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JPA Criteria API - Selecting User defined Objects using CriteriaBuilder#construct()

[Updated: Aug 4, 2018, Created: Aug 4, 2018]

To return user defined objects (not entities) as query results we can use CriteriaBuilder#construct(). Following is its signature:

<Y> CompoundSelection<Y> construct(Class<Y> resultClass, Selection<?>... selections);

For example:

    CriteriaQuery<EmployeeInfo> query = criteriaBuilder.createQuery(EmployeeInfo.class);
    Root<Employee> employee = query.from(Employee.class);
    query.select(criteriaBuilder.construct(EmployeeInfo.class, employee.get(Employee_.name),
                                                             employee.get(Employee_.salary)));
    List<EmployeeInfo> resultList = entityManager.createQuery(query).getResultList();

Criteriabuilder#construct() is equivalent to JPQL constructor expressions.

Example

Entity

@Entity
public class Employee {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  private double salary;
  private String dept;
    .............
}

Non-entity user defined class

public class EmployeeInfo {
  private String name;
  private double salary;

  public EmployeeInfo(String name, double salary) {
      this.name = name;
      this.salary = salary;
  }
    .............
}

Using construct() method

public class ExampleMain {
  private static EntityManagerFactory entityManagerFactory =
          Persistence.createEntityManagerFactory("example-unit");

  public static void main(String[] args) {
      try {
          persistEmployees();
          findAllEmployeeEntities();
          findEmployeeInfo();
      } finally {
          entityManagerFactory.close();
      }
  }

  public static void persistEmployees() {
      Employee employee1 = Employee.create("Diana", 2000, "IT");
      Employee employee2 = Employee.create("Rose", 4500, "Admin");
      Employee employee3 = Employee.create("Denise", 2000, "Admin");
      Employee employee4 = Employee.create("Mike", 4000, "IT");
      Employee employee5 = Employee.create("Linda", 4500, "Sales");
      EntityManager em = entityManagerFactory.createEntityManager();
      em.getTransaction().begin();
      em.persist(employee1);
      em.persist(employee2);
      em.persist(employee3);
      em.persist(employee4);
      em.persist(employee5);
      em.getTransaction().commit();
      em.close();
  }

  private static void findAllEmployeeEntities() {
      System.out.println("-- finding Employee entities - entity selection --");
      EntityManager em = entityManagerFactory.createEntityManager();
      CriteriaBuilder cb = em.getCriteriaBuilder();
      CriteriaQuery<Employee> query = cb.createQuery(Employee.class);
      Root<Employee> employee = query.from(Employee.class);
      query.select(employee);
      TypedQuery<Employee> typedQuery = em.createQuery(query);
      List<Employee> resultList = typedQuery.getResultList();
      resultList.forEach(System.out::println);
      em.close();
  }

  private static void findEmployeeInfo() {
      System.out.println("-- EmployeeInfo - using construct() --");
      EntityManager em = entityManagerFactory.createEntityManager();
      CriteriaBuilder cb = em.getCriteriaBuilder();
      CriteriaQuery<EmployeeInfo> query = cb.createQuery(EmployeeInfo.class);
      Root<Employee> employee = query.from(Employee.class);
      query.select(cb.construct(EmployeeInfo.class, employee.get(Employee_.name),
              employee.get(Employee_.salary)));
      TypedQuery<EmployeeInfo> typedQuery = em.createQuery(query);
      List<EmployeeInfo> resultList = typedQuery.getResultList();
      resultList.forEach(System.out::println);
      em.close();
  }
}
-- finding Employee entities - entity selection --
Employee{id=1, name='Diana', salary=2000.0, dept='IT'}
Employee{id=2, name='Rose', salary=4500.0, dept='Admin'}
Employee{id=3, name='Denise', salary=2000.0, dept='Admin'}
Employee{id=4, name='Mike', salary=4000.0, dept='IT'}
Employee{id=5, name='Linda', salary=4500.0, dept='Sales'}
-- EmployeeInfo - using construct() --
EmployeeInfo{name='Diana', salary=2000.0}
EmployeeInfo{name='Rose', salary=4500.0}
EmployeeInfo{name='Denise', salary=2000.0}
EmployeeInfo{name='Mike', salary=4000.0}
EmployeeInfo{name='Linda', salary=4500.0}

Example Project

Dependencies and Technologies Used:

  • h2 1.4.197: H2 Database Engine.
  • hibernate-core 5.3.2.Final: Hibernate's core ORM functionality.
    Implements javax.persistence:javax.persistence-api version 2.2
  • hibernate-jpamodelgen 5.3.2.Final: Annotation Processor to generate JPA 2 static metamodel classes.
  • JDK 1.8
  • Maven 3.5.4

CriteriaBuilder#construct() Example Select All Download
  • jpa-criteria-api-construct
    • src
      • main
        • java
          • com
            • logicbig
              • example
                • ExampleMain.java
          • resources
            • META-INF

    See Also