Many to Many Mapping : Many to Many Mapping « Hibernate « Java Tutorial

1. Java Tutorial
2. Hibernate
3. Many to Many Mapping

File: Main.java

import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;

public class Main {
  public static void main(String[] args) throws Exception {
    HibernateUtil hibernateUtil = new HibernateUtil();
    hibernateUtil.executeSQLCommand("create table event(event_id int,event_date date, title varchar)");
    hibernateUtil.executeSQLCommand("create table person(person_id int,firstname varchar, lastname varchar,age int)");
    hibernateUtil.executeSQLCommand("create table PERSON_EVENT(event_id int, person_id int)");
    Session session = hibernateUtil.getSession();
    
    Event event1 = new Event();
    event1.setTitle("event1");
    Event event2 = new Event();
    event2.setTitle("event2");
    
    Person person1 = new Person();
    person1.setFirstname("person1");
    Person person2 = new Person();
    person2.setFirstname("person2");
    
    person1.getEvents().add(event1);
    person1.getEvents().add(event2);
    
    person2.getEvents().add(event1);
    person2.getEvents().add(event2);
    
    session.save(event1);
    session.save(event2);
    
    session.save(person1);
    session.flush();
    
    person1.getEvents().clear();
    
    session.save(person1);
    session.flush();
    
    session.close();
    hibernateUtil.checkData("select * from event");
    hibernateUtil.checkData("select * from person");
    hibernateUtil.checkData("select * from PERSON_EVENT");
  }
}

File: Person.java

import java.util.HashSet;
import java.util.Set;

public class Person {

    private Long id;
    private int age;
    private String firstname;
    private String lastname;

    public Person() {}

    private Set events = new HashSet();

    public Set getEvents() {
        return events;
    }

    public void setEvents(Set events) {
        this.events = events;
    }

    public int getAge() {
      return age;
    }

    public void setAge(int age) {
      this.age = age;
    }

    public String getFirstname() {
      return firstname;
    }

    public void setFirstname(String firstname) {
      this.firstname = firstname;
    }

    public Long getId() {
      return id;
    }

    public void setId(Long id) {
      this.id = id;
    }

    public String getLastname() {
      return lastname;
    }

    public void setLastname(String lastname) {
      this.lastname = lastname;
    }

}

File: Person.hbm.xml

<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 2.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>

 <class name="Person" table="person">
        <id name="id" column="person_id">
            <generator class="increment"/>
        </id>
        <property name="age"/>
        <property name="firstname"/>
        <property name="lastname"/>
      <set name="events" table="PERSON_EVENT">
          <key column="PERSON_ID"/>
          <many-to-many column="EVENT_ID" class="Event"/>
      </set>
        
    </class>



</hibernate-mapping>

File: Event.java

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

public class Event {
    private Long id;

    private String title;
    private Date date;

    private Set persons = new HashSet();
    
    public Event() {}

    public Long getId() {
        return id;
    }

    private void setId(Long id) {
        this.id = id;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public Set getPersons() {
      return persons;
    }

    public void setPersons(Set persons) {
      this.persons = persons;
    }
    
}

File: Event.hbm.xml

<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 2.0//EN" 
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>

    <class name="Event" table="event">
        <id name="id" column="event_ID">
            <generator class="increment"/>
        </id>
        <property name="date" type="timestamp" column="event_date"/>
        <property name="title"/>
        
      <set name="persons" table="PERSON_EVENT">
          <key column="PERSON_ID"/>
          <many-to-many column="EVENT_ID" class="Event"/>
      </set>        
    </class>

</hibernate-mapping>

File: HibernateUtil.java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
  Session session;
  Statement st;
  Configuration config;
  
  public HibernateUtil() throws Exception{
    config = new Configuration().configure();

    
    SessionFactory sessionFactory = config.buildSessionFactory();
    session = sessionFactory.openSession();

    // Load the JDBC driver.
    Class.forName("org.hsqldb.jdbcDriver");
    System.out.println("Driver Loaded.");
    // Establish the connection to the database.
    String url = "jdbc:hsqldb:data/tutorial";

    Connection conn = DriverManager.getConnection(url, "sa", "");
    System.out.println("Got Connection.");
    st = conn.createStatement();
  }
  public Configuration getConfiguration(){
    return config;
  }
  public Session getSession(){
    return session;
  }
  public void executeSQLCommand(String sql) throws Exception {
    st.executeUpdate(sql);
  }

  public void checkData(String sql) throws Exception {
    ResultSet rs = st.executeQuery(sql);
    ResultSetMetaData metadata = rs.getMetaData();

    for (int i = 0; i < metadata.getColumnCount(); i++) {
      System.out.print("\t"+ metadata.getColumnLabel(i + 1)); 
    }
    System.out.println("\n----------------------------------");

    while (rs.next()) {
      for (int i = 0; i < metadata.getColumnCount(); i++) {
        Object value = rs.getObject(i + 1);
        if (value == null) {
          System.out.print("\t       ");
        } else {
          System.out.print("\t"+value.toString().trim());
        }
      }
      System.out.println("");
    }
  }
}

File: hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- Database connection settings -->
        <property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
        <property name="connection.url">jdbc:hsqldb:data/tutorial</property>
        <property name="connection.username">sa</property>
        <property name="connection.password"></property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.HSQLDialect</property>
        <!-- Enable Hibernate's current session context -->
        <property name="current_session_context_class">org.hibernate.context.ManagedSessionContext</property>

        <property name="hibernate.cache.use_second_level_cache">false</property>
        <property name="hibernate.cache.use_query_cache">false</property>
        
        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>        
        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>
        
        <!-- Mapping files -->
        <mapping resource="Event.hbm.xml"/>
        <mapping resource="Person.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

21.6.Many to Many Mapping
	21.6.1.	Many to Many Mapping
	21.6.2.	Many To Many Mapping Cascade Delete
	21.6.3.	Many To Many Mapping: set collection