An Object/relational mapping are usually defined in an XML document. This mapping file instructs Hibernate — how to map the defined class or classes to the database tables?
Though many Hibernate users choose to write the XML by hand, but a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA for the advanced Hibernate users.
Let us consider our previously defined POJO class whose objects will persist in the table defined in next section.

public class Employee {
private int id;
private String firstName; 
private String lastName; 
private int salary;

public Employee() {}

public Employee(String fname, String lname, int salary) {
this.firstName = fname;
this.lastName = lname;
this.salary = salary;
}

public int getId() {
return id;
}

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

public String getFirstName() {
return firstName;
}

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

public String getLastName() {
return lastName;
}

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

public int getSalary() {
return salary;
}

public void setSalary( int salary ) {
this.salary = salary;
}
}

There would be one table corresponding to each object you are willing to provide persistence. Consider above objects need to be stored and retrieved into the following RDBMS table −

create table EMPLOYEE (
id INT NOT NULL auto_increment,
first_name VARCHAR(20) default NULL,
last_name VARCHAR(20) default NULL,
salary INT default NULL,
PRIMARY KEY (id)
);

Based on the two above entities, we can define following mapping file, which instructs Hibernate how to map the defined class or classes to the database tables.

<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name = "Employee" table = "EMPLOYEE">

<meta attribute = "class-description">
This class contains the employee detail. 
</meta>

<id name = "id" type = "int" column = "id">
<generator class="native"/>
</id>

<property name = "firstName" column = "first_name" type = "string"/>
<property name = "lastName" column = "last_name" type = "string"/>
<property name = "salary" column = "salary" type = "int"/>

</class>
</hibernate-mapping>

You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml.
Let us see understand a little detail about the mapping elements used in the mapping file −
• The mapping document is an XML document having <hibernate-mapping> as the root element, which contains all the <class> elements.
• The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
• The <meta> element is optional element and can be used to create the class description.
• The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
• The <generator> element within the id element is used to generate the primary key values automatically. The class attribute of the generator element is set to native to let hibernate pick up either identity, sequence, or hilo algorithm to create primary key depending upon the capabilities of the underlying database.
• The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
There are other attributes and elements available, which will be used in a mapping document and I would try to cover as many as possible while discussing other Hibernate related topics.

Hibernate – Mapping Types

When you prepare a Hibernate mapping document, you find that you map the Java data types into RDBMS data types. The types declared and used in the mapping files are not Java data types; they are not SQL database types either. These types are called Hibernate mapping types, which can translate from Java to SQL data types and vice versa.
This chapter lists down all the basic, date and time, large object, and various other builtin mapping types.

Primitive Types

Mapping type	Java type	ANSI SQL Type
integer	int or java.lang.Integer	INTEGER
long	long or java.lang.Long	BIGINT
short	short or java.lang.Short	SMALLINT
float	float or java.lang.Float	FLOAT
double	double or java.lang.Double	DOUBLE
big_decimal	java.math.BigDecimal	NUMERIC
character	java.lang.String	CHAR(1)
string	java.lang.String	VARCHAR
byte	byte or java.lang.Byte	TINYINT
boolean	boolean or java.lang.Boolean	BIT
yes/no	boolean or java.lang.Boolean	CHAR(1) (‘Y’ or ‘N’)
true/false	boolean or java.lang.Boolean	CHAR(1) (‘T’ or ‘F’)

Date and Time Types

Mapping type	Java type	ANSI SQL Type
date	java.util.Date or java.sql.Date	DATE
time	java.util.Date or java.sql.Time	TIME
timestamp	java.util.Date or java.sql.Timestamp	TIMESTAMP
calendar	java.util.Calendar	TIMESTAMP
calendar_date	java.util.Calendar	DATE

Binary and Large Object Types

Mapping type	Java type	ANSI SQL Type
binary	byte[]	VARBINARY (or BLOB)
text	java.lang.String	CLOB
serializable	any Java class that implements java.io.Serializable	VARBINARY (or BLOB)
clob	java.sql.Clob	CLOB
blob	java.sql.Blob	BLOB

JDK-related Types

Mapping type	Java type	ANSI SQL Type
class	java.lang.Class	VARCHAR
locale	java.util.Locale	VARCHAR
timezone	java.util.TimeZone	VARCHAR
currency	java.util.Currency	VARCHAR

Hibernate – O/R Mappings

So far, we have seen very basic O/R mapping using hibernate, but there are three most important mapping topics, which we have to learn in detail.
These are −
• Mapping of collections,
• Mapping of associations between entity classes, and
• Component Mappings.

Collections Mappings

If an entity or class has collection of values for a particular variable, then we can map those values using any one of the collection interfaces available in java. Hibernate can persist instances of java.util.Map, java.util.Set, java.util.SortedMap, java.util.SortedSet, java.util.List, and any array of persistent entities or values.

Sr.No.	Collection type & Mapping Description
1	java.util.Set This is mapped with a <set> element and initialized with java.util.HashSet
2	java.util.SortedSet This is mapped with a <set> element and initialized with java.util.TreeSet. The sort attribute can be set to either a comparator or natural ordering.
3	java.util.List This is mapped with a <list> element and initialized with java.util.ArrayList
4	java.util.Collection This is mapped with a <bag> or <ibag> element and initialized with java.util.ArrayList
5	java.util.Map This is mapped with a <map> element and initialized with java.util.HashMap
6	java.util.SortedMap This is mapped with a <map> element and initialized with java.util.TreeMap. The sort attribute can be set to either a comparator or natural ordering.

Arrays are supported by Hibernate with <primitive-array> for Java primitive value types and <array> for everything else. However, they are rarely used, so I am not going to discuss them in this tutorial.
If you want to map a user defined collection interfaces, which is not directly supported by Hibernate, you need to tell Hibernate about the semantics of your custom collections, which is not very easy and not recommend to be used.

Association Mappings

The mapping of associations between entity classes and the relationships between tables is the soul of ORM. Following are the four ways in which the cardinality of the relationship between the objects can be expressed. An association mapping can be unidirectional as well as bidirectional.

Sr.No.	Mapping type & Description
1	Many-to-One Mapping many-to-one relationship using Hibernate
2	One-to-One Mapping one-to-one relationship using Hibernate
3	One-to-Many Mapping one-to-many relationship using Hibernate
4	Many-to-Many Mapping many-to-many relationship using Hibernate

Component Mappings

It is very much possible that an Entity class can have a reference to another class as a member variable. If the referred class does not have its own life cycle and completely depends on the life cycle of the owning entity class, then the referred class hence therefore is called as the Component class.
The mapping of Collection of Components is also possible in a similar way just as the mapping of regular Collections with minor configuration differences. We will see these two mappings in detail with examples.

Sr.No.

Mapping type & Description

Component Mappings

Mapping for a class having a reference to another class as a member variable.

So, this brings us to the end of blog. This Tecklearn ‘Mapping in Hibernate’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Hibernate and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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Java Collections

• Wrapper Classes and Inner Classes: Integer, Character, Boolean, Float etc
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Java Database Connectivity (JDBC)

• Introduction to SQL: Connect, Insert, Update, Delete, Select
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• Batch Processing Transaction
• Management: Commit and Rollback

Java Enterprise Edition – Servlets

• Introduction to J2EE
• Client Server architecture
• URL, Port Number, Request, Response
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• Servlet fundamentals
• Setting up a web project in Eclipse
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• GET and POST request in web application with demo
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• Fundamentals of Java Server Page
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• Servlet Chaining
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Hibernate

• Introduction to Hibernate
• Introduction to ORM
• ORM features
• Hibernate as an ORM framework
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• Setting up a project with Hibernate framework
• Basic APIs needed to do CRUD operations with Hibernate
• Hibernate Architecture

POJO (Plain Old Java Object)

• POJO (Plain Old Java Object)
• Persistent Objects
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