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https://issues.apache.org/jira/browse/OPENJPA-1804?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12912811#action_12912811
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Willis Blackburn commented on OPENJPA-1804:
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Donald,
Maybe I'm not using the right terminology. I understand that I can use OpenJPA
in one of four ways. First, I can run the class enhancer during the build
process. Second, I can define a Java agent when I run the JVM that will
enhance the classes as they're loaded. Third, if I'm using Java 6, OpenJPA
will install a classloader hook of some kind that will also enhance the classes
as they're loaded. And fourth, if I'm running Java 5, OpenJPA will dynamically
build persistent subclasses of my entity classes. In all cases we ultimately
get classes that extend PersistenceCapable, but in the first three strategies,
the PersistenceCapable classes are the application's own classes, while in the
last case, they're subclasses of the application's classes. I understand that
the fourth method (dynamically-generated subclasses) has certain drawbacks,
some by necessity (it not being possible to intercept field set operations for
example), and some because it is still experimental. I believe that I'm using
the third method, Java 6 with the classloader hook.
That said, I did have to set openjpa.runtimeUnenhancedClasses=supported in
order to get the program to run at all. Am I in fact using the Java 5 dynamic
subclassing strategy?
> NPE in MappingInfo.java line 1514
> ---------------------------------
>
> Key: OPENJPA-1804
> URL: https://issues.apache.org/jira/browse/OPENJPA-1804
> Project: OpenJPA
> Issue Type: Bug
> Components: Enhance, UnenhancedClasses, usability
> Affects Versions: 2.0.1
> Reporter: Willis Blackburn
>
> I am encountering a NPE at MappingInfo.java line 1514. I'm sorry I don't
> have time to prepare a test case right now. I'm not even 100% sure what is
> causing the problem, but it seems to be something like this:
> I have classes Organization, Group, User, and AddressBook.
> Group has a reference (many-to-one) to Organization.
> I'm using field-level access and the Java 6 runtime enhancer. (NOT the
> build-time enhancer or the agent.)
> In ManagedClassSubclasser.prepareUnenhancedClasses, the code receives a list
> of Class objects--the ones in persistence.xml. They are not in the same
> order as they are in persistence.xml, but I don't know if that's important.
> The first one is Group. During the processing of Group, the code invokes
> ClassMapping.setTable to set the table name, which is "group_table." This
> comes from the @Table annotation of Group. So far, so good. Then the code
> does some additional processing on the relationship mappings of Group, which
> include the reference to Organization. During the processing of that
> relationship, the code calls MappingInfo.mergeJoinColumn. On line 1367, the
> method calls rel.getTable. The "rel" variable points to the ClassMapping for
> Organization. Remember that prepareUnenhancedClasses hasn't gotten to
> Organization yet--it's still working on Group--and so I assume that this
> Organization mapping was created on-demand. However, rel.getTable returns
> null. The Organization class has a @Table attribute, and I can see that the
> table name has been loaded in the ClassMappingInfo instance attached to the
> ClassMapping for Organization, however the table name has not yet been copied
> from ClassMappingInfo to ClassMapping. Later, the mergeJoinColumn method
> attempts to dereference the null table name and generates the NPE.
> Bottom line: When using the Java 6, runtime enhancer, if class A has a
> reference to class B, and both classes have @Table annotations (at least),
> and class A is processed first, then it produces a NPE.
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