JavaServer Pages, 3rd Edition

Memory Usage Considerations

You should be aware that all objects you save in the application and session scopes take up memory in the server process. It’s easy to calculate how much memory is used for the application scope because you have full control over the number of objects you place there. But the total number of objects in the session scope depends on the number of concurrent sessions, so in addition to the size of each object, you also need to know how many concurrent users you have and how long a session lasts. Let’s look at an example.

The CartBean used in this chapter is small. It stores only references to ProductBean instances, not copies of the beans. An object reference in Java is 8 bytes, so with three products in the cart we need 24 bytes. The java.util.Vector object used to hold the references adds some overhead, say 32 bytes. All in all, we need 56 bytes per shopping cart bean with three products.

If this is a site with a modest amount of customers, you may have 10 users shopping per hour. The default timeout for a session is 30 minutes, so let’s say that at any given moment, you have 10 active users and another 10 sessions that aren’t active but have not timed out yet. This gives a total of 20 sessions times 56 bytes per session, a total of 1,120 bytes. In other words, roughly 1 KB—nothing to worry about.

Now let’s say your site becomes extremely popular, with 2,000 customers per hour. Using the same method to calculate the number of concurrent sessions as before, ...