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Entity lifecycle listeners
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method checks to see if the initial bid amount set for the item to be inserted or updated is above the threshold specified by the ItemMonitor.MONITORING_ THRESHOLD variable and sends the ActionBazaar admin an e-mail alert if it is. As you might have guessed by examining listing 9.11, entity listener callback methods follow the form void <METHOD>(Object). The single method parameter of type Object specifies the entity instance for which the lifecycle event was generated. In our case, this is the Item entity. If the lifecycle callback method throws a runtime exception, the intercepted persistence operation is aborted. This is an extremely important feature to validate persistence operations. For example, if you have a listener class to validate that all entity data is present before persisting an entity, you could abort the persistence operation if needed by throwing a runtime exception. Listing 9.12 shows an example listener class that can be used to validate an entity state before an entity is persisted. You can build validation logic in a PrePersist callback, and if the callback fails the entity will not be persisted. For example, ActionBazaar sets a minimum price for the initialPrice for items being auctioned, and no items are allowed to be listed below that price.
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Listing 9.12 ItemVerifier, which validates the price set for an item
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public class ItemVerifier{ public ItemVerifier() { } @PrePersist public void newItemVerifier(Item item){ if (item.getInitialPrice()<MIN_INITIAL_PRICE) throw new ItemException( "Item Price is lower than "+ " Minimum Price Allowed"); } }
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All entity listeners are stateless in nature and you cannot assume that there is a one-to-one relationship between an entity instance and a listener instance.
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One great drawback of entity listener classes is that they do not support dependency injection. This is due to the fact that entities may be used outside containers, where DI is not available.
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Manipulating entities with EntityManager
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For crosscutting concerns like logging or auditing, it is inconvenient to have to specify listeners for individual entities. Given this problem, JPA enables you to specify default entity listeners that receive callbacks from all entities in a persistence unit. Let s take a look at this mechanism next.
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9.4.2 Default listener classes
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ActionBazaar audits all changes made to entities. You can think of this as an ActionBazaar version of a transaction log. This feature can be implemented using a default listener like the following:
public class ActionBazaarAuditor { ... @PrePersist @PostPersist ... @PostRemove public void logOperation(Object object) { Logger.log("Performing Persistence Operation on: " + object.getName());
The ActionBazaarAuditor listens for all persistence operations for all entities in the ActionBazaar persistence unit, and logs the name of the entity that the callback was generated on. Unfortunately, there is no way to specify default entity listeners using annotations, and we must resort to using the persistence. xml deployment descriptor. Since we have not yet fully described the persistence deployment descriptor, we ll simply note the relevant descriptor snippet here, leaving a detailed analysis to chapter 11:
<persistence-unit name="actionBazaar"> ... <default-entity-listeners> actionbazaar.persistence.ActionBazaarAuditor.class </default-entity-listeners> ...
In this snippet, the default-entity-listeners element lists the default entity listeners for the actionBazaar persistence unit. Again, do not worry too much about the specific syntax at the moment, as we ll cover it in greater detail later. This brings us to the interesting question of what happens if there is both a default listener and an entity-specific listener for a given entity, as in the case of our Item entity. The Item entity now has two lifecycle listeners: the default ActionBazaarAuditor listener and the ItemMonitor listener. How do you think they interact Moreover, since entities are POJOs that can inherit from other entities, both the superclass and subclass may have attached listeners. For example, what if the
Entity lifecycle listeners
User entity has an attached listener named UserMonitor and the Seller subclass also has an attached listener, SellerMonitor How these listeners relate to each
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