Tutorials : Server-Side Web Applications Using Servlets and JSP :
: Session Tracking

The Session Tracking API

Using sessions in servlets is quite straightforward, and involves looking up the session object associated with the current request, creating a new session object when necessary, looking up information associated with a session, storing information in a session, and discarding completed or abandoned sessions.

2.1 Looking up the HttpSession object associated with the current request.

This is done by calling the getSession method of HttpServletRequest. If this returns null, you can create a new session, but this is so commonly done that there is an option to automatically create a new session if there isn't one already. Just pass true to getSession. Thus, your first step usually looks like this:

  HttpSession session = request.getSession(true);

2.2 Looking up Information Associated with a Session.

HttpSession objects live on the server; they're just automatically associated with the requester by a behind-the-scenes mechanism like cookies or URL-rewriting. These session objects have a builtin data structure that let you store any number of keys and associated values. In version 2.1 and earlier of the servlet API, you use getValue("key") to look up a previously stored value. The return type is Object, so you have to do a typecast to whatever more specific type of data was associated with that key in the session. The return value is null if there is no such attribute. In version 2.2, getValue is deprecated in favor of getAttribute, both because of the better naming match with setAttribute (the match for getValue is putValue, not setValue), and because setAttribute lets you use an attached HttpSessionBindingListener to monitor values, while putValue doesn't. Nevertheless, since few commercial servlet engines yet support version 2.2, I'll use getValue in my examples. Here's one representative example, assuming ShoppingCart is some class you've defined yourself that stores information on items being purchased.

  HttpSession session = request.getSession(true);
  ShoppingCart previousItems =
    (ShoppingCart)session.getValue("previousItems");
  if (previousItems != null) {
    doSomethingWith(previousItems);
  } else {
    previousItems = new ShoppingCart(...);
    doSomethingElseWith(previousItems);
  }

In most cases, you have a specific attribute name in mind, and want to find the value (if any) already associated with it. However, you can also discover all the attribute names in a given session by calling getValueNames, which returns a String array. In version 2.2, use getAttributeNames, which has a better name and which is more consistent in that it returns an Enumeration, just like the getHeaders and getParameterNames methods of HttpServletRequest.

Although the data that was explicitly associated with a session is the part you care most about, there are some other pieces of information that are sometimes useful as well.

• getId. This method returns the unique identifier generated for each session. It is sometimes used as the key name when there is only a single value associated with a session, or when logging information about previous sessions.
• isNew. This returns true if the client (browser) has never seen the session, usually because it was just created rather than being referenced by an incoming client request. It returns false for preexisting sessions.
• getCreationTime. This returns the time, in milliseconds since the epoch, at which the session was made. To get a value useful for printing out, pass the value to the Date constructor or the setTimeInMillis method of GregorianCalendar.
• getLastAccessedTime. This returns the time, in milliseconds since the epoch, at which the session was last sent from the client.
• getMaxInactiveInterval. This returns the amount of time, in seconds, that a session should go without access before being automatically invalidated. A negative value indicates that the session should never timeout.

2.3 Associating Information with a Session

As discussed in the previous section, you read information associated with a session by using getValue (or getAttribute in version 2.2 of the servlet spec). To specify information, you use putValue (or setAttribute in version 2.2), supplying a key and a value. Note that putValue replaces any previous values. Sometimes that's what you want (as with the referringPage entry in the example below), but other times you want to retrieve a previous value and augment it (as with the previousItems entry below). Here's an example:

  HttpSession session = request.getSession(true);
  session.putValue("referringPage", request.getHeader("Referer"));
  ShoppingCart previousItems =
    (ShoppingCart)session.getValue("previousItems");
  if (previousItems == null) {
    previousItems = new ShoppingCart(...);
  }
  String itemID = request.getParameter("itemID");
  previousItems.addEntry(Catalog.getEntry(itemID));
  // You still have to do putValue, not just modify the cart, since
  // the cart may be new and thus not already stored in the session.
  session.putValue("previousItems", previousItems);

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