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Best practices for modularity?
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Best practices for modularity?
by polly.c.chang
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Hi everyone,
I'm back. :) I have been using EclipseLink for OXM, and it has succeeded extremely well for my prototype. Thank you guys for all your help! And congrats on the 1.0 release. EclipseLink is awesome. Now I need to figure out a general strategy for using EclipseLink OXM in our enterprise. I'm thinking that we will have some mappings that are considered "common" and then others that are application-specific and not shared. The common vs. application mappings will be in different projects and deployed as different jars. I can see that it's possible to partition the mappings from different projects so that they each have their own project.xml fiile, and then you can have one session.xml file that is able to work with multiple projects.xml files (we want to define as much as we can in the project xml files for easier maintenance). My questions are: 1. What does it mean to have a "primary" project versus "additional" projects in the session? The documentation talks about how to configure these but doesn't really go into the ramifications. Does the order of the projects matter? 2. Is is possible for an application's mappings to override the ones from the common project? 3. Is is possible for the common project to define mappings that are "abstract" or based on interfaces? I see that there is an XMLChoiceMapping, but I am not sure how to use that because I won't know ahead of time what application-specific class I need to use. I would like to have a common POJO that contains composite objects that are defined in the application-specific project. Is that possible? 4. Is is possible to map different root elements to the same POJO? Or can I use a regular expression when matching the root element name? Our schema is defined such that we have root elements with different names that have very similar content, so I'd like to be able to map them to the same POJO. From what I've seen from the code, it looks like I will not be able to do this directly. So I think my alternative is to come up with a transformation to apply before unmarshalling and after marshalling to change the root element name. Or do you have other suggestions? 5. Can you share any other best practices for implementing a modular design with these mappings? Thanks for your help! Polly |
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Re: Best practices for modularity?
by polly.c.chang
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Does anybody know the answers? I tried hacking some stuff together but haven't met any success. I really need the answers to 3 and 4 the most.
Thanks, Polly
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Re: Best practices for modularity?
by Blaise Doughan
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Hi Polly,
I'm glad to hear you are enjoying your experience with EclipseLink, in particular the OXM mapping. Below is my first stab at an answer based on some things we have done in the past. 1. What does it mean to have a "primary" project versus "additional" projects in the session? The documentation talks about how to configure these but doesn't really go into the ramifications. Does the order of the projects matter? I never really thought of it as primary vs additional projects. Each project is equal, the order is only significant if you have multiple objects mapped to the same default root element. As a default root element must correspond to only one descriptor. --- 2. Is is possible for an application's mappings to override the ones from the common project?I'll describe a strategy we use to unmarshal our own metadata. We use the Java version of the metadata and create a subclass of the project for each version that we distribute. Each descriptor is built in its own method, and when new features are added the corresponding mapping project overrides the corresponding buildDescriptor method. In Root Project public RootProject() { super(); this.addDescriptor(buildFooDescriptor); } protected XMLDescriptor buildFooDescriptor() { XMLDescriptor fooDescriptor = new XMLDescriptor(); // set up the descriptor return fooDescriptor; } In Child Project (Which Extends Root Project) protected XMLDescriptor buildFooDescriptor() { XMLDescriptor fooDescriptor = super.buildFooDescriptor(); // add to the descriptor return fooDescriptor; } This strategy is demonstrated in the EclipseLink source code in the following classes: org.eclipse.persistence.internal.sessions.factories.ObjectPersistenceRuntimeXMLProject.java org.eclipse.persistence.internal.sessions.factories.ObjectPersistenceRuntimeXMLProject_11_1_1.java org.eclipse.persistence.internal.sessions.factories.EclipseLinkObjectPersistenceRuntimeXMLProject.java --- 3. Is is possible for the common project to define mappings that are "abstract" or based on interfaces? I see that there is an XMLChoiceMapping, but I am not sure how to use that because I won't know ahead of time what application-specific class I need to use. I would like to have a common POJO that contains composite objects that are defined in the application-specific project. Is that possible?I have seen users make use of our Any mappings for this. Their scenario is usually the following, they have one OXM project that corresponds to the message envelope, and many OXM projects that correspond to possible payloads. To implement this they use an XMLAnyObjectMapping to map the message body property on their message object, and then in the payload projects they ensure that all of objects that form the root of the body have default root elements set. You will need to ensure that the XMLContext is created with both the message and payload projects. --- 4. Is is possible to map different root elements to the same POJO? Or can I use a regular expression when matching the root element name? Our schema is defined such that we have root elements with different names that have very similar content, so I'd like to be able to map them to the same POJO. From what I've seen from the code, it looks like I will not be able to do this directly. So I think my alternative is to come up with a transformation to apply before unmarshalling and after marshalling to change the root element name. Or do you have other suggestions?You can map multiple root elements to the same POJO. Currently this can not be done in the UI, but you can create an "After Load" method and modify the descriptor by hand. You can call "setDefaultRootElement(String)" multiple times on the descriptor. All of these names will be used to recognize the object during unmarshalling, but the last defaultRootElement set will be used for marshalling. If you want to use a different root element, you can wrap your object in an instance of org.eclipse.persistence.oxm.XMLRoot of javax.xml.bind.JAXBElement (if your are using JAXB). --- 5. Can you share any other best practices for implementing a modular design with these mappings?I mentioned a couple of our most common strategies above. Can you share more details about the type of modularity you would like to see? -Blaise amphoras wrote: Does anybody know the answers? I tried hacking some stuff together but haven't met any success. I really need the answers to 3 and 4 the most. Thanks, Polly amphoras wrote: _______________________________________________ eclipselink-users mailing list eclipselink-users@... https://dev.eclipse.org/mailman/listinfo/eclipselink-users |
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Re: Best practices for modularity?
by polly.c.chang
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Hi Blaise,
Thanks for your detailed and helpful response. I just have a few more questions. > > 1. What does it mean to have a "primary" project versus "additional" > > projects in the session? The documentation talks about how to configure > > these but doesn't really go into the ramifications. Does the order of the > > projects matter? > I never really thought of it as primary vs additional projects. Each project is equal, the order is only > significant if you have multiple objects mapped to the same default root element. As a default root > element must correspond to only one descriptor. What I was trying to do was create an incomplete/abstract mapping for a class in my "common" project.xml file. Then I created another mapping for the same class in my "app" project.xml file that fills in the missing pieces and overrides certain settings from the "common" mapping. My session.xml file loads both project files. I found that since both mappings are for the same class, whichever one that I set as the "primary" in the session was wiped out by the second one. Actually, all settings were replaced except for one. The "common" and "app" versions of the class mappings have different "default root element" settings, and I found that both were available for unmarshalling. So the result was cumulative. It sounds like you expect that the default root element should have been replaced by the second class mapping? --- > > 2. Is is possible for an application's mappings to override the ones from > > the common project? > I'll describe a strategy we use to unmarshal our own metadata. We use > the Java version of the metadata and create a subclass of the project > for each version that we distribute. Each descriptor is built in its > own method, and when new features are added the corresponding mapping > project overrides the corresponding buildDescriptor method. > ... yes, that is the exact behavior that I'm looking for! Is it possible to do this using the deployment project.xml files? I am trying to do as much as I can using the xml files. I am ok with creating event listeners like the amendment method, but I am trying to minimize use of Java for ease of changing the mappings. --- > > 3. Is is possible for the common project to define mappings that are > > "abstract" or based on interfaces? I see that there is an > > XMLChoiceMapping, but I am not sure how to use that because I won't know > > ahead of time what application-specific class I need to use. I would like > > to have a common POJO that contains composite objects that are defined in > > the application-specific project. Is that possible? > I have seen users make use of our Any mappings for this. Their scenario > is usually the following, they have one OXM project that corresponds to > the message envelope, and many OXM projects that correspond to possible > payloads. To implement this they use an XMLAnyObjectMapping to map the > message body property on their message object, and then in the payload > projects they ensure that all of objects that form the root of the body > have default root elements set. You will need to ensure that the > XMLContext is created with both the message and payload projects. yes, exactly. I have an element like this: <Foo> <Action></Action> <Data></Data> <Data></Data> ... </Foo> "Action" is a class hierarchy of different kinds of actions, and "Data" is a collection of any type of element. So I configured "Action" with an XMLChoiceMapping and "Data" as XMLAnyCollectionMapping, and that works! I did try to set "Action" to XMLAnyObjectMapping, but EclipseLink got confused. So I guess you can't use two "any" mappings next to one another, which makes sense. The XMLChoiceMapping works fine for the "Action". The only problem is when I try to go roundtrip more than once. I can unmarshall the document and marshall it back out, but the Action element is output with the attribute 'xsi:type="ns1:ProcessType"'. I don't think this attribute is allowed by the schema, and when I unmarshall the document again, I get: [Fatal Error] :178:72: The prefix "xsi" for attribute "xsi:type" associated with an element type "ns1:Process" is not bound. I noticed that the "xsi" namespace is not defined in the output file even though it was in the original one. So my questions are: a. Can I suppress the xsi:type attribute from being generated? b. Failing that, I need to get this attribute added to the schema, correct? c. How do I get the "xsi" namespace to be generated for the output? --- > > 4. Is is possible to map different root elements to the same POJO? Or > > can I use a regular expression when matching the root element name? Our > > schema is defined such that we have root elements with different names > > that have very similar content, so I'd like to be able to map them to the > > same POJO. From what I've seen from the code, it looks like I will not be > > able to do this directly. So I think my alternative is to come up with a > > transformation to apply before unmarshalling and after marshalling to > > change the root element name. Or do you have other suggestions? > You can map multiple root elements to the same POJO. Currently this can > not be done in the UI, but you can create an "After Load" method and > modify the descriptor by hand. You can call > "setDefaultRootElement(String)" multiple times on the descriptor. All > of these names will be used to recognize the object during > unmarshalling, but the last defaultRootElement set will be used for > marshalling. Hmm, my scenario is that the class mapping is in the "common" project, and I won't know what the possible default root elements are ahead of time. Only the "app" project will know that, and it needs to define multiple root elements (around 5). So I cannot call setDefaultRootElement() from the amendment method while the "common" project is loading. This is similar to the "data" element in question 3 that was solved by the "any" mapping. I need to go roundtrip using these many root elements, so marshalling using the last defined root element also won't work. > If you want to use a different root element, you can wrap > your object in an instance of org.eclipse.persistence.oxm.XMLRoot of > javax.xml.bind.JAXBElement (if your are using JAXB). can you clarify this? I've never actually worked with JAXB directly. So far I've been very happy to let EclipseLink hide JAXB under the covers. ;) If I can marshall to those different root elements using a callback method, that would be fine. I can figure out what root element to use based on data inside the object. Or... would the "preserve document" setting help in this case? --- > > 5. Can you share any other best practices for implementing a modular > > design with these mappings? > I mentioned a couple of our most common strategies above. Can you share > more details about the type of modularity you would like to see? sure, I'll summarize from my response above: * ability to allow the project.xml files to inherit and override mappings like you are doing with Java. * ability to defer defining root elements to another project.xml file. * ability to marshall using different root elements using a callback method or some way to "remember" what was unmarshalled. Thanks for your help! This is much easier than debugging random EclipseLink source code in hope of finding a hint. :) --Polly |
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Re: Best practices for modularity?
by polly.c.chang
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Hi Blaise,
OK, I Googled the JAXB API and experimented. I think you're saying that I need to do this: public Object fromXML(final File file) throws Exception { JAXBContext jc = JAXBContext.newInstance(MyGenericObject.class.getPackage().getName()); Unmarshaller u = jc.createUnmarshaller(); DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); dbf.setNamespaceAware(true); DocumentBuilder db = dbf.newDocumentBuilder(); Document doc = db.parse(file); Element rootElem = doc.getDocumentElement(); JAXBElement<MyGenericObject> jaxbElem = u.unmarshal( rootElem, MyGenericObject.class); return jaxbElem.getValue(); } public String toXML(final MyGenericObject obj) throws Exception { JAXBContext jc = JAXBContext.newInstance(MyGenericObject.class.getPackage().getName()); Marshaller m = jc.createMarshaller(); JAXBElement jaxbElem = new JAXBElement( new QName("http://www.foo.com/bar/baz","RootNodeFoo"), MyGenericObject.class, obj); StringWriter writer = new StringWriter(); m.marshal(jaxbElem, writer ); return writer.toString(); } Is that right? This seems a bit messy. It's not nearly as elegant as the generic marshalling and unmarshalling code that I'm able to use with EclipseLink. :( Is this what you recommend? Thanks, Polly |
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Re: Best practices for modularity?
by Blaise Doughan
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Hi Polly,
I'm going to try and develop this a little more but here are some current thoughts: We use MOXy to map our XML mapping data to our programmatic metadata. This means that you could use MOXy to unmarshal your XML metadata files into the programmatic metadata: import java.io.File; import org.eclipse.persistence.internal.sessions.factories.EclipseLinkObjectPersistenceRuntimeXMLProject; import org.eclipse.persistence.oxm.*; import org.eclipse.persistence.sessions.Project; ... EclipseLinkObjectPersistenceRuntimeXMLProject project = new EclipseLinkObjectPersistenceRuntimeXMLProject(); XMLContext xmlContext = new XMLContext(project); XMLUnmarshaller unmarshaller = xmlContext.createUnmarshaller(); Project baseProject = (Project) unmarshaller.unmarshal(new File("your_base_metadata.xml")); Project extensionProject = (Project) unmarshaller.unmarshal(new File("your_extension_metadata.xml")); Then you could pull data from extensionProject into the baseProject, useful methods here are the following: XMLDescriptor customerDescriptor = (XMLDescriptor) project.getClassDescriptor(Customer.class); XMLDirectMapping firstNameMapping = (XMLDirectMapping) customerDescriptor.getMappingForAttributeName("firstName"); Finally you would create an XMLContext on your combined project, if you moved everything into your base project, then this would be done as follows: XMLContext xmlContext2 = new XMLContext(baseProject); --- Other Items:
-Blaise amphoras wrote: Hi Blaise, Thanks for your detailed and helpful response. I just have a few more questions. _______________________________________________ eclipselink-users mailing list eclipselink-users@... https://dev.eclipse.org/mailman/listinfo/eclipselink-users |
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Re: Best practices for modularity?
by polly.c.chang
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ah, I see. This is an interesting strategy. It gives me a lot of food for thought, and I'll definitely keep it in mind.
Thanks, Polly
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