newTransformer returns null

[Richard Jones <ri...@hp.com>]

Hi Folks,

I'm having a problem with the following code:

TransformerFactory tFactory = TransformerFactory.newInstance();
Reader reader = new
FileReader("/home/richard/workspace/dspace-trunk/foresite/src/main/resources/rdfxml2rdfa.xsl");
StreamSource xslt = new StreamSource(reader);
Transformer transformer = tFactory.newTransformer(xslt);

and am finding that "transformer" is null.  This is confusing, as the
javadocs are explicit on the subject:


      newTransformer

public abstract Transformer <http://xml.apache.org/xalan-j/apidocs/javax/xml/transform/Transformer.html> *newTransformer*(Source <http://xml.apache.org/xalan-j/apidocs/javax/xml/transform/Source.html> source)
                                    throws TransformerConfigurationException <http://xml.apache.org/xalan-j/apidocs/javax/xml/transform/TransformerConfigurationException.html>

    *Returns:*
        A |Transformer| object that may be used to perform a
        transformation in a single |Thread|, never |null|.

Am I missing something, or is this a bug?  What problems with my code
might be able to generate this null?  The specified file does exist, and
as far as I can tell everything up to the last line is working fine.

I'm at a bit of a loss to know where to go next with this, so any help
gratefully appreciated,

Cheers,

Richard

Re: newTransformer returns null

[Henry Zongaro <zo...@ca.ibm.com>]

Hi, Richard.

Richard Jones <ri...@hp.com> wrote on 2008-06-24 11:54:08 AM:
> Transformer transformer = tFactory.newTransformer(xslt);
> 
> and am finding that "transformer" is null.  This is confusing, as the
> javadocs are explicit on the subject:
> 
> 
>       newTransformer
> 
> public abstract Transformer <http://xml.apache.org/xalan-
> j/apidocs/javax/xml/transform/Transformer.html> *newTransformer*(Source 
<
> http://xml.apache.org/xalan-j/apidocs/javax/xml/transform/Source.html> 
source)
>                                     throws 
TransformerConfigurationException <
> http://xml.apache.org/xalan-
> j/apidocs/javax/xml/transform/TransformerConfigurationException.html>
> 
>     *Returns:*
>         A |Transformer| object that may be used to perform a
>         transformation in a single |Thread|, never |null|.

This is a bug.  See XALANJ-2376.[1]  The source of the problem was that 
earlier versions of JAXP stated that the default ErrorListener must simply 
report any error in System.err output stream, but never throw exceptions, 
even in the case of a fatalError.  But how could a processor possibly 
return something useful from newTransformer if it had suffered a fatal 
error?

The Xalan-J developers saw only two choices - throw the exception, despite 
the fact that the ErrorListener is supposed to dictate whether an 
exception is thrown, or return null, so that the user would at least be 
alerted to the problem by the subsequent NullPointerException.  The JAXP 
Expert Group considered the problem, and decided that the processor should 
be permitted to throw the exception in the event of a fatal error, even if 
the installed ErrorListener chose not to throw it.  So we made the wrong 
guess on the outcome, and we haven't yet fixed it.

The work-around is to install an ErroListener that always throws the 
reported exception from its fatalError method.

I hope that helps.

Thanks,

Henry
[1] https://issues.apache.org/jira/browse/XALANJ-2376
------------------------------------------------------------------
Henry Zongaro
XML Transformation & Query Development
IBM Toronto Lab   T/L 313-6044;  Phone +1 905 413-6044
mailto:zongaro@ca.ibm.com