In the last tutorial we used Criteria API along with string based attribute references, i.e.
Root<Employee> employee = query.from(Employee.class);
query.select(employee)
.where(cb.equal(employee.get("dept"), "Admin"));
Here we have another option, JPA allows to automatically generate metamodel class for each entity class. If entity is T then it's metamodel class is generated as T_ which can be used to reference attributes in a strongly typed manner:
Root<Employee> employee = query.from(Employee.class);
query.select(employee)
.where(cb.equal(employee.get(Employee_.dept), "Admin"));
Example
Entity
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
private double salary;
private String dept;
.............
}
Additional maven dependencies
pom.xml<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-jpamodelgen</artifactId>
<version>5.3.2.Final</version>
</dependency>
Hibernate JPA Metamodel Generator
We also need to specify Hibernate annotation processor which is responsible to generate the metamodel for the entities:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.7.0</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
<encoding>UTF-8</encoding>
<compilerArguments>
<processor>org.hibernate.jpamodelgen.JPAMetaModelEntityProcessor</processor>
</compilerArguments>
</configuration>
</plugin>
</plugins>
</build>
Compiling
We need to compile our entity class for above processor to work. Employee_ will be generated at following location (screenshot from IntelliJ):
The generated class Employee_
package com.logicbig.example;
import javax.persistence.metamodel.SingularAttribute;
import javax.persistence.metamodel.StaticMetamodel;
@StaticMetamodel(Employee.class)
public abstract class Employee_ {
public static volatile SingularAttribute<Employee, String> name;
public static volatile SingularAttribute<Employee, Long> id;
public static volatile SingularAttribute<Employee, String> dept;
public static volatile SingularAttribute<Employee, Double> salary;
public static final String NAME = "name";
public static final String ID = "id";
public static final String DEPT = "dept";
public static final String SALARY = "salary";
public Employee_() {
}
}
Using Criteria API
public class ExampleMain {
private static EntityManagerFactory entityManagerFactory =
Persistence.createEntityManagerFactory("example-unit");
public static void main(String[] args) {
try {
persistEmployees();
findAdmins();
} 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();
em.close();
}
private static void findAdmins() {
System.out.println("-- All employees with salary greater than 3000 --");
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)
.where(cb.gt(employee.get(Employee_.salary), 3000d));
TypedQuery<Employee> typedQuery = em.createQuery(query);
typedQuery.getResultList()
.forEach(System.out::println);
em.close();
}
} -- All employees with salary greater than 3000 -- Employee{id=2, name='Rose', salary=3500.0, dept='Admin'} Employee{id=4, name='Mike', salary=4000.0, dept='IT'} Employee{id=5, name='Linda', salary=4500.0, dept='Sales'}
Example ProjectDependencies 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
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