[jira] [Commented] (NIFI-1752) EvaluateXPath invalid when starting processor

["James Wing (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/NIFI-1752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15235704#comment-15235704 ] 

James Wing commented on NIFI-1752:
----------------------------------

The underlying exception, "javax.xml.xpath.XPathFactoryConfigurationException: No XPathFactory implementation found for the object model: http://saxon.sf.net/jaxp/xpath/om" is thrown by XPathValidator while executing on EvaluateXPath.java:436:

{code}
XPathFactory factory = XPathFactory.newInstance(NamespaceConstant.OBJECT_MODEL_SAXON); 
{code}

>From watching in the debugger, *sometimes* the exception is thrown, but also *sometimes not* - and validation continues to compile the expression successfully and validate the property (seen by watching {{evaluator.compile(input);}} on line 441).

One possible explanation would be multi-threading.  There are several validation operations in progress at the time the exceptions occur.  And the [documentation for XPathFactory|https://docs.oracle.com/javase/7/docs/api/javax/xml/xpath/XPathFactory.html] states that it is not thread safe.

bq. The XPathFactory class is not thread-safe. In other words, it is the application's responsibility to ensure that at most one thread is using a XPathFactory object at any given moment. Implementations are encouraged to mark methods as synchronized to protect themselves from broken clients.

I made an experimental change to synchronize the creation and use of XPathFactory, and that seems to resolve the issue on my machine.  But I'm not sure how multi-threading would relate to changes introduced in 0.6.0.  Proving that multi-threading is or is not the actual problem and that a {{synchronize}} block is the right fix is the next step.

> EvaluateXPath invalid when starting processor
> ---------------------------------------------
>
>                 Key: NIFI-1752
>                 URL: https://issues.apache.org/jira/browse/NIFI-1752
>             Project: Apache NiFi
>          Issue Type: Bug
>          Components: Extensions
>    Affects Versions: 0.6.0
>            Reporter: Joseph Witt
>             Fix For: 1.0.0, 0.7.0, 0.6.1
>
>
> Guillaume Pool:
> Hi,
>  
> If anyone could help I would appreciate it:
>  
> After upgrading to 0.6.0 all my EvaluateXMLPath processors report as invalid with nothing specific reported as the error.
>  
> I use flowfile attribute as the destination, xpath query similar to /message/address
>  
> Return type auto-detect / string
>  
> Error:
>  
> 2016-04-11 17:37:12,465 INFO [NiFi Web Server-64629] org.apache.nifi.web.filter.RequestLogger Attempting request for (anonymous) PUT http://sv-hdpdf1.hdp.supergrp.net:8443/nifi-api/controller/process-groups/d02a0bbc-c142-37ba-adaa-ee191b98790d/processors/e49acf40-76d1-3b6e-a67d-dc40153605ad (source ip: 172.27.0.97)
> 2016-04-11 17:37:12,474 INFO [NiFi Web Server-64629] o.a.n.w.a.c.IllegalStateExceptionMapper java.lang.IllegalStateException: EvaluateXPath[id=e49acf40-76d1-3b6e-a67d-dc40153605ad] is not in a valid state. Returning Conflict response.
>  
> Reports this even if you drop a new processor in.
> James Wing:
> I am able to see the same EvaluateXPath issue Guillame Pool reported on NiFi 0.6.0.  I created a template gist at https://gist.github.com/jvwing/7e7948d8eb5ad3643f38966b4ba3ce2e that illustrates this for me.  You don't even need to run the whole thing, just try to start the EvaluateXPath processor.  I see a pop-up error trying to start it "EvaluateXPath[id=b481c570-686a-4b6e-859c-9ebe27398fe1] is not in a valid state".
> Trying to troubleshoot this, I attached a debugger and found an IllegalStateException thrown by StandardProcessorNode.verifyCanStart() on line 1141.  The error text was:
> 'address' validated against '/message/address' is invalid because Unable to initialize XPath engine due to javax.xml.xpath.XPathFactoryConfigurationException: No XPathFactory implementation found for the object model: http://saxon.sf.net/jaxp/xpath/om
> I haven't figured out what change is the root cause, but the trail seems to lead into validating the dynamic XPath attribute. 



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