In JPQL, a constructor can be used in the SELECT clause to return an instance of a non-entity Java object. For example:
Query query = em.createQuery("SELECT NEW com.example.MyClass(e.attribute1, e.attribute2...) FROM MyEntity e");
List<MyClass> resultList = query.getResultList();
Example
Entity
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
private long salary;
private String dept;
.............
}
Object to be selected
public class EmployeeNameSalaryPair {
private String name;
private long salary;
public EmployeeNameSalaryPair(String name, long salary) {
this.name = name;
this.salary = salary;
}
.............
}
Executing Query with Constructor Expression
Following example retrieves employee name-salary pairs both as Object[] and as EmployeeNameSalaryPair:
public class ExampleMain {
private static EntityManagerFactory entityManagerFactory =
Persistence.createEntityManagerFactory("example-unit");
public static void main(String[] args) {
try {
persistEmployees();
getNameSalaryPairsAsObjectArray();
getNameSalaryPairsViaConstructorExpression();
} finally {
entityManagerFactory.close();
}
}
public static void persistEmployees() {
Employee employee1 = Employee.create("Diana", 2000, "IT");
Employee employee2 = Employee.create("Rose", 3500, "Admin");
Employee employee3 = Employee.create("Denise", 2500, "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();
System.out.println("-- employee persisted --");
em.createQuery("Select e from Employee e")
.getResultList()
.forEach(System.out::println);
em.close();
}
private static void getNameSalaryPairsAsObjectArray() {
System.out.println("-- Employee name salary pair as Object[] --");
EntityManager em = entityManagerFactory.createEntityManager();
Query query = em.createQuery("SELECT e.name, e.salary FROM Employee e");
List<Object[]> resultList = query.getResultList();
resultList.forEach(arr -> System.out.println(Arrays.toString(arr)));
em.close();
}
private static void getNameSalaryPairsViaConstructorExpression() {
System.out.println("-- Employee name salary pair as EmployeeNameSalaryPair --");
EntityManager em = entityManagerFactory.createEntityManager();
Query query =
em.createQuery("SELECT NEW com.logicbig.example.EmployeeNameSalaryPair(e.name, e.salary)"
+ " FROM Employee e");
List<EmployeeNameSalaryPair> resultList = query.getResultList();
resultList.forEach(System.out::println);
em.close();
}
} -- employee persisted -- Employee{id=1, name='Diana', salary=2000, dept='IT'} Employee{id=2, name='Rose', salary=3500, dept='Admin'} Employee{id=3, name='Denise', salary=2500, dept='Admin'} Employee{id=4, name='Mike', salary=4000, dept='IT'} Employee{id=5, name='Linda', salary=4500, dept='Sales'} -- Employee name salary pair as Object[] -- [Diana, 2000] [Rose, 3500] [Denise, 2500] [Mike, 4000] [Linda, 4500] -- Employee name salary pair as EmployeeNameSalaryPair -- EmployeeNameSalaryPair{name='Diana', salary=2000} EmployeeNameSalaryPair{name='Rose', salary=3500} EmployeeNameSalaryPair{name='Denise', salary=2500} EmployeeNameSalaryPair{name='Mike', salary=4000} EmployeeNameSalaryPair{name='Linda', salary=4500}
Example ProjectDependencies and Technologies Used: - h2 1.4.197: H2 Database Engine.
- hibernate-core 5.3.1.Final: Hibernate's core ORM functionality.
Implements javax.persistence:javax.persistence-api version 2.2 - JDK 1.8
- Maven 3.5.4
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