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From Jon Wingfield <jon.wingfi...@mkodo.com>
Subject Re: Listener vs. load-on-startup
Date Tue, 24 Apr 2007 11:55:37 GMT
The order of events is:
Startup:
All listeners get called.
All filters, load-on-startup servlets get inited.

Shutdown:
All filters, servlets get destroyed.
All listeners get called.

So, it is possible for the combination of load-on-startup init +
contextDestroyed to do the same job as just using one
ServletContextListener. But why do it? IMO load-on-startup was a hack in
the early spec versions so that application data could be set up.
ServletContextListener was added as the way forward ;)

Regards,

Jon

lizhg wrote:
> 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 <drevil@sidereal.kz>
>> Reply-To: Tomcat Users List <tomcat-user@jakarta.apache.org>
>> To: tomcat-user@jakarta.apache.org
>> 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
> 
> 



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