Hibernate Assosiations :Mapping Relations

Many to One Relation

Let us take the case of Employee and Company , its a typical case for many to one relation. Many employees can belong to a single Company.

Now this relation is unidirectional

@Entity
public class Employee
{

@Id
@GeneratedValue
@Column(name = "F_EMP_ID")
private int empId;

@ManyToOne(optional = true,fetch=FetchType.EAGER)
@JoinColumn
(name = "F_EMP_CMPNY_ID",referencedColumnName="F_CMPNY_ID")
private Company company;

private String name ;
.....
.....



getter and setters for all the values ...

}

@Id : shows its primary key

@GeneratedValue : asks hibernate to generate the primary keys

@Column : to give a java name for a corresponding DB field name

@ManyToOne : shows the many to one realtionship with field company
<Company>

optional : says if this field can be null for Employee row ( true is the default value)

fetch : whether to eagerly fetch the dependent rows (of company ) when fetching employee

(LAZY if the default value , we can change it to FetchType.EAGER)

@JoinColumn - how to join the column for the realtionship

name database field of the current entity which is used for joining

referencedColumnName data base field of the joining entity used for joining
If this field is not mentioned the primary key of the joing entity is taken hence in the above example we ned not mention this value

The other entity in the relationship

@Entity
public class Company
{

@Id
@GeneratedValue
@Column(name = "F_CMPNY_ID")
private int cmpnyId;

private String cmpnyName ;

.....

getter and setters for all the values ...
}


Now in the above example the mapping is complete .Now we can use the entitymanager to fetch any row of Emlpoye entity
(as showm in the previous blog)

entityManager.find(Employee.class, id) --> gives the corresponding Employee

(Note : This statement will come under the scope of @Transactional which manages the session)

Now we can use Employee.getCompany() -> which gives the coresponding Company value

Note:

This is possible because we used fetch=FetchType.EAGER while mentioning the manytoone relation.This ensure that when the request is made for Employee the corresponding Company is also fetched from the DB.

If this option(fetch=FetchType.EAGER) was not mentioned the default one would be used ,that is (fetch=FetchType.LAZY)

Lazy initilization commands hibernate that when Employee is fetched the corresponding , the coresponding Company is not fetched . This is called lazy initilization.Hibernate tries to fetch Company only when i first request is made to the company field in the Employee instance.


Now if this request for company is made outide the scope of @Transaction , that is when no session is open , hibernate gibes a error saying it could not Lazily initialize Company because of Lazy init.

Hence care should be taken to see to that the first request to the depending Entity(in this Company )in the case of FetchType.EAGER should be made inside a valid session.

There is an alternative to get all the dependent rows , when we use Fetch.LAZY . this is by explicitly initializing hibernate.

//include this in the service which gets employee

Employee e = entityManager.find(Employee.class, id);
//this ensures that corresponding company is also fetched along with employee
Hibernate.initialize(employee.getCompany());
//employee.getCompany() -- i sthe first request to Company
return e;



Now the above relation is Unidirectional , that is employee gets Company .Now what if we want to get all the employees in a Company.

We can achive this by making it Bidirectional

We will make small chages in Company Entity to make it bidirectional

@Entity
public class Company
{

@Id
@GeneratedValue
@Column(name = "F_CMPNY_ID")
private int cmpnyId;

private String cmpnyName ;

@OneToMany(mappedBy = "company" ,fetch =FetchType.EAGER)
private List<Employee> employees;
.....



getter and setters for all the values ...
}

@OneToMany - > indicates that the relationship is onetomany with List of employess , that is this company instance can have list of employees

mappedBy -> indicates that the other side is the owner of the relation.

That is a corresponding relation is defined in Employee entity and the

Employee entity had a member variable of Company by name company

fetch =FetchType.EAGER - > defines the type of fetch (Described above)


Now this relation is bidirectional that is when we fetch a Company insatnce we will get all the employees working for it by

Company c = entityManager.find(Company.class, id);

c.getEmployees(); - will return all the emplyees working in that company



One to One mapping


NO\ow to show a one2one mapping lets take a case of Address Entity

public class Address
{

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


.....

// Getters and Setters ...
}


@Entity
public class Company
{

@Id
@GeneratedValue
@Column(name = "F_CMPNY_ID")
private int cmpnyId;

private String cmpnyName ;

@OneToOne(optional = false, fetch = FetchType.EAGER)
@JoinColumn(name = "F_CMPNY_ID", referencedColumnName = "F_ADDRS_ID")
private Address address;


getter and setters for all the values ...
}


The above relation is similar to the above mentioned many to one relation , and all the attributes for many to one also apply to this.

This relation in Unidirectional and can be made bidirectional by declaring a corresponding relation on Address Entity.

Some Interesting notes on Hibernate

When a primitive value is mapped from a java class to db table column remeber to assign default values to these in the DB ,else when null is
inserted to the column ,and when we retrieve this its gives an error saying null canot be casted to primitive members.Hence for example if null
is present in the DB to retrieve this Wrapper classes such as Integer should be used instead of int.

Though we mention relation(involving primary key) in the classes using annotaions it is also helpful to mention the primary again with indertable and
updatebale equlas false , because in certain cases we need to retrive a row(or certain fileds only) in the db just based on this value and it can be '
hence mentioned in the HQL query directly .

// @Column(name = "F_PRODUCT_CATEGORY_ID" , insertable = false , updatable = false)
// private int pSampleId;\



Still to Follow .....

Many To Many Relation ..

Hibernate Annotations & EntityManager With JPA and Spring

Hibernate is an Open Source Object Relational mapping tool which provides transparent persistence for POJOs. Object-relational mapping is used to map object-oriented programming objects to relational databases managed by Oracle, DB2, Sybase, and other relational database managers (RDBMSs)

Hibernate allows you to express queries in its own portable SQL extension (HQL), as well as in native SQL, or with an object-oriented Criteria.

Other popular ORM solutions are iBatis, JDO and TopLink.

JPA(Java Persistence API)

Java Persistence API was included as part of the Java EE 5 platform. This new API simplifies the development of Java EE and Java SE applications using data persistence. This also gets the entire Java community behind a single, standard persistence API.The Java Persistence API draws upon the best ideas from persistence technologies such as Hibernate, TopLink, and JDO.

Hibernate implements the Java Persistence object/relational mapping and persistence management interfaces with the Hibernate Annotations and Hibernate EntityManager modules, on top of the mature and powerful Hibernate Core.

Since JPA's are used for persistence in the application the ORM providers can be easily changed, in the below example it is easy to replace Hibernate with TopLink , since it will be only configuration changes. This is because we have used JPA's in our entity classes and used entitymanager and we have not stuck to any hibernate specific API's

Hibernate Annotations

Hibernate, like all other object/relational mapping tools, requires metadata that governs the transformation of data from one representation to the other (and vice versa). As an option, you can now use JDK 5.0 annotations for object/relational mapping with Hibernate 3.2. You can use annotations in addition to or as a replacement of XML mapping metadata.

You can use Hibernate extension annotations on top of standardized Java Persistence annotations to utilize all native Hibernate features.

Hence in all the below sample entities you can use of javax.persistence.* classes which are the JPA's used to indicate hibernate about the mapping it has to perform in the Database.

In the basic format of hibernate without annotations,we need to define the mapping between between a class and the Db tables using hibernate config files.

Hibernate EntityManager

It implements:

* The standard Java Persistence management API
* The standard Java Persistence Query Language
* The standard Java Persistence object lifecycle rules
* The standard Java Persistence configuration and packaging

Hibernate EntityManager wraps the powerful and mature Hibernate Core. You can fall back to Hibernate native APIs, native SQL,and native JDBC whenever necessary.

Spring For Hibernate

When used with hibernate this reduces a lot of work like obtaining session for each and every transaction .Instead we mention this using @Transaction annotation that the entitymanger needs to be transaction enabled and spring takes care of managing the session using
AOP.Spring also helps in easy configuraion of the DB and ORM into the entitymanager.

Now since we are familiar with the technologies to be used lets build a sample application

Configuration in spring.xml

Assuming we use mysql as the DB following will be the configuration.

Declaring the Datasource

<bean id="dataSource"
class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost/<DB_NAME>" />
<property name="username" value="<USER_NAME>" />
<property name="password" value="<PASSWORD>" />
</bean>


BeanPostProcessor that processes PersistenceUnit and PersistenceContext annotations, for injection of the corresponding JPA resources EntityManagerFactory and EntityManager. Any such annotated fields or methods in any Spring-managed object will automatically be injected.

This post-processor will inject sub-interfaces of EntityManagerFactory and EntityManager if the annotated fields or methods are declared as such.

PersistenceAnnotationBeanPostProcessor only supports @PersistenceUnit and @PersistenceContext

<bean
class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor" />

This creates a JPA EntityManagerFactory according to JPA's standard container bootstrap contract. This is the most powerful way to set up a shared JPA EntityManagerFactory in a Spring application context

As with LocalEntityManagerFactoryBean, configuration settings are usually read in from a META-INF/persistence.xml config file, residing in the class path, according to the general JPA configuration contract.

The datasource is given as a parameter to the ORM(hibernate) which later if given as a parameter to JPA factory to create the entitymanager.

<bean id="entityManagerFactory"
class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="jpaVendorAdapter">
<bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
<property name="database" value="MYSQL" />
<property name="showSql" value="true" />
</bean>
</property>
</bean>


This defines a transaction manager which is responsible for opening and closing sessions when an entity manager is used.

<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
<property name="entityManagerFactory" ref="entityManagerFactory" />
</bean>


This ensures that session management for all classes/methods marked with @Transactional are taken care by the transaction manager, that is it will take care of opening the sessions and closing them.

<tx:annotation-driven transaction-manager="transactionManager" />

This is a directive to spring to enable annotations.

<context:annotation-config/>

JPA configuration

1. Create a folder named "META-INF" under the "src" folder.
2. Create a file named "persistence.xml" under the "META-INF" folder and set its content to:

persistence.xml

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
version="1.0">
<persistence-unit name="punit">
</persistence-unit>
</persistence>

Now lets create a sample class in Java(Class mapping to DB table)

package test;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

@Entity
@Table(name = "T_STUDENT")
public class Student {
@Id
@GeneratedValue
@Column(name = "F_ID")
private Integer id;
private String lastName;
private String firstName;

public String getFirstName() {
return firstName;
}

public void setFirstName(String firstName) {
this.firstName = firstName;
}

public String getLastName() {
return lastName;
}

public void setLastName(String lastName) {
this.lastName = lastName;
}

public Integer getId() {
return id;
}

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

in the above class

@Entity indicates that this class will be mapped to a table in database

@Table indicates the corresponding table name in DB , if this is not mentioned the default class name will be taken

@Id Indicates the primary key to the table

@GeneratedValue - Indicates the strategy used to generate primary key for the table

@Column - indicates the corresponding column name for the filed in the DB.


Now Lets write a service to access these

Interface

package test;

import java.util.List;

public interface IStudentService {

public List<Student> findAll();

public void save(Student student);

public void remove(int id);

public Student find(int id);
}


Implementation

package test;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

import org.springframework.transaction.annotation.Transactional;


@Transactional
public class StudentService implements IStudentService {


private EntityManager em;

//This is injected by spring
@PersistenceContext
public void setEntityManager(EntityManager em) {
this.em = em;
}


//gets all the student tuples from the DB
@SuppressWarnings("unchecked")
public List<Student> findAll() {
Query query = getEntityManager().createQuery("select s FROM Student s");
return query.getResultList();
}


//saves a touple int DB , if its already exists will update it
public void save(Student student) {
if (student.getId() == null) {
// new
em.persist(student);
} else {
// update
em.merge(student);
}
}

//deletes a touple from teh DB
public void remove(int id) {
Student student = find(id);
if (person != null) {
em.remove(student);
}
}

private EntityManager getEntityManager() {
return em;
}

//gets a tuple based on the primarykey
public Student find(int id) {
return em.find(Student.class, id);
}

}


Now the above service can be injected using spring to any action class and used to do all the required DB operations.