Hi Ron,
I like the generic Binder API, I had been thinking that this would
apply nicely to DataObjects as well as XML and POJOs.
My interpretation of the proposal is that new Types are not created
during load operations. Instead the DataObjects that are created
correspond to Types that already exist within the scope of the
HelperContext. If an external representation cannot be directly linked
to a type then an algorithm like section 9.10 (from SDO 2.1) is used.
Is this correct?
With respect to the uniform handling of different standard
representations. I did not intent to argue that this was a bad thing,
only that it is not strictly a requirement since enough information is
available to do specific bindings.
-Blaise
Barack, Ron wrote:
9279AFBA5302884AB7169C4C51B6FDF70165A8E3@dewdfe1a.wdf.sap.corp"
type="cite">
Hi Blaise,
Let's address the fundamental
disagreements first:
1. Can JAXB marshal an
arbitrary POJO?
Actually, I don't know how
relevant this discussion is to SDO, and I don't exactly
relish argueing about JAXB with a member of the expert group, and I
want to first of all make clear that
a) I do not see SDO 3.0 as a
means of fixing any sort of problems or deficiencies in JAX-B.
b) I do not see SDO 3.0 as
competing in any way with JAX-B
Perhaps my choice of the word
"arbitrary" was wrong, perhaps "unannotated" would have been better. For the record, though, I
believe this is actually listed as a non-goal of JAXB, at least that
how I interpret the statement that "mechanisms
to traverse a graph of JavaBean objects will not be addressed". And of
course there's section 8.2.7, which states that the determination of
which relationships are containment is the responsibility of the
programmer, who must annotate with @XmlID and @XmlIDRef appropriately.
In any case, since our resolution of SDO-124, I
believe SDO 3.0 to be capable of mashalling an unannotated object
graph to XML.
Staying on the subject of integration with JAXB,
the is nothing in my proposal that would involve the creation of a mapping
between all JAXB annotations to SDO metadata. If we have use-cases where we need to generate
metadata from JAXBs, which I'm not sure we do, but if, then we would
certainly want to use something like
"XSDHelper.define(JAXBContext.generateSchema())", rather than parsing
the annotations ourselves.
2. Is it desireable to handle the different
standard representations (SDO, POJO, JAXB, DOM) uniformly.
I can't believe you're arguing that this would
be a bad thing, the question is, is the work involved justified by the
use-cases. This maybe brings us to the Binder<T> API. Here is a
first pass, simply adding generics to your API, plus a getHelperContext
method.
Generic Binder
/**
* The Binder is used to scope when a external data representation is
linked to a DataObject. */
public interface Binder<T> {
/**
* Any DataObjects created
will have types that are defined in this HelperContext.
*/
HelperContext
getHelperContext();
/**
* DataObject customer1DO = binder.load(customer);
* DataObject customer2DO = binder.load(customer);
* customer1DO == customer2DO
*/
DataObject load(T externalRepresentation);
/**
* Object customer1 = binder.save(customerDO);
* Object customer2 = binder.save(customerDO);
* customer1 == customer2
*/
T save(DataObject dataObject);
/**
* Get the DataObject linked with is Object.
* DataObjects are linked with Objects through both the load and
update operations.
*/
DataObject getDataObject(T externalRepresentation);
/**
* Get the Object linked with is DataObject.
* Objects are linked with DataObjects through both the load and
update operations.
*/
T getExternalRepresentation(DataObject dataObject);
/**
* Use this method if you want the changes made to the Object to be
reflected in the corresponding DataObject.
*/
DataObject updateDataObject(T externalRepresentation);
/**
* Use this method if you want the changes made to the DataObject
to be reflected in the corresponding Object.
*/
T updateExternalRepresentation(DataObject dataObject);
}
The next question is, how do you
get a Binder: my first idea would be through
interface HelperContext {
...
<T> Binder<T>
createBinder<Class<T> representationClass);
...
}
Where this would be required to
return binders for org.w3c.dom.Node, for java.lang.Object (POJOs) and
for commonj.sdo.DataObject (in this case, the functionality would be
"projection", maybe we would remove projection and replace it with this
functionality. I'm still doing some evaluation of this approach, but I
want to throw it out there to see if you think it is something to which
you could potentially agree.
Best Regards,
Ron
Hi Ron,
I see that there are fundamental areas in which we disagree:
- <Ron>One reason is that I can't use the
JAXB marshaller on an arbitrary POJO, because the arbitrary POJO will
have non-containment references that are not annotated with @IDREF.
</Ron>
From the Oracle perspective you can use JAXB to marshal an arbitrary
POJO, that is one of its primary goals. If the default rules are not
sufficient then you customize the binding through the use of
annotations until the XML representation truly reflects the data.
Implementations such as TopLink/EclipseLink even improve upon this
functionality. SDOs are meant to be an alternative SCA paramater type
specifically designed for use in a disconnected environment.
- <Ron>In
my SOA infrastructure, it's pretty desireable to handle these different
representations uniformly. </Ron>
SCA components know their parameter types, and therefore can easily
know the appropriate binding mechanism (JAXB, SDO, XML). I don't see a
strong case for having a uniform API, or the need to convert everything
to DataObjects internally.
<Ron>It's not a small undertaking
like Mt Everest isn't a small hill.</Ron>
I'm not against hard work, but if I
interpret your proposal correctly the work would involve the creation
of a mapping between all JAXB annotations to SDO metadata. The
proposal may also include the creation of a mapping between all JPA
annotations (including XML metadata) to SDO metadata. The view from Mt
Everest may be better than the view from the hill, but building my
house on the hill made the commute to work much easier ;).
<Ron>I'm try to understand why we need
PojoHelper in addition to PojoBinder.
POJOHelper
creates DataObjects from POJOs, and is required just like XMLHelper to
ensure that all the relationships are preserved during the load/save
process. In other words it should ensure the following is true.
customerPojo ==
customerPojo.getOrders().get(23).getCustomer()
DataObject customerDataObject =
PojoHelper.INSTANCE.load(customerPojo)
customerDataObject ==
customerDataObject.getList("orders").get(23).get("customer")
Note that if POJOHelper was used a second time to create DataObjects
from POJOs the DataObjects would not be identical to the DataObjects
created the first time:
DataObject
customerDataObject2ndConversion = PojoHelper.INSTANCE.load(customerPojo)
customerDataObject
!= customerDataObject2ndConversion
This is where POJOBinder comes into play. POJOBinder
forms a link between the POJO and the DataObject. With this link
formed you can make changes on one side and then apply them to the
other side.
POJOBinder
pojoBinder = PojoHelper.INSTANCE.createPOJOBinder();
DataObject customerDataObject =
pojoBinder.load(customerPojo)
customerDataObject ==
customerDataObject.getList("orders").get(23).get("customer")
DataObject customerDataObject2ndConversion
= pojoBinder.load(customerPojo)
customerDataObject
== customerDataObject2ndConversion
<Ron>Sorry,
I don't get your point here. Are you saying that
the persistent objects are disconnected before handed to the PojoHelper</Ron>
My intention here is to have SDO work with JPA as well as possible. In
areas like converting objects that are JPA entities with lazy loading
to SDOs the default conversion may involve triggering the lazy
relationships. Currently the serialization of JPA entities with JAXB
causes this. If we want to do better I would rather petition the JPA
group (a number of SDO 3.0 members have JPA spec group contacts) for
standard hooks than introducing too many SDO tricks to hack this in.
<Ron>What worries me more
about the approach is the potential side effects (e.g., modifying the
POJO would cause DataObject.get() to return the new values, and
vice-versa). But this seems to be
something we can document, make our users aware of, and even sell as a
"feature, not a bug".</Ron>
One of the fundamental decisions to be made is if modifying the POJO
directly affects the DataObject, and if modifying the DataObject
directly affects the POJO. Although it is easy for SDO to push all
changes to the POJO level, it is not possible (without something like
byte code manipulation) to have ALL changes made to the POJO model
immediately reflected in the corresponding DataObjects (although it is
possible to reflect SOME changes).
<Ron>Are you saying that you are no longer
interested in introspecting POJOs as a source of metadata?</Ron>
I think the sources of metadata will become clearer once other issues
have been decided on. This is an area where compromise is possible.
<Ron>You mean public <T> Binder<T>
createBinder(Class<T> dataRepresentation), right? This is
probably an API we could work on to get the uniformity wrt standard
representations that I want, without requiring that the entire
HelperContext be re-implemented for every such representation (which
you mention below as an objection to the HelperContext<Object>
approach). The idea would be to move "project" from DataFactory to
Binder<DataObject>...and rename it to load, or load to project,
or whatever. Potential for compromise here, I hope. Let me work on
the API and see how it could fit in to our infrastructure.
</Ron>
We'll discuss the API proposal when it is available.
-Blaise
Barack, Ron wrote:
9279AFBA5302884AB7169C4C51B6FDF701622F70@dewdfe1a.wdf.sap.corp"
type="cite">
Hi Blaise,
SDO
3 is finally getting fun. I've been thinking about your ealier
question
Why do (in the future):
HelperProvider.getPojoContext().getXMLHelper().save(pojo);
When you can do (today):
JAXBContext.newInstance().createMarshaller().marshal(pojo);
One reason is that I can't use the JAXB
marshaller on an arbitrary POJO, because the arbitrary POJO will have
non-containment references that are not annotated with @IDREF. Now
that SDO 3.0 can serialize to non-closed graphs (via orphan properties)
I have reason to prefer SDO's marshaller (in some cases... esp, where
XML fidelity does not matter).
Another
reason is because I cannot say JAXBContext.newInstance().createMarshaller().marshal(DataObject)
or
JAXBContext.newInstance().createMarshaller().marshal(DOMNode) or
JAXBContext.newInstance().createMarshaller().marshal(whatever-other-structure-for-which-I-can-have-a-binder).
In my SOA
infrastructure, it's pretty desireable to handle these different
representations uniformly. Of course, this functionality is really
also just a convenience function. Internally, the different structures
would be converted to DataObjects.
more comments inline...
Ron
PS: Does everyone's
mail client handle the different fonts that I am using to keep my voice
seperate from Blaise's? It seems like Frank's client converts
everything to plain text... Is it OK to continue the discussion in
this mode?
Hi Ron,
The POJOHelper proposal is focussed on POJO-to-SDO mapping rather than
deriving SDO types from JPA and/or JAXB annotations (the latter is not
a small undertaking).
It's not a small undertaking like Mt
Everest isn't a small hill. What do you think about the idea of
deriving types from JAXB by using something like
XSDHelper.define(JAXBContext.generateSchema())? I know real code would
be more complex, but as pseudocode you get the idea. Of course, if the
JAXBs were generated from an XSD, it would be much, much better to have
the XSD available as a direct source of metadata, and I think we should
encourage this. But in cases where the XSD is not available, this is
the best we can do, I guess.
Yes the following would be expected to work using POJOHelper:
customerPojo ==
customerPojo.getOrders().get(23).getCustomer()
DataObject customerDataObject =
PojoHelper.INSTANCE.load(customerPojo)
customerDataObject ==
customerDataObject.getList("orders").get(23).get("customer")
I'm try to understand
why we need PojoHelper in addition to PojoBinder. In order to
implement the hooking up of non-containment references you need to
maintain a map from the POJO to the corresponding SDO. This is the
functionality that the PojoBinder provides. It seems like PojoHelper
would be implemented by creating a binder, doing the conversion, and
then throwing the binder away. That is, PojoHelper is simply a
convenience function; the real work is done by PojoBinder. I'm not
saying it's not a desireable convenience function, but we should focus
first on the methods/ interfaces that do the real work.
When creating SDOs from POJOs that happen to be JPA entities,
it is important to consider the lazy loading aspect. Until (if ever)
JPA offers a means to introspect if a relationship has not been
realized then it will be difficult for SDO implementations working
against standard JPA to accomplish this, although vendors offering both
SDO & JPA implementations can do this in a proprietary way.
Sorry, I don't get your point here. Are you saying that
the persistent objects are disconnected before handed to the PojoHelper ?
---
I do see the POJO to SDO step as a copy rather than a wrap operation.
This is do to the disconnected nature of SDO.
I would like
to consider relaxing the "disconnected nature of SDO" in some cases,
but it's really not necessary, here. It is the POJOs that are
(potentially) connected.
Wrap Approach
While the wrap approach does have the advantage of not requiring a copy
step, a mechanism will need to be provided to deal with how to re-sync
the DataObject with the POJO (after serialization).
POJO --WRAP-STEP--> DataObject(WRAPPING POJO) --SERIALIZE-->
XML --DE-SERIALIZE--> DataObject(WITHOUT POJO)
there's no
reason why the story has to stop here. Whether or not the DataObject
is a wrapper or uses some other structure to hold values, the
PojoBinder (or HelperContext<Object>.getDataFactory()!) would
still be in the capable of converting from the DataObject to a POJO.
It's just that, if it is a wrapper, then obviously we can convert to
POJO very quickly, easilly, and with a very small memory footprint.
The wrapper approach is simply a performance optimization, it doesn't
really add or remove functionality...wrapping is allowed, but never
required. If we imagine BusinessProcess, Rules, and Mapping engines
that are all implemented using SDO, then the chain
POJO --WRAP-STEP-->
DataObject(WRAPPING POJO) -- SDO-based component --
DataObject(Wrapping POJO, Modified) -- UNWRAP -- POJO
Is
something that will meet my KPIs, where as a copy step may be too
expensive.
What worries me more about the
approach is the potential side effects (e.g., modifying the POJO would
cause DataObject.get() to return the new values, and vice-versa). But this seems to be something we can document,
make our users aware of, and even sell as a "feature, not a bug".
Copy
Approach
Here is how the copy approach can be used in a disconnected scenario:
Step #1 - Retrieve SDO
Information From Server
Database --JPA--> POJO --POJOHelper--> DataObject
--SERIALIZE--> XML
Step
#2 - Receive SDO on Client Modify SDO and Return to
Server
XML --DE-SERIALIZE--> DataObject --MODIFY--> DataObject(MODIFIED)
Step #3 - Receive SDO on Server
XML --DE-SERIALIZE--> DataObject(MODIFIED) --POJOHelper--> POJO'
--JPA-MERGE--> POJO --JPA--> Database
as I said,
this works for an implementation that allows wrapping just as for an
implementation that has a copy step. No difference here.
---
Question #1 - To what does XMLBinder bind?
Your assumptions are correct:
- XMLBinder would bind DataObjects to DOM nodes, although like
JAXB we could offer XMLBinder<XmlNode> instead.
- XMLHelper would have a method called createXMLBinder() and/or
public <T> createBinder(java.lang.Class<T> domType)
You mean
public <T> Binder<T> createBinder(Class<T>
dataRepresentation), right? This is probably an API we could work on
to get the uniformity wrt standard representations that I want, without
requiring that the entire HelperContext be re-implemented for
every such representation (which you mention below as an objection to
the HelperContext<Object> approach). The idea would be to move
"project" from DataFactory to Binder<DataObject>...and rename it
to load, or load to project, or whatever. Potential for compromise
here, I hope. Let me work on the API and see how it could fit in to our infrastructure.
---
Question #2 -
What is the type of the DataObject that is created through
PojoHelper.load()?
The mapping between DataObjects and
POJOs is defined much like the mapping between DataObjects and XML,
through setting properties on Type & Property or through
annotations on the XML schema. The mapping is established on the SDO
side not the POJO side.
Are you saying that you are no longer interested in
introspecting POJOs as a source of metadata?
---
Question #3 -
If a PojoHelper is being used to translate to and from JAXBs, doesn't
it need a handle to the JAXB context?
The POJO helper
is used to translate to/from POJOs and not JAXB objects. Therefore
only a SDO-to-Object mapping is required on the already defined types
and would not require a JAXB context.
As you mention you could sync between JAXB and SDO through the use of
an XML binder on each side. The purpose of POJOHelper however is to
expose any object so that it can traverse an SCA system as a
DataObject, and then be returned to object form to work with things
like JPA.
---
Difference Between POJOHelper & POJOContext?
I think the difference
between these two concepts is quite large. POJOContext requires all
the helpers to work with POJOs, while POJOHelper focuses specifically
on converting DataObjects to POJOs. This matches what we did with XML,
we have an XMLHelper and not an XMLContext.
I
understand, but I see the Helpers mostly as simple convenience
functions over an internal XX-to-DataObject adaptation.
SCA implementations already need to address SDOs and POJOs as
parameters. I don't see having one API to handle everything being
necessary, especially in terms of XML handling the SDO API still has a
ways to go to catch JAXB.
Projection is not the only means of
moving data across the wire. In SDO projection can be leveraged when
appropriate, but in a typical disconnected environment a solution is
requirement that survives serialization (see the Copy Approach example
above). As I wrote above, I don't see the problem here.
-Blaise
Barack, Ron wrote:
9279AFBA5302884AB7169C4C51B6FDF7016228ED@dewdfe1a.wdf.sap.corp"
type="cite">
Hi Blaise, Rick,
Some additional questions
and comments...
If I assume a bunch of POJOs
that are connected in a net, with cycles, etc, like POJOs tend to be
connected, and I say, for instance, that
customerPojo ==
customerPojo.getOrders().get(23).getCustomer()
And then I create a SDO by
loading from the PojoHelper
DataObject
customerDataObject = PojoHelper.INSTANCE.load(customerPojo)
Then I would expect
customerDataObject ==
customerDataObject.getList("orders").get(23).get("customer")
Is that the way you expect
things to work even when using the PojoHelper instead of the
PojoBinder? I'm asking because this behaviour implies that at least
during the creation of the data objects, some sort of binder was there,
maintaining the map of POJO objects we've already seen and the
associated DataObjects. It's true that the binder could have been
thrown away after the call to PojoHelper.load has finished, but this
then precludes the possibility that the conversion from POJO to
DataObject occurs lazilly... it means that that PojoHelper.load must
translate the entire transitive closure in a single call. I my
opinion, in order to handle integration with JPA, it's important that
at least non-containment references be traversed lazilly.
The other question I have
concerns a possible semantic difference between the
HelperContext<Object> and the PojoHelper approaches. When I
think of converting a Pojo to a DataObject using the project method,
I'm thinking that the DataObject would in fact be a wrapper around the
Pojo, the Pojo would (or at least could) be the "underlying data store"
we talk about when discussing SDO-66. Is this how you imagine
PojoHelper/Binder working, or do you envision a copy step occuring
during the translation?
Best Regards,
Ron
Hi Blaise,
I'm encouraged by the agreement
on semantics. We all seem to agree
1) That we need to round trip
between POJO and DataObject "views" of the data.
2) That we need to maintain
object identity between the different views, that is, we need the
binder.
First question: to what does
XMLBinder bind? Am I correct in assuming DOM nodes? And that
XMLHelper would get the method createXmlBinder?
Second question: what is the
type of the DataObject that is created through PojoHelper.load()? Are
you assuming that the class is introspected, and used to derive an SDO
type? Do we need PojoHelper.define(Class)?
I hope I did not give the
impression that I want to reinvent the wheel or compete in any way with
JAXB. For all the reasons that you list, that is out of the question
for SDO. We want to integrate with JAXB. And, at least as important,
we want to integrate with JPA. When it comes to deriving SDO types
through introspection, we have a disconnect here. One
problem we have is that the natural interpretation of a POJO in JPA is
that relationships are non-containment (unless they are marked as
embedded), whereas the interpretation of an unannotated POJO in JAXB
would be that there is a containment relationship. Another problem
that we face is that JAXB (since it has such good schema coverage) is
very complex indeed, with getter methods returning JAXBElements, DOM
nodes, or simple values depending on the details of the schema and
XML. Going from JPA to SDO is compatitively straightforward. This
seems to lead to the idea that we need to support seperate strategies,
that is, we need a JaxB PojoHelper and a plain/JPA PojoHelper. Both
could have the same interface, they would just have different
implementations (especially for "define").
Third question: If a PojoHelper
is being used to translate to and from JAXBs, doesn't it need a handle
to the JAXB context? So the API to get a JAXB helper would be
something like
PojoHelper
HelperContext.getPojoHelper(JAXBContext)
Actually, I've spent a lot of
time thinking about how to do a robust integration between JAXB and SDO
(simple cases are simple, but complex cases are very hard), and I've
come to the conclusion that it's best to go over DOM nodes, with
"binders" on each side. Do you agree? If so, do we really need a
PojoHelper for JAXB integration, isn't what you call the XMLBinder
enough? (If only the RI/ JSE 6 versions fully implemented this
functionality...BTW is eclipseLink better in this regard?).
I
think once we get these small points settled, the differences between having a
PojoHelper and a PojoHelperContext will be pretty small, as far as the
functionality that they offer. The difference is mostly in the
preferred style of the API. I can see where you'd say that
HelperContext<Object> is a "heavy" solution to the problem. On
the other hand, for the SCA use case, it's pretty liberating to be able
to say that I have a HelperContext on both sides of the wire. It could
be a HelperContext<Object>, or a HelperContext<DataObject>,
but in any case, when moving data over the wire, all I do is project
from one context into the other. I could achive the same thing with
PojoHelper, it would just take alot more code and configuration. In
this case, HelperContext<Object> has clear advantages.
What are the use cases where
PojoHelper is "lighter"?
Best Regards,
Ron
Hello All,
The introduction of a POJOHelperContext is quite a heavy weight
solution to this problem.
Why do (in the future):
HelperProvider.getPojoContext().getXMLHelper().save(pojo);
When you can do (today):
JAXBContext.newInstance().createMarshaller().marshal(pojo);
JAXB 2.0 already has the following over SDO:
- JAXB 2.0 is already included in Java SE 6.
- Marshaller & Unmarshaller are a richer API than SDO's
XMLHelper.
- Supports 100% of XML schema concepts
- Integrates with Java EE standards such as JAX-WS (including
support for binary attachments)
- Well defined name mangling algorithm, meaning portability
across vendors.
Instead of re-inventing the wheel, we can deal specifically with
converting POJOs to/from DataObjects. My proposal is that we address
this much like we handled converting XML to/from DataObjects by
creating a new POJOHelper (see interface below). If you intend on
working with both the POJOs and DataObjects and want to to update the
other accordingly then I propose introducing a new class called
POJOBinder (see interface below). POJOBinder maintains a link between
the POJO and the corresponding DataObject and allows you to apply the
changes from one to the other.
JAXB 2.0 has a similar concept called XMLBinder. It was introduced in
JAXB to address XML fidelity issues. In addition I propose adding a
XMLBinder to XMLHelper, (in a manner similar to POJOBinder &
POJOHelper). This can be used to address many (if not all) of the SDO
XML fidelity issues.
POJO Helper
/**
* POJOHelper is a mechanism to convert DataObjects to/from POJOs.
* DataObjects are linked to the corresponding POJOs by specifying a
property (to be defined) on a Type,
* or through an annotation (to be defined) in the XML schema.
*/
public interface POJOHelper {
/**
* DataObject customer1DO = POJOHelper.INSTANCE.load(customer);
* DataObject customer2DO = POJOHelper.INSTANCE.load(customer);
* customer1DO != customer2DO
*/
DataObject load(Object object);
/**
* Object customer1 = POJOHelper.INSTANCE.save(customerDO);
* Object customer2 = POJOHelper.INSTANCE.save(customerDO);
* customer1 != customer2
*/
Object save(DataObject dataObject);
POJOBinder createPOJOBinder();
}
POJO Binder
/**
* The POJOBinder is used to scope when a POJO is linked to a
DataObject.
* The Binder concept could easily be applied to XMLHelper to address
many (if not all)
* of the XML fidelity issues.
*/
public interface POJOBinder {
/**
* POJOBinder pojoBinder = POJOHelper.INSTANCE.createPOJOBinder();
* DataObject customer1DO = pojoBinder.load(customer);
* DataObject customer2DO = pojoBinder.load(customer);
* customer1DO == customer2DO
*/
DataObject load(Object object);
/**
* POJOBinder pojoBinder = POJOHelper.INSTANCE.createPOJOBinder();
* Object customer1 = pojoBinder.save(customerDO);
* Object customer2 = pojoBinder.save(customerDO);
* customer1 == customer2
*/
Object save(DataObject dataObject);
/**
* Get the DataObject linked with is Object.
* DataObjects are linked with Objects through both the load and
update operations.
*/
DataObject getDataObject(Object object);
/**
* Get the Object linked with is DataObject.
* Objects are linked with DataObjects through both the load and
update operations.
*/
Object getPOJO(DataObject);
/**
* Use this method if you want the changes made to the Object to be
reflected in the corresponding DataObject.
*/
DataObject updateDataObject(Object object);
/**
* Use this method if you want the changes made to the DataObject
to be reflected in the corresponding Object.
*/
Object updatePOJO(DataObject dataObject);
}
-Blaise
Barack, Ron wrote:
9279AFBA5302884AB7169C4C51B6FDF7015A4A43@dewdfe1a.wdf.sap.corp"
type="cite">
Hi Frank,
We agree on the main point, which is that projection should be possible in both directions. Do you also agree to the semantics, that calling
helperContext.project(pojo) always returns the same DataObject, and that calling (in your API) dataObject.project(Pojo.class) always returns the same object? That is, whatever the API, in this sense projecting to and from the POJO world works just like projecting between SDO contexts?
Putting all POJOs in a virtual HelperContext gives you two things:
1. It allows you to determine the rules of the projection. For instance, we could have a JaxBHelperContext that projects to JaxB using a different algorithm than is normally used to project to POJOs. (Note: JaxB classes are not necessarily annotated).
2. It gives you a place to manage the mapping between POJOs and DataObjects. Otherwise, the only way I can think of managing the association is by using weak hash maps. Maybe there's nothing wrong with managing such an association through weak hash maps...that's what hash maps were invented for. But it seems like a poor design to me.
Maybe we should start talking use-cases. As usual, I'm thinking about local wires in an SCA composite. I'm thinking about a SOA landscape where some components are written in Java and use POJO data objects, there is some kind of XML based process engine (say BPEL), and some components are written using SDO data objects. Maybe there are some components that are also DOM based. Anyway, we don't want to say that in order to attach your application into the landscape you have to write it so that it uses static SDOs. This would mean that every "new" has to be replaces with a DataFactory.create().
We've established HelperContext.project as the way to move data between two SDO based applications. That is, each application has ist own HelperContext, and we project the data representation from one to the other. The question is, why should it be different when an application uses POJOs? By having POJO helper contexts I can say that each application has it's own HelperContext. Even the POJO application has a HelperContext, just a HelperContext<Object> instead of a HelperContext<DataObject>. But the way to move data between them is consistent, through HelperContext.project.
Plus the idea of taking a POJO and calling HelperProvider.getPojoContext().getXMLHelper().save(pojo) is just really cool. ;-)
Best Regards,
Ron
-----Ursprüngliche Nachricht-----
Von: Frank Budinsky [mailto:frankb@ca.ibm.com]
Gesendet: Donnerstag, 3. Juli 2008 23:58
An: sdo@lists.oasis-open.org
Betreff: Re: [sdo] Re: ISSUE 130: New DataObject method: cast()
Hi Ron,
Thanks for your reply ... some interesting ideas.
As you may have guessed, I'm very much in favor of changing the argument
of HelperContext.project() to Object and using it as the API to convert
from POJO to DataObject.
I'm not so sure, however, about using HelperContext.project() as the API
to go from DataObject to POJO. I've given it quite a bit of thought and I
can't convince myself that it's the right way to handle this. In my mind,
it seems too complex to try to think of every Java Object as being in an
SDO context. It seems simpler to me to think of a POJO as being in "no
context", but by using HelperContext.project(), you can "bring it into a
context" and then work with it as a DataObject.
Your generic POJO HelperContext, below, helps to highlight what I mean:
HelperContext<Object> getPojoContext();
As you've realized, the only class that can be used for the generic
HelperContext argument is java.lang.Object, since it's the only base class
for any arbitrary POJO class (e.g., Company, Employee, Department, etc.).
Given this, it seems quite odd to think that any java class is
conceptually in the "Object" context - for example, java.lang.Integer
would fall into this category. Given that the generic POJO context is
really only capable of working with "proper SDO-capable objects", the
various SDO helper methods, e.g., XMLHelper.save(), CopyHelper.copy(),
would need to fail, if the Object passed in isn't really an SDO-capable
object. If instead, we said that POJOs are in no context, but you can
project() them into a context if you want to use them with SDO helpers,
then the only possible failure would be if
HelperContext.project(someObject) fails. For example
someContext.project(new Integer()) would probably fail - because it's not
really a SDO-capable POJO. On the other hand, someContext.project(new
Company()) would probably succeed. Once you have the DataObject in the
target context, you can then use SDO helpers, e.g., CopyHelper, XMLHelper,
and know that they will work.
Whether or not we can convince ourselves that HelperContext.projsec() is a
good way to convert an SDO to a POJO, it seems that it still isn't as user
friendly as the API I've proposed. It would still require a Java cast:
Company company =
(Company)HelperProvider.getPojoContext().project(myDataObject);
I would think that the DataObject method I suggested would still be a
cleaner API:
Company company = myDataObject.cast(Company.class);
Even if, the implementation of this method would simply delegate to a POJO
context (although, as I said above, I'm not sure that's the right way to
do it anyway).
This discussion, has however, made me start to think that the
DataObject.cast() method I've proposed would be better named
DataObject.project(), for consistency with HelperContext.project() which I
think is the right API for the reverse operation.
I hope my thoughts are clear enough here. This is a fairly complicated
topic to discuss in email.
Thanks,
Frank
"Barack, Ron" <ron.barack@sap.com>
07/01/2008 04:38 AM
To
Frank Budinsky/Toronto/IBM@IBMCA, <sdo@lists.oasis-open.org>
cc
Subject
[sdo] Re: ISSUE 130: New DataObject method: cast()
Hi Frank,
As you point out, for the case of getSequence() there is no loss of
functionality here. Not so wrt casting between DataObject and a static
SDO. In this case, we would be taking a functionality that allows
"conversion" in both directions with a functionality that allows
"conversion" in only one direction. I don't think we should consider a
resolution of the issue that leaves SDO 3.0 weaker than SDO 2.1, and then
hope that the missing functionality is added back in through the
resolution of a seperate issue.
One of the selling points of SDO is that clients can use the (type safe,
business logic oriented) static SDO to make changes, and then send the
modified data, complete with change summary back to the server. The
server can then use the ChangeSummary, rich metadata model, XML
serialization and other functionality provided by the SDO infrastructure.
In order to make this work in any sort of reasonable way, we need to
maintain the association between the static SDOs and the corresponding
DataObjects so that identity is maintained. When we return the DataObject
that corresponds to a particular POJO, we must always return the same
DataObject. (Note here the correspondence with the behavior when
projecting between contexts...this supports the ideas that I will be
proposing later in this email.) In SDO 2.1, association between the
static SDO and the DataObject is maintained because the instance
implements both. Potentially, we could weaken this to requiring that the
association be maintained through references (that is, that the POJO could
have a reference to the corresponding DataObject). This doesn't help us
when dealing with POJOs that are not "SDO-ready", that is, that are simple
JavaBean data containers that are neither castable nor contain references
to DataObject. In this case, the association must be maintained
externally. The only way I can think of doing this is to have some sort
of (weak) map from the POJO to the DataObject. Of course, maintaining
such a map introduces its own set of problems and costs, made more
critical because the conversion between POJO and DataObject is probably
something we want to do lazilly. Clients that are in the position to work
with static SDO should continue to work just as efficiently as before.
Therefore, I wouldn't want to see the new functionality, the ability to
move between POJO (and potentially also DOM) data representations and
DataObjects, as a replacement for static SDO. We are talking about a new
functionality here: converting from a POJO (or other) data representation
to DataObject and back.
The most obvious API for converting from a POJO object to an SDO is to
reuse the project method from SDO-66. That is, to change the signature
from project(commonj.sdo.DataObject) to project(java.lang.Object). The
behavior is simply to consider the POJO object as if it were a static SDO.
This is the key idea behind everything I will propose here. Of course,
to make this work we need something like what I proposed in SDO-5, a way
to introspect the classes and generate an SDO metamodel. If we have that,
then we can consider every POJO object a potential SDO. That is, the POJO
behaves exactly like the DataObject for which it could be the static
representation.
If we are going to consider the POJOs "potential" DataObjects, then I
think it follows to consider them to be defined in some HelperContext. If
there is a HelperContext that manages the POJOs the way that normal
HelperContexts manage DataObjects, then to get from a DataObject to a POJO
is simply a matter of projecting from the DataObject into the POJO
context.
I think the approach could be just as extensible as the proposed cast
method. For instance, we could have a DOM context. Projecting into this
context returns a Node.
The approach also give the user the chance to control the POJO ->
DataObject map, in that he can control the HelperContext that maintains
it. In a way, the helperContext works like the JaxB Binder.
Users could obtain a POJO helper context from the HelperProvider.
Something like: HelperProvider.getPojoContext(ClassLoader). The user
could then define types in this context using the API proposed in SDO-5.
Alternatively, we could be a bit more JAX-B like, and have
HelperProvider.getPojoContext(Class ...) or
HelperProvider.getPojoContext(Package).
What about the behavior of the Helpers within the HelperContext?
Logically, the behavior of CopyHelper, EqualityHelper, even XMLHelper are
all clear from the concept stated above, the POJOs behave just like static
SDOs. The APIs, however, don't match... Everywhere where
commonj.sdo.DataObject appears the API becomes unusable. Here is a place
where Generics could be used. We could make all our Helpers generic, eg,
XMLHelper<T> {
...
String save(T dataObject, String uri, String localName);
...
}
We would then have
HelperProvider {
HelperContext<DataObject> getDefaultContext();
HelperContext<Object> getPojoContext();
<T> HelperContext<T> getOtherContext(Class<T> t); // Probably
Node
...
}
I think this approach has a lot of potential. It would be pretty neat to
be able to e.g. serialize POJOs to XML using the SDO rules (including
orphan properties, etc). Normally, I have a working prototype before I
make a proposal like this. In this case, I'd like to get a little
feedback from the group first. We don't have to go all the way to
accepting the API changes to make this all work, in fact, I'd rather leave
the API changes to a general refactoring of the API (for which we have an
entire scope item).
What do you think?
Ron
-----Ursprüngliche Nachricht-----
Von: Frank Budinsky [mailto:frankb@ca.ibm.com]
Gesendet: Freitag, 13. Juni 2008 20:43
An: sdo@lists.oasis-open.org
Betreff: [sdo] Re: ISSUE 130: New DataObject method: cast()
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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