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Taken from
http://www.hibernate.org/hib_docs/v3/reference/en/html_single
Lazy loading
By default, Hibernate3 uses lazy select fetching for collections and lazy proxy fetching for single-valued associations. These defaults make sense for almost all associations in almost all applications.
Initializing collections and proxies
A LazyInitializationException will be thrown by Hibernate if an uninitialized collection or proxy is accessed outside of the scope of the Session, ie. when the entity owning the collection or having the reference to the proxy is in the detached state.
Sometimes we need to ensure that a proxy or collection is initialized before closing the Session. Of course, we can alway force initialization by calling cat.getSex() or cat.getKittens().size(), for example. But that is confusing to readers of the code and is not convenient for generic code.
The static methods Hibernate.initialize() and Hibernate.isInitialized() provide the application with a convenient way of working with lazily initialized collections or proxies. Hibernate.initialize(cat) will force the initialization of a proxy, cat, as long as its Session is still open. Hibernate.initialize( cat.getKittens() ) has a similar effect for the collection of kittens.
Another option is to keep the Session open until all needed collections and proxies have been loaded. In some application architectures, particularly where the code that accesses data using Hibernate, and the code that uses it are in different application layers or different physical processes, it can be a problem to ensure that the Session is open when a collection is initialized.
There are two basic ways to deal with this issue:
- In a web-based application, a servlet filter can be used to close the Session only at the very end of a user request, once the rendering of the view is complete (the Open Session in View pattern). Of course, this places heavy demands on the correctness of the exception handling of your application infrastructure. It is vitally important that the Session is closed and the transaction ended before returning to the user, even when an exception occurs during rendering of the view. See the Hibernate Wiki for examples of this "Open Session in View" pattern.
- In an application with a separate business tier, the business logic must "prepare" all collections that will be needed by the web tier before returning. This means that the business tier should load all the data and return all the data already initialized to the presentation/web tier that is required for a particular use case. Usually, the application calls Hibernate.initialize() for each collection that will be needed in the web tier (this call must occur before the session is closed) or retrieves the collection eagerly using a Hibernate query with a FETCH clause or a FetchMode.JOIN in Criteria. This is usually easier if you adopt the Command pattern instead of a Session Facade.
- You may also attach a previously loaded object to a new Session with merge() or lock() before accessing uninitialized collections (or other proxies). No, Hibernate does not, and certainly should not do this automatically, since it would introduce ad hoc transaction semantics!
Sometimes you don't want to initialize a large collection, but still need some information about it (like its size) or a subset of the data.
You can use a collection filter to get the size of a collection without initializing it:
( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0) ).intValue()
The createFilter() method is also used to efficiently retrieve subsets of a collection without needing to initialize the whole collection:
s.createFilter( lazyCollection, "").setFirstResult(0).setMaxResults(10).list();
Using batch fetching
Hibernate can make efficient use of batch fetching, that is, Hibernate can load several uninitialized proxies if one proxy is accessed (or collections. Batch fetching is an optimization of the lazy select fetching strategy. There are two ways you can tune batch fetching: on the class and the collection level.
Batch fetching for classes/entities is easier to understand. Imagine you have the following situation at runtime: You have 25 Cat instances loaded in a Session, each Cat has a reference to its owner, a Person. The Person class is mapped with a proxy, lazy="true". If you now iterate through all cats and call getOwner() on each, Hibernate will by default execute 25 SELECT statements, to retrieve the proxied owners. You can tune this behavior by specifying a batch-size in the mapping of Person:
<class name="Person" batch-size="10">...</class>
Hibernate will now execute only three queries, the pattern is 10, 10, 5.
You may also enable batch fetching of collections. For example, if each Person has a lazy collection of Cats, and 10 persons are currently loaded in the Sesssion, iterating through all persons will generate 10 SELECTs, one for every call to getCats(). If you enable batch fetching for the cats collection in the mapping of Person, Hibernate can pre-fetch collections:
<class name="Person">
<set name="cats" batch-size="3">
…
</set>
</class>
With a batch-size of 8, Hibernate will load 3, 3, 3, 1 collections in four SELECTs. Again, the value of the attribute depends on the expected number of uninitialized collections in a particular Session.
Batch fetching of collections is particularly useful if you have a nested tree of items, ie. the typical bill-of-materials pattern. (Although a nested set or a materialized path might be a better option for read-mostly trees.)
