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From "Chris Cheshire" <cheshira...@gmail.com>
Subject Re: [digester] WAS: FactoryCreateRule help NOW: Custom Rule help
Date Wed, 13 Sep 2006 21:05:28 GMT
Thanks Simon, I figured this would be the case, but I wanted to make sure.

The solution I have come up with is to make a custom rule that parses
that element into a helper class, then uses the VariableFactory to
instantiate the right Variable subclass.

Just to make sure I have this right in begin() I am pushing an
instance of the helper class (DefaultVariable) on the digester stack.

In end() I am popping it, creating a new subclass and adding it to the
list (which was one above on the stack). At the end of end() should I
be pushing this new Variable back on the stack after I add it to the
list (top of the stack), or is it correct to just consume it?

Thanks

Chris

On 9/13/06, Simon Kitching <skitching@apache.org> wrote:
> Hi Chris,
>
> On Tue, 2006-09-12 at 18:01 -0700, Chris Cheshire wrote:
> > I have to process an XML file that contains (in part) the following structure
> > <variables>
> >   <Variable>
> >     <ID>...</ID>
> >     <Name>...</Name>
> >     <Instance>...</Instance>
> >     <Value>...</Value>
> >   </Variable>
> > </variables>
> >
> > I have a predefined list of variables based upon their ID, an abstract
> > Variable class, and then concrete subclasses of these for each valid
> > ID.
> >
> > Variable contains a static method Variable getInstance(int ID) that
> > creates a new variable subclass based upon the ID (the id-subclass
> > mapping is based upon a properties file).
> >
> > The FactoryCreateRule allows for creation of classes based via a
> > factory providing the identifiying parameter is an XML attribute. In
> > this case it is not. It is a sub-element, I have no control over that.
> >
> > How can I use the FactoryCreateRule to call getInstance(int ID) in my
> > Variable class when I need to extract the ID from a nested element not
> > an attribute?
>
> This is quite a common question, and really deserves an FAQ entry.
>
> In short, you can't. Digester is a SAX-based system, and with SAX there
> is no "look ahead" option.
>
> One possible solution is to use an XSLT stylesheet to pre-process your
> xml to move all the data needed to determine the object type into
> attributes of the element that triggers the object creation. Then the
> FactoryCreateRule can be used as normal.
>
> It might be possible to create a variant of the FactoryCreateRule that
> creates a hashmap or similar when the start element is found, store
> child objects in that hashmap until the end element is found, then looks
> into the hashmap to get the data to determine the type of object to
> instantiate and copy the hashmap contents into properties of the created
> object. It all seems rather clumsy/fragile though. If you do wish to try
> this, and get it to work then please put that info in the digester wiki
> as you're not the first person to ask about this.
>
> Regards,
>
> Simon
>
>
>
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