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Subject: AW: [sdo] Re: ISSUE 130: New DataObject method: cast()
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. --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. You may a link to this group and all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. You may a link to this group and all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. You may a link to this group and all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. You may a link to this group and all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
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