Entity Manager

---

layout: default title: EntityManager

---

Use an EntityManager to retrieve or store an entity in the database.

• An EntityManager is an interface provided by the javax.persistence class, which is used to interact with the Persistence Unit's Persistence Context.

persistence context

The collection of entities known to the persistence unit.

• What this means to you is, if you want to interact with a managed entity (data persisted in SQL), you will use the EntityManager to do so.

In Java SE, you must use an EntityManagerFactory to obtain the EntityManager.

public class CustomerClient {
  public static void main(String[] args) {
    EntityManagerFactory emf =
      Persistence.createEntityManagerFactory("VideoStore");
    EntityManager em = emf.createEntityManager();
  }
}

• Use the Persistence class to get the factory for the named persistence unit.

Use EntityManager's find() method to retrieve an entity instance loaded with data from the database.

public class CustomerClient {
  public static void main(String[] args) {
    EntityManagerFactory emf =
      Persistence.createEntityManagerFactory("VideoStore");
    EntityManager em = emf.createEntityManager();

    // returns a Customer entity where id = 1
    Customer cust = em.find(Customer.class, 1);
    /*
      Behind the scenes, the 'em.find(Customer.class, 1);' is translated into a
      SQL query like:

        SELECT * FROM Customer WHERE id = 1;

      The returned data is then returned as an Object, and cast to the indicated
      e.g. Customer.class
    */
  }
}

• Pass the expected datatype as a Class literal in the first argument and the primary key value as the second argument.

Be sure to close an EntityManager that is obtained from an EntityManagerFactory.

• Also, close the EntityManagerFactory if you obtained it from the Persistence class.

public class CustomerClient {
  public static void main(String[] args) {
    EntityManagerFactory emf =
      Persistence.createEntityManagerFactory("VideoStore");
    EntityManager em = emf.createEntityManager();

    Customer cust = em.find(Customer.class, 1);

    // No memory leaks...
    em.close();
    emf.close();
  }
}

Drill

• Create a new package called com.example.jpavideostore.client and in that package create a class called CustomerClient.

  • Add a main method to your new class and add code to find a Customer using an EntityManager.

  • Print out the returned Customer.

  • If everything went well, you will see the output from your Customer’s toString implementation.

---

Prev -- Up -- Next