[
https://issues.apache.org/jira/browse/XALANJ-2540?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13085727#comment-13085727
]
Michael Glavassevich commented on XALANJ-2540:
----------------------------------------------
Element e = (Element)
document.getElementsByTagName("SomeElementName").item(0);
String result = ((Element) e).getTextContent();
is very fast because Xerces' implementation of getElementsByTagName() is lazy.
It immediately returns a NodeList which is initially empty and fills it as you
call methods on it. If you only call item(0) on it, it only walks up to the
first element which matches the tag name you specified. If that happens to be
the root element or one very early in the document it will return quickly.
> Very inefficient default behaviour for looking up DTMManager
> ------------------------------------------------------------
>
> Key: XALANJ-2540
> URL: https://issues.apache.org/jira/browse/XALANJ-2540
> Project: XalanJ2
> Issue Type: Improvement
> Security Level: No security risk; visible to anyone(Ordinary problems in
> Xalan projects. Anybody can view the issue.)
> Components: DTM, XPath
> Affects Versions: 2.7.1, 2.7
> Reporter: Lukas Eder
>
> I have analysed an issue that has been bothering me for some time. When
> executing XPath evaluations, it looks like a very significant amount of time
> is spent in the initialisation of the XPathContext. I have asked this
> question on Stack Overflow and answered it myself:
> http://stackoverflow.com/questions/6340802/java-xpath-apache-jaxp-implementation-performance
> I think the default behaviour of
> org.apache.xml.dtm.ObjectFactory.lookUpFactoryClassName() is quite
> sub-optimal and should be improved, statically. I imagine, it is unlikely
> that this configuration is going to change once classes have been loaded.
> Hence, the fallback lookup of META-INF/service/org.apache.xml.dtm.DTMManager
> should only be done once.
> For reference, here's the question and answer again in JIRA:
> ----
> I have come to an astonishing conclusion that this:
> Element e = (Element)
> document.getElementsByTagName("SomeElementName").item(0);
> String result = ((Element) e).getTextContent();
> Seems to be an incredible 100x faster than this:
> // Accounts for 30%, can be cached
> XPathFactory factory = XPathFactory.newInstance();
> // Negligible
> XPath xpath = factory.newXPath();
> // Accounts for 70% (caching a compiled expression doesn't change much...)
> String result = (String) xpath.evaluate(
> "//SomeElementName", document, XPathConstants.STRING);
> I'm using the JVM's default implementation of JAXP:
> org.apache.xpath.jaxp.XPathFactoryImpl
> org.apache.xpath.jaxp.XPathImpl
> I'm really confused, because it's easy to see how JAXP could optimise the
> above XPath query to actually execute a simple getElementsByTagName()
> instead. But it doesn't seem to do that. This problem is limited to around
> 5-6 frequently used XPath calls, that are abstracted and hidden by an API.
> Those queries involve simple paths (e.g. /a/b/c, no variables, conditions)
> against an always available DOM Document only. So, if an optimisation can be
> done, it will be quite easy to achieve.
> ----
> I have debugged and profiled my test-case and Xalan/JAXP in general. I
> managed to identify the big major problem in
> org.apache.xml.dtm.ObjectFactory.lookUpFactoryClassName()
> It can be seen that every one of the 10k test XPath evaluations led to the
> classloader trying to lookup the DTMManager instance in some sort of default
> configuration. This configuration is not loaded into memory but accessed
> every time. Furthermore, this access seems to be protected by a lock on the
> ObjectFactory.class itself. When the access fails (by default), then the
> configuration is loaded from the xalan.jar file's
> META-INF/service/org.apache.xml.dtm.DTMManager
> configuration file. Every time!:
> Fortunately, this behaviour can be overridden by specifying a JVM parameter
> like this:
> -Dorg.apache.xml.dtm.DTMManager=
> org.apache.xml.dtm.ref.DTMManagerDefault
> or
> -Dcom.sun.org.apache.xml.internal.dtm.DTMManager=
> com.sun.org.apache.xml.internal.dtm.ref.DTMManagerDefault
> So here's a performance improvement overview for 10k consecutive XPath
> evaluations of //SomeNodeName against a 90k XML file (measured with
> System.nanoTime():
> measured library : Xalan 2.7.0 | Xalan 2.7.1 | Saxon-HE 9.3 | jaxen
> 1.1.3
> --------------------------------------------------------------------------------
> without optimisation : 10400ms | 4717ms | |
> 25500ms
> reusing XPathFactory : 5995ms | 2829ms | |
> reusing XPath : 5900ms | 2890ms | |
> reusing XPathExpression : 5800ms | 2915ms | 16000ms |
> 25000ms
> adding the JVM param : 1163ms | 761ms | n/a |
--
This message is automatically generated by JIRA.
For more information on JIRA, see: http://www.atlassian.com/software/jira
---------------------------------------------------------------------
To unsubscribe, e-mail: xalan-dev-unsubscr...@xml.apache.org
For additional commands, e-mail: xalan-dev-h...@xml.apache.org
