XOM API for Processing XML with Java

[Elliotte Harold <elharo AT metalab.unc.edu>]

Wolfgang has asked me to demonstrate whether it's really necessary to 
use equality (equals()) comparison instead identity (==) comparison when 
looking for namespace URIs in Verifier's cache. Herewith are my results. 
The tests I'm showing here are performed with the 
nu.xom.benchmarks.Reproducer class run on http://www.rddl.org/. This 
document only uses three namespaces. The parser is Xerces 2.6.x. The VM 
is Apple's latest 1.4 VM. The test was run inside Eclipse, which 
shouldn't be a big deal since I'm only looking to see whether we're 
hitting th cache or not, not at the raw performance, but I'll mention 
that just in case it proves to be significant later.

First I added some echo printing to the URICache contains() method to 
check whether XOM is hitting the cache or not. Only testing for equals() 
here's what I see:

Cache miss: http://www.xom.nu/fakeRoot
Cache miss: http://www.w3.org/1999/xhtml
Cache miss: http://www.w3.org/1999/xlink
Cache miss: http://www.rddl.org/
Cache miss: http://www.rddl.org/
Cache miss: http://www.w3.org/1999/xhtml
Cache miss: http://www.w3.org/XML/1998/namespace
Cache miss: http://www.w3.org/1999/xhtml
Cache miss: http://www.w3.org/1999/xhtml
a lot more misses (mostly of http://www.w3.org/1999/xhtml)...
Cache miss: http://www.w3.org/1999/xhtml
Cache miss: http://www.rddl.org/
Cache miss: http://www.w3.org/1999/xlink
Cache miss: http://www.w3.org/1999/xhtml

The total was almost 700 cache misses. Now, using equals() comparison:

Cache miss: http://www.xom.nu/fakeRoot
Cache miss: http://www.w3.org/1999/xhtml
Cache miss: http://www.w3.org/1999/xlink
Cache miss: http://www.rddl.org/
Cache miss: http://www.w3.org/XML/1998/namespace

It misses exactly five times, once per URI.

Curious, I decided to see how often we're hitting the cache. I adjusted 
the echo printing and found we were hitting the cache 1033 times with 
simply an == comparison, so it's not that every namespace URI reported 
is a new object. Some of them are indeed interned. If I turn equals() 
testing back on, the cache is hit 1712 times, and missed 5 times. This 
is as expected (1033+684-5=1712).

This is all with a cache size of 6. Doubling the cache size to 12 only 
reduces the cache misses with == comparison to 656, not a significant 
drop. It does not appear that they're just a few extra nonidentical 
strings.

This is all quite surprising. It may be a bug in Xerces. It may be some 
unintended effect of code in XOM. It may be something I haven't yet 
conceived of. I will investigate further.

--
Elliotte Rusty Harold  elharo AT metalab.unc.edu
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/
http://www.amazon.com/exec/obidos/ISBN=0596007647/cafeaulaitA/ref=nosim