how to generate barcode in vb.net 2008 Introducing the EntityManager in Java

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Introducing the EntityManager
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The persistence context scope is the equivalent of airport radar range. It is critical to understand how the persistence context works to use managed entities effectively. We ll examine the relationship between the persistence context, its scope, and the EntityManager in the next section.
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9.1.3 Persistence contexts, scope, and the EntityManager
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The persistence context plays a vital role in the internal functionality of the EntityManager. Although we perform persistence operations by invoking methods on the EntityManager, the EntityManager itself does not directly keep track of the lifecycle of an individual entity. In reality, the EntityManager delegates the task of managing entity state to the currently available persistence context. In a very simple sense, a persistence context is a self-contained collection of entities managed by an EntityManager during a given persistence scope. The persistence scope is the duration of time a given set of entities remains managed. The best way to understand this is to start by examining what the various persistence scopes are and what they do and then backtracking to the meaning of the term. We ll explain how the persistence context and persistence scope relates to the EntityManager by first exploring what the persistence context is. There are two different types of persistence scopes: transaction and extended. Transaction-scoped EntityManager An EntityManager associated with a transaction-scoped persistence context is known as a transaction-scoped EntityManager. If a persistence context is under transaction scope, entities attached during a transaction are automatically detached when the transaction ends. (All persistence operations that may result in data changes must be performed inside a transaction, no matter what the persistence scope is.) In other words, the persistence context keeps managing entities while the transaction it is enclosed by is active. Once the persistence context detects that a transaction has either been rolled back or committed, it will detach all managed entities after making sure that all data changes until that point are synchronized with the database. Figure 9.3 depicts this relationship between entities, the transaction persistence scope, and persistence contexts. Extended EntityManager The life span of the extended EntityManager lasts across multiple transactions. An extended EntityManager can only be used with stateful session beans and lasts as long as the bean instance is alive. Therefore, in persistence contexts with extended scope, how long entities remain managed has nothing to do with transaction
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Manipulating entities with EntityManager
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Figure 9.3 Transaction-scoped persistence contexts only keep entities attached within the boundaries of the enclosing transaction.
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boundaries. In fact, once attached, entities pretty much stay managed as long as the EntityManager instance is around. As an example, for a stateful session bean, an EntityManager with extended scope will keep managing all attached entities until the EntityManager is closed as the bean itself is destroyed. As figure 9.4 shows, this means that unless explicitly detached through a remove method to end the life of the stateful bean instance, entities attached to an extended persistence context will remain managed across multiple transactions.
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Figure 9.4 For an extended persistence context, once an entity is attached in any given transaction, it is managed for all transactions in the lifetime of the persistence context.
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Introducing the EntityManager
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The term scope is used for persistence contexts in the same manner that it is used for Java variable scoping. It describes how long a particular persistence context remains active. Transaction-scoped persistence contexts can be compared to method local variables, in the sense that they are only in effect within the boundaries of a transaction. On the other hand, persistence contexts with extended scope are more like instance variables that are active for the lifetime of an object they hang around as long as the EntityManager is around. At this point, we ve covered the basic concepts needed to understand the functionality of the EntityManager. We are now ready to see the EntityManager itself in action.
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9.1.4 Using the EntityManager in ActionBazaar We ll explore the EJB 3 EntityManager interface by implementing an ActionBazaar component. We ll implement the ItemManagerBean stateless session bean used to provide the operations to manipulate items. As listing 9.1 demonstrates, the session bean provides methods for adding, updating, and removing Item entities using the JPA EntityManager.
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