how to generate barcode in vb.net 2008 ItemManagerBean using the container-managed, transaction-scoped entity manager in Java

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Listing 9.1 ItemManagerBean using the container-managed, transaction-scoped entity manager
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@Stateless public class ItemManagerBean implements ItemManager { @PersistenceContext(unitName="actionBazaar") private EntityManager entityManager; public ItemManagerBean() {}
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Injects EntityManager instance
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public Item addItem(String title, String description, byte[] picture, double initialPrice, long sellerId) { Item item = new Item(); item.setTitle(title); item.setDescription(description); Retrieves entity item.setPicture(picture); using primary key item.setInitialPrice(initialPrice); Seller seller = entityManager.find(Seller.class, sellerId); item.setSeller(seller); entityManager.persist(item); Persists entity instance
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return item; } public Item updateItem(Item item) { entityManager.merge(item); return item; }
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Merges changes to database
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Manipulating entities with EntityManager
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public Item undoItemChanges(Item item) { entityManager.refresh(entityManager.merge(item)); return item; Refreshes entity }
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Removes entity from database
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public void deleteItem(Item item) { entityManager.remove(entityManager.merge(item)); } }
ItemManagerBean is a pretty good representation of the most common ways the EntityManager API is used. First, an instance of the EntityManager is injected using the @PersistenceContext annotation b and is used in all the session bean methods that manipulate entities. As you might imagine, the addItem method is used by the presentation layer to add an item posted by the seller to the database. The persist method is used by addItem to add a new Item entity to the database D. The addItem method also uses the EntityManager s find method to retrieve the Seller of the Item using the entity s primary key C. The retrieved Seller entity is set as an association field of the newly instantiated Item entity along with all other item data. The updateItem method updates the Item entity data in the database using the merge method E. This method could be invoked from an administrative interface that allows a seller to update a listing after an item is posted. The EntityManager s refresh method is used in the undoItemChanges method to discard any changes made to an Item entity and to reload it with the data stored in the database F. The undoItemChanges method could be used by the same administrative interface that uses the updateItem method to allow the user to start over with modifying a listing (think of an HTTP form s reset button). Lastly, an Item entity is removed from the database using the remove method
G. This method could be used by an ActionBazaar administrator to remove an
offending listing. Now that we ve surface-scanned the code in listing 9.1, we re ready to start our in-depth analysis of the EntityManager API. We ll start from the most logical point: making an EntityManager instance available to the application.
9.2 Creating EntityManager instances
EntityManager is like the conductor of an orchestra who manages the show. Say
you d like to bring the orchestra to your town for a performance. You first get in touch with and attempt to hire the conductor. Similarly, the first obvious step to performing any persistence operation is obtaining an instance of an EntityManager.
Creating EntityManager instances
In listing 9.1, we do this by injecting an instance using the @PersistenceContext annotation. If you are using a container, this is more or less all you will need to know about getting an instance of an EntityManager. All EntityManager instances injected using the @PersistenceContext annotation are container managed. This means that the container takes care of the mundane task of looking up, opening, and closing the EntityManager behind the scenes. In addition, unless otherwise specified, injected EntityManagers have transaction scope. Just as you aren t limited to using the transaction scope, you are not limited to using a container-managed EntityManager either. JPA fully supports creating application-managed EntityManagers that you explicitly create, use, and release, including controlling how the EntityManager handles transactions. This capability is particularly important for using JPA outside the container. In this section we ll explore how to create and use both container- and application-managed EntityManagers.
9.2.1 Container-managed EntityManagers As you saw earlier, container-managed EntityManagers are injected using the @PersistenceContext annotation. Let s take a look at the definition of the annotation to start exploring its features:
@Target({TYPE, METHOD, FIELD}) @Retention(RUNTIME) public @interface PersistenceContext { String name() default ""; String unitName() default ""; PersistenceContextType type default TRANSACTION; PersistenceProperty[] properties() default {}; }
The first element of the annotation, name, specifies the JNDI name of the persistence context. This element is used in the unlikely case you have to explicitly mention the JNDI name for a given container implementation to be able to look up an EntityManager. In most situations, leaving this element empty is fine, except when you use @PersistenceContext at the class level to establish reference to the persistence context. The unitName element specifies the name of the persistence unit. A persistence unit is essentially a grouping of entities used in an application. This idea is useful when you have a large Java EE application and would like to separate it into several logical areas (think Java packages). For example, ActionBazaar entities could be grouped into general and admin units.
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