Google App Engine是否支持带有“对象”字段的JDO数据对象？

Question

我的一个JDO类中有一个“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;
   ...
}

当我尝试访问此字段时，我遇到以下异常：

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/或如何解决此问题？

谢谢！

Answer 1

谢谢！而不是使用@Value，我应该这样做：

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

Answer 2

根据他们的网页，Google App-Engine使用DataNucleus来支持JDO。从DataNucleus的网站看，似乎支持@Value注释，但在'type'属性上它显示“虽然属性允许多个值，DataNucleus目前仅支持第一个类型值”

http://www.datanucleus.org/products/accessplatform_1_1/jdo/annotations.html#Value

附录，似乎@Value仅用于地图。

Answer 3

如果使用Object类型的字段，则DN文档中的页面为 http://www.datanucleus.org/products/accessplatform_1_1/jdo/orm/objects.html 这显然支持RDBMS，XML，LDAP，ODBMS等数据存储。 Google的BigTable插件是否支持它我不知道。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......

- 安迪（DataNucleus）