Monday 26 December 2022

Hibernate6: uni directional many to many assoiciation

@ManyToMany association maintain a link table to join two entities. Like the @OneToMany, @OneToOne associations, @ManyToMany can be either unidirectional or bidirectional.

 

Unidirectional @ManyToMany association

@Entity
@Table(name = "project")
public class Project {
    @Id
    private int projectId;

    private String projectName;

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

@Entity
@Table(name = "employee")
public class Employee {
    @Id
    private int id;

    private String name;

    @ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE })
    private List<Project> projects = new ArrayList<>();

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

 

Above snippet generates following DDL.

  create table employee (
       id integer not null,
        name varchar(255),
        primary key (id)
    )

    create table employee_project (
        Employee_id integer not null,
        projects_projectId integer not null
    )

    create table project (
        projectId integer not null,
        projectName varchar(255),
        primary key (projectId)
    )

    alter table if exists employee_project 
       add constraint FK84if10lpuqoq270kixummx39k 
       foreign key (projects_projectId) 
       references project

    alter table if exists employee_project 
       add constraint FKcfge3ley7bbpogw8dur1p5q35 
       foreign key (Employee_id) 
       references employee

 

Find the below working application.

 

Step 1: Create new maven project ‘hibernate-uni-directional-many-to-many’.

 

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-uni-directional-many-to-many</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 entity classes.

 

Project.java

 

package com.sample.app.entity;

import java.util.Objects;

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.Table;

@Entity
@Table(name = "project")
public class Project {
    @Id
    private int projectId;

    private String projectName;

    public Project() {
    }

    public Project(int projectId, String projectName) {
        this.projectId = projectId;
        this.projectName = projectName;
    }

    public int getProjectId() {
        return projectId;
    }

    public void setProjectId(int projectId) {
        this.projectId = projectId;
    }

    public String getProjectName() {
        return projectName;
    }

    public void setProjectName(String projectName) {
        this.projectName = projectName;
    }

    @Override
    public int hashCode() {
        return Objects.hash(projectId);
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Project other = (Project) obj;
        return projectId == other.projectId;
    }

    @Override
    public String toString() {
        return "Project [projectId=" + projectId + ", projectName=" + projectName + "]";
    }

}

 

Employee.java

package com.sample.app.entity;

import java.util.ArrayList;
import java.util.List;

import jakarta.persistence.CascadeType;
import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.ManyToMany;
import jakarta.persistence.Table;

@Entity
@Table(name = "employee")
public class Employee {
    @Id
    private int id;

    private String name;

    @ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE })
    private List<Project> projects = new ArrayList<>();

    public Employee() {
    }

    public Employee(int id, String name) {
        this.id = id;
        this.name = name;
    }

    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 List<Project> getProjects() {
        return projects;
    }

    public void setProjects(List<Project> projects) {
        this.projects = projects;
    }

    @Override
    public String toString() {
        return "Employee [id=" + id + ", name=" + name + "]";
    }

}

 

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" />
        <mapping class="com.sample.app.entity.Project" />

    </session-factory>

</hibernate-configuration>

Step 5: Define main application class.

 

App.java

package com.sample.app;

import java.util.List;

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.Project;

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();
            Employee emp = new Employee(1, "Krishna");
            emp.getProjects().add(new Project(11, "Chat server"));
            emp.getProjects().add(new Project(12, "Intelligent Sync"));
            session.persist(emp);
            session.flush();
            session.getTransaction().commit();

            List<Employee> employees = loadAllData(Employee.class, session);
            for (Employee emp1 : employees) {
                System.out.println(emp1);
                
                List<Project> projects = emp.getProjects();
                for(Project pjt: projects) {
                    System.out.println(pjt);
                }
            }

        }

    }
}

Total project structure looks like below.

 


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

 

 

Sep 12, 2022 12:01:56 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate ORM core version 6.1.2.Final
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using built-in connection pool (not intended for production use)
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: Loaded JDBC driver class: org.postgresql.Driver
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001012: Connecting with JDBC URL [jdbc:postgresql://127.0.0.1:5432/test]
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {password=****, user=postgres}
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH10001115: Connection pool size: 20 (min=1)
Sep 12, 2022 12:01:56 PM org.hibernate.engine.jdbc.dialect.internal.DialectFactoryImpl logSelectedDialect
INFO: HHH000400: Using dialect: org.hibernate.dialect.PostgreSQLDialect
Hibernate: 
    
    alter table if exists employee_project 
       drop constraint if exists FK84if10lpuqoq270kixummx39k
Sep 12, 2022 12:01:57 PM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@1c7350b0] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: SQL Warning Code: 0, SQLState: 00000
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: relation "employee_project" does not exist, skipping
Hibernate: 
    
    alter table if exists employee_project 
       drop constraint if exists FKcfge3ley7bbpogw8dur1p5q35
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: SQL Warning Code: 0, SQLState: 00000
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: relation "employee_project" does not exist, skipping
Hibernate: 
    
    drop table if exists employee cascade
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: SQL Warning Code: 0, SQLState: 00000
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: table "employee" does not exist, skipping
Hibernate: 
    
    drop table if exists employee_project cascade
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: SQL Warning Code: 0, SQLState: 00000
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: table "employee_project" does not exist, skipping
Hibernate: 
    
    drop table if exists project cascade
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: SQL Warning Code: 0, SQLState: 00000
Sep 12, 2022 12:01:57 PM org.hibernate.engine.jdbc.spi.SqlExceptionHelper$StandardWarningHandler logWarning
WARN: table "project" does not exist, skipping
Hibernate: 
    
    create table employee (
       id integer not null,
        name varchar(255),
        primary key (id)
    )
Sep 12, 2022 12:01:57 PM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@795f8317] 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_project (
       Employee_id integer not null,
        projects_projectId integer not null
    )
Hibernate: 
    
    create table project (
       projectId integer not null,
        projectName varchar(255),
        primary key (projectId)
    )
Hibernate: 
    
    alter table if exists employee_project 
       add constraint FK84if10lpuqoq270kixummx39k 
       foreign key (projects_projectId) 
       references project
Hibernate: 
    
    alter table if exists employee_project 
       add constraint FKcfge3ley7bbpogw8dur1p5q35 
       foreign key (Employee_id) 
       references employee
Sep 12, 2022 12:01:57 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
        employee
        (name, id) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        project
        (projectName, projectId) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        project
        (projectName, projectId) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        employee_project
        (Employee_id, projects_projectId) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        employee_project
        (Employee_id, projects_projectId) 
    values
        (?, ?)
Hibernate: 
    select
        e1_0.id,
        e1_0.name 
    from
        employee e1_0
Employee [id=1, name=Krishna]
Project [projectId=11, projectName=Chat server]
Project [projectId=12, projectName=Intelligent Sync]

 

Query the database to confirm DDL.

test=# \d+ employee;
                                                Table "public.employee"
 Column |          Type          | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
--------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
 id     | integer                |           | not null |         | plain    |             |              | 
 name   | character varying(255) |           |          |         | extended |             |              | 
Indexes:
    "employee_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "employee_project" CONSTRAINT "fkcfge3ley7bbpogw8dur1p5q35" FOREIGN KEY (employee_id) REFERENCES employee(id)
Access method: heap

test=# 
test=# 
test=# 
test=# \d+ project;
                                                   Table "public.project"
   Column    |          Type          | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
-------------+------------------------+-----------+----------+---------+----------+-------------+--------------+-------------
 projectid   | integer                |           | not null |         | plain    |             |              | 
 projectname | character varying(255) |           |          |         | extended |             |              | 
Indexes:
    "project_pkey" PRIMARY KEY, btree (projectid)
Referenced by:
    TABLE "employee_project" CONSTRAINT "fk84if10lpuqoq270kixummx39k" FOREIGN KEY (projects_projectid) REFERENCES project(projectid)
Access method: heap

test=# 
test=# 
test=# 
test=# 
test=# \d+ employee_project
                                          Table "public.employee_project"
       Column       |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description 
--------------------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
 employee_id        | integer |           | not null |         | plain   |             |              | 
 projects_projectid | integer |           | not null |         | plain   |             |              | 
Foreign-key constraints:
    "fk84if10lpuqoq270kixummx39k" FOREIGN KEY (projects_projectid) REFERENCES project(projectid)
    "fkcfge3ley7bbpogw8dur1p5q35" FOREIGN KEY (employee_id) REFERENCES employee(id)
Access method: heap

 

Query tables to get the data.

test=# SELECT * FROM employee;
 id |  name   
----+---------
  1 | Krishna
(1 row)

test=# 
test=# 
test=# 
test=# SELECT * FROM project;
 projectid |   projectname    
-----------+------------------
        11 | Chat server
        12 | Intelligent Sync
(2 rows)

test=# 
test=# 
test=# SELECT * FROM employee_project;
 employee_id | projects_projectid 
-------------+--------------------
           1 |                 11
           1 |                 12
(2 rows)

 

You can download this application from this link.


 

 

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