Is there a way in Hibernate to obtain an entity's loaded PersistentCollection without loading the entire entity object first?

问题

This is a puzzler! :D

Is there a way to force Hibernate to load a collection for an entity without loading the entire entity first?

Let m explain better. I have a Role entity annotated this way:

@Entity(name="Role")
@Table(name = "ROLES")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
@javax.persistence.TableGenerator(
 name="GENERATED_IDS",
 table="GENERATED_IDS",
    valueColumnName = "ID"
)
public abstract class Role implements Serializable {
 private static final long serialVersionUID = 1L;


 /**
  * The id of this role. Internal use only.
  * 
  * @since 1.0
  */
 @Id @GeneratedValue(strategy = GenerationType.TABLE, generator="GENERATED_IDS")
 @Column(name = "ROLE_ID")
 protected long id;


 /**
  * Set of permissions granted to this role.
  * 
  * @since 1.0
  */
 @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE }, mappedBy="sourceRole")
 protected Set<Permission> permissions = new HashSet<Permission>();

...

}

When I access the permissions collection by doing:

Role role = ...
Set permissions = role.getPermission();

Hibernate wraps the returned collection with one of its subclasses of PersistentCollection. I can then use Hibernate.initialize(permissions); to force the collection to be initialized.

However, what I need is a way to accomplish the same without first loading the role entity. I know the id for the entity I need the permissions collections for, and the Role for the collection (temp.pack.Role.permissions).

Is there a way to do that? I wold like to avoid hitting the database to retrieve all fields of the Role object (many!) just to get the collection and discard them all.

I could use a join, but that givens me access to the permission objects themselves, not the actual PersistentCollection wrapper which the one I need.

I tried this:

Session session = connectionInfoProvider.getSession(); 

// this is just "permissions", the collection field name
String attributeName = role.substring(role.lastIndexOf(".")+1, role.length()); 

// this is the Role class name, temp.pack.Role
String entityClassName = role.substring(0, role.lastIndexOf("."));
Class<?> roleClass = Class.forName(entityClassName);

Field collectionField = roleClass.getDeclaredField(attributeName);
collectionField.setAccessible(true);
Hibernate.initialize(collection);

But didn't work. The collection I get is just a regular empty set and nothing is loaded.

I also tried this:

Session session = connectionInfoProvider.getSession();

// this is just "permissions", the collection field name
String attributeName = role.substring(role.lastIndexOf(".")+1, role.length());

// this is the Role class name, temp.pack.Role
String entityClassName = role.substring(0, role.lastIndexOf("."));
Class<?> roleClass = Class.forName(entityClassName);

Field collectionField = roleClass.getDeclaredField(attributeName);
collectionField.setAccessible(true);
Object object = session.load(roleClass, id);
ProxyObject objectProxy = (ProxyObject)object;
LazyInitializer lazyInitializer = (LazyInitializer)objectProxy.getHandler();

Object objectImpl = lazyInitializer.getImplementation();
Object collection = collectionField.get(objectImpl);
Hibernate.initialize(collection);

But also didn't work, it fails with java.lang.IllegalArgumentException: unknown handler key.

I also tried:

PersistenceContext context = ((SessionImplementor)currSession).getPersistenceContext();
CollectionPersister persister = ((SessionFactoryImplementor) sessFactory) .getCollectionPersister(role);
CollectionKey key = new CollectionKey(persister, id, EntityMode.POJO);
// collection - contains set containing actual data
PersistentCollection collection = context.getCollection(key);

But also fails with java.lang.IllegalArgumentException: unknown handler key

Any ideas on how to accomplish this?

回答1:

Well, you could make things complicated and use lazy fetching of properties (which requires bytecode instrumentation). But let me quote the documentation:

19.1.7. Using lazy property fetching

Hibernate3 supports the lazy fetching of individual properties. This optimization technique is also known as fetch groups. Please note that this is mostly a marketing feature; optimizing row reads is much more important than optimization of column reads. However, only loading some properties of a class could be useful in extreme cases. For example, when legacy tables have hundreds of columns and the data model cannot be improved.

So before to take this path, I would first make sure that loading these properties is really a concern.

来源：https://stackoverflow.com/questions/4060126/is-there-a-way-in-hibernate-to-obtain-an-entitys-loaded-persistentcollection-wi

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java

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internals