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Managing persistence operations
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always detached, just like the newly created Item entity returned by the ItemManager s addItem method:
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public Item addItem(String title, String description, byte[] picture, double initialPrice, long sellerId) { Item item = new Item(); item.setTitle(title); ... entityManager.persist(item); return item; }
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At some point we ll want to reattach the entity to a persistence context to synchronize it with the database. The EntityManager s merge method is designed to do just that (see figure 9.8). You should remember that like all attached entities, the entity passed to the merge method is not necessarily synchronized with the database immediately, but it is guaranteed to be synchronized with the database sooner or later. We use the merge method in the ItemManager bean in the most obvious way possible, to update the database with an existing Item:
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public Item updateItem(Item item) { entityManager.merge(item); return item; }
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As soon as the updateItem method returns, the database is updated with the data from the Item entity. The merge method must only be used for an entity that exists in the database. An attempt to merge a nonexistent entity will result in an IllegalArgumentException. The same is true if the EntityManager detects that the
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Figure 9.8 An entity instance can be detached and serialized to a separate tier where the client makes changes to the entity and sends it back to the server. The server can use a merge operation to attach the entity to the persistence context.
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Manipulating entities with EntityManager
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entity you are trying to merge has already been deleted through the remove method, even if the DELETE statement has not been issued yet. Merging relationships By default, entities associated with the entity being merged are not merged as well. For example, the Seller, Bid and Category entities related to the Item are not merged when the Item is merged in the previous code snippet. However, as mentioned in section 9.3.1, this behavior can be controlled using the cascade element of the @OneToOne, @OneToMany, @ManyToOne, and @ManyToMany annotations. If the element is set to either ALL or MERGE, the related entities are merged. For example, the following code will cause the Seller entity related to the Item to be merged since the cascade element is set to MERGE:
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public class Item { @ManyToOne(cascade=CascadeType.MERGE) public Seller getSeller() {
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Note that as in most of the EntityManager s methods, the merge method must be called from a transactional context or it will throw a TransactionRequiredException. We ll now move on to the final element of the CRUD sequence: deleting an entity.
Detached entities and the DTO anti-pattern
If you have spent even a moderate amount of time using EJB 2, you are probably thoroughly familiar with the Data Transfer Object (DTO) anti-pattern. In a sense, the DTO anti-pattern was necessary because of entity beans. The fact that EJB 3 detached entities are nothing but POJOs makes the DTO anti-pattern less of a necessity of life. Instead of having to create separate DTOs from domain data just to pass back and forth between the business and presentation layers, you may simply pass detached entities. This is exactly the model we follow in this chapter. However, if your entities contain behavior, you might be better off using the DTO pattern anyway, to safeguard business logic from inappropriate usage outside a transactional context. In any case, if you decide to use detached entities as a substitute to DTOs, you should make sure they are marked java.io.Serializable.
9.3.4 Deleting entities
The deleteItem method in the ItemManagerBean in listing 9.1 deletes an Item from the database. An important detail to notice about the deleteItem method
Managing persistence operations
(repeated next) is that the item to be deleted was first attached to the EntityManager using the merge method:
public void deleteItem(Item item) { entityManager.remove(entityManager.merge(item)); }
This is because the remove method can only delete currently attached entities and the Item entity being passed to the deleteItem method is not managed. If a detached entity is passed to the remove method, it throws an IllegalArgumentException. Before the deleteItem method returns, the Item record will be deleted from the database using a DELETE statement like this:
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