java applet qr code reader Caching fundamentals in Java

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Caching fundamentals
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Choosing a cache provider For now, Hibernate forces you to choose a single cache provider for the whole application. Providers for the following open source products are built into Hibernate:
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EHCache is a cache provider intended for a simple process scope cache in a single JVM. It can cache in memory or on disk, and it supports the optional Hibernate query result cache. (The latest version of EHCache now supports clustered caching, but we haven t tested this yet.) OpenSymphony OSCache is a service that supports caching to memory and disk in a single JVM, with a rich set of expiration policies and query cache support. SwarmCache is a cluster cache based on JGroups. It uses clustered invalidation but doesn t support the Hibernate query cache. JBoss Cache is a fully transactional replicated clustered cache also based on the JGroups multicast library. It supports replication or invalidation, synchronous or asynchronous communication, and optimistic and pessimistic locking. The Hibernate query cache is supported, assuming that clocks are synchronized in the cluster.
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It s easy to write an adaptor for other products by implementing org.hibernate. cache.CacheProvider. Many commercial caching systems are pluggable into Hibernate with this interface. Not every cache provider is compatible with every concurrency strategy! The compatibility matrix in table 13.1 will help you choose an appropriate combination. Setting up caching involves two steps: First, you look at the mapping metadata for your persistent classes and collections and decide which cache concurrency
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Table 13.1 Cache concurrency strategy support Read-only X X X X Nonstrictread-write X X X X Read-write X X Transactional
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Concurrency strategy cache provider EHCache OSCache SwarmCache JBoss Cache
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Optimizing fetching and caching
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strategy you d like to use for each class and each collection. In the second step, you enable your preferred cache provider in the global Hibernate configuration and customize the provider-specific settings and physical cache regions. For example, if you re using OSCache, you edit oscache.properties, or for EHCache, ehcache.xml in your classpath. Let s enable caching for the CaveatEmptor Category, Item, and Bid classes.
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13.4 Caching in practice
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First we ll consider each entity class and collection and find out what cache concurrency strategy may be appropriate. After we select a cache provider for local and clustered caching, we ll write their configuration file(s).
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13.4.1 Selecting a concurrency control strategy
The Category has a small number of instances and is updated rarely, and instances are shared between many users. It s a great candidate for use of the second-level cache. Start by adding the mapping element required to tell Hibernate to cache Category instances.
<class name="auction.model.Category" table="CATEGORY"> <cache usage="read-write"/> <id ... </class>
The usage="read-write" attribute tells Hibernate to use a read-write concurrency strategy for the auction.model.Category cache. Hibernate now hits the second-level cache whenever you navigate to a Category or when you load a Category by identifier. If you use annotations, you need a Hibernate extension:
@Entity @Table(name = "CATEGORY") @org.hibernate.annotations.Cache(usage = org.hibernate.annotations.CacheConcurrencyStrategy.READ_WRITE ) public class Category { ... }
You use read-write instead of nonstrict-read-write because Category is a highly concurrent class, shared between many concurrent transactions. (It s clear that a read committed isolation level is good enough.) A nonstrict-read-write
Caching in practice
would rely only on cache expiration (timeout), but you prefer changes to categories to be visible immediately. The class caches are always enabled for a whole hierarchy of persistent classes. You can't only cache instances of a particular subclass. This mapping is enough to tell Hibernate to cache all simple Category property values, but not the state of associated entities or collections. Collections require their own <cache> region. For the items collection you use a read-write concurrency strategy:
<class name="auction.model.Category" table="CATEGORY"> <cache usage="read-write"/> <id ... <set name="items"> <cache usage="read-write"/> <key ... </set> </class>
The region name of the collection cache is the fully qualified class name plus the collection property name, auction.model.Category.items. The @org.hibernate.annotations.Cache annotation can also be declared on a collection field or getter method. This cache setting is effective when you call aCategory.getItems() in other words, a collection cache is a region that contains which items are in which category. It s a cache of identifiers only; there is no actual Category or Item data in that region. If you require the Item instances themselves to be cached, you must enable caching of the Item class. A read-write strategy is especially appropriate. Your users don t want to make decisions (placing a bid, for example) based on possibly stale Item data. Let s go a step further and consider the collection of bids: A particular Bid in the bids collection is immutable, but the collection of bids is mutable, and concurrent units of work need to see any addition or removal of a collection element without delay:
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