Re: error messages and documentation, was: RE: Strange null pointer e xceptions

[Jan Berkel]

Oliver,

[EMAIL PROTECTED] wrote:

since the NPE is due to missing fields in the metadata,
I assume that a workaround is to add field-descriptors.
     

I added field-descriptors to the class descriptor of the Interface 
(Template) and it worked fine, thanks.

Actually you need field-descriptors in your Interface and 
implementing 
class, not in the base class itself (as there can be no instances ?).

I think the documentation on the ojb website is not very 
clear or even 
erroneous on this point.
   

I agree that the OJB documentation is not sufficient.
Personally, I consider valuable error messages (as
opposed to mysterious NPEs) even more important.
We have to work on that. 

I dug a bit deeper in the code to find out why OJB is throwing NPEs 
instead of giving a bit more of a valuable hint.
ClassDescriptor.java:getPkFields()
  
            if (m_isInterface)
            {
               ... // look for implementor classes and return their 
PkFields
            }
           // 1.b if not an interface The classdescriptor must have 
FieldDescriptors
           else
           {
               fields = getFieldDescriptions();
           }

           // now collect all PK fields
           for (int i = 0; i < fields.length; i++)
                                            ^^^ NPE
actually it was taking the interface for a real class and tried to get 
its field descriptors, which obviously failed.
m_isInterface is set by RepositoryXmlHandler:

            if (table == null)
            {
                      m_CurrentCLD.setIsInterface(true);
            }
So if there is no table definition for a class-descriptor entry in 
repository_user.xml, the class will be automatically declared as an 
interface.

In my example the xdoclet-ojb module automatically creates the 
table="..." entry unless you specify generate-table-info="false".

To avoid confusion, I have some suggestions:
1) make repository_user more explicit regarding the difference between 
class/interface
   or
2) make xdoclet-ojb more intelligent (e.g. never generate table info for 
interfaces or issue a warning when doing so)
3) state explicitely in the doc that including table infos for an 
interface will result in errors

I am willing to work on a patch for 2) but I haven't checked if the 
xdoclet code is already in the CVS.

I suggest moving the thread to ojb-dev.

regards,
  Jan
 

<SNIP from http://db.apache.org/ojb/tutorial3.html#extents>
   1.  A declaration for InterfaceArticle, defining which classes 
implement this interface:

<!-- Definitions for org.apache.ojb.broker.InterfaceArticle -->
   <class-descriptor class="org.apache.ojb.broker.InterfaceArticle">
      <extent-class class-ref="org.apache.ojb.broker.Article" />
      <extent-class class-ref="org.apache.ojb.broker.BookArticle" />
      <extent-class class-ref="org.apache.ojb.broker.CdArticle" />
   </class-descriptor>
</SNIP>
It doesn't give a hint to include field-descriptors for the 
interface,  or does my setup just happen to be special ?
   



 

Moreover, having the obligation to add field descriptors to the 
interface is not very practical when using the xdoclet-ojb 
module (where 
you have to define the mappings on the instance fields), or 
is there a workaround ?
   

Sorry, I have no experience with the xdoclet-ojb module.
You might start a new thread with that question.
Olli

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