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Re: PROV-ISSUE-648: Can schema be made a bit more jaxb friendly? [XML Serialization]

From: Stephan Zednik <zednis@rpi.edu>
Date: Thu, 21 Mar 2013 09:49:32 -0600
Cc: public-prov-wg@w3.org
Message-Id: <9D39552B-06D5-48DA-BA90-5C5FA98C104C@rpi.edu>
To: Luc Moreau <l.moreau@ecs.soton.ac.uk>


On Mar 21, 2013, at 5:26 AM, Luc Moreau <l.moreau@ecs.soton.ac.uk> wrote:

> H Hook,
> 
> Thanks for this analysis.
> 
> In this specific instance, I think that it is the element
>  <xs:any namespace="##other" processContents="lax" minOccurs="0" />
> occurring inside
>  <xs:sequence maxOccurs="unbounded">
> that causes these jaxb elements to be generated.
> 
> If you were to remove xsd:any there, jaxbElements would no longer be generated.
> 
> While we want to allow the possibility of elements from other schemas, do we
> really want to allow them any where inside a document/bundle?

We want to provide for elements from other schemas but I don't think we formally identified what areas we intend to allow non-prov elements in before we added this functionality to the schema.

What if we made a FAQ entry about OXM mappings with PROV-XML and created a customized schema or bindings file specifically for JAXB code generation?  This would allow us to work on asynchronously with the document and past the note publication, it would also allow us to introduce JAXB-specific solutions that I do not think make sense in the official schema or note.

--Stephan

> 
> Luc
> 
> 
> On 03/21/2013 11:09 AM, Hua, Hook (388C) wrote:
>> Hi Luc,
>> 
>> I'm using jaxb-ri-2.2.6 against our latest prov*.xsd and seeing slightly
>> different bindings with JAXBElement:
>> 
>> public class Document {
>>     @XmlElementRefs({
>>         @XmlElementRef(name = "hadPrimarySource", namespace =
>> "http://www.w3.org/ns/prov#", type = JAXBElement.class, required = false),
>>         @XmlElementRef(name = "agent", namespace =
>> "http://www.w3.org/ns/prov#", type = JAXBElement.class, required = false),
>>         @XmlElementRef(name = "activity", namespace =
>> "http://www.w3.org/ns/prov#", type = JAXBElement.class, required = false),
>>         @XmlElementRef(name = "organization", namespace =
>> "http://www.w3.org/ns/prov#", type = JAXBElement.class, required = false),
>>         @XmlElementRef(name = "softwareAgent", namespace =
>> "http://www.w3.org/ns/prov#", type = JAXBElement.class, required = false),
>> ....
>> 
>> 
>> 
>> Some findings:
>> 
>> 
>> (1) JAXB's generation of JAXBElement<T> classes seems to be a wrapper
>> approach to preserve sufficient information in the schema for round-trip
>> marshaling & unmarshalling of values in XML instances. More specifically,
>> it wraps the data with a QName and a nillable flag [1].
>> 
>> It appears that the a frequent cause of JAXB producing JAXBElement<T> is
>> its attempt to preserve elements with both minOccurs=0 and nillable=true.
>> JAXB needs to distinguish between the two cases where:
>> 
>>   a. element missing, minOccurs=0, then jaxbElement==null
>>   b. element present, xsi:nil=true, then jaxbElement.isNil()==true
>> 
>> It would not be possible to distinguish between these two states if the
>> bindings were the raw types.
>> 
>> 
>> 
>> (2) It would be possible to customize the JAXB bindings [2] to ignore the
>> full round-trip requirement. The "generateElementProperty=false"
>> customization option "can be used to generate an alternate developer
>> friendly but lossy binding" [3].
>> 
>> I tried variations of a "bindings.xjb" customization file:
>> 
>> <jaxb:bindings version="2.1"
>>   xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
>>   xmlns:xjc="http://java.sun.com/xml/ns/jaxb/xjc"
>>   xmlns:xs="http://www.w3.org/2001/XMLSchema">
>>   <jaxb:bindings schemaLocation="prov-core.xsd"
>>     node="//xs:complexType[@name='Document']">
>>     <jaxb:globalBindings generateElementProperty="false" />
>>   </jaxb:bindings>
>> </jaxb:bindings>
>> 
>> $ xjc.sh -d BINDINGS -b bindings.xjb prov.xsd
>> 
>> But none truly eliminated the JAXBElement<T> from the bindings.
>> 
>> 
>> 
>> (3) Nowhere in our prov-core.xsd do we define minOccurs=0 in conjunction
>> with nillable=true. In my attempts with JAXB, I'm seeing JAXBElements
>> appearing in the bindings for the (a) Document class and (b)
>> BundledConstructor class. Both types leverage the prov:documentElements
>> grouping.
>> 
>>   <xs:element name="document" type="prov:Document" />
>> <xs:complexType name="Document">
>>   <xs:sequence maxOccurs="unbounded">
>>     <xs:group ref="prov:documentElements" minOccurs="0"/>
>>     <xs:element name="bundleContent" type="prov:BundleConstructor"
>> minOccurs="0"/>
>>     <xs:any namespace="##other" processContents="lax" minOccurs="0" />
>>   </xs:sequence>
>>   </xs:complexType>
>> 
>> It's unclear if there is some nillable-like affect that triggers JAXB to
>> generate the JAXBElements.
>> 
>> 
>> 
>> (4) On the upside, JAXB does provide an ObjectFactory class as part of the
>> generated bindings that define creational factory methods to generate the
>> JAXBElement instances. For example:
>> 
>>   public JAXBElement<Usage> createUsed(Usage value)
>> 
>> Still, I agree that it is not as clean.
>> 
>> 
>> --Hook
>> 
>> 
>> [1] http://docs.oracle.com/javaee/5/api/javax/xml/bind/JAXBElement.html
>> [2]
>> http://docs.oracle.com/cd/E17802_01/webservices/webservices/docs/1.5/tutori
>> al/doc/JAXBUsing4.html#wp148515
>> [3]
>> http://docs.oracle.com/cd/E17802_01/webservices/webservices/reference/tutor
>> ials/wsit/doc/DataBinding5.html
>> 
>> 
>> 
>> 
>> 
>> 
>> On 3/8/13 4:20 AM, "Provenance Working Group Issue Tracker"
>> <sysbot+tracker@w3.org> wrote:
>> 
>>> PROV-ISSUE-648: Can schema be made a bit more jaxb friendly? [XML
>>> Serialization]
>>> 
>>> http://www.w3.org/2011/prov/track/issues/648
>>> 
>>> Raised by: Luc Moreau
>>> On product: XML Serialization
>>> 
>>> 
>>> Hi
>>> 
>>> I have ported the ProvToolbox and the ProvValidator to the new XML schema.
>>> I just wanted to report on my experience with the schema and JAXB.
>>> Obviously, others may have better experience with JAXB and may be able
>>> to help on some of the issues I encountered.
>>> 
>>> Everything worked fine, except:
>>> - <xs:element ref="prov:internalElement abstract=true/>
>>> - extensibility <xs:any namespace="##other"/> in Document and Bundle
>>> 
>>> 
>>> These two constructs, while processable by JAXB, are not JAXB-friendly.
>>> 
>>> Indeed, JAXB compiles the schema in a list containing all possible
>>> statements.
>>> 
>>>    protected List<Object> entityAndActivityAndWasGeneratedBy;
>>> 
>>> However, the presence on an abstract element and an <any/> element result
>>> in the
>>> content of that list to be of type:
>>> 
>>> 
>>>    @XmlElementRefs({
>>>        @XmlElementRef(name = "used", namespace =
>>> "http://www.w3.org/ns/prov#", type = JAXBElement.class),
>>>        @XmlElementRef(name = "wasAssociatedWith", namespace =
>>> "http://www.w3.org/ns/prov#", type = JAXBElement.class),
>>>        @XmlElementRef(name = "person", namespace =
>>> "http://www.w3.org/ns/prov#", type = JAXBElement.class),
>>>        @XmlElementRef(name = "entity", namespace =
>>> "http://www.w3.org/ns/prov#", type = JAXBElement.class),
>>>        @XmlElementRef(name = "wasInfluencedBy", namespace =
>>> "http://www.w3.org/ns/prov#"
>>> ....
>>>    })
>>> 
>>>    @XmlAnyElement(lax = true)
>>>    protected List<Object> entityAndActivityAndWasGeneratedBy;
>>> 
>>> where all data structures are wrapped up in this unpleasant JAXBElement.
>>> 
>>> Without these features, we get a much more natural mapping:
>>>    @XmlElements({
>>>        @XmlElement(name = "entity", namespace =
>>> "http://www.w3.org/ns/prov#", type = Entity.class),
>>>        @XmlElement(name = "activity", namespace =
>>> "http://www.w3.org/ns/prov#", type = Activity.class),
>>>        @XmlElement(name = "wasGeneratedBy", namespace =
>>> "http://www.w3.org/ns/prov#", type = WasGeneratedBy.class),
>>>        @XmlElement(name = "used", namespace =
>>> "http://www.w3.org/ns/prov#", type = Used.class),
>>>        @XmlElement(name = "wasInformedBy", namespace =
>>> "http://www.w3.org/ns/prov#", type = WasInformedBy.class),
>>>    ...
>>> })
>>> 
>>> So, how I did I solve the problem?  I inserted the extension schemas into
>>> the schema file, and hence got rid of the abstract element.  I am ok with
>>> this. We could possible provide the utility to that transformation.
>>> 
>>> For the extensibility, I used a different definition. It happens to
>>> parse prov-xml compliant xml. When serializing, it  puts all
>>> extensibility elements at the end.  This is not a satisfactory
>>> solution, and is likely to be dependent of the jaxb implementation
>>> (though I am not entirely sure).
>>> 
>>> 
>>>   <xs:complexType name="Document">
>>>     <xs:sequence>
>>>       <xs:choice maxOccurs="unbounded">
>>>         <xs:group ref="prov:documentElements"/>
>>>         <xs:element name="bundleContent" type="prov:NamedBundle"/>
>>>       </xs:choice>
>>>       <xs:any namespace="##other" processContents="lax" minOccurs="0"
>>> maxOccurs="unbounded"/>
>>>     </xs:sequence>
>>>   </xs:complexType>
>>> 
>>> Can something be done to make the XML schema a bit more jaxb friendly,
>>> while still keeping the same flexibility?  Thoughts welcome.
>>> 
>>> Cheers,
>>> Luc
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> 
> 
> -- 
> Professor Luc Moreau
> Electronics and Computer Science   tel:   +44 23 8059 4487
> University of Southampton          fax:   +44 23 8059 2865
> Southampton SO17 1BJ               email: l.moreau@ecs.soton.ac.uk
> United Kingdom                     http://www.ecs.soton.ac.uk/~lavm
> 
> 
> 
Received on Thursday, 21 March 2013 15:49:59 UTC

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