Re: [xmlblaster] server side exception: Can't process XPath expression

[Andrew Yinger <andrew.yinger at gmail.com>]

with version 2.1.0 of the server, i still (although rarely) see the
server-side error.  it seems to be encountered only during unit
testing, when many simultaneous publish/subscribe events are
triggered.  is it possible that there is an XPathFactory instance
somewhere else that is not properly synchronized?  (i am not positive
that the error is caused by an XPathFactory synchronization issue --
i'm just basing that diagnosis on earlier post regarding sporadic,
invalid, server-side xpath expression errors).

regards,
andrew


On Sat, Aug 25, 2012 at 2:07 AM, Marcel Ruff <mr at marcelruff.info> wrote:
> Am 24.08.2012 05:01, schrieb Andrew Yinger:
>
>> i believe this issue was identified before, but i occasionally see a
>> server-side exception that starts off like:
>>
>>     errorCode=user.query.invalid message=#exported Sorry, can't access,
>> query syntax of
>>     ....
>>     XmlBlasterException errorCode=[resource.configuration]
>> serverSideException=true location=[XmlNotPortable]
>>      message=[#exported Can't process XPath expression .....
>>
>>
>> it was first suggested that this was a parsing issue -- a possible xml
>> parser library conflict.  (i have verified that i do *not* have any
>> extraneous xml parsing libs in my 'java\lib\endorsed' directory.)
>>
>> later, a user noticed that a similar error was happening during unit
>> testing, and that it was a bug on the server:
>>
>> The issues are:
>> 1. The null check (method_newXPath == null) is outside the
>> synchronization block. Thread A and thread B could both pass through
>> the null check at the same time. Thread A will create the xpath
>> objects while B waits. Then thread B will go through and recreate
>> xpath objects while A is using them.
>>
>> 2. XPathFactory is, in general, not thread-safe (see
>> http://java.sun.com/javase/6/docs/api/javax/xml/xpath/XPathFactory.html),
>> and a method on that object is invoked outside the synchro block.
>
> In the current code it looks like this:
>
>             Object xpath = null;
>             synchronized(XmlNotPortable.class) {
>                if (method_newXPath == null) {
>                   clazz_XPathFactory =
> java.lang.Class.forName("javax.xml.xpath.XPathFactory");
>                   Class[] paramCls = new Class[0];
>                   Object[] params = new Object[0];
>                   java.lang.reflect.Method method =
> clazz_XPathFactory.getMethod("newInstance", paramCls);
>                   instance_XPathFactory = method.invoke(clazz_XPathFactory,
> params);
>                   method_newXPath = clazz_XPathFactory.getMethod("newXPath",
> paramCls);
>
>                   clazz_XPath =
> java.lang.Class.forName("javax.xml.xpath.XPath");
>
>                   Class clazz_XPathConstants =
> java.lang.Class.forName("javax.xml.xpath.XPathConstants");
>                   clazz_QName =
> java.lang.Class.forName("javax.xml.namespace.QName");
>                   java.lang.reflect.Field field =
> clazz_XPathConstants.getDeclaredField("NODESET");
>                   field_NODESET = field.get(null);
>                   paramCls_StringDocument = new Class[] {
>                                    java.lang.String.class,
>                                    java.lang.Object.class, //
> org.w3c.dom.Document.class,
>                                    clazz_QName };
>                }
>                xpath = method_newXPath.invoke(instance_XPathFactory, new
> Object[0]);
>             }
>
> so it seems to be correctly synchronized, or do I overlook your issue?
>
> Thank you
> Marcel
>
>>
>> has this bug been fixed?  if so, what version should i be using now?
>> (i am currently using xmlBlaster version 1.6.2 2007-12-03
>> 'Openroads'.)
>>
>> a response to this ongoing issue would be greatly appreciated, as it
>> is impacting our automated unit tests.
>>
>>
>> thanks,
>> andrew yinger
>>
>
>