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From "Geir Magnusson Jr." <ge...@optonline.net>
Subject To Singleton, or not to Singlton, that is the question...
Date Mon, 16 Jul 2001 03:45:24 GMT
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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