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Executing the queries
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For a complex application, you ll probably have multiple named queries. In that case, you can use the @javax.persistence.NamedQueries annotation to specify multiple named queries like this:
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@Entity @NamedQueries({ @NamedQuery( name = "findCategoryByName", query = "SELECT c FROM Category c WHERE c.categoryName LIKE :categoryName order by c.categoryId" ), @NamedQuery( name = "findCategoryByUser", query = "SELECT c FROM Category c JOIN c.user u WHERE u.userId = 1" )}) @Table(name = "CATEGORIES") public class Category implements Serializable { }
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Keep in mind that a named query is scoped with a persistence unit and therefore must have a unique name. We recommend that you devise a naming convention for your applications that will avoid duplicate names for named queries.
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So now you know the difference between named and dynamic queries. Next we ll show you how to execute your queries.
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10.2 Executing the queries
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If you ve used Hibernate or TopLink, you ll note many similarities between the query APIs in those frameworks and the EJB 3 Java Persistence API. As you saw in section 10.1.2, there are three steps to running a query in EJB 3:
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Create an instance of the EntityManager. Create an instance of the query. Execute the query.
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In chapter 9, you saw how to create an instance of EntityManager. (To recap, you can either inject an instance of a container-managed EntityManager or create an application-managed EntityManager from an EntityManagerFactory.) In this section we focus on the last two steps. First we ll look at creating a query
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Using the query API and JPQL to retrieve entities
instance and then explore the methods in the Query interface designed for executing queries.
10.2.1 Creating a query instance
As our first example showed, before you can execute a named or dynamic query using JPQL, you must create the query instance to retrieve persistent data. The EntityManager interface provides methods for creating a query instance, as well as methods for creating native SQL queries. Table 10.2 lists the methods.
Table 10.2 The EntityManager interface provides several methods to create queries using either JPQL or native SQL statements. Method public Query createQuery(String qlString); Purpose Creates a dynamic query using a JPQL statement. Creates a query instance based on a named query. This method can be used for both JPQL and native SQL queries. Creates a dynamic query using a native SQL statement with UPDATE or DELETE. Creates a dynamic query using a native SQL statement that retrieves a single entity type. Creates a dynamic query using a native SQL statement that retrieves a result set with multiple entity types.
public Query createNamedQuery(String name);
public Query createNativeQuery( String sqlString);
public Query createNativeQuery( String sqlString,Class result-class);
public Query createNativeQuery( String sqlString,String result-setMapping);
In table 10.2, some of the methods for creating query instances use JPQL and others use with native SQL queries. Section 10.3 explores JPQL, and section 10.4 shows you how to work with native SQL queries. In this section, the sample queries are JPQL based. We suggest you use native SQL only as a last resort.
NOTE
You do not need an active transaction to create or execute a query; if one does not exist, the retrieved entities become detached instances.
Executing the queries
Creating a named query instance As we discussed earlier, named queries are globally scoped. You can create a named query instance from any component that has access to the persistence unit to which the entity belongs. You must have an open EntityManager instance to create a named query instance. To use a named query stored on the entity, you invoke the EntityManager.createNamedQuery method, passing the name of the desired named query as the parameter. In the previous example when we created the stored query, we stored the findAllCategories named query in the Category entity. Creating a named query from that stored query is as easy as this:
Query query = em.createNamedQuery("findAllCategories");
The EntityManager instance em takes care of all the details of fetching our named query findAllCategories and returning a reference, which we assign to the query object. Creating a dynamic query instance A dynamic query can be created wherever the EntityManager is available. This includes using it in session beans, MDBs, web applications, or even outside the container (as long as you can you can access the EntityManager). EJB 2 did not support dynamic queries, and many developers found that to be a significant limitation. We can use the EntityManager.createQuery method to create a dynamic query. The only requirement is to pass a valid JPQL statement. It makes no difference whether the EntityManager is a container- or application-managed entity manager. The following shows how to create a dynamic query:
Query query = em.createQuery("SELECT i FROM Item i");
You can see that the JPQL syntax resembles SQL, but JPA recommends that you use JPQL. The differences in notation between SQL and JPQL will be more evident later in section 10.3, when we explore JPQL by itself. Let s recap where we are now. We ve created an instance of the EntityManager, and we ve created an instance of the query. The next step is the actual execution of the query. The Query interface provides the methods we need.
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