Hi Eitan and all,
I was also thinking about integration of JiBX and XPath recently. I was
looking for a possibility to run XPath queries over Java object trees
(JOTs), which are "somehow" connected to XML data ("somehow" means, that
this could for example be described by a JiBX binding.) While jaxen and
JXPath claim to support this, in both cases I wasn't satisfied with the
exisiting techniques. The JXPath support for JOT works, but the XPath
expression for querying an object tree is not necessarily equal to the
expression for querying the according XML document if you want to obtain
equal results. Jaxen's support for JOT seems not to be implemented
completely, and there were some remarks on the jaxen developer list
about some problems which need to be solved beforehand. One of these
mentioned problems was that in JOTs a set of the XPath axes is not
clearly defined. This especially counts for the "parent" axis, and all
axes which are based on the document order, like "descendant" and
"ascendant" axes.
This is where a binding framework like JiBX comes into play, because a
binding as in JiBX exactly defines the axes missing in Java. With a
binding definition you always know the "document place" of a member in a
JOT. To run XPath on the JOT using the same expressions as if you would
query the XML document, you would not even need an attribute
"xpath='...'" like Eitan proposed (Or did I miss something in that idea?)
I am wondering about possible use cases of the discussed techniques. In
our current project we are using XUpdate (including XPath) for a
protocol to synchronize distributed instances of XML documents which are
represented in JOTs. Here we really need an integration of
(un)marshalling and XPath/XUpdate functionality for JOTs. We do not use
JiBX currently (because I wasn't the one to decide which data binding
framework to use :) but a combination of JDOM and handmade code for
(un)marshalling and a prototypical extension for jaxen to run
XPath/XUpdate over bound JOTs. While this works quite OK, the code is
rather blown up and the performance is of course weak (our prototypical
binding framework for XPath queries uses Java reflection API).
Which use cases could other people on this list imagine? One could
argue, that when using XML techniques like XPath and XUpdate to access
your data, (un)marshalling to a customized JOT becomes obsolete. One
could also just use a common DOM implementation to represent your data.
(I think the article mentioned by Eitan points in this direction.) Is
there the common need for "native" Java access to data
(Object.getMemeber()) combined with XPath access (/node/@attribute) to
the same data?
Regards,
Tilman
Eitan Suez wrote:
hi all,
while on developerworks reading dennis' articles, i also
came across this one:
http://www-128.ibm.com/developerworks/java/library/x-pracdb8.html
entitled: 'xpath as a data binding tool, part 1'
i have to admit i just skimmed the article but it grabbed
my attention. i've been thinking for a while that adding
xpath to jibx would be a good move.
more specifically what i mean is that the jibx binding
vocabulary could be extended nicely with xpath support.
here's an example binding file:
<binding name="binding1">
<mapping name="customer" class="simple.Customer">
<structure field="name">
<value name="first-name" field="firstName"/>
<value name="last-name" field="lastName"/>
</structure>
<value name="street" field="street1"/>
<value name="city" field="city"/>
<value name="state" field="state"/>
<value name="zip" field="zip"/>
<value name="phone" field="phone"/>
</mapping>
</binding>
in jibx we use the attribute 'name' to bind a specific
element or attribute name to a java field. i think that could
be a perfect spot in the jibx binding file for introducing
an alternative to name="" : xpath=""
as in:
<value xpath="name/first-name" field="firstName" />
i know this is not a complete description of how such an
enhancement would work or fit in to jibx. but it's a starting
point for consideration, and definitely for discussion.
/ eitan
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