Servlet Session Tracking Persistent information A server site typically needs to maintain two kinds of persistent remembered information Information about the session A session starts when the user logs in or otherwise identifies himself herself and continues until the user logs out or completes the transaction for example makes a purchase Information about the user User information must generally be maintained much longer than session information for example remembering a purchase This information must be stored on the server for example on a file or in a database 2 Server capabilities Servlets like Applets can be trusted or untrusted A servlet can use a unique ID to store and retrieve information about a given session User information usually requires a login ID and a password Since servlets don t quit between requests any servlet can maintain information in its internal data structures as long as the server keeps running A trusted servlet can read and write files on the server hence can maintain information about sessions and users even when the server is stopped and restarted An untrusted servlet will lose all information when the servlet or server stops for any reason This is sometimes good enough for session information This is almost never good enough for user information 3 Session tracking HTTP is stateless When it gets a page request it has no memory of any previous requests from the same client This makes it difficult to hold a conversation Typical example Putting things one at a time into a shopping cart then checking out each page request must somehow be associated with previous requests The server must be able to keep track of multiple conversations with multiple users Session tracking is keeping track of what has gone before in this particular conversation Since HTTP is stateless it does not do this for you You have to do it yourself in your servlets You can do this by maintaining a session ID for each user 4 Session tracking solutions Hidden form fields can be used to store a unique ID for the session Cookies are small files that the servlet can store on the client computer and retrieve later URL rewriting You can append a unique ID after the URL to identify the user Java s Session Tracking API can be used to do most of the work for you 5 Hidden form fields input type hidden name sessionID value Advantages All you need to know is how to read servlet parameters Efficient Minimizes repeated calls to the server Disadvantages String sessionID getParameter sessionID out println input type hidden name sessionID value sessionID Information is lost when browser quits or goes to another page Useless for maintaining persistent information about a user Can be spoofed Since the session ID must be incorporated into every HTML page every HTML page must be dynamically generated Hidden fields are good for session tracking holding a conversation with the user they re simple and efficient 6 Using hidden form fields I The very first request that the user sends you will typically have null for the value of your hidden field Each subsequent request will include this hidden field When your servlet sees the null it can assign a unique session ID and include it in a hidden field in the response The servlet can keep session information in some data structure of its own keyed by the session ID This is feasible because the servlet does not quit between requests so it can maintain information in its memory You cannot assume the user will end the session the way you think she should say by logging off If the session data is sufficiently old you need to assume the user isn t coming back and discard the session data 7 Using hidden form fields II The session ID does not have to be the only hidden field You can have other fields in addition to or instead of a session ID field This might be a good way to keep track of small amounts of simple information during a session Hidden fields are not particularly well suited to holding complex or structured information In all cases hidden form fields are good only for storing session information Information in servlet data structures will eventually be lost when the servlet quits or get old and be discarded 8 Cookies A cookie is a small bit of text sent to the client that can be read again later Limitations for the protection of the client Not more than 4KB per cookie more than enough in general Not more than 20 cookies per site Not more than 300 cookies total Cookies are not a security threat Cookies can be a privacy threat Cookies can be used to customize advertisements Outlook Express allows cookies to be embedded in email A servlet can read your cookies Incompetent companies might keep your credit card info in a cookie Netscape and Firefox let you refuse cookies to sites other than that to which you connected 9 Using cookies import javax servlet http Constructor Cookie String name String value Assuming request is an HttpServletRequest and response is an HttpServletResponse response addCookie cookie Cookie cookies request getCookies String name cookies i getName String value cookies i getValue There are of course many more methods in the HttpServletRequest HttpServletResponse and Cookie classes in the javax servlet http package 10 Some more Cookie methods public void setComment String purpose public String getComment public void setMaxAge int expiry public int getMaxAge Max age in seconds after which cookie will expire If expiry is negative delete when browser exits If expiry is zero delete cookie immediately setSecure boolean flag public boolean getSecure Indicates to the browser whether the cookie should only be sent using a secure protocol such as HTTPS or SSL 11 What cookies are good for Advantages Java s Session Tracking API to be discussed makes cookies dead simple to use Cookies can easily contain more data than hidden fields Data is stored on the client computer not on yours Disadvantages Data is stored on the client computer not on yours This saves space on the server May let you avoid keeping track of multiple session data structures This means the data is neither safe nor secure Should not be used for user data cookies may be discarded or the user may contact the server from another computer Users can tell their browser to turn cookies off Cookies are good for keeping session data not user data 12 Java s session tracking API I Here s how you get a session ID from the request Here s what this does for you HttpSession session request
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