JPA Named Queries

A named query is a statically defined query with a predefined unchangeable query string. Using named queries instead of dynamic queries may improve code organization by separating the JPQL query strings from the Java code. It also enforces the use of query parameters rather than embedding literals dynamically into the query string and results in more efficient queries.

@NamedQuery and @NamedQueries Annotations

The following @NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language. annotation defines a query whose name is "Country.findAll" that retrieves all the Country objects in the database:

@NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language.(namejavax.persistence.NamedQuery.name - JPA Annotation Attribute(Required) The name used to refer to the query with the {@link EntityManager}
 methods that create query objects.="Country.findAll", queryjavax.persistence.NamedQuery.query - JPA Annotation Attribute(Required)
 The query string in the Java Persistence query language.="SELECT c FROM Country c")

The @NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language. annotation contains four elements - two of which are required and two are optional. The two required elements, namejavax.persistence.NamedQuery.name - JPA Annotation Attribute(Required) The name used to refer to the query with the {@link EntityManager} methods that create query objects. and queryjavax.persistence.NamedQuery.query - JPA Annotation Attribute(Required) The query string in the Java Persistence query language. define the name of the query and the query string itself and are demonstrated above. The two optional elements, lockModejavax.persistence.NamedQuery.lockMode - JPA Annotation Attribute(Optional) The lock mode type to use in query execution. and hintsjavax.persistence.NamedQuery.hints - JPA Annotation Attribute(Optional) Query properties and hints., provide static replacement for the setLockModeQuery.setLockMode(lockMode) - JPA MethodSet the lock mode type to be used for the query execution. and setHintQuery.setHint(hintName,value) - JPA MethodSet a query property or hint. methods.

Every @NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language. annotation is attached to exactly one entity class or mapped superclass - usually to the most relevant entity class. But since the scope of named queries is the entire persistence unit, names should be selected carefully to avoid collision (e.g. by using the unique entity name as a prefix).

It makes sense to add the above @NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language. to the Country entity class:

@Entityjavax.persistence.Entity - JPA AnnotationSpecifies that the class is an entity.
@NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language.(namejavax.persistence.NamedQuery.name - JPA Annotation Attribute(Required) The name used to refer to the query with the {@link EntityManager}
 methods that create query objects.="Country.findAll", queryjavax.persistence.NamedQuery.query - JPA Annotation Attribute(Required)
 The query string in the Java Persistence query language.="SELECT c FROM Country c") 
public class Country {
  ...
}

Attaching multiple named queries to the same entity class requires wrapping them in a @NamedQueriesjavax.persistence.NamedQueries - JPA AnnotationSpecifies multiple named Java Persistence query language queries. annotation, as follows:

@Entityjavax.persistence.Entity - JPA AnnotationSpecifies that the class is an entity.
@NamedQueries({
    @NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language.(namejavax.persistence.NamedQuery.name - JPA Annotation Attribute(Required) The name used to refer to the query with the {@link EntityManager}
 methods that create query objects.="Country.findAll",
                queryjavax.persistence.NamedQuery.query - JPA Annotation Attribute(Required)
 The query string in the Java Persistence query language.="SELECT c FROM Country c"),
    @NamedQueryjavax.persistence.NamedQuery - JPA AnnotationSpecifies a static, named query in the Java Persistence query language.(namejavax.persistence.NamedQuery.name - JPA Annotation Attribute(Required) The name used to refer to the query with the {@link EntityManager}
 methods that create query objects.="Country.findByName",
                queryjavax.persistence.NamedQuery.query - JPA Annotation Attribute(Required)
 The query string in the Java Persistence query language.="SELECT c FROM Country c WHERE c.name = :name"),
}) 
public class Country {
  ...
}

Using Named Queries at Runtime

Named queries are represented at runtime by the same Queryjavax.persistence.Query - JPA InterfaceInterface used to control query execution.javax.jdo.Query - JDO InterfaceThe Query interface allows applications to obtain persistent instances, values, and aggregate data from the data store. and TypedQueryjavax.persistence.TypedQuery - JPA InterfaceInterface used to control the execution of typed queries. interfaces but different EntityManagerjavax.persistence.EntityManager - JPA InterfaceInterface used to interact with the persistence context. factory methods are used to instantiate them. The createNamedQueryEntityManager.createNamedQuery(name,resultClass) - JPA MethodCreate an instance of TypedQuery for executing a Java Persistence query language named query. method receives a query name and a result type and returns a TypedQueryjavax.persistence.TypedQuery - JPA InterfaceInterface used to control the execution of typed queries. instance:

  TypedQueryjavax.persistence.TypedQuery - JPA InterfaceInterface used to control the execution of typed queries.<Country> query =
      em.createNamedQueryEntityManager.createNamedQuery(name,resultClass) - JPA MethodCreate an instance of TypedQuery for executing a
 Java Persistence query language named query.("Country.findAll", Country.class);
  List<Country> results = query.getResultListTypedQuery.getResultList() - JPA MethodExecute a SELECT query and return the query results
 as a typed List.();

Another form of createNamedQueryEntityManager.createNamedQuery(name) - JPA MethodCreate an instance of Query for executing a named query (in the Java Persistence query language or in native SQL). receives a query name and returns a Queryjavax.persistence.Query - JPA InterfaceInterface used to control query execution.javax.jdo.Query - JDO InterfaceThe Query interface allows applications to obtain persistent instances, values, and aggregate data from the data store. instance:

  Queryjavax.persistence.Query - JPA InterfaceInterface used to control query execution.javax.jdo.Query - JDO InterfaceThe Query interface allows applications to obtain persistent
 instances, values, and aggregate data
 from the data store. query = em.createNamedQueryEntityManager.createNamedQuery(name) - JPA MethodCreate an instance of Query for executing a named query
 (in the Java Persistence query language or in native SQL).("Country.findAll");
  List results = query.getResultListQuery.getResultList() - JPA MethodExecute a SELECT query and return the query results
 as an untyped List.();

One of the reasons that JPA requires the listing of managed classes in a persistence unit definition is to support named queries. Notice that named queries may be attached to any entity class or mapped superclass. Therefore, to be able to always locate any named query at runtime a list of all these managed persistable classes must be available.

ObjectDB makes the definition of a persistence unit optional. Named queries are automatically searched for in all the managed classes that ObjectDB is aware of, and that includes all the entity classes that have objects in the database. However, an attempt to use a named query still might fail if that named query is defined on a class that is still unknown to ObjectDB.

As a workaround, you may introduce classes to ObjectDB before accessing named queries, by using the JPA 2 Metamodeljavax.persistence.metamodel.Metamodel - JPA InterfaceProvides access to the metamodel of persistent entities in the persistence unit. interface. For example:

  em.getMetamodelEntityManager.getMetamodel() - JPA MethodReturn an instance of Metamodel interface for access to the
 metamodel of the persistence unit.().managedTypeMetamodel.managedType(cls) - JPA MethodReturn the metamodel managed type representing the
  entity, mapped superclass, or embeddable class.(MyEntity.class);

Following the above code ObjectDB will include MyEntity in searching named queries.