JPA EntityManager Interface with Example

Overview

A connection to a database is represented by an EntityManager instance, which also provides functionality for performing operations on a database. Many applications require multiple database connections during their lifetime. For instance, in a web application, it is common to establish a separate database connection, using a separate EntityManager instance, for every HTTP request.

An EntityManager instance is associated with a persistence context. A persistence context is a set of entity instances in which for any persistent entity identity there is a unique entity instance. Within the persistence context, the entity instances and their lifecycle are managed.

The EntityManager API is used to create and remove persistent entity instances, to find entities by their primary key, and to query over entities.

The set of entities that can be managed by a given EntityManager instance is defined by a persistence unit. A persistence unit defines the set of all classes that are related or grouped by the application, and which must be colocated in their mapping to a single database.

EntityManager Interface - Class Diagram

The below class diagram shows a list of methods that EntityManager Interface provides.

EntityManager Interface Example

EntityManager Interface provides persist() method to store an entity into a database. In this example, we will demonstrate the usage of EntityManager Interface via persisting an entity into a database.

Steps to persist an entity object

Step 1: Creating an entity manager factory object

The EntityManagerFactory interface present in java.persistence package is used to provide an entity manager.

EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("PERSISTENCE");

• Persistence - The Persistence is a bootstrap class which is used to obtain an EntityManagerFactory interface.
• createEntityManagerFactory() method - The role of this method is to create and return an EntityManagerFactory for the named persistence unit. Thus, this method contains the name of the persistence unit passed in the Persistence.xml file.

Step 2: Obtaining an entity manager from a factory

EntityManager entityManager = entityManagerFactory.createEntityManager();

• EntityManager - An EntityManager is an interface
• createEntityManager() method - It creates new application-managed EntityManager

Step 3: Intializing an entity manager.

entityManager.getTransaction().begin();

• getTransaction() method - This method returns the resource-level EntityTransaction object.
• begin() method - This method is used to start the transaction.

Step 4: Persisting a data into the relational database.

entityManager.persist(student); 

• persist() - This method is used to make an instance managed and persistent. An entity instance is passed within this method.

Step 5: Closing the transaction

entityManager.getTransaction().commit();

Step 6: Releasing the factory resources

entityManager.close();
entityManagerFactory.close();

• close() - This method is used to releasing the factory resources.

EntityManager Interface Complete Example

private static void insertEntity() {
    EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("PERSISTENCE");
    EntityManager entityManager = entityManagerFactory.createEntityManager();
    entityManager.getTransaction().begin();

    Student student = new Student("Ramesh", "Fadatare", "rameshfadatare@javaguides.com");
    entityManager.persist(student);
    entityManager.getTransaction().commit();
    entityManager.close();
    entityManagerFactory.close();
}