Lesson 3	Creating Entities with JPA
Objective	Learn how to create and persist entities in JPA, and retrieve them by primary key.

Creating Entities in JPA

In JPA, creating an entity instance is straightforward. Instead of EJB-specific lifecycle methods like ejbCreate(), you define a plain Java class annotated with @Entity and use EntityManager.persist() to save it to the database.

Persisting an Entity

Suppose you have a Customer entity with a primary key of type Long. You create a new instance, set its fields, and call persist() inside a transaction.


@Entity
public class Customer {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    // getters, setters, constructors
}


@Transactional
public void createCustomer(String name) {
    Customer customer = new Customer();
    customer.setName(name);
    em.persist(customer);
    // customer.id is generated after flush/commit
}

The persistence provider automatically inserts the row into the database and manages the primary key generation.

Finding an Entity by Primary Key

In modern JPA, you retrieve an entity using EntityManager.find():


Customer customer = em.find(Customer.class, 123L);
if (customer != null) {
    // Found
} else {
    // Not found
}

This replaces legacy ejbFindByPrimaryKey() methods and avoids manual JDBC code for lookups.

Lifecycle Hooks

JPA offers lifecycle callbacks like @PrePersist and @PostPersist to perform logic before and after saving an entity. These replace ejbPostCreate() in legacy EJB code.


@PrePersist
public void beforeInsert() {
    System.out.println("About to insert customer: " + name);
}