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Optimizing fetching and caching
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Figure 13.2 Proxies and collection wrappers represent the boundary of the loaded graph.
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perspective: the identifier values are all foreign key columns in the item s row. Collections also aren t loaded right away, but we use the term collection wrapper to describe this kind of placeholder. Internally, Hibernate has a set of smart collections that can initialize themselves on demand. Hibernate replaces your collections with these; that is why you should use collection interfaces only in your domain model. By default, Hibernate creates placeholders for all associations and collections, and only retrieves value-typed properties and components right away. (This is unfortunately not the default fetch plan standardized by Java Persistence; we ll get back to the differences later.)
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Does lazy loading of one-to-one associations work Lazy loading for one-to-one associations is sometimes confusing for new Hibernate users. If you consider one-to-one associations based on shared primary keys (chapter 7, section 7.1.1, Shared primary key associations ), an association can be proxied only if it s constrained="true". For example, an Address always has a reference to a User. If this association is nullable and optional, Hibernate first would have to hit the database to find out whether a proxy or a null should be applied the purpose of lazy loading is to not hit the database at all. You can enable lazy loading through bytecode instrumentation and interception, which we ll discuss later.
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A proxy is initialized if you call any method that is not the identifier getter method, a collection is initialized if you start iterating through its elements or if you call any of the collection-management operations, such as size() and contains(). Hibernate provides an additional setting that is mostly useful for large
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Defining the global fetch plan
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collections; they can be mapped as extra lazy. For example, consider the collection of bids of an Item:
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<class name="Item" table="ITEM"> ... <set name="bids" lazy="extra" inverse="true"> <key column="ITEM_ID"/> <one-to-many class="Bid"/> </set> </class>
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The collection wrapper is now smarter than before. The collection is no longer initialized if you call size(), contains(), or isEmpty() the database is queried to retrieve the necessary information. If it s a Map or a List, the operations containsKey() and get() also query the database directly. A Hibernate extension annotation enables the same optimization:
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@OneToMany @org.hibernate.annotations.LazyCollection( org.hibernate.annotations.LazyCollectionOption.EXTRA ) private Set<Bid> bids = new HashSet<Bid>();
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Let s define a fetch plan that isn t completely lazy. First, you can disable proxy generation for entity classes.
13.1.4 Disabling proxy generation
Proxies are a good thing: They allow you to load only the data that is really needed. They even let you create associations between objects without hitting the database unnecessarily. Sometimes you need a different plan for example, you want to express that a User object should always be loaded into memory and no placeholder should be returned instead. You can disable proxy generation for a particular entity class with the lazy="false" attribute in XML mapping metadata:
<class name="User" table="USERS" lazy="false"> ... </class>
Optimizing fetching and caching
The JPA standard doesn t require an implementation with proxies; the word proxy doesn t even appear in the specification. Hibernate is a JPA provider that relies on proxies by default, so the switch that disables Hibernate proxies is available as a vendor extension:
@Entity @Table(name = "USERS") @org.hibernate.annotations.Proxy(lazy = false) public class User { ... }
Disabling proxy generation for an entity has serious consequences. All of these operations require a database hit:
User user = (User) session.load(User.class, new Long(123)); User user = em.getReference(User.class, new Long(123));
A load() of a User object can t return a proxy. The JPA operation getReference() can no longer return a proxy reference. This may be what you desired to achieve. However, disabling proxies also has consequences for all associations that reference the entity. For example, the Item entity has a seller association to a User. Consider the following operations that retrieve an Item:
Item item = (Item) session.get(Item.class, new Long(123)); Item item = em.find(Item.class, new Long(123));
In addition to retrieving the Item instance, the get() operation now also loads the linked seller of the Item; no User proxy is returned for this association. The same is true for JPA: The Item that has been loaded with find() doesn t reference a seller proxy. The User who is selling the Item must be loaded right away. (We answer the question how this is fetched later.) Disabling proxy generation on a global level is often too coarse-grained. Usually, you only want to disable the lazy loading behavior of a particular entity association or collection to define a fine-grained fetch plan. You want the opposite: eager loading of a particular association or collection.
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