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SessionFactory.evict( Category.class, new Long(123) );
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You can also evict all elements of a certain class or only evict a particular collection role:
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SessionFactory.evict("org.hibernate.auction.model.Category");
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You ll rarely need these control mechanisms.
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5.4 Summary
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This chapter was dedicated to concurrency control and data caching. You learned that for a single unit of work, either all operations should be completely successful or the whole unit of work should fail (and changes made to persistent state should be rolled back). This led us to the notion of a transaction and the ACID attributes. A transaction is atomic, leaves data in a consistent state, and is isolated from concurrently running transactions, and you have the guarantee that data changed by a transaction is durable. You use two transaction concepts in Hibernate applications: short database transactions and long-running application transactions. Usually, you use read committed isolation for database transactions, together with optimistic concurrency control (version and timestamp checking) for long application transactions. Hibernate greatly simplifies the implementation of application transactions because it manages version numbers and timestamps for you. Finally, we discussed the fundamentals of caching, and you learned how to use caching effectively in Hibernate applications. Hibernate provides a dual-layer caching system with a first-level object cache (the Session) and a pluggable second-level data cache. The first-level cache is always active it s used to resolve circular references in your object graph and to optimize performance in a single unit of work. The (process or cluster scope) second-level cache on the other hand is optional and works best for read-mostly candidate classes. You can configure a non-volatile second-level cache for reference (read-only) data or even a second-level cache with full transaction isolation for critical data. However, you have to carefully examine whether the performance gain is worth the effort. The second-level cache can be customized fine-grained, for each persistent class and even for each collection and class association. Used correctly and thoroughly tested, caching in Hibernate gives you a level of performance that is almost unachievable in a hand-coded data access layer.
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Advanced mapping concepts
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This chapter covers
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The Hibernate type system Custom mapping types Collection mappings One-to-one and many-to-many associations
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Advanced mapping concepts
In chapter 3, we introduced the most important ORM features provided by Hibernate. You ve met basic class and property mappings, inheritance mappings, component mappings, and one-to-many association mappings. We now continue exploring these topics by turning to the more exotic collection and association mappings. At various places, we ll warn you against using a feature without careful consideration. For example, it s usually possible to implement any domain model using only component mappings and one-to-many (occasionally one-to-one) associations. The exotic mapping features should be used with care, perhaps even avoided most of the time. Before we start to talk about the exotic features, you need a more rigorous understanding of Hibernate s type system particularly of the distinction between entity and value types.
6.1 Understanding the Hibernate type system
In chapter 3, section 3.5.1, Entity and value types, we first distinguished between entity and value types, a central concept of ORM in Java. We must elaborate that distinction in order for you to fully understand the Hibernate type system of entities, value types, and mapping types. Entities are the coarse-grained classes in a system. You usually define the features of a system in terms of the entities involved: the user places a bid for an item is a typical feature definition that mentions three entities. Classes of value type often don t appear in the business requirements they re usually the fine-grained classes representing strings, numbers, and monetary amounts. Occasionally, value types do appear in feature definitions: the user changes billing address is one example, assuming that Address is a value type, but this is atypical. More formally, an entity is any class whose instances have their own persistent identity. A value type is a class that doesn t define some kind of persistent identity. In practice, this means entity types are classes with identifier properties, and valuetype classes depend on an entity. At runtime, you have a graph of entity instances interleaved with value type instances. The entity instances may be in any of the three persistent lifecycle states: transient, detached, or persistent. We don t consider these lifecycle states to apply to the value type instances. Therefore, entities have their own lifecycle. The save() and delete() methods of the Hibernate Session interface apply to instances of entity classes, never to value type instances. The persistence lifecycle of a value type instance is completely tied to the lifecycle of the owning entity instance. For example, the username
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