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Using the Criteria API to Create Queries in Java Maker 39 barcode in Java Using the Criteria API to Create Queries

22 Using the Criteria API to Create Queries generate, create code 3 of 9 none on java projects Microsoft's .NET Framework Using the Criteria API and Metamodel API to Create Basic Typesafe Queries Using Metamodel Classes Metamodel class barcode code39 for Java es that correspond to entity classes are of the following type:. javax.persistence.metamodel.EntityType<T> Metamodel class jdk 3 of 9 es are typically generated by annotation processors either at development time or at runtime. Developers of applications that use Criteria queries may generate static metamodel classes by using the persistence provider s annotation processor or may obtain the metamodel class by either calling the getModel method on the query root object or first obtaining an instance of the Metamodel interface and then passing the entity type to the instance s entity method. The following code snippet shows how to obtain the Pet entity s metamodel class by calling Root<T>.

getModel:. EntityManager e m = ...

; CriteriaBuilder cb = em.getCriteriaBuilder(); CriteriaQuery cq = cb.createQuery(Pet.

class); Root<Pet> pet = cq.from(Pet.class); EntityType<Pet> Pet_ = pet.

getModel();. The following c j2ee USS Code 39 ode snippet shows how to obtain the Pet entity s metamodel class by first obtaining a metamodel instance by using EntityManager.getMetamodel and then calling entity on the metamodel instance:. EntityManager e m = ...

; Metamodel m = em.getMetamodel(); EntityType<Pet> Pet_ = m.entity(Pet.

class);. Using the Criteria API and Metamodel API to Create Basic Typesafe Queries The basic seman tics of a Criteria query consists of a SELECT clause, a FROM clause, and an optional WHERE clause, similar to a JPQL query. Criteria queries set these clauses by using Java programming language objects, so the query can be created in a typesafe manner..

Creating a Criteria Query The javax.persistence.criteria.CriteriaBuilder interface is used to construct Criteria queries Selections Expressions The Java EE 6 Tutorial: Basic Concepts Using the Criteria API and Metamodel API to Create Basic Typesafe Queries Predicates Ordering To obtain an in jvm bar code 39 stance of the CriteriaBuilder interface, call the getCriteriaBuilder method on either an EntityManager or an EntityManagerFactory instance. The following code shows how to obtain a CriteriaBuilder instance by using the EntityManager.getCriteriaBuilder method.

. EntityManager e m = ...

; CriteriaBuilder cb = em.getCriteriaBuilder();. Criteria querie bar code 39 for Java s are constructed by obtaining an instance of the following interface:. javax.persistence.criteria.CriteriaQuery CriteriaQuery o Code 39 Full ASCII for Java bjects define a particular query that will navigate over one or more entities. Obtain CriteriaQuery instances by calling one of the CriteriaBuilder.createQuery methods.

For creating typesafe queries, call the CriteriaBuilder.createQuery method as follows:. CriteriaQuery&l t;Pet> cq = cb.createQuery(Pet.class);.

The CriteriaQue barcode 39 for Java ry object s type should be set to the expected result type of the query. In the preceding code, the object s type is set to CriteriaQuery<Pet> for a query that will find instances of the Pet entity. In the following code snippet, a CriteriaQuery object is created for a query that returns a String:.

CriteriaQuery&l t;String> cq = cb.createQuery(String.class);.

Query Roots For a particula r CriteriaQueryobject, the root entity of the query, from which all navigation originates, is called the query root. It is similar to the FROM clause in a JPQL query. Create the query root by calling the from method on the CriteriaQuery instance.

The argument to the from method is either the entity class or an EntityType<T> instance for the entity. The following code sets the query root to the Pet entity:. CriteriaQuery&l tomcat ANSI/AIM Code 39 t;Pet> cq = cb.createQuery(Pet.class); Root<Pet> pet = cq.

from(Pet.class);. 22 Using the Criteria API to Create Queries Using the Criteria API and Metamodel API to Create Basic Typesafe Queries The following c ode sets the query root to the Pet class by using an EntityType<T> instance:. EntityManager e swing 3 of 9 barcode m = ...

; Metamodel m = em.getMetamodel(); EntityType<Pet> Pet_ = m.entity(Pet.

class); Root<Pet> pet = cq.from(Pet_);. Criteria querie s may have more than one query root. This usually occurs when the query navigates from several entities. The following code has two Root instances:.

CriteriaQuery&l Code 3 of 9 for Java t;Pet> cq = cb.createQuery(Pet.class); Root<Pet> pet1 = cq.

from(Pet.class); Root<Pet> pet2 = cq.from(Pet.

class);. Querying Relationships Using Joins For queries tha Code 39 for Java t navigate to related entity classes, the query must define a join to the related entity by calling one of the From.join methods on the query root object or another join object. The join methods are similar to the JOIN keyword in JPQL.

The target of the join uses the Metamodel class of type EntityType<T> to specify the persistent field or property of the joined entity. The join methods return an object of type Join<X, Y>, where X is the source entity and Y is the target of the join. In the following code snippet, Pet is the source entity, and Owner is the target:.

CriteriaQuery&l t;Pet> cq = cb.createQuery(Pet.class); Metamodel m = em.

getMetamodel(); EntityType<Pet> Pet_ = m.entity(Pet.class); Root<Pet> pet = cq.

from(Pet.class); Join<Pet, Owner> owner = pet.join(Pet_.

owners);. Joins can be ch applet barcode code39 ained together to navigate to related entities of the target entity without having to create a Join<X, Y> instance for each join:. CriteriaQuery&l t;Pet> cq = cb.createQuery(Pet.class); Metamodel m = em.

getMetamodel(); EntityType<Pet> Pet_ = m.entity(Pet.class); EntityType<Owner> Owner_ = m.

entity(Owner.class); Root<Pet> pet = cq.from(Pet.

class); Join<Owner, Address> address = cq.join(Pet_.owners).

join(Owner_.addresses);.
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