Tags: Hibernate , one-to-many

Before we see the complete code, lets first understand how to define relationship in code.

Country.java, State.java


Source code (Country.java)

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.OneToOne;
import javax.persistence.Table;

/**
 * @author javaQuery
 * @date 11th April, 2017
 * @Github: https://github.com/javaquery/Examples
 */
@Entity
@Table(name = "country")
public class Country implements Serializable {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long id;

    @Column(name = "name")
    private String name;

    @OneToMany(mappedBy = "country" /*, fetch = FetchType.LAZY*/)
    private Set<State> states = new HashSet<State>();
    
    //getter-setter
}

Source code (State.java)

import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

/**
 * @author javaQuery
 * @date 15th June, 2017
 * @Github: https://github.com/javaquery/Examples
 */
@Entity
@Table(name = "state")
public class State implements Serializable{

    @Id
    @GeneratedValue
    @Column(name = "id")
    private Long id;
    
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "country_id")
    private Country country;

    @Column(name = "name")
    private String name;
    
    //getter-setter
}

Source code (OneToManyMappingExample.java)

import com.javaquery.bean.Country;
import com.javaquery.bean.State;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/**
 * Hibernate one to many example.
 * 
 * @author javaQuery
 * @date 15th June, 2017
 * @Github: https://github.com/javaquery/Examples
 */
public class OneToManyMappingExample {

    public static void main(String[] args) {
        try {
            /* Create hibernate configuration. */
            Configuration configuration = new Configuration();
            configuration.configure("com\\javaquery\\database\\hibernate\\hibernate.cfg.xml");

            /* Open session and begin database transaction for database operation. */
            SessionFactory sessionFactory = configuration.buildSessionFactory();
            Session session = sessionFactory.openSession();

            Country country = session.load(Country.class, 1L);
            if(!country.getStates().isEmpty()){
                for (State state: country.getStates()) {
                    System.out.println(state.getName());
                }
            }else{
                System.out.println("No states found!");
            }
            
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output

Hibernate: 
    select
        country0_.id as id1_1_0_,
        country0_.name as name2_1_0_ 
    from
        country country0_ 
    where
        country0_.id=?
Hibernate: 
    select
        states0_.country_id as country_3_3_0_,
        states0_.id as id1_3_0_,
        states0_.name as name2_3_1_ 
    from
        state states0_ 
    where
        states0_.country_id=?
  
Banglore
Gujarat
Mumbai

Tags: Hibernate , one-to-many