java - 执行实体管理器工厂创建语句时出现此异常

Question

当执行这个java代码时，从数据存储中进行选择，由于第一行出现了异常，那么我该如何解决这个异常？ 异常(exception):

javax.persistence.PersistenceException: Explicit persistence provider error(s) occurred for "transactions-optional" after trying the following discovered implementations: org.datanucleus.api.jpa.PersistenceProviderImpl from provider: org.datanucleus.api.jpa.PersistenceProviderImpl

代码:

PrintWriter out = resp.getWriter();
try {
    EntityManagerFactory emfactory = Persistence
            .createEntityManagerFactory("transactions-optional");
    EntityManager entitymanager = emfactory.createEntityManager();
    javax.persistence.Query query = entitymanager.createQuery("Select u from test u");
    List<test> list = (List<test>) query.getResultList();
    out.print(list.size());
} catch (Exception e) {
    out.println(e.toString());
} finally {
    out.close();
}

持久性.xml:

	<persistence-unit name="transactions-optional">
		<provider>
		org.datanucleus.store.appengine.jpa.DatastorePersistenceProviderImpl
</provider>
		<properties>
			<property name="datanucleus.NontransactionalRead" value="true" />
			<property name="datanucleus.NontransactionalWrite" value="true" />
			<property name="datanucleus.ConnectionURL" value="appengine" />
		</properties>
	</persistence-unit>
</persistence>

Answer 1

正如您在评论(!)中发布的异常所说

org.datanucleus.metadata.InvalidClassMetaDataException: Class s.follows has application-identity and no objectid-class specified yet has 0 primary key fields. Unable to use SingleFieldIdentity. 

但是您没有发布此类(为什么？!您不认为人们可能想看到它，因为它是异常(exception)的内容吗？)。

解决方案:将字段标记为 @Id 就像所有 JPA 文档所说的那样。