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From "Dmitri Plotnikov" <dmi...@plotnix.com>
Subject Re: [jxpath] CompiledExpression [was: New features, bug fixes and documentation]
Date Sun, 28 Apr 2002 13:42:35 GMT
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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