Re: xpath, dom and multi threading

"Mike Schilling" <>
Tue, 18 May 2010 17:34:23 -0700
FrenKy wrote:

On 17.5.2010 15:48, Lew wrote:

What part do you want to be thread safe, parsing the XML document or
accessing the DOM that results?

The former is almost certainly not practicable. The second boils
down to what you do to make any object model thread safe.


I'm building the DOM in a single thread and then I'm reading it in
several threads (usually not more then 20, depending on number of
CPUs, e.g. I'm running it sometimes on 100+ CPU machines).
But sometimes (seldom) I get NullPointer exception on most unexpected
locations during read operations... But _always_ when I've already
built XML. Threads are started after xml file is built. So I figured
I'm doing something wrong with multithreading and sync. Same
application ran in single threading mode does not throw

Are you using Xerces for XPath? It builds another representation of the DOM
(called a DTM) on which to run the XPath expressions, and it builds it
incrementally. Thus, even if the DOM is fully built, and thus safe to
travese, running XPath on it in two threads can result in exceptions. You
should be OK if you

1. Don't access a DOM until it's fully built.
2. Synchronize all use of XPath.

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