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* Bob Foster <bob@objfac.com> [2004-12-31 18:03]:
> Yes, thanks for that.
>
> I have a question, though. What is the guaranteed lifetime of an object
> appearing in a SAX event, like an Attributes object, and any objects
> used to implement it? If, for example, Attributes were implemented as a
> collection of lightweight Attribute objects that were re-used for
> subsequent events, the event data could not be passed directly to
> parallel threads without copying it. (Or by joining at the end of every
> event, which would rather limit the parallelism.)
Xerces recycles Attributes structures for each call to
startElement.
In my library, I keep a stack of attribute structures. The
attribute structures on the stack are recycled for each element
depth, not actually popped and reallocated.
I copy over the values in SAX Attributes to an attributes
structure on this stack, but SAX Attributes are all Strings and
in Java Strings are immutable, so this is really a bunch of
pointer assignments (and the adjustment of an array length parameter).
Not too expensive to keep that stack around.
(Because of this, I've come to see streaming problems as SAX
connected stacks of elements. If I need to transform a
document, I chain SAX Strategy Handlers. This, rather
than allow a Strategy to fiddle with its stack within
the handler.)
The characters event is interesting, becuase it is an index into
the parse buffer (in theory, and on Xerces indeed), but a
characters evet is only ever at the top of the stack. I only
ever need one.
In SAX Strategy, all of the lexemes in the events have a
getImmutable() method that will return an immutable copy (or
return itself it it is immutable) for when a series of events
needs to be recoreded.
(Not yet implemented, but if one was buffering and releasing
nodes, they could use the mutable lexemes and events to
implement a cache.)
I need to look harder, but I suspect that the handful of
workhorse SAX ContentHandlers I use, that I get from outside my
library, are probably self contained. Things like DOM4J's
ContentBuilder, and the SAXTransformers of Saxon, via TRAX.
--
Alan Gutierrez - alan@engrm.com
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