Google App Engine 是否支持带有“Object”的 JDO 数据对象？领域？

Question

我的 JDO 类之一中有一个“Object”字段，如下所示：

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable = "true")
class Foo {
   ...
   @Persistent
   @Value(types={java.lang.Boolean.class, java.lang.String.class})
   Object bar;
   ...
}

当我尝试访问该字段时，出现以下异常：

Field "Foo.bar" is declared as a reference
type (interface/Object) but no implementation classes of "java.lang.Object" have been found!
org.datanucleus.exceptions.NucleusUserException: Field "Foo.bar" is declared as a reference type (interface/Object) but no implementation classes of "java.lan
g.Object" have been found!
        at org.datanucleus.metadata.MetaDataUtils.getImplementationNamesForReferenceField(MetaDataUtils.java:531)
        at org.datanucleus.store.mapped.mapping.ReferenceMapping.createPerImplementationColumnsForReferenceField(Re
ferenceMapping.java:279)
        at org.datanucleus.store.mapped.mapping.ReferenceMapping.prepareDatastoreMapping(ReferenceMapping.java:205)
        at org.datanucleus.store.mapped.mapping.ReferenceMapping.initialize(ReferenceMapping.java:114)
        at org.datanucleus.store.mapped.mapping.MappingFactory.createMapping(MappingFactory.java:97)
        at org.datanucleus.store.mapped.mapping.AbstractMappingManager.getMapping(AbstractMappingManager.java:248)
        at org.datanucleus.store.appengine.DatastoreTable.initializeNonPK(DatastoreTable.java:301)
        at org.datanucleus.store.appengine.DatastoreTable.buildMapping(DatastoreTable.java:276)
        at org.datanucleus.store.appengine.DatastoreManager.newStoreData(DatastoreManager.java:329)
        at org.datanucleus.store.AbstractStoreManager.addClasses(AbstractStoreManager.java:788)
        at org.datanucleus.store.AbstractStoreManager.addClass(AbstractStoreManager.java:759)
        at org.datanucleus.store.mapped.MappedStoreManager.getDatastoreClass(MappedStoreManager.java:358)
        at org.datanucleus.store.appengine.DatastoreManager.getDatastoreClass(DatastoreManager.java:555)
        at org.datanucleus.store.appengine.query.DatastoreQuery.performExecute(DatastoreQuery.java:200)
        at org.datanucleus.store.appengine.query.JDOQLQuery.performExecute(JDOQLQuery.java:84)
        at org.datanucleus.store.query.Query.executeQuery(Query.java:1489)
        at org.datanucleus.store.query.Query.executeWithArray(Query.java:1371)
        at org.datanucleus.store.query.Query.execute(Query.java:1344)
        at org.datanucleus.jdo.JDOQuery.execute(JDOQuery.java:221)

有谁知道 Google App Engine 是否支持“@Value”注释和/或如何解决这个问题吗？

谢谢！

Answer 1

谢谢！我不应该使用@Value，而是应该这样做：

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable = "true")
class Foo {
   ...
   @Persistent(types={java.lang.Boolean.class, java.lang.String.class})
   Object bar;
   ...
}

Answer 2

根据他们的页面，Google App-Engine 使用 DataNucleus 来支持 JDO。从 DataNucleus 的网站来看，似乎支持 @Value 注释，但在“type”属性上，它显示“虽然该属性允许多个值，但 DataNucleus 目前仅支持第一个类型值

” datanucleus.org/products/accessplatform_1_1/jdo/annotations.html#Value" rel="nofollow noreferrer">http://www.datanucleus.org/products/accessplatform_1_1/jdo/annotations.html#Value

附录， @Value 似乎仅用于地图。

Answer 3

如果使用对象类型的字段，则 DN 文档中的页面是
http://www.datanucleus.org/products/accessplatform_1_1/jdo/ orm/objects.html
它明确支持 RDBMS、XML、LDAP、ODBMS 等数据存储。我不知道Google的BigTable插件是否支持它。也许如果您发布该异常的堆栈跟踪，您可以向人们展示它来自哪里...

--Andy (DataNucleus)