How to do it…

To provide hibernate mapping based on XML, perform the following steps:

How it works…

Here, <hibernate-mapping> is a root element that contains all the <class> elements. The <class> tag contains the following attributes:

The <id> tag defines the primary key column for the database table. It contains the following attributes:

Usually, we create a mapping file called hbm (hibernate mapping). It is a normal XML schema file that contains custom hibernate XML tags. This helps the hibernate engine to map the class to the table and the class field to the table column, along with the given attributes.

All the mapping definitions for hibernate are bundled under the <hibernate-mapping> ... </hibernate-mapping> tag. In <hibernate-mapping> ... </hibernate-mapping>, we can add any number of class-to-table mapping definitions.

There's more…

Now, let's create the XML mapping for the POJO having a reference with another POJO. Here, we will create two different mapping files. To achieve this using an XML-based mapping, we have to create different class mappings for each POJO that has a dependency.

The following is a code that represents the mapping for the Department class. The mapping is in the Department.hbm.xml file:

…
<hibernate-mapping>
  <class name="Department" table="department">
    <id name="id" type="long" column="id">
      <generator class="auto" />
    </id>

    <property column="deptName" name="deptName" />
    <!-- other properties mapping -->
  </class>  
</hibernate-mapping>
…

Next, we will create a mapping for the Employee class. Its definition is present in the Employee.hbm.xml file:

…
<hibernate-mapping>
  <class="Employee" table="employee">
    <id name="id" type="long" column="id">
      <generator class="auto" />
    </id>

    <property column="firstName" name="firstName" />
    <property column="salary" name="salary" />
 <many-to-one name="department" class="Department" >
 <column name="department"/>
 </many-to-one>
    <!-- other properties mapping-->
  </class>
</hibernate-mapping>
…

In the preceding example, we mapped the Department entity with the Employee entity. This will refer to the department column in the employee table. This means that it will create a foreign key that is referenced to the department table.

Here, we will use the <many-to-one> relationship, which means that either many employees are connected with one department, or one department is used by many employees.

The properties are as follows:

The two possible options for lazy are true and false. In our example, Employee is a parent class, whereas Department is a child of the Employee class. Now, while fetching, if we set lazy as true, it means that it will only fetch employee records. No child records will be fetched with Employee, and hibernate will use a separate query if we try to access a child record, which is employee.getDepartment(). If we set lazy as false, hibernate will fetch the child records with the parent, which means that the department information will also be fetched, along with that of the employee. Hibernate will use a join query to fetch the child records.