Hi all,I have some problem with Listener and load-on-startup,I serch on google
and I find this article below,it do a lot good to me, but I still want to ask ,
using the init() of a servlet and the contextDestroyed() of a listener are
equal to Iusing the contexInitialized() and the contextDestroyed() of a
listener?
On 9 Nov 2001, Dr. Evil wrote:
> Date: 9 Nov 2001 07:43:17 -0000
> From: Dr. Evil <[EMAIL PROTECTED]>
> Reply-To: Tomcat Users List <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED]
> Subject: API 2.3: Listener vs. load-on-startup
>
>
> I have a few classes that need to be loaded by Tomcat before it starts
> to serve any requests. One of these opens a database pool, another
> opens a logger, etc. The traditional way to do this is with
> load-on-startup like this:
>
> <servlet>
> <servlet-name>startlogging</servlet-name>
> <servlet-class>startlogging</servlet-class>
> <load-on-startup>1</load-on-startup>
> </servlet>
>
> which calls the init() method of the class.
>
> However, I need to have an object installed into the ServletContext
> object before any requests can be serviced. The init() method of a
> servlet seems to have no access to the ServletContext object (is this
> correct?).
You have access to the servlet context via the getServletContext() method
of the servlet -- it is pre-initialized to work for you. However, using a
listener is the better way to do this in a Servlet 2.3 environment.
> Because of this, I am thinking of using the new <listener>
> declaration to achieve this same goal. I can declare a listener like
> this:
>
> <listener>
> <listener-class>dostartupstuff</listener-class>
> </listener>
>
> and then in the dostartupstuff class, I declare a method:
>
> public void contexInitialized(ServletContextEvent e) { ... }
>
> which will be called as soon as the context is created.
>
> Is this the right way to do this?
Yes. That is exactly what context listeners are designed to do.
Why is it better than a servlet init() method? Because the container
gives you *no* guarantee that it will keep a servlet loaded for the
lifetime of the application (although many of them do), so if you clean up
your resources in the destroy() method -- such as closing database
connections -- this might happen to you at a bad time. The
contextDestroyed() method of your listener will not get called until the
application is really being shut down.
> And is there a way I can control
> the order in which listeners are called?
>
At startup time, listeners are called in the order they are defined in the
deployment descriptor -- at shutdown time, they are called in reverse
order. For more info, see the 2.3 spec:
http://java.sun.com/products/servlet/download.html
> Thanks
>
Craig
