how to generate barcode in 2008 Introducing the EntityManager in Java

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Introducing the EntityManager
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freshness. If the EntityManager finds that any of the entity s data has changed, it automatically synchronizes the changes with the database. The EntityManager stops managing the entity when the entity is either deleted or moves out of persistence provider s reach. An entity can become attached to the EntityManager s context when you pass the entity to the persist, merge, or refresh method. Also an entity becomes attached when you retrieve using the find method or a query within a transaction. The state of the entity determines which method you will use. When an entity is first instantiated as in the following snippet, it is in the new or transient state since the EntityManager doesn t know it exists yet:
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Bid bid = new Bid();
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Hence the entity instance is not managed yet. It will become managed if the EntityManager s persist method creates a new record in the database corresponding to the entity. This would be the most natural way to attach the Bid entity in the previous snippet to the EntityManager s context:
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A managed entity becomes detached when it is out of scope, removed, serialized, or cloned. For example, the instance of the Bid entity will become detached when the underlying transaction commits. Unlike entities explicitly created using the new operator, an entity retrieved from the database using the EntityManager s find method or a query is attached if retrieved within a transactional context. A retrieved instance of the entity becomes detached immediately if there is no associated transaction. The merge and refresh methods are intended for entities that have been retrieved from the database and are in the detached state. Either of these methods attaches entities to the entity manager. EntityManager.merge updates the database with the data held in the entity, and refresh does the opposite it resets the entity s state with data from the database. We ll discuss these methods in much greater detail in section 9.3. Detached entities A detached entity is an entity that is no longer managed by the EntityManager and there is no guarantee that the state of the entity is in synch with the database. Detachment and merge operations become handy when you want to pass an entity across application tiers. For example, you can detach an entity and pass it
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Manipulating entities with EntityManager
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to the web tier, then update it and send it back to the EJB tier, where you can merge the detached entity to the persistence context. The usual way entities become detached is a little subtler. Essentially, an attached entity becomes detached as soon as it goes out of the EntityManager context s scope. Think of this as the expiration of the invisible link between an entity and the EntityManager at the end of a logical unit of work or a session. An EntityManager session could be limited to a single method call or span an arbitrary length of time. (Reminds you of session beans, doesn t it As you ll soon see, this is not entirely an accident.) For an EntityManager whose session is limited to a method call, all entities attached to it become detached as soon as a method returns, even if the entity objects are used outside the method. If this is not absolutely crystal clear right now, it will be once we talk about the EntityManager persistence context in the next section. Entity instances also become detached through cloning or serialization. This is because the EntityManager quite literally keeps track of entities through Java object references. Since cloned or serialized instances don t have the same object references as the original managed entity, the EntityManager has no way of knowing they exist. This scenario occurs most often in situations where entities are sent across the network for session bean remote method calls. In addition, if you call the clear method of EntityManager, it forces all entities in the persistence context to be detached. Calling the EntityManager s remove method will also detach an entity. This makes perfect sense since this method removes the data associated with the entity from the database. As far as the EntityManager is concerned, the entity no longer exists, so there is no need to continue managing it. For our Bid entity, this would be an apt demise :
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We ll return to this discussion on detachment and merge operations in section 9.3.3. A good way to remember the entity lifecycle is through a convenient analogy. Think of an entity as an aircraft and the EntityManager as the air traffic controller. While the aircraft is outside the range of the airport (detached or new), it is not guided by the air traffic controller. However, when it does come into range (managed), the traffic controller manages the aircraft s movement (state synchronized with database). Eventually, a grounded aircraft is guided into takeoff and goes out of airport range again (detached), at which point the pilot is free to follow her own flight plan (modifying a detached entity without state being managed).
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