Re: ISSUE 130: New DataObject method: cast()

[Frank Budinsky <frankb@ca.ibm.com>]

Hi Ron,

Sorry I didn't provide the problem statement. This is what I had in mind:

<BEGIN DESCRIPTION>

An SDO Data Object is a data structure composed of named properties. The 
DataObject interface provides the primary standard Dynamic API for reading 
and manipulating the underlying data structure. However, a number of other 
interfaces provide alternate "views" of the same data structure:

1) Sequence.class - if the object's type isSequenced
2) a custom Java interface (e.g., Company.class) - if the type is static 
SDO

Another possible view of a data object (currently 
implementation-dependent, but a possible future addition to the spec) 
would be a DOM view:

3) Node.class - if one wants to provide a 100% XML fidelity view of the 
underlying XML structure using an XML standard API

The current approach for accessing alternate views is using specific APIs 
for each view:

1) DataObject.getSequence() to get the Sequence view
2) Java cast - e.g., (Company)myDO - for a static SDO view
3) If we want to get a Node view, then we'll need to add some new API for 
that - e.g., XMLHelper.getNode(myDO)

This proposal suggests to instead provide a single uniform API for 
accessing any alternate view. This will have the advantage of simplicity 
(1 method for all) as well as extensibility for future supported (or even 
implementation dependent) views.

<END DESCRIPTION>

Now, to answer your questions (from below) Ron:

1) I'm not sure if we need to cast back from every possible view - if so, 
then we may want to add one or more APIs for that as well, but I think 
it's a separate issue. Note that Sequence currently doesn't provide a 
method to get back to the DataObject view - i.e., there is no 
Sequence.getDataObject() method.

2) I'm just saying this will "probably" be useful when we want to 
integrate with JAXB/JPA. It opens up the possibility of implementations 
supporting static using DataObject proxies, or similar, instead of 
byte-code insertion or specialized code generation to make sure that 
everything is done in a single instance.

3) JAXB is a static solution. If we want Dynamic SDO with 100% XML 
fidelity, we need something like Node. Note that I said "something like" - 
I'm open to other suggestions, but I think that since Node is "the 
standard XML API", we should consider using it.

I hope this helps to clarify things.

Thanks,
Frank.




"Barack, Ron" <ron.barack@sap.com> 
06/12/2008 09:36 AM

To
Frank Budinsky/Toronto/IBM@IBMCA, <sdo@lists.oasis-open.org>
cc

Subject
ISSUE 130: New DataObject method: cast()






http://www.osoa.org/jira/browse/SDO-130 

Hi Frank,

I had a little trouble entering this into the JIRA:  you make a proposal 
without stating what problem you would like to solve.  Casting to Node 
isn't part of the proposal, so XMLFidelity isn't addressed, so what is the 
problem we are trying to solve?  From your text I have extrapolated the 
following DESCRIPTION:

Under SDO 2.1, clients obtain a reference to the static SDO by casting the 
DataObject, that is, the static SDO must be the same instance as the 
DataObject.  This will be a problem when we do JPA/ JAXB integration.

Is this correct?

Now for some questions:

1) don't we need a symetric operation, one that allows you to go from the 
static SDO back to the DataObject (casting always worked in both 
directions)?

2) what is your justification is for the last assertion, that we need this 
for JPA/ JAXB integration?

3) Regarding casting to Node...Assuming we get JAXB integration to work 
(by the way, I think JAXB integration is much harder than JPA), doesn't 
that give us all the XMLFidelity we need?  Which technology should we 
focus on integrating?



Best Regards,
Ron

-----Ursprüngliche Nachricht-----
Von: Frank Budinsky [mailto:frankb@ca.ibm.com] 
Gesendet: Donnerstag, 12. Juni 2008 00:06
An: sdo@lists.oasis-open.org
Betreff: [sdo] [NEW ISSUE] New DataObject method: cast()

Hi Guys,

I would like to propose adding the following method to the DataObject 
interface:

    <T> T cast(Class<T> targetClass)

I'm not sure if the name should really be "cast" or "project" or something 

else, but I'm thinking "cast" may be best, since the intent is to get some 

other interface to work with and view the DataObject.

An implementation of this method will look something like this:

    public <T> T cast(Class<T> targetClass)
    { 
        if (targetClass.isInstance(this)) return (T)this;
        if (targetClass == Sequence.class && getType().isSequenced() 
return getSequence();
        // TBD other required or optional casts (e.g., maybe Node.class)
        // TBD implementation specific casts
        return null; //TBD maybe instead we should throw 
ClassCastException
    }

The idea behind this method is that it provides a single API for 
converting a DataObject to any other possible interface "view".

To get the Sequence view of a DataObject, a user would call:

    Sequence sequence = myDO.cast(Sequence.class);

To get the interface of a static SDO, you would call:

    Company company = myDO.cast(Company.class);

Using this API instead of simply using Java cast - (Company)myDO - has the 

advantage that we've opened up the door for different implementations of 
static SDO (e.g., a corresponding POJO) in the future where the static 
object and the DataObject are not required to be the same instance. This 
will be important when we get to the JAXB/JPA integration discussions.

Once this API is in place, we can think about other (required or optional) 

uses for it, such as using it to cast to Node.class (or something else) as 

a catch all for the XML Fidelity corner cases that we don't want to handle 

in SDO directly.

If we agree to add this new method, I think we should also deprecate 
getSequence(), which will have the added benefit of this issue not 
actually increasing the number of methods in the DataObject interface.

Thanks,
Frank.

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