CS651

Servlet Basic Structure

Lifecycle

IMPORTANT: each time the same servlet is invoked, ONLY a new thread is created, NOT a new independent process.

First Time Servlet Invoked:

Nth Time Servlet Invoked (N>1):

Servlet Instance is going to be illiminated by server:

Standard Methods of the HttpServlet class

init()

init(ServletConfig c)

You will have only one of the init methods implemented in your servlet clas
Invoked only the first time that the servlet is invoked.
Used like init() method of applet to initialize parameters, possilby setup persistent connections, perform one-time compuations, etc.
Version of init with ServletConfig parameter used to read server-specific parameters.

Server-Specific parameters stored in a pre-specified file (see documentation for your server).

example of a server file containing such parameters or inside of web.xml file
example code of a simple servlet using the file above.
location of such a file is dependent on the server configuration, see documentation

Use the getInitParameter method to retrieve a parameter value from this file. See API, use like getParameter method of Applet class.
generally do only for a minimal number of parameters as using by default a shared server file with other programmers.
If do not like the concept of editing this server-side file (which by default is one file shared with everyone), can simple perform File I/O with a file located anywhere on the server your servlet would have access to.

NOTE: if you are implementing the init with the ServletConfig parameter, you MUST make the call super.init(config) as the first line of your method!!! This is because the ServletConfig object is used elsewhere in the servlet and the default (parent) method will register it for use.

service(HttpServletRequest req, HttpServletResponse res)

This method is called whenever the servlet is invoked, the first or Nth time.
It is responsible for checking the HTTP request type (GET,POST, PUT, DELETE, OPTIONS, TRACE, etc.) and calling the appropriate servlet method doGet, doPost, doPut, doDelete (see below).
TIP: In general you do not want to override the service method....instead implement the methods it will call (doGet, etc). Otherwise, you must do this HTTP request type parsing yourself.
WARNING: as you can have multiple requests (simultaneous or near) for a servlet, note that you can have the default service method calling the appropriate doGet, etc. methods and these can execute at the same time. Hence in ALL the doXXX methods you MUST be careful to synchronize access to shared data!!!!!

To allow only one servlet instance (thread) to run at a time have your servlet implement the SingleThreadModel interface. This queues up servlet requests and allows only one to run at a time. BUT, YOU STILL MUST synchronize access to to shared data, as threads can swap running.

doGet(HttpServletRequest req, HttpServletResponse res)

GET requests generated by: user typing in a URL on address line of browser, following a link from a Web-page, or submits an HTML form that either specifies GET as its METHOD or does not specify a MEHTOD type.

System uses status line and header settings of doGet to answer HEAD (HTTP type) requests. System returns whatever headers and status code doGet sets, but omits the web-page body.

doPost(HttpServletRequest req, HttpServletResponse res)

POST requests generated by: user submits an HTML form that specifies POST as its METHOD

doDelete

Called by default service method to handle HTTP DELETE requests made by user.

doOptions

Called by default service method to handle HTTP OPTIONS requests made by user.

doTrace

Called by default service method to handle HTTP TRACE requests made by user.
TRACE is an HTTP request type used for client debugging. It returns the HTTP request headers back to the client

destroy

Whenever the server decides to remove a servlet instance (because requested to do so, or servlet is idle for a long time), this method is called.
Allows you to clean up transactions, close-down connections, etc. Like the destroy method for the Applet class.

getServletContext

returns a ServletContext object that can be used to store persistent data and this is shared by ALL servlets currently running in the server's servlet engine (this means not just your servlet or its many instances but, any servlet in the same engine can have access to this info)
ServletContent has the following useful methods: