Servlet life cycle, and Java Servlet variable scope
Archive - Originally posted on "The Horse's Mouth" - 2009-05-16 07:31:01 - Graham Ellis
If you write a 'traditional' program and run it ... then it is cleared from memory when you finish running it. It does its job then exits. And so it is with a java program that you run from the command line under the java command.But if you're running a web server, where lots of users all call up the same page, you've got a different requirement / scenario. You really don't want a piece of code to keep dropping out of memory to be reloaded yet again a few milliseconds later as your next user comes along, as that would be hugely inefficient. So on a web server, when running Java, you may use the servlet approach.
A Servlet is loaded into a container (we provide training on the Tomcat container) and remains loaded - i.e. running as a daemon on service. Each time a user submits a GET request to the web server / servlet (i.e. follows a link to the URL that points to the servlet, or sends a default setup form to it), the doGet method is run which processes a request object into a response object. And each time the user POSTs to the servlet's URL, the doPost method is used to convert a request to a response.
When you're writing the class ("code") for a Java Servlet, then, you need to be very careful that you don't have one user leave behind variables / settings which the next user could pick up. On rare occasions, this "global scope" IS useful - for example to hold the highest bid so far for an item in an auction - but more usually, you'll want to use a "session scope" where a separate set of variables / settings are used for each user as they mover from page to page, adding things to their shopping cart. They you'll also have "local scope" - variables that are only used within an individual request.
Global scope variables may be defined as statics within the servlet class as a whole, session scope variables within a session object, and local variables within the methods of the servlet.
Init and finalise on my diagram show methods used to initialise a servlet when it's first loaded into the container, and used to flush it out when it's no longer needed after a long period of inactivity