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From piero de salvia <pie...@yahoo.com>
Subject Re: To Singleton, or not to Singlton, that is the question...
Date Mon, 16 Jul 2001 14:08:54 GMT
Hi Geir,

I was, as some of you may remember, a big fan of the
instantiable-at-will engine, so my vote is a
resounding "YES". i will start testing away as soon as
possible. 

In my opinion this gives a developer new and exciting
ways to use Velocity, which after all is a template
engine and not just a web page generator.

Velocity developers ROCK!

piero de salvia


--- "Geir Magnusson Jr." <geirm@optonline.net> wrote:
> Once Brian finishes the CVS migration (Thanks,
> Brian!) to the new
> server, I will put into whiteboard/geir a new jar,
> source tgz and
> example proggie that is a successful attempt to
> convert Velocity to
> allow multiple concurrent instances of the template
> engine in a given
> program.
> 
> I won't commit changes to CVS until people agree
> this is a good way to
> go.  I don't want to have to back this change out :)
>  It's very broad
> and touches most if not all parts of Velocity.
> 
> Summary
> =======
> 
> You now have the option of how you want to use the
> Velocity template
> engine in your applications :
> 
> 1) The current model of using the Velocity class to
> access the singlton
> pattern is still valid - this jar should drop in and
> work anywhere the
> exsting production or nightly jars work.
> 
> 2) There is a new class
> org.apache.velocity.app.VelocityEngine that can
> be used in the way you suspect
> 
> VelocityEngine ve = new VelocityEngine();
> 
> and VelocityEngine currently has the same interface
> as Velocity, so
> everything you can do now with the singleton will
> work with the
> instance.
> 
> Details
> =======
> 
> The basic idea is that all parts of velocity
> depended upon Runtime for
> core services in that they would call things like 
> Runtime.error( msg )
> to log, for example.  So, the class Runtime was
> renamed RuntimeInstance,
> a new interface RuntimeServices was created (that
> has most of the old
> Runtime methods), and a referece to RuntimeInstance
> (which implements
> RuntimeServices) is passed into the various
> subcomponents of Velocity
> for use to replace using the singleton for services.
>  To do this, any
> other singleton classes are now instance classes
> (ResourceManager, for
> example) as well, and a few things had to be
> slightly modified to break
> singleton assumptions.
> 
> So, the meta API of Velocity is :
> 
> VelocityEngine : new class that is a new-able class
> to provide separate
> instances, delegates all calls to a RuntimeInstance.
>  With it you can :
> 
> VelocityEngine ve1 = new VelocityEngine();
> VelocityEngine ve2 = new VelocityEngine();
> 
> and then configure and init() each separately, as
> you would hope for...
> 
> Velocity : the old Singleton application helper
> class. Nothing has
> changed. Delegates calls to the RuntimeSingleton
> class.  You can and
> should continue to use this for the Singleton model.
> 
> RuntimeSingleton : renamed Runtime class, this is
> the static class for
> Singleton usage.  
> 
> Runtime : new deprecated wrapper class, delegates to
> RuntimeSingleton. 
> I want to get rid of this if we go forward as I
> think that
> RuntimeSingleton is a more descriptive class name. 
> Anyway, you
> shouldn't be accessing the class directly in your
> applications - that's
> what Velocity is for.
> 
> The nice thing is that you can continue to use the
> Singleton model, or
> even use both models concurrently.
> 
> I converted Anakia to use an instance, but the
> structure of Texen
> requires more work than I cared to do at the time,
> so it still requires
> the singleton model.  This can be converted later,
> or left as is.  It's
> up to the Texen tribe.  I will note that the
> failures we have been
> seeing from Gump lately are due to a new classloader
> scheme in Ant that
> Sam is trying which I believe could be solved by
> converting Texen to the
> new instance model.
> 
> The only real change is that anything implementing
> LogSystem must now
> add an init( RuntimeServices) method, as the logger
> factory made this
> necessary to pass the RuntimeServices.  I don't
> think this will be
> something that affects many people at all.  It would
> only affect you if
> you created your own personal logger class that is
> instantiated.  If you
> have a logger class that you pass to Velocity as a
> living instance (yes,
> you can do that - see the logger test and examples),
> all is ok.  We can
> discuss this - if it's too large a burden, we can
> try to redo, but I
> think that it would be counter productive in the
> long run.
> 
> So, take a look at the code, and more importantly,
> test it out in
> existing apps to make sure all is still healthy, and
> in new apps that
> you were planning. There has been more and more
> interest in seeing this
> solution, so lets give this a try.
> 
> Note that there is no performance penalty for this
> (it actually sped up
> as I found some spurious initialization of the
> syntax tree when going
> through this :)
> 
> Also, the example program is called InstanceExample,
> and is very very
> simple. It was inspired by the needs of Turbine (or
> what I understand
> their needs to be...) in that you setup two engines,
> and give each a
> different template path, and then ask for the same
> template from each -
> so you can have a default top level template
> index.vm for each of your
> webapps, and don't really have to worry other than
> at setup time where
> the templates are - you can make the same request
> into any engine...
> 
> geir
> 
> 
> 
> -- 
> Geir Magnusson Jr.                          
> geirm@optonline.net
> System and Software Consulting
> Developing for the web?  See
> http://jakarta.apache.org/velocity/
> You have a genius for suggesting things I've come a
> cropper with!


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