Saturday 24 December 2022

Hibernate 6: model Composite identifiers using @EmbeddedId

Entity identifiers are used to uniquely identify a row in a table. Hibernate and JPA (Jakarta Persistence API), assumes following rules must be followed by the identifier.

 

a.   Identifier must be unique in a table.

b.   Identifier must not be null. If you use a composite key as an identifier, then no other part can be null.

c.    Once a value assigned to identifier property, the value must not be changed on any circumstances.

 

Types of identifiers

An identifier can be

a.   Simple

b.   Composite

 

Let’s discuss how to model a composite identifier using @EmbeddedId annotation.

 

Step 1: Define an embeddable class. As per the documentation, the primary key class must be serializable.

@Embeddable
public class EmployeePK implements Serializable {
    private int empId;
    private int deptId;

    ........
    ........
}

 

Step 2: Model the variable of EmployeePK as primary key in the actual Employee entity class.

@Entity
@Table(name = "employees")
public class Employee {
    @EmbeddedId
    private EmployeePK empPK;

    ..........
    ..........
}

Find the below working application.

 

Step 1: Create new maven project ‘hibernate-composite-id-by-embedded-id’.

 

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-composite-id-by-embedded-id</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: Model Employee entity class.

 

EmployeePK.java

package com.sample.app.entity;

import java.io.Serializable;

import jakarta.persistence.Embeddable;

@Embeddable
public class EmployeePK implements Serializable {
    private int empId;
    private int deptId;

    public int getEmpId() {
        return empId;
    }

    public void setEmpId(int empId) {
        this.empId = empId;
    }

    public int getDeptId() {
        return deptId;
    }

    public void setDeptId(int deptId) {
        this.deptId = deptId;
    }

    @Override
    public String toString() {
        return "EmployeePK [empId=" + empId + ", deptId=" + deptId + "]";
    }

}

Employee.java

package com.sample.app.entity;

import jakarta.persistence.EmbeddedId;
import jakarta.persistence.Entity;
import jakarta.persistence.Table;

@Entity
@Table(name = "employees")
public class Employee {
    @EmbeddedId
    private EmployeePK empPK;
    
    private String firstName;
    private String lastName;

    public EmployeePK getEmpPK() {
        return empPK;
    }

    public void setEmpPK(EmployeePK empPK) {
        this.empPK = empPK;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    @Override
    public String toString() {
        return "Employee [empPK=" + empPK + ", firstName=" + firstName + ", lastName=" + lastName + "]";
    }

}

 

Step 4: 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 5: Define main application class.

 

App.java

 

package com.sample.app;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

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.entity.Employee;
import com.sample.app.entity.EmployeePK;

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[]) {

        Map<String, Object> config = new HashMap<>();
        config.put("maxThreads", 10);
        config.put("storagePath", "/Users/krishna/my-docs");

        EmployeePK empPk = new EmployeePK();
        empPk.setDeptId(1);
        empPk.setEmpId(23);

        Employee emp = new Employee();
        emp.setEmpPK(empPk);
        emp.setFirstName("Lakshmana Rao");
        emp.setLastName("Gurram");

        try (Session session = sessionFactory.openSession()) {
            session.beginTransaction();
            session.persist(emp);

            session.flush();
            session.getTransaction().commit();

            List<Employee> emps = loadAllData(Employee.class, session);
            for (Employee emp1 : emps) {
                System.out.println(emp1);

            }
        }

    }
}

 

Total project structure looks like below.

 

 


Run App.java, you will see below messages in the console.

 

Aug 23, 2022 4:53:35 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate ORM core version 6.1.2.Final
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using built-in connection pool (not intended for production use)
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: Loaded JDBC driver class: org.postgresql.Driver
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001012: Connecting with JDBC URL [jdbc:postgresql://127.0.0.1:5432/test]
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {password=****, user=postgres}
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH10001115: Connection pool size: 20 (min=1)
Aug 23, 2022 4:53:35 PM org.hibernate.engine.jdbc.dialect.internal.DialectFactoryImpl logSelectedDialect
INFO: HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
Aug 23, 2022 4:53:35 PM org.hibernate.mapping.RootClass checkCompositeIdentifier
WARN: HHH000038: Composite-id class does not override equals(): com.sample.app.entity.EmployeePK
Aug 23, 2022 4:53:35 PM org.hibernate.mapping.RootClass checkCompositeIdentifier
WARN: HHH000039: Composite-id class does not override hashCode(): com.sample.app.entity.EmployeePK
Hibernate: 
    
    drop table if exists employees cascade
Aug 23, 2022 4:53:36 PM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@32b0876c] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Aug 23, 2022 4:53:36 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: SQL Warning Code: 0, SQLState: 00000
Aug 23, 2022 4:53:36 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: table "employees" does not exist, skipping
Hibernate: 
    
    create table employees (
       deptId integer not null,
        empId integer not null,
        firstName varchar(255),
        lastName varchar(255),
        primary key (deptId, empId)
    )
Aug 23, 2022 4:53:36 PM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@606f81b5] 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.
Aug 23, 2022 4:53:36 PM 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
        employees
        (firstName, lastName, deptId, empId) 
    values
        (?, ?, ?, ?)
Hibernate: 
    select
        e1_0.deptId,
        e1_0.empId,
        e1_0.firstName,
        e1_0.lastName 
    from
        employees e1_0
Employee [empPK=EmployeePK [empId=23, deptId=1], firstName=Lakshmana Rao, lastName=Gurram]

 

Query PostgreSQL to confirm the generated DDL.

test=# \d+ employees;
                                                 Table "public.employees"
  Column   |          Type          | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
-----------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
 deptid    | integer                |           | not null |         | plain    |             |              | 
 empid     | integer                |           | not null |         | plain    |             |              | 
 firstname | character varying(255) |           |          |         | extended |             |              | 
 lastname  | character varying(255) |           |          |         | extended |             |              | 
Indexes:
    "employees_pkey" PRIMARY KEY, btree (deptid, empid)
Access method: heap

test=#

 You can download this application from this link.



 

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