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Managing persistence operations
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Using JPA in a web container and the ThreadLocal pattern If you are using an application-managed entity manager in a web container such as Tomcat or Jetty, some persistence providers such as Hibernate recommend that you use the ThreadLocal pattern. This is widely known as the ThreadLocal session pattern in the Hibernate community. It associates a single instance of the EntityManager with a particular request. You have to bind the EntityManager to a thread-local variable and set the EntityManager instance to the associated thread, as shown here:
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private static EntityManagerFactory emf; public static final ThreadLocal<EntityManager> _ threadLocal = new ThreadLocal<EntityManager>();
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Stores EntityManager in ThreadLocal variable
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public static EntityManagerFactory getEntityManagerFactory() { if (emf == null) { emf = Persistence.createEntityManagerFactory("actionBazaar"); } return emf; }
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public static EntityManager getEntityManager() { EntityManager entityManager = _threadLocal.get(); if (entityManager == null) { EntityManager entityManager = emf.createEntityManager(); _threadLocal.set(entityManager); Associates EntityManager } with a thread return entityManager; }
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Check your persistence provider s documentation if it requires you to use the ThreadLocal pattern. Next, we ll tackle the most important part of this chapter: EntityManager operations.
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9.3 Managing persistence operations
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The heart of the JPA API lies in the EntityManager operations, which we ll discuss in upcoming sections. As you might have noted in listing 9.1, although the EntityManager interface is small and simple, it is pretty complete in its ability to provide an effective persistence infrastructure. In addition to the CRUD (Create,
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Manipulating entities with EntityManager
Read, Update, and Delete) functionality we introduced in listing 9.1, we ll cover a few less-commonly used operations like flushing and refreshing. Let s start our coverage in the most logical place: persisting new entities into the database.
9.3.1 Persisting entities
Recall that in listing 9.1, the addItem method persists an Item entity into the database. Since listing 9.1 was quite a few pages back, we ll repeat the addItem method body as reviewed in listing 9.4. Although it is not obvious, the code is especially helpful in understanding how entity relationships are persisted, which we ll look at in greater detail in a minute. For now, let s concentrate on the persist method itself.
Listing 9.4 Persisting entities
public Item addItem(String title, String description, byte[] picture, double initialPrice, long sellerId) { Item item = new Item(); item.setTitle(title); item.setDescription(description); item.setPicture(picture); item.setInitialPrice(initialPrice); Seller seller = entityManager.find(Seller.class, sellerId); item.setSeller(seller); entityManager.persist(item);
Persists entity
return item; }
A new Item entity corresponding to the record being added is first instantiated in the addItem method. All of the relevant Item entity data to be saved into the database, such as the item title and description, is then populated with the data passed in by the user. As you ll recall from chapter 7, the Item entity has a manyto-one relationship with the Seller entity. The related seller is retrieved using the EntityManager s find method and set as a field of the Item entity. The persist method is then invoked to save the entity into the database, as shown in figure 9.6. Note that the persist method is intended to create new entity records in the database and not update existing ones. This means that you should make sure the identity or primary key of the entity to be persisted doesn t already exist in the database.
Managing persistence operations
Figure 9.6 Invoking the persist method on the EntityManager interface makes an entity instance managed. When the transaction commits, the entity state is synchronized with the database.
If you try to persist an entity that violates the database s integrity constraint, the persistence provider will throw javax.persistence.PersistenceException, which wraps the database exception. As we noted earlier, the persist method also causes the entity to become managed as soon as the method returns. The INSERT statement (or statements) that creates the record corresponding to the entity is not necessarily issued immediately. For transaction-scoped EntityManagers, the statement is typically issued when the enclosing transaction is about to commit. In our example, this means the SQL statements are issued when the addItem method returns. For extendedscoped (or application-managed) EntityManagers, the INSERT statement is probably issued right before the EntityManager is closed. The INSERT statement can also be issued at any point when the EntityManager is flushed. We ll discuss automatic and manual flushing in more detail shortly. For now, you just need to know that under certain circumstances, either you or the EntityManager can choose to perform pending database operations (such as an INSERT to create a record), without waiting for the transaction to end or the EntityManager to close. The INSERT statement corresponding to listing 9.4 to save the Item entity could look something like this:
INSERT INTO ITEMS (TITLE, DESCRIPTION, SELLER_ID, ...) VALUES ("Toast with the face of Bill Gates on it", "This is an auction for...", 1, ...)
As you may have noticed, the ITEM_ID primary key that is the identity for the Item entity is not included in the generated INSERT statement. This is because the keygeneration scheme for the itemId identity field of the entity is set to IDENTITY. If the scheme were set to SEQUENCE or TABLE instead, the EntityManager would have generated a SELECT statement to retrieve the key value first and then include the retrieved key in the INSERT statement. As we mentioned earlier, all persistence operations that require database updates must be invoked within the scope of a
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