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Subject Re: Avalon explainations
Date Thu, 10 Aug 2000 09:06:48 GMT
On Thu, 10 Aug 2000, Eric SCHAEFFER wrote:
> I work on FOP, and I'd like to make it Avalon aware. Can someone explain to
> me how Avalon configuration framework should be used ?

Unfortunately at this current moment the Avalon list is
going through it. There are some parties who would remove
the Configuration interface completely and do everything
through JNDI. 

I am trying to stop that but unfortunately I am lone in my
venture so it looks like all Configuration and
ComponentManager could be dropped soon. 

In which case it may be best to hold off converting FOP just
yet to see the outcome. 

> I've got several questions:
> 1- Does the component manager should create a new instance of the requested
> component on each getComponent() method call, or does it should only return
> an already
> created instance (during initialization) ?

A ComponentManager just associates particular component
roles with a component. Usually that means a simple lookup
in a hashtable or iteration over already existing
components. The avalon SimpleComponentManager simply looks
in a hastable and if found it returns it. Otherwise it
checks if it has a parent ComponentManager and returns the
role in it's parent's context. This allows a hierarchy of
component managers.

So really it should only return created instances but if
there is a particular reason to not behave this way it is
possible for you to change this. Cocoon itself is the
ComponentManager and it dynamically creates new components
during lookup IIRC. However ideally you should populate it
before you pass it to the component.  

> 2- What is the difference between Component and NamedComponent, which one to
> choose and why ?

NamedComponent is useful when you have multiple components
that have same role. For instance Cocoon has multiple
generators. A normal ComponentManager can't determine the
difference between all of them, but if you give them names
you can extract a particular named component from manager.

> In fact, my problem is that, during the FO -> "internal representation"
> process (layouting process), I need to use a factory class instance. This
> factory instance is initialized by a configuration file, but it would be
> stupid to create a new instance of this class each time I want to use it...

Sounds like it is crying out for the lazy singlton design
pattern. ie

class MyClass {

 protected static MyClass singleton;
 public MyClass getMyClass() {
   if( null == singleton ) {
     singleton = createMyClass();
   return singleton;

> On the other hand, I understand that for a "parser" component, it would be
> better that
> getComponent() return a new instance...

ComponentManager wouldn't be the right place to get a parser
then. The ComponentManager may return a factory that you can
use to create Parsers and that would be an acceptable way to
use it.

> 3- If you want to give parameters to a component, you write, in the config
> file, something like
> <parameter name="..." value="..."/>
> But if you want to give it a list of classes, without "names", what is the
> "good" way of doing it ?
> Ex.:
>     ImageReaders are classes that read image headers, check if it is an
> image type it can handle, and read the width and height of the image.
>   <component role="image-reader-factory"
> class="org.apache.fop.image.analyser.ImageReaderFactory">
>     <image-reader class="org.apache.fop.image.analyser.GIFReader"/>
>     <image-reader class="org.apache.fop.image.analyser.JPEGReader"/>
>     <image-reader class="org.apache.fop.image.analyser.PNGReader"/>
>     <image-reader class="org.apache.fop.image.analyser.BMPReader"/>
>   </component>
> This factory needs parameters, but it's just a list of classes, and not
> named parameters.

Unfortunately there is no "good" way of doing it. Actually
there is very little way of doing it all with
ComponentManagers. I need this functionality for my app so I
am trying to get this put into a ComponentManager but I keep
getting told it is bad design practice :<. Thou I will still
work on them to try and get it allowed :D.



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