Many to One XML

Introduction

There are different relations that are maintained to establish a link between different database tables in relational database models. There can be different types of such relations. A similar concept is there in Hibernate. 

Hibernate works to link the Java language to the database table. Along with this link,  mappings can be established. These mappings are used to navigate through the database. This mapping is defined in the XML sheet.  

So, in this article, we will discuss the Many-to-One relationship in Hibernate using XML. 

Many-to-One in Hibernate using XML 

The first importance here is that Many-to-One is the same as One-to-Many mapping in Hibernate. 

In Many-to-One mapping, various attributes are referred to as one attribute only.

For this article, we will use an example, where one customer can own many cars and one car can belong to only one customer. Here, Many-to-One mapping can be performed using XML.

Persistent classes for mappings 

There are two persistent classes Customer.java and Cars.java. Customer class contains Cars class reference and vice versa.

Customer.java 

package codingninjas;  
  
public class Customer
{  
	private int customer_id;  
	private String name
	private int phone_numer;  
	private Cars cars;  
	
	// setters and getters methods
}  

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Cars.java

package codingninjas;  
  
public class Cars
{  
	private int car_id;  
	private String state;  
	private int number;  
	private Customer customer;   


	// setters and getters methods
}  

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Mapping files for persistent classes

The two mapping files are customer.hbm.xml and cars.hbm.xml.

customer.hbm.xml 

This is the mapping file for the Customer class, which is a Many-to-One mapping type. 

<?xml version='1.0' encoding='UTF-8'?>  
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 5.3//EN" "http://hibernate.sourceforge.net/hibernate-mapping-5.3.dtd">  
<hibernate-mapping>
	<class name="codingninjas.Customer" table="cust">  
		<id name="customer_id">  
          	<generator class="increment"></generator>    
        </id>  
        <property name="name"></property>  
        <property name="phone_number"></property>  
     </class>         
</hibernate-mapping>  

cars.hbm.xml 

This is the simple mapping file for the Cars class. Note that we are using the generator class.

<?xml version='1.0' encoding='UTF-8'?>  
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 5.3//EN""http://hibernate.sourceforge.net/hibernate-mapping-5.3.dtd">  
  
<hibernate-mapping>  
<class name="codingninjas.cars" table="carst">  
	<id name="car_id">  
		<generator class="increment"></generator>   
	</id>  
	<property name="state"></property>  
	<property name="number"></property>  
                     
	<many-to-one name="customer"></one-to-one>  
</class>          
</hibernate-mapping>    

Configuration File

This file has the information about the database and mapping file. 

hibernate_file.cfg.xml

<?xml version='1.0' encoding='UTF-8'?>    
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 5.3//EN" "http://hibernate.sourceforge.net/hibernate-configuration-5.3.dtd">    
     
<hibernate-configuration>    
    
    <session-factory>    
        <property name="hbm2ddl.auto">update</property>    
        <property name="dialect">org.hibernate.dialect.Oracle9Dialect</property>    
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>    
        <property name="connection.username">system</property>    
        <property name="connection.password">jtp</property>    
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>    
    <mapping resource="cars.hbm.xml"/>    
    <mapping resource="customer.hbm.xml"/>    
    </session-factory>    
    
</hibernate-configuration>   

User classes for storing and fetching the data

Store.java

package codingninjas;    
    
import org.hibernate.*;  
import org.hibernate.boot.Metadata;  
import org.hibernate.boot.MetadataSources;  
import org.hibernate.boot.registry.StandardServiceRegistry;  
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;    
    
public class Store 
{    
	public static void main(String[] args) 
	{    
      
    	StandardServiceRegistry ssr=new StandardServiceRegistryBuilder().configure("hibernate_file.cfg.xml").build();  
    	Metadata meta=new MetadataSources(ssr).getMetadataBuilder().build();  
      
    	SessionFactory factory=meta.getSessionFactoryBuilder().build();  
    	Session session=factory.openSession();  
      
    	Transaction t=session.beginTransaction();   
      
    	Customer cust1=new Customer();    
    	cust1.setName("Abc Def");    
    	cust1.setPhno(999999999);     
        
    	Cars car1=new Cars();    
    	car1.setState("Kerala");    
    	car1.setNumber(7654);     
   
    	Cars car2=new Cars();    
    	car2.setState("Bihar");    
    	car2.setNumber(4321);   
         
   		car1.setCustomer(cust1);    
   		car2.setCustomer(cust1);
        
    	session.persist(car1);  
    	session.persist(car2);   
    	t.commit();    
        
    	session.close();    
    	System.out.println("Done");    
 	}    
}    

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Output

Done

Fetch.java

package codingninjas;    
  
import java.util.*;  
import javax.persistence.TypedQuery;  
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;   
    
public class Fetch 
{    
	public static void main(String[] args) 
	{    
    	StandardServiceRegistry ssr=new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();  
    	Metadata meta=new MetadataSources(ssr).getMetadataBuilder().build();  
      
    	SessionFactory factory=meta.getSessionFactoryBuilder().build();  
    	Session session=factory.openSession();  
        
    	TypedQuery query=session.createQuery("from Customer c");    
    	List<Customer> list=query.getResultList();    
        
    	Iterator<Customer> itr=list.iterator();    
    	while(itr.hasNext())
    	{    
			Customer c1=itr.next();    
     		System.out.println(c1.getCustomerId()+" "+emp.getName()+" "+emp.getNumber());    
     		Cars car1=c1.getCars();    
     		System.out.println(car1.getCarId()+" "+car1.getState()+" "+    
        	car1.getNumber());    
    	}     
    
    	session.close();    
    	System.out.println("Done");    
	}    
}    

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Output 

1 Abc Def 999999999
Kerala 7654
1 Abc Def 999999999
Bihar 4321 
Done 

Must Read:  Java List Iterator