A one-to-one relationships occurs when one entity is related to exactly one occurrence in another entity.

In this tutorial, we show you how to work with one-to-one table relationship in Hibernate, via XML mapping file (hbm).

Tools and technologies used in this tutorials :

  1. Hibernate 3.6.3.Final
  2. MySQL 5.1.15
  3. Maven 3.0.3
  4. Eclipse 3.6

Project Structure

See the final project structure of this tutorial.

one to one project structure

Project Dependency

Get hibernate.jar from JBoss repository, Maven will take care all the related dependencies for you.

File : pom.xml

<project ...>
 
	<repositories>
		<repository>
			<id>JBoss repository</id>
			<url>http://repository.jboss.org/nexus/content/groups/public/</url>
		</repository>
	</repositories>
 
	<dependencies>
 
		<!-- MySQL database driver -->
		<dependency>
			<groupId>mysql</groupId>
			<artifactId>mysql-connector-java</artifactId>
			<version>5.1.15</version>
		</dependency>
 
		<dependency>
			<groupId>org.hibernate</groupId>
			<artifactId>hibernate-core</artifactId>
			<version>3.6.3.Final</version>
		</dependency>
 
		<dependency>
			<groupId>javassist</groupId>
			<artifactId>javassist</artifactId>
			<version>3.12.1.GA</version>
		</dependency>
 
	</dependencies>
</project>

1. “One-to-one” table relationship

A one-to-one relationship table design, a STOCK table contains exactly one record in STOCK_DETAIL table. Both tables have the same Stock_Id as primary key. In STOCK_DETAIL table, Stock_Id is the primary key and also a foreign key to STOCK table. This is the common way of define “one-to-one” table relationship.

one to one relationship

To get the STOCK and STOCK_DETAIL table script, refer to this “one to one table relationship in MySQL” article.

2. Hibernate Model Class

Create two model classes – Stock.java and StockDetail.java, to represent the above tables.

File : Stock.java

package com.mkyong.stock;
 
public class Stock implements java.io.Serializable {
 
	private Integer stockId;
	private String stockCode;
	private String stockName;
	private StockDetail stockDetail;
 
	//constructor & getter and setter methods
 
}

File : StockDetail.java

package com.mkyong.stock;
 
public class StockDetail implements java.io.Serializable {
 
	private Integer stockId;
	private Stock stock;
	private String compName;
	private String compDesc;
	private String remark;
	private Date listedDate;
 
	//constructor & getter and setter methods
 
}

3. Hibernate XML Mapping

Now, create two Hibernate mapping files (hbm) – Stock.hbm.xml and StockDetail.hbm.xml.

File : Stock.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 25 April 2011 7:52:33 PM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
	<class name="com.mkyong.stock.Stock" table="stock" catalog="mkyongdb">
		<id name="stockId" type="java.lang.Integer">
			<column name="STOCK_ID" />
			<generator class="identity" />
		</id>
		<property name="stockCode" type="string">
			<column name="STOCK_CODE" length="10" not-null="true" unique="true" />
		</property>
		<property name="stockName" type="string">
			<column name="STOCK_NAME" length="20" not-null="true" unique="true" />
		</property>
		<one-to-one name="stockDetail" class="com.mkyong.stock.StockDetail"
			cascade="save-update"></one-to-one>
	</class>
</hibernate-mapping>

File : StockDetail.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 25 April 2011 7:52:33 PM by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
	<class name="com.mkyong.stock.StockDetail" table="stock_detail"
		catalog="mkyongdb">
		<id name="stockId" type="java.lang.Integer">
			<column name="STOCK_ID" />
			<generator class="foreign">
				<param name="property">stock</param>
			</generator>
		</id>
		<one-to-one name="stock" class="com.mkyong.stock.Stock"
			constrained="true"></one-to-one>
		<property name="compName" type="string">
			<column name="COMP_NAME" length="100" not-null="true" />
		</property>
		<property name="compDesc" type="string">
			<column name="COMP_DESC" not-null="true" />
		</property>
		<property name="remark" type="string">
			<column name="REMARK" not-null="true" />
		</property>
		<property name="listedDate" type="date">
			<column name="LISTED_DATE" length="10" not-null="true" />
		</property>
	</class>
</hibernate-mapping>
Note
The main difficulty in this one-to-one relationship is ensuring both are assigned the same primary key. In StockDetail.hbm.xml, a special foreign identifier generator is declared, it will know get the primary key value from STOCK table. With constrained=”true”, it ensure the Stock must exists.

4. Hibernate Configuration File

Puts Stock.hbm.xml and StockDetail.hbm.xml in your Hibernate configuration file, and also MySQL connection details.

File : 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>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/mkyongdb</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">password</property>
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="show_sql">true</property>
    <mapping resource="com/mkyong/stock/Stock.hbm.xml" />
    <mapping resource="com/mkyong/stock/StockDetail.hbm.xml" />
</session-factory>
</hibernate-configuration>

5. Run It

Run it, Hibernate will insert a row into the STOCK table and a row into the STOCK_DETAIL table.

File : App.java

package com.mkyong;
 
import java.util.Date;
import org.hibernate.Session;
import com.mkyong.stock.Stock;
import com.mkyong.stock.StockDetail;
import com.mkyong.util.HibernateUtil;
 
public class App {
	public static void main(String[] args) {
		System.out.println("Hibernate one to one (XML mapping)");
		Session session = HibernateUtil.getSessionFactory().openSession();
 
		session.beginTransaction();
 
		Stock stock = new Stock();
 
		stock.setStockCode("4715");
		stock.setStockName("GENM");
 
		StockDetail stockDetail = new StockDetail();
		stockDetail.setCompName("GENTING Malaysia");
		stockDetail.setCompDesc("Best resort in the world");
		stockDetail.setRemark("Nothing Special");
		stockDetail.setListedDate(new Date());
 
		stock.setStockDetail(stockDetail);
		stockDetail.setStock(stock);
 
		session.save(stock);
		session.getTransaction().commit();
 
		System.out.println("Done");
	}
}

Output

Hibernate one TO one (XML mapping)
Hibernate: INSERT INTO mkyongdb.stock (STOCK_CODE, STOCK_NAME) VALUES (?, ?)
Hibernate: INSERT INTO mkyongdb.stock_detail 
(COMP_NAME, COMP_DESC, REMARK, LISTED_DATE, STOCK_ID) VALUES (?, ?, ?, ?, ?)
Done
Hibernate Annotation
For one-to-one in Hibernate annotation, please refer to this example
Download it – Hibernate-one-to-one-xml-mapping.zip (10KB)

Reference

  1. Hibernate Documentation – one to one relationship
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