Behavior check of Web Appllication for multiple requests

I have a web application running of Tomcat server. I want to test the behavior of the code that I have written for the simultaneous requests hitting the server. Say, I have two functionalities A and B in my web application. Now, I have to test the behavior of the code for multiple simultaneous requests for A and similarly for B.(Multiple requests from one user or multiple users) I have googled about how to test webApp for simultaneous but could not find any concrete. Could someone please tell how can I achieve this?