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16.2.1 Building JPA EAOs for Spring
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In chapter 12 you learned that the Entity Access Object design pattern improves code maintainability by decoupling the persistence logic from the business logic. Spring provides EAO (Spring still calls it DAO) for JPA and many O/R mapping frameworks such as Hibernate and TopLink. Spring provides two ways to access and manipulate entities in building Spring EAOs: using the JPA API directly or using JpaTemplate. In chapter 12 we used EAOs to call the EntityManager API directly. We ll now demonstrate how to change the implementation classes to use JPA from Spring applications with the Spring JpaTemplate. Listing 16.1 shows the ActionBazaar EAO implementation classes you need when using JPA from Spring applications. Note that the Spring classnames
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Using JPA with Spring
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continue to use the DAO naming convention instead of the revised EAO naming that we re featuring in this book.
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Listing 16.1 EAO implementation when using JPA from Spring applications
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package actionbazaar.persistence.eao; import org.springframework.orm.jpa.support.JpaDaoSupport; public abstract class BasicSpringEAO extends JpaDaoSupport { }
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Extends JpaDaoSupport
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public class BidSpringEAO extends BasicSpringEAO implements BidEAO { public Bid addBid(Item item, String bidderId, double bidPrice) { Bid bid = new Bid(); ... getJpaTemplate().persist(bid); Uses JpaTemplate to persist return bid; }
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public Bid cancelBid(Long bidId) { Bid bid = (Bid)getJpaTemplate().find(Bid.class, bidId); bid.setBidStatus(BidStatus.CANCELLED); getJpaTemplate().merge(bid); Uses JpaTemplate to merge return bid; }
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The class that implements the EAO interface must extend JpaDaoSupport b. Instead of using the EntityManager API, you use JpaTemplate. In listing 16.1, we ve used the persist and merge methods c and d to persist or merge entity instances. Using JpaTemplate How are exceptions handled with regard to JpaTemplate We re glad you asked. You need to be aware that JpaTemplate does not throw any persistence API exceptions. Instead, it throws Spring s DataAccessException. The primary benefit of this to developers is that by translating exceptions into those provided by the Spring framework, the persistence mechanism is neutral. This means you can swap out persistence mechanisms and your application code won t have to change to support the new framework s error handling. This makes it easier to migrate from one persistence toolkit to another, or to support multiple toolkits if you are a tool vendor. Table 16.2 describes some of the important methods in JpaTemplate.
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EJB 3 and Spring
Table 16.2 Important JpaTemplate methods provided by Spring JpaTemplate Methods persist(Object entity) remove(Object entity) merge(T entity) refresh(Object entity) <T> T find(Class<T> entityClass, Object Id) List find(String queryString) List find(String queryString, Object values) findByNamedQuery(String queryName) findByNamedQuery(String queryName, Map<String,Object> params) Description Persists an entity instance Removes an entity instance Merges a detached entity instance Refreshes an entity instance from the database Retrieves an entity instance by primary key Retrieves a list of entities using a dynamic query Restricts a list of entities using a dynamic query with positional parameters Retrieves a list of entities using a named query Retrieves a list of entities using a named query with named parameters
You can use the JpaTemplate methods to access entities. Spring limits some of the repetitive use of the EJB 3 JPA. For example, if you want to you use a dynamic query to retrieve all Bidders with Gold status, then JpaTemplate will yield the following:
List bidders = getJpaTemplate().find( "SELECT b FROM Bidder b WHERE status = 1", "Gold");
The equivalent code with the EntityManager API will look like this:
List bidders = em.createQuery( "SELECT b FROM Bidder b WHERE status = 1") .setParameter(1, "Gold") .getResultList();
This code makes it evident that Spring makes using the EJB 3 JPA simpler. We encourage you to explore other JpaTemplate methods. The only problem we see with JpaTemplate is that it does not provide finegrained access to the EntityManager API s methods. JPA EAO in your service beans Spring s service beans are similar to the EJB 3 session beans that you work with to implement business logic. In our example the BidServiceBean is used to place a
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