Sometimes, you may want to deal with the embeddable data associated with an entity.
For example,
a. An employee might have one or more communication addresses
b. A person might have some alias/nick names.
In both the cases, the collection of addresses or collection of nick names do not make any sense outside of employee object. In this case, we can model, these nick names as embeddable collections inside an entity and no need to model these nick names as another entity.
Example
@ElementCollection
private Set<NickName> nickNames;
You can even specify the sort order of set elements in two ways.
Approach 1: By implementing Comparable interface.
@Embeddable
public class NickName implements Comparable<NickName> {
}
@Entity
@Table(name = "employee")
public class Employee {
@ElementCollection
@SortNatural
private SortedSet<NickName> nickNames;
}
Since NickName is defined as Comparable, its # compareTo method will be used to sort the elements in this set.
Approach 2: Using custom comparator.
@ElementCollection
@SortComparator( NickNameComparator.class )
private SortedSet <NickName> names;
In approach 2, the explicit NickNameComparator will be used instead to sort the data.
Find the below working application.
Step 1: Create new maven project ‘hibernate-map-set’.
Step 2: Update pom.xml with maven dependencies.
pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.sample.app</groupId>
<artifactId>hibernate-map-set</artifactId>
<version>1</version>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<java.version>15</java.version>
<maven.compiler.source>${java.version}</maven.compiler.source>
<maven.compiler.target>${java.version}</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>org.postgresql</groupId>
<artifactId>postgresql</artifactId>
<version>42.4.1</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>6.1.2.Final</version>
</dependency>
</dependencies>
</project>
Step 3: Define NickName class.
NickName.java
package com.sample.app.dto;
import jakarta.persistence.Embeddable;
@Embeddable
public class NickName implements Comparable<NickName> {
private String name;
public NickName() {
}
public NickName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "NickName [name=" + name + "]";
}
@Override
public int compareTo(NickName nickName) {
if (nickName == null || nickName.name == null)
return 0;
return nickName.name.compareTo(this.name);
}
}
Step 4: Define Employee entity class.
Employee.java
package com.sample.app.entity;
import java.util.SortedSet;
import org.hibernate.annotations.SortNatural;
import com.sample.app.dto.NickName;
import jakarta.persistence.Column;
import jakarta.persistence.ElementCollection;
import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.Table;
@Entity
@Table(name = "employee")
public class Employee {
@Id
@Column(name = "employee_id")
private int id;
private String name;
@ElementCollection
@SortNatural
private SortedSet<NickName> nickNames;
public Employee() {
}
public Employee(int id, String name, SortedSet<NickName> nickNames) {
this.id = id;
this.name = name;
this.nickNames = nickNames;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public SortedSet<NickName> getNickNames() {
return nickNames;
}
public void setNickNames(SortedSet<NickName> nickNames) {
this.nickNames = nickNames;
}
@Override
public String toString() {
return "Employee [id=" + id + ", name=" + name + ", nickNames=" + nickNames + "]";
}
}
Step 5: Create hibernate.cfg.xml file under src/main/resources folder.
hibernate.cfg.xml
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<!-- Database Connection settings -->
<property name="connection.driver_class">org.postgresql.Driver</property>
<property name="connection.url">jdbc:postgresql://127.0.0.1:5432/test</property>
<property name="connection.username">postgres</property>
<property name="connection.password">postgres</property>
<!-- Enable the logging of all the generated SQL statements to the console -->
<property name="show_sql">true</property>
<!-- Format the generated SQL statement to make it more readable, -->
<property name="format_sql">true</property>
<!-- Hibernate will put comments inside all generated SQL statements to
hint what’s the generated SQL trying to do -->
<property name="use_sql_comments">false</property>
<!-- This property makes Hibernate generate the appropriate SQL for the
chosen database. -->
<property name="dialect">org.hibernate.dialect.PostgreSQLDialect</property>
<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">create</property>
<!-- mappings for annotated classes -->
<mapping class="com.sample.app.entity.Employee" />
</session-factory>
</hibernate-configuration>
Step 6: Define main application class.
App.java
package com.sample.app;
import java.util.*;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import com.sample.app.dto.NickName;
import com.sample.app.entity.Employee;
import jakarta.persistence.criteria.CriteriaBuilder;
import jakarta.persistence.criteria.CriteriaQuery;
public class App {
private static SessionFactory sessionFactory = buildSessionFactory();
private static <T> List<T> loadAllData(Class<T> clazz, Session session) {
final CriteriaBuilder builder = session.getCriteriaBuilder();
final CriteriaQuery<T> criteria = builder.createQuery(clazz);
criteria.from(clazz);
return session.createQuery(criteria).getResultList();
}
private static SessionFactory buildSessionFactory() {
try {
if (sessionFactory == null) {
StandardServiceRegistry standardRegistry = new StandardServiceRegistryBuilder()
.configure("hibernate.cfg.xml").build();
Metadata metaData = new MetadataSources(standardRegistry).getMetadataBuilder().build();
sessionFactory = metaData.getSessionFactoryBuilder().build();
}
return sessionFactory;
} catch (Throwable ex) {
throw new ExceptionInInitializerError(ex);
}
}
public static void main(String args[]) {
try (Session session = sessionFactory.openSession()) {
session.beginTransaction();
TreeSet<NickName> nickNamesSet = new TreeSet<>();
nickNamesSet.add(new NickName("Vasu dev"));
nickNamesSet.add(new NickName("Rishikesha"));
nickNamesSet.add(new NickName("Keshava"));
nickNamesSet.add(new NickName("Sri Krishna"));
nickNamesSet.add(new NickName("Nandakumara"));
Employee emp = new Employee(1, "Krishna", nickNamesSet);
session.persist(emp);
session.flush();
session.getTransaction().commit();
List<Employee> employees = loadAllData(Employee.class, session);
for (Employee emp1 : employees) {
System.out.println(emp1);
}
}
}
}
Total project structure looks like below.
Sep 13, 2022 11:36:56 AM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate ORM core version 6.1.2.Final
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using built-in connection pool (not intended for production use)
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: Loaded JDBC driver class: org.postgresql.Driver
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001012: Connecting with JDBC URL [jdbc:postgresql://127.0.0.1:5432/test]
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {password=****, user=postgres}
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH10001115: Connection pool size: 20 (min=1)
Sep 13, 2022 11:36:57 AM org.hibernate.engine.jdbc.dialect.internal.DialectFactoryImpl logSelectedDialect
INFO: HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
Hibernate:
alter table if exists Employee_nickNames
drop constraint if exists FK5ahwp3hworqds1shqyvktsyg7
Sep 13, 2022 11:36:57 AM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@f1dd5b4] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Hibernate:
drop table if exists employee cascade
Hibernate:
drop table if exists Employee_nickNames cascade
Hibernate:
create table employee (
employee_id integer not null,
name varchar(255),
primary key (employee_id)
)
Sep 13, 2022 11:36:57 AM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@48eb001a] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Hibernate:
create table Employee_nickNames (
Employee_employee_id integer not null,
name varchar(255)
)
Hibernate:
alter table if exists Employee_nickNames
add constraint FK5ahwp3hworqds1shqyvktsyg7
foreign key (Employee_employee_id)
references employee
Sep 13, 2022 11:36:57 AM org.hibernate.engine.transaction.jta.platform.internal.JtaPlatformInitiator initiateService
INFO: HHH000490: Using JtaPlatform implementation: [org.hibernate.engine.transaction.jta.platform.internal.NoJtaPlatform]
Hibernate:
insert
into
employee
(name, employee_id)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
delete
from
Employee_nickNames
where
Employee_employee_id=?
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
insert
into
Employee_nickNames
(Employee_employee_id, name)
values
(?, ?)
Hibernate:
select
e1_0.employee_id,
e1_0.name
from
employee e1_0
Employee [id=1, name=Krishna, nickNames=[NickName [name=Vasu dev], NickName [name=Sri Krishna], NickName [name=Rishikesha], NickName [name=Nandakumara], NickName [name=Keshava]]]
From the output, you can confirm that the Nick names are sorted by their name in descendin order.
Query database to confirm DDL.
test=# \d+ employee;
Table "public.employee"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
-------------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
employee_id | integer | | not null | | plain | | |
name | character varying(255) | | | | extended | | |
Indexes:
"employee_pkey" PRIMARY KEY, btree (employee_id)
Referenced by:
TABLE "employee_nicknames" CONSTRAINT "fk5ahwp3hworqds1shqyvktsyg7" FOREIGN KEY (employee_employee_id) REFERENCES employee(employee_id)
Access method: heap
test=#
test=#
test=#
test=#
test=# \d+ employee_nicknames;
Table "public.employee_nicknames"
Column | Type | Collation | Nullable | Default | Storage | Compression | Stats target | Description
----------------------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
employee_employee_id | integer | | not null | | plain | | |
name | character varying(255) | | | | extended | | |
Foreign-key constraints:
"fk5ahwp3hworqds1shqyvktsyg7" FOREIGN KEY (employee_employee_id) REFERENCES employee(employee_id)
Access method: heap
Query tables to confirm the data.
test=# SELECT * FROM employee;
employee_id | name
-------------+---------
1 | Krishna
(1 row)
test=#
test=#
test=# SELECT * FROM employee_nicknames;
employee_employee_id | name
----------------------+-------------
1 | Vasu dev
1 | Sri Krishna
1 | Rishikesha
1 | Nandakumara
1 | Keshava
(5 rows)
You can download the application from this link.
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