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From "Ivelin Ivanov" <ive...@apache.org>
Subject Re: [jxpath] CompiledExpression [was: New features, bug fixes and documentation]
Date Mon, 29 Apr 2002 13:02:12 GMT

Thanks Dmitri. I think understand the difference now.

----- Original Message -----
From: "Dmitri Plotnikov" <dmitri@plotnix.com>
To: "Ivelin Ivanov" <ivelin@apache.org>; "Ovidiu Predescu"
<ovidiu@apache.org>
Cc: <cocoon-dev@xml.apache.org>
Sent: Sunday, April 28, 2002 8:42 AM
Subject: Re: [jxpath] CompiledExpression [was: New features, bug fixes and
documentation]


> Ivelin,
>
> > So how does the CompiledExpression differ from a JXPointer?
> >
> > When I needed to cache a dictionary of paths I dused a Map.Entry where
the
> > key is the path string (JXPointer.asPath() )and
> > the value is a JXPointer (which has the getValue() setValue() methods).
>
> CompiledExpression is the result of the _compilation_ of an XPath, while
> Pointer is the result of an _evaluation_ of an XPath, augmented with
> information about where that value was found.
>
> For example,  let's say your xpath looks like this: "//person[@name =
'John
> Doe']/phone". The corresponding Pointer.toString() may yield something
like
> "/addressBook/people/person[13]/phone".
>
> When you call Pointer.getValue(), it does not reevaluate the path - just
> gives you the result.  Also you can say Pointer.setValue("5551234") and
does
> not reevaluate the path - just changes the corresponding value.
>
> Regards,
>
> - Dmitri
>
>
> > ----- Original Message -----
> > From: "Dmitri Plotnikov" <dmitri@plotnix.com>
> > To: "Ovidiu Predescu" <ovidiu@apache.org>; "Ivelin Ivanov"
> > <ivelin@apache.org>
> > Cc: <cocoon-dev@xml.apache.org>
> > Sent: Saturday, April 27, 2002 11:46 PM
> > Subject: Re: [jxpath] CompiledExpression [was: New features, bug fixes
and
> > documentation]
> >
> >
> > > Ovidiu,
> > >
> > > I have added to JXPath the explicit caching you proposed in your
earlier
> > > submission:
> > >
> > > [... snip...]
> > >
> > > > > >
> > > > > > > I've got a small suggestion for an optimization, if
> > > > > > > you don't mind. In
> > > > > > > the implementation I have, 0.1-dev from Jakarta's
> > > > > > > CVS, I noticed
> > > > > > > there's no way to compile an XPath, before using it.
> > > > > > > Having a way to
> > > > > > > compile the XPaths to be used later, would
> > > > > > > considerably improve the
> > > > > > > performance in repeated operations.
> > > > > > >
> > > > > > > E.g. right now you have something like this:
> > > > > > >
> > > > > > >   Object bean = ...;
> > > > > > >   JPathContext context =
> > > > > > > JPathContext.newContext(bean);
> > > > > > >   context.getValue("some XPath");
> > > > > > >
> > > > > > > I'd like to also have the ability to do:
> > > > > > >
> > > > > > >   Object bean = ...;
> > > > > > >   JPathContext context =
> > > > > > > JPathContext.newContext(bean);
> > > > > > >   XPathObject xpath = new XPathObject("some XPath");
> > > > > > >   context.getValue(xpath);
> > > > > > >
> > > > > > > With this approach, I can setup a dictionary of
> > > > > > > already compiled XPath
> > > > > > > expressions, and use compiled XPaths, instead of
> > > > > > > having Xalan
> > > > > > > interpret the XPath all the time.
> > > > > > >
> > > > > > > Last time I checked, Xalan did have the ability to
> > > > > > > precompile XPaths
> > > > > > > in its internal format, so maybe this is easy to
> > > > > > > achieve.
> > > > > > Actually JXPath is already doing something like that
> > > > > > transparently to you. It maintains a static soft cache
> > > > > > of parse trees (which are that internal representation
> > > > > > you are talking about). The reason I am using soft
> > > > > > cache as opposed to a regular map is that some
> > > > > > applications will generate XPaths dynamically, in
> > > > > > which case the size of the cache has a chance of going
> > > > > > through the roof.  The soft cache is cleared by
> > > > > > garbage collector, which removes the possibility of
> > > > > > running out of memory.
> > > > > >
> > > > > > I have been thinking about improving that mechanism a
> > > > > > little bit.  I now want to have a regular map of a
> > > > > > limited size in addition to the soft cache.
> > > > > > Everything that spills over the hard cache will still
> > > > > > be stored in the soft cache.  I think that will cover
> > > > > > the 90% case.
> > >
> > > I now see the need for caching external to JXPath:  you may want to
> > > precompile some expression at application start time, you may want to
> > check
> > > syntax before you use an expression etc.
> > >
> > > I called "XPathObject" "CompiledExpression". You make an explicit
call:
> > > context.compile(xpath) to get a CompiledExpression.  That method will
in
> > > fact check the internal cache before it proceeds with the compilation.
> > >
> > > Thanks again for the suggestion,
> > >
> > > Regards,
> > > - Dmitri
> > >
> >
> >
>


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