Let's consider one scenario. We have an Employee and Department relationship where each employee has only one department, but each department can be used multiple times for different employees. Add the following code to the respective files:

Source file: Employee.java

@Entity
@Table
public class Employee{

    @Id
    @GeneratedValue
    private long id;

    @Column
    private String name;

    @ManyToOne
    @JoinColumn
    private Department department;

    // getters and setters

}

Source file: Department.java

@Entity
@Table
public class Department{

    @Id
    @GeneratedValue
    private long id;

    @Column
    private String name;

    // getters and setters

}

Now, using the criteria of the Employee class, we want to access a field name of the Department class. The following code shows the same.

First, we will take a look at what would happen if we did not use an alias. Execute the following code:

Criteria criteria = session.createCriteria(Employee.class);
criteria.add(Restrictions.eq("department.name", "account"));
List list = criteria.list();

The preceding code throws an error similar to could not resolve property: department.name of: Employee.

Now, to resolve the error, we need to create an alias for the Department class. Execute the following code:

/* Line 1 */ Criteria criteria = session.createCriteria(Employee.class);
/* Line 2 */ criteria.createAlias("department", "dept");
/* Line 3 */ criteria.add(Restrictions.eq("dept.name", "account"));
/* Line 4 */ List list = criteria.list();

Now it will work as expected.

Here, in Line 2, we created an alias in the second code snippet. The createAlias(String, String) method accepts both the parameters as string: the first is the instance variable of the referenced class (here, the Department class), and the other is an alias name (here, dept), which will be used further.

In Line 3, we used the alias dept to refer the name field of the Department class using the dept.name code.

Line 4 actually makes the call to the database to get the data.

Let's consider a department that also refers to another class called Location. In this case, let's take a look at how to refer to an attribute of the Location class using the criteria of Employee. Here, we will create a three-level hierarchy to make the example a bit more complex. Add the following code to the respective files:

Source file: Location.java

@Entity
@Table
public class Location{
  @Id
  @GeneratedValue
  private long id;

  @Column
  private String name;
  // getters and setters

}

Source file: Department.java

@Entity
@Table
public class Department{
  @Id
  @GeneratedValue
  private long id;
  
  @Column
  private String name;
  
  @ManyToOne
  @JoinColumn
  private Location location;
  // getters and setters

}

Source file: Employee.java

@Entity
@Table
public class Employee{
  @Id
  @GeneratedValue
  private long id;
  
  @Column
  private String name;
  
  @ManyToOne
  @JoinColumn
  private Department department;
  // getters and setters
  
}

Let's directly go to the code side:

/* Line 1 */ Criteria criteria = session.createCriteria(Employee.class);

/* Line 3 */ criteria.createAlias("department", "dept");
/* Line 4 */ criteria.createAlias("dept.location", "loc");

/* Line 5 */ criteria.add(Restrictions.eq("loc.name", "AHD"));
List list = criteria.list();

Here, we created a chain of aliases to refer to the subclasses. In Line 3, we created an alias of the Department class, which is dept, and in Line 4, we created an alias, "loc", which refers to the location class using the previous alias, "dept".