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The Java Persistence Query Language in Java Encoder ANSI/AIM Code 39 in Java The Java Persistence Query Language




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21 The Java Persistence Query Language use awt code 39 extended implement tobuild code 3/9 with java Office Word This page intentionally left blank C H A P T E R Using the Criteria API to Create Queries The Criteria API is used to define queries for entities and their persistent state by creating query-defining objects. Criteria queries are written using Java programming language APIs, are typesafe, and are portable. Such queries work regardless of the underlying data store.

The following topics are addressed here:. Overview of the Criteria a Code 39 for Java nd Metamodel APIs on page 415 Using the Metamodel API to Model Entity Classes on page 417 Using the Criteria API and Metamodel API to Create Basic Typesafe Queries on page 418. Overview of the Criteria and Metamodel APIs Similar to JPQL, the Criter jvm Code 39 Full ASCII ia API is based on the abstract schema of persistent entities, their relationships, and embedded objects. The Criteria API operates on this abstract schema to allow developers to find, modify, and delete persistent entities by invoking Java Persistence API entity operations. The Metamodel API works in concert with the Criteria API to model persistent entity classes for Criteria queries.

The Criteria API and JPQL are closely related and are designed to allow similar operations in their queries. Developers familiar with JPQL syntax will find equivalent object-level operations in the Criteria API. The following simple Criteria query returns all instances of the Pet entity in the data source:.

EntityManager em = ...

; Cri teriaBuilder cb = em.getCriteriaBuilder(); CriteriaQuery<Pet> cq = cb.createQuery(Pet.

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

select(pet);. Overview of the Criteria and Metamodel APIs TypedQuery<Pet> q = e m.createQuery(cq); List<Pet> allPets = q.getResultList();.

The equivalent JPQL query is:. SELECT p FROM Pet p This query demonstrates the swing barcode 3 of 9 basic steps to create a Criteria query: 1. Use an EntityManager instance to create a CriteriaBuilder object. 2.

Create a query object by creating an instance of the CriteriaQuery interface. This query object"s attributes will be modified with the details of the query. 3.

Set the query root by calling the from method on the CriteriaQuery object. 4. Specify what the type of the query result will be by calling the select method of the CriteriaQuery object.

5. Prepare the query for execution by creating a TypedQuery<T> instance, specifying the type of the query result. 6.

Execute the query by calling the getResultList method on the TypedQuery<T> object. Because this query returns a collection of entities, the result is stored in a List. The tasks associated with each step are discussed in detail in this chapter.

To create a CriteriaBuilder instance, call the getCriteriaBuilder method on the EntityManager instance:. CriteriaBuilder cb = em.get CriteriaBuilder();. The query object is created Code 3/9 for Java by using the CriteriaBuilder instance:. CriteriaQuery<Pet> cq = cb.createQuery(Pet.class);.

The query will return insta Code 39 Full ASCII for Java nces of the Pet entity, so the type of the query is specified when the CriteriaQuery object is created to create a typesafe query. The FROM clause of the query is set, and the root of the query specified, by calling the from method of the query object:. Root<Pet> pet = cq.fr om(Pet.class);.

The SELECT clause of the qu 3 of 9 barcode for Java ery is set by calling the select method of the query object and passing in the query root:. cq.select(pet);. The query object is now use jsp Code 39 d to create a TypedQuery<T> object that can be executed against the data source. The modifications to the query object are captured to create a ready-to-execute query:. The Java EE 6 Tutorial: Basic Concepts Using the Metamodel API to Model Entity Classes TypedQuery<Pet> q = e m.createQuery(cq);. This typed query object is ANSI/AIM Code 39 for Java executed by calling its getResultList method, because this query will return multiple entity instances. The results are stored in a List<Pet> collection-valued object..

List<Pet> allPets = q .getResultList();. Using the Metamodel API to Model Entity Classes The Metamodel API is used t o create a metamodel of the managed entities in a particular persistence unit. For each entity class in a particular package, a metamodel class is created with a trailing underscore and with attributes that correspond to the persistent fields or properties of the entity class. The following entity class, com.

example.Pet, has four persistent fields: id, name, color, and owners:. package com.example; ..

. @E USS Code 39 for Java ntity public class Pet { @Id protected Long id; protected String name; protected String color; @ManyToOne protected Set<Person> owners; ..

. }. The corresponding Metamodel class is:. package com.example; ..

. @S barcode code39 for Java tatic Metamodel(Pet.class) public class Pet_ { public public public public } static static static static volatile volatile volatile volatile SingularAttribute<Pet, Long> id; SingularAttribute<Pet, String> name; SingularAttribute<Pet, String> color; SetAttribute<Pet, Person> owners;.

The metamodel class and its attributes are used in Criteria queries to refer to the managed entity classes and their persistent state and relationships..
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