Lesson 5	Finding by Primary Key
Objective	Use the primary key to retrieve a persistent entity with JPA.

Finding by Primary Key

In modern applications, entity beans have been replaced by the Java Persistence API (JPA). The recommended way to retrieve an entity is with the EntityManager.find() method. This method takes the entity class and its primary key and returns the managed entity instance or null if no match is found.

Java EE 8 (JPA 2.2)

In Java EE 8, the JPA API resides in the javax.persistence package. Here’s how you can find a Customer entity by its primary key:


  import javax.persistence.EntityManager;
  import javax.persistence.PersistenceContext;

  @Stateless
  public class CustomerService {

      @PersistenceContext
      private EntityManager em;

      public Customer findCustomerById(Long id) {
          return em.find(Customer.class, id);
      }
  }

Jakarta EE 10 (JPA 3.1)

Jakarta EE 10 moved JPA to the jakarta.persistence package. The functionality is the same, but the imports change:


  import jakarta.persistence.EntityManager;
  import jakarta.persistence.PersistenceContext;
  import jakarta.ejb.Stateless;

  @Stateless
  public class CustomerService {

      @PersistenceContext
      private EntityManager em;

      public Customer findCustomerById(Long id) {
          return em.find(Customer.class, id);
      }
  }

The following two diagrams illustrate how entity retrieval evolved in Java:

The legacy findByPrimaryKey() EJB call chain used in J2EE applications.
The streamlined JPA EntityManager.find() flow used in Java EE 8 and Jakarta EE 10.

Legacy EJB `findByPrimaryKey()` Call Chain

JPA `EntityManager.find()` Flow (Java EE 8 / Jakarta EE 10)

JPA EntityManager.find flow: client EntityManager.find → persistence context cache → database (if needed) → managed entity or null — The modern JPA approach calls `EntityManager.find()`, which checks the persistence context first and queries the database if necessary, returning a managed entity instance or `null`.

How It Works

EntityManager.find() queries the persistence context first, then the database if necessary.
If the entity is found, it becomes a managed instance within the persistence context.
If not found, the method returns null.
Unlike legacy Entity Beans, you don’t need to handle FinderException — just check for null.

Example Usage


  Customer customer = customerService.findCustomerById(1001L);
  if (customer != null) {
      System.out.println("Found: " + customer.getName());
  } else {
      System.out.println("Customer not found.");
  }

Beyond Primary Keys

In addition to find(), JPA provides query mechanisms such as JPQL and the Criteria API to locate entities by non-primary key attributes. These will be covered in later modules.

Finding Entities - Quiz

Click the quiz link below to test your understanding of using JPA to find entities.
Finding Entities - Quiz