Here is what you can try. Since Nagios has tcp and http plugins which you can use to monitor your web application.
The command which you can use to monitor the port is :
/usr/local/nagios/libexec/check_http -I ip_address_of_localhost -p port_address -u url_of_webapp
This will help you to check your web application.
I hope the above information will be helpful to you.