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Lazy fetching lets you decide how much of the object graph is loaded in the first database hit and which associations should be loaded only when they re first accessed. Lazy fetching is a foundational concept in object persistence and the first step to attaining acceptable performance. We recommend that, to start with, all associations be configured for lazy (or perhaps batched lazy) fetching in the mapping file. This strategy may then be overridden at runtime by queries that force eager fetching to occur. Eager (outer join) fetching Lazy association fetching can help reduce database load and is often a good default strategy. However, it s a bit like a blind guess as far as performance optimization goes. Eager fetching lets you explicitly specify which associated objects should be loaded together with the referencing object. Hibernate can then return the associated objects in a single database request, utilizing an SQL OUTER JOIN. Performance optimization in Hibernate often involves judicious use of eager fetching for particular transactions. Hence, even though default eager fetching may be declared in the mapping file, it s more common to specify the use of this strategy at runtime for a particular HQL or criteria query. Batch fetching Batch fetching isn t strictly an association fetching strategy; it s a technique that may help improve the performance of lazy (or immediate) fetching. Usually, when you load an object or collection, your SQL WHERE clause specifies the identifier of the object or object that owns the collection. If batch fetching is enabled, Hibernate looks to see what other proxied instances or uninitialized collections are referenced in the current session and tries to load them at the same time by specifying multiple identifier values in the WHERE clause. We aren t great fans of this approach; eager fetching is almost always faster. Batch fetching is useful for inexperienced users who wish to achieve acceptable performance in Hibernate without having to think too hard about the SQL that will be executed. (Note that batch fetching may be familiar to you, since it s used by many EJB2 engines.) We ll now declare the fetching strategy for some associations in our mapping metadata.
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4.4.6 Selecting a fetching strategy in mappings
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Hibernate lets you select default association fetching strategies by specifying attributes in the mapping metadata. You can override the default strategy using features of Hibernate s query methods, as you ll see in chapter 7. A minor caveat: You don t have to understand every option presented in this section immediately; we recommend that you get an overview first and use this section as a reference when you re optimizing the default fetching strategies in your application. A wrinkle in Hibernate s mapping format means that collection mappings function slightly differently than single-point associations; so, we ll cover the two cases separately. Let s first consider both ends of the bidirectional association between Bid and Item. Single point associations For a <many-to-one> or <one-to-one> association, lazy fetching is possible only if the associated class mapping enables proxying. For the Item class, we enable proxying by specifying lazy="true":
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<class name="Item" lazy="true">
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Now, remember the association from Bid to Item:
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<many-to-one name="item" class="Item">
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When we retrieve a Bid from the database, the association property may hold an instance of a Hibernate generated subclass of Item that delegates all method invocations to a different instance of Item that is fetched lazily from the database (this is the more elaborate definition of a Hibernate proxy). Hibernate uses two different instances so that even polymorphic associations can be proxied when the proxied object is fetched, it may be an instance of a mapped subclass of Item (if there were any subclasses of Item, that is). We can even choose any interface implemented by the Item class as the type of the proxy. To do so, we declare it using the proxy attribute, instead of specifying lazy="true":
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