on what I should do? Thanks!

EDIT: I think the root cause for the first problem is the action generated by JSF for my h:form. It is always generated with /forum/.

Copyright Notice：Content Author:「qxlab」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/30769891/nginx-and-tomcat-subdomain-pointing-to-subdirectory