Entity Class

An entity is a Plain Old Java Object (POJO) whose fields represent a row in a database table.

• An entity MUST have a public (or protected) no-argument constructor.

• Fields must not be marked public or final.

The @Entity annotation is used to declare a class as an entity:

• If your class name matches the table name, JPA will find the table by itself.

If your class name does not match the table name, then identify the table name with the @Table annotation.

You must annotate the field that maps to your primary key using the @Id annotation:

@Entity
public class Dog {
  @Id
  private int id;

  // ...
}

Practice Exercise

@Entity and @Id are the only two required annotations for a valid entity class.

Including Entities In your Persistence Unit

In order for the Persistence Unit to know about the Entities it manages, we need to include them in the persistence.xml within <class></class> tags.

Entities are included with the convention package.class within the Persistence Unit:

<persistence-unit name="PetStore">
  <!-- List Entities Here -->
  <class>entities.Dog</class>

  <!-- ... -->
</persistence-unit>

Drill

• A JPA Entity is the Java equivalent of a SQL table.
• The corresponding table for the Customer entity looks like:

  mysql > desc customer;
  +------------+-------------+------+-----+---------+----------------+
  | Field      | Type        | Null | Key | Default | Extra          |
  +------------+-------------+------+-----+---------+----------------+
  | id         | int(11)     | NO   | PRI | NULL    | auto_increment |
   ...
  +------------+-------------+------+-----+---------+----------------+
  

• Create a new class that represents the customer table as an Entity.

• Name the class Customer and place it in a package named com.example.jpavideostore.entities. For now, add a field for:

  • id

• Annotate the Customer class with @Id and @Entity.

• Generate a no-arg constructor (you must have one in every JPA entity).

• Generate getters and setters for the fields.

• Add the class to your persistence.xml

• NOTE your entity class will need its full package and class name: <class>com.example.jpavideostore.entities.Customer</class>

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