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Entity lifecycle listeners
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entityManager.refresh(item); return item; }
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After the persist method is invoked, the EntityManager is flushed immediately so that the INSERT statement is executed and the generated values are set by the database. The entity is then refreshed so that we get the most up-to-date data from the database and populate it into the inserted Item instance (including the postingDate field). In most cases you should try to avoid using default or generated values with JPA due to the slightly awkward nature of the code just introduced. Luckily, this awkward code is not necessary while using fields that use the JPA @GeneratedValue annotation since the persist method correctly handles such fields. Before we wrap up this chapter, we ll introduce entity lifecycle-based listeners.
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9.4 Entity lifecycle listeners
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You saw in earlier chapters that both session and message-driven beans allow you to listen for lifecycle callbacks like PostConstruct and PreDestroy. Similarly, entities allow you to receive callbacks for lifecycle events like persist, load, update, and remove. Just as in session and message-driven beans, you can do almost anything you need to in the lifecycle callback methods, including invoking an EJB, or using APIs like JDBC or JMS. In the persistence realm, however, lifecycle callbacks are typically used to accomplish such tasks as logging, validating data, auditing, sending notifications after a database change, or generating data after an entity has been loaded. In a sense, callbacks are the database triggers of JPA. Table 9.5 lists the callbacks supported by the API.
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Table 9.5 Callbacks supported by JPA and when they are called When It Is Performed Before the EntityManager persists an entity instance After an entity has been persisted After an entity has been loaded by a query, find, or refresh operation Before a database update occurs to synchronize an entity instance After a database update occurs to synchronize an entity instance Before EntityManager removes an entity After an entity has been removed
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Lifecycle Method PrePersist PostPersist PostLoad PreUpdate PostUpdate PreRemove PostRemove
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Manipulating entities with EntityManager
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Entity lifecycle methods need not be defined in the entity itself. Instead, you can choose to define a separate entity listener class to receive the lifecycle callbacks. We highly recommend this approach, because defining callback methods in the entities themselves will clutter up the domain model you might have carefully constructed. Moreover, entity callbacks typically contain crosscutting concerns rather than business logic directly pertinent to the entity. For our purposes, we ll explore the use of entity callbacks using separate listener classes, default listeners, and the execution order of entity listeners if you have multiple listeners.
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9.4.1 Using an entity listener
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Let s see how entity lifecycle callbacks look by coding an entity listener on the Item entity that notifies an ActionBazaar admin if an item s initial bid amount is set higher than a certain threshold. It is ActionBazaar policy to scrutinize items with extremely high initial prices to check against possible fraud, especially for items such as antiques and artwork. Listing 9.11 shows the code.
Listing 9.11 ItemMonitor entity listener
public class ItemMonitor { ... public ItemMonitor() {} @PrePersist Specifies callbacks @PreUpdate public void monitorItem(Item item) { if (item.getInitialBidAmount() > ItemMonitor.MONITORING_THRESHOLD) { notificationManager.sendItemPriceEmailAlert(item); } } } @Entity @EntityListeners(actionbazaar.persistence.ItemMonitor.class) public class Item implements Serializable {
Registers listener
As you can see in listing 9.11, our listener, ItemMonitor, has a single method, monitorItem, which receives callbacks for both the PrePersist and PreUpdate events. The @EntityListeners annotation on the Item entity specifies ItemMonitor to be the lifecycle callback listener for the Item entity. It s worth noting that the listener callbacks can be defined on an entity class or mapped superclass. All we have to do to receive a callback is to annotate our method with a callback annotation such as @PrePersist, @PostPersist, @PreUpdate, and so on. The monitorItem
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