Attributes in Servlet

An attribute is an object set into one of three other servlet API objects
1. ServletContext: All the servlet/JSP can set/get data. It is not thread safe.
2. HttpServletRequest (or ServletRequest):Servlet can set data and JSP view can get data
3. HttpSession: For each session, servlets/JSP have access. Session can persists across multiple requests from the same client. Session attribute is not thread safe. For example: Shopping cart.

As ServletContext is not thread safe, it is recommended to make call to get/set method under synchronized block.
To be noted that, all other servlet should synchronizes on the ServletContext.

synchronized(getServletContext())  
{  
  getServletContext().setAttribute("adminEmail","admin@email.com");  
  out.println(getServletContext().getAttribute("adminEmail"));  
} 

As Session is not thread safe, it is recommended to make call to get/set method under synchronized block.
To be noted that, all other servlet should synchronizes on the Session.

HttpSession session=request.getSession();
synchronized(session)  
{  
  session.setAttribute("supportEmail","support@email.com");  
  out.println(session.getAttribute("supportEmail"));  
}