Ans. Sometimes you have two entities and there\'s a relationship between them. For example, you might have an entity called Organization and another entity called Employee.

The Organization entity might have some basic properties such as id, name, address, etc. as well as a property called students:

public class Organization {
private String id;
private String name;
private String address;
private List<employee> employees;

// setters and getters
}
Now when you load a Organization from the database, JPA loads its id, name, and address fields for you. But you have two options for employees: to load it together with the rest of the fields (i.e. eagerly) or to load it on-demand (i.e. lazily) when you call the Organization\'s getEmployees() method.

When a organization has many employees it is not efficient to load all of its employees with it when they are not needed. So in suchlike cases, you can declare that you want employees to be loaded when they are actually needed. This is called lazy loading.

Any time you see a @OneToOne, @OneToMany or @ManyToMany annotations, you’ve got a relationship. What’s important to note is that the fetch type should be specified within those annotations, if you don’t specify one then it defaults to FetchType.LAZY.

Posted on Jul 28, 2014 by Arindam Ghosh