How to make struts2 web application thread -safe?

I am using struts2 framework for creating a java web application.There i am calling a method in a class to perform appointment operation for users. My problem is , when the time slot in the page is clicked by multiple users, the appointments method is called and appointment are made for all the users who clicked on the same slot and values of the users are stored in the database also , but the slot must be alloted for the single user only? How to make the method to be thread -safe ?

Please help me to solve the problem... ,