JPA Tutorial - JPA Introduction

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Object-Relational Mapping

The domain model has a class. The database has a table. JPA is a simple way to convert one to the other automatically.

The technique of bridging the gap between the object model and the relational model is known as object-relational mapping, or O-R mapping or simply ORM.

Creating an Entity

Regular Java classes can be transformed into entities by annotating them.

Let's start by creating a regular Java class for an employee.

public class Employee {
  private int id;
  private String name;
//from w  ww  . j a  v  a2  s.  c  om
  public Employee() {
  }

  public Employee(int id) {
    this.id = id;
  }

  public int getId() {
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

This class resembles a JavaBean-style class with two properties: id and name.

Each of these properties is represented by a pair of accessor methods to get and set the property, and is backed by a member field.

To turn Employee into an entity, we first annotate the class with @Entity. It is a marker annotation to indicate to the persistence engine that the class is an entity.

Then we use @Id annotation to mark a field as the primary key.

The following code shows the entity class.

@Entity
public class Employee {
  @Id
  private int id;
  private String name;

  public Employee() {
  }

  public Employee(int id) {
    this.id = id;
  }

  public int getId() {
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

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