HttpSession interface

The HttpSession object represents a user session. A user session contains information about the user across multiple HTTP requests.

When a user enters your site for the first time, the user is given a unique ID to identify his session by. This ID is typically stored in a cookie or in a request parameter.

Here is how you access the session object:

• public HttpSession getSession():Returns the current session associated with this request, or if the request does not have a session, creates one.
• public HttpSession getSession(boolean create):Returns the current HttpSession associated with this request or, if there is no current session and create is true, returns a new session.

protected void doPost(HttpServletRequest request,
    HttpServletResponse response)
        throws ServletException, IOException {

    HttpSession session = request.getSession();
}

You can store values in the session object, and retrieve them later. First, let's see how you can store values in the session object:

session.setAttribute("userName", "theUserName");

To read the value again, you do this:

String userName = (String) session.getAttribute("userName");

Values stored in the session object are stored in the memory of the servlet container.

An object of HttpSession can be used to perform two tasks:

1. bind objects
2. view and manipulate information about a session, such as the session identifier, creation time, and last accessed time.

Commonly used methods of HttpSession interface

1. public String getId(): Returns a string containing the unique identifier value.
2. public long getCreationTime(): Returns the time when this session was created, measured in milliseconds since midnight January 1, 1970 GMT.
3. public long getLastAccessedTime(): Returns the last time the client sent a request associated with this session, as the number of milliseconds since midnight January 1, 1970 GMT.
4. public void invalidate(): Invalidates this session then unbinds any objects bound to it.

Sessions and Clusters

If you have an architecture with 2 web servers in a cluster, keep in mind that values stored in the session object of one server, may not be available in the session object on the other server. So, if a user's requests are divided evenly between the two servers, sometimes session values may be missing.

The solution to this problem would be one of:

1. Do not use session attributes.
2. Use a session database, into which session attributes are written, and from which it is read.
3. Use sticky session, where a user is always sent to the same server, throughout the whole session.

Servlet Related Posts

1. Java Servlets Overview
2. Servlet Life Cycle
3. Servlet Example
4. Difference between ServletConfig and ServletContext
   
5. Difference between GenericServlet and HttpServlet
   
6. What is web application?
7. Advantages of Servlets over CGI
8. GenericServlet Example
9. RequestDispatcher Example
10. ServletConfig
11. ServletContext
12. Servlet Filter Example
13. Database Access Example using Sevlet
14. File Uploading Example using Servlet