Hibernate mapping file

Map the file

An object/relationship map is generally defined in an XML file. T he mapping file indicates how Hibernate corresponds a defined class or group of classes to a table in the database.

Although some Hibernate users choose to write XML files by hand, there are many tools you can use to generate mapping files for advanced Hibernate users. S uch tools include XDoclet, Middlegen, and AndroMDA.

Let's consider our previously defined POJO class, where the objects are extended to the table defined in the 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;
    }
}

For every object you want to provide persistence, you need a table to match it. C onsider that the above objects need to be stored and retrieved into the following RDBMS tables:

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 these two entities, we can define the following mapping files to indicate how Hibernate matches defined classes or groups of classes to 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'll need to save the mapping file in the format <classname>.hbm.xml W e save the mapping file in the Employee.hbm .xml file. L et's take a look at some of the labels used in the mapping file in detail:

• The mapping file is an XML file with the root element of the <hibernate-mapping> which <class>
• <class> a specific map from a Java class to a database table. The class name of Java is represented by the name property, and the database is represented by the table property.
• <meta> can be used to decorate classes.
• <id> ID property in the class with the primary key in the database table. T he name property in the id element refers to the nature of the class, and the column property refers to the column of the database table name. The type property holds the type of Hibernate map, which is converted from Java to SQL data type.
• The label in the id <generator> generate primary key values. Setting the class property in the generator label sets the native so that Hibernate can use the identity, sequence, or hilo algorithm to create primary keys based on the underlying database.
• <property> of the Java class to the columns of the database table. T he name property in the label refers to the nature of the class, and the column property refers to the columns of the database table. The type property holds the type of Hibernate map, which is converted from Java to SQL data type.

There are other properties and elements available in the mapping file, and I'll cover as much as possible in other topics related to Hibernate.