关闭持久性管理器后,如何使查询结果可用

Question

我正在学习GAE并且有点卡住了.如果我使用以下内容,最后确保持久性管理器已关闭,那么在尝试实际读取Note对象时会出现异常:

public class Notes {
    public List<Note> getAll() {
    PersistenceManager pm = PMF.instance().getPersistenceManager();

    try {
        Query query = pm.newQuery("select from com.uptecs.google1.model.Note order by subject");
        return (List<Note>) query.execute();
    } finally {
        pm.close();
    }
    }
}

我得到的例外是:

Object Manager has been closed
org.datanucleus.exceptions.NucleusUserException: Object Manager has been closed
    at org.datanucleus.ObjectManagerImpl.assertIsOpen(ObjectManagerImpl.java:3876)
    at org.datanucleus.ObjectManagerImpl.getFetchPlan(ObjectManagerImpl.java:376)
    at org.datanucleus.store.query.Query.getFetchPlan(Query.java:497)

Answer 1

尝试使用以下方法从图中分离对象detachable="true":

@PersistenceCapable(identityType = IdentityType.APPLICATION, detachable="true")
public class Note {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Long key;
...
}

注意:我完全理解需要这样做,有时你需要在控制器中检索对象和列表,关闭控制器中的PM,然后将模型传递给视图.在我了解更好的解决方案之前,这就是我在JDO/GAE上做到这一点,到目前为止没有任何问题.

列表:

在我看来,如果你想在PM关闭后能够使用它们,你将不得不分离列表中的所有项目.我会用它来获取特定的项目列表.一个完整的getAll()可以是非常大的.

public List<Note> getList(){
    List<Note> detachedList=null, list=null;
    try {
        String query = "select from " + Note.class.getName();
        pm = PMF.get().getPersistenceManager();
        list = (List<Note>)pm.newQuery(query).execute();            
        detachedList = new ArrayList<Note>();
        for(Note obj : list){
            detachedList.add(pm.detachCopy(obj));
        }

    } finally {
        pm.close();
    }
    return detachedList;

}

按键:

public Note findByKey(Long key) {
    Note detachedCopy=null, object=null;
    try{
        pm= PMF.get().getPersistenceManager();
        object = pm.getObjectById(Note.class,key);
        detachedCopy = pm.detachCopy(object);
    }catch (JDOObjectNotFoundException e) {
        return null; // or whatever
    } 
    finally {
        pm.close(); // close here
    }
    return detachedCopy;

}

结束后,你有一个独立的副本,你可以使用它.

参考:http://www.datanucleus.org/products/accessplatform_1_1/jdo/attach_detach.html