What does entity manager means???
If i am a naive programmer i could simply interpret something which manages entity & Indeed it means the same.
An Entity Manager is been instantiated with the help of an Entity Manager Factory. A connection to a database is managed by the entity manager i.e. it provides functionality for performing operations on a database. Therefore we could say if an application needs multiple database connections, an EntityManagerFactory will be constructed for a specific database which provides an efficient way to construct multiple EntityManager instances(if required, even single instance of entity manager can the job depending upon the requirement you may opt for multiple instances) for that database required for every HTTP request. We shall understand this with the help of an example. Suppose we have a
Database : A , having a relational tables B and C.
So for A, an instance of entity manager factory will be instantiated. Now we if ever want to perform any update to table B & lets say delete operation for table C, either two different entity manager could be instantiated or same the entity manager instance can be utilized for both.
The instantiation of Entity Manager Factory itself is considered to be less efficient but since it's a one time activity therefore it's manageable task because Entity Manager Factory once instantiated, it will serve the entire application
The entity manager instantiated is associated with a persistence context.
@PersistenceUnit(unitName = "MyDatabase")
EntityManagerFactory emf;
EntityManager entityManager = emf.createEntityManager();
or
@PersistenceContext(unitName = "MyDatabase")
private EntityManager entityManager;
PersistenceUnit injects an EntityManagerFactory, and PersistenceContext injects an EntityManager. It's generally better to use PersistenceContext unless you really need to manage the EntityManager lifecycle manually.
EntityManagerFactory defines another method for instantiation of EntityManager that, like the factory, takes a map of properties as an argument. This form is useful when a user name and a password other than the EntityManagerFactory's default user name and password have to specified:
Map properties = new HashMap();
properties.put("javax.persistence.jdbc.user", "kashyap");
properties.put("javax.persistence.jdbc.password","kashyap");
EntityManager em = emf.createEntityManager(properties);
Within the persistence context, the entity instances and their lifecycle are managed. By Entity instance, we mean instance of an entity & each entity designates relational table in the database. Entity Manager is actually an interface which provides methods to create and remove persistent entity instances, to find entities by their primary key, and to query over entities so together these functionalities are grouped under operations we perform. Operations that modify the content of a database require active transactions. Transactions are managed by an Entity Transaction instance obtained from the EntityManager.
Precise Definition :-
An Entity Manager 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.
Below i'm writing a code snippet for better understanding :-
try {
em.getTransaction().begin();
// Operations that modify the database should come here.
em.getTransaction
/**
*getTransaction() EntityManager's method Return the resource-level EntityTransaction object. See JavaDoc Reference Page
*/
em.getTransaction().commit();
}
finally {
if (em.getTransaction().isActive())
em.getTransaction().rollback();
}
Lets proceed as per JPA Specification :-
1) Extended vs Transactional - Scoped :
By Default the Transactional Persistence Context is in used which means all changes are flushed and all managed entities become detahced when the current transaction commits.
The Extended scope is available only for Stateful EJBs & it even makes a perfect sense since stateful beans can save the state therefore one can say end of
one business method doesn't necessary means the end of the transaction.
With the Stateless beans, it has a different aspect - We have business method that must end when the business method finishes.
===> One Method = One Transaction;
Only Transactional-Scoped Entity Manager is allowed for Stateless Beans
You can control if the EntityManager is extended or transactional during the EntityManager Injection :-
@PersistenceContext (type = javax.persistence.PersistenceContextType.EXTENDED)
EntityManager emng;
By Default it's javax.persistence.PersistenceContextType.TRANSACTION
Extended and Transaction Scoped PersistenceContext are allowed only in case of container-managed EntityManagers.
Time to step up a bit with: Container-managed vs Application-managed
2) Container-managed vs Application-managed :
@PersistenceContext
EntityManager emng;
Above statement authorises Container to inject the entity manager for you, hence Container-Managed.
Alternatively, you can create an EntityManager by yourself using EntityManagerFactory But this time the injection will be bit different -
@PersistenceUnit
EntityManagerFactory emf;
Now to get the EntityManager you need to invoke
emf.createEntityManager();
And here it is - you're using the application managed Persistence Context. Now you're responsible for creation and removal of EntityManagers.
Focus before you read the next para because that's what the tangled context , i'm trying to resolve-
You might use createEntityManager if you want to have control over created EM - e.g. if you need to move the created EntityManager across multiple beans involved in the trasaction - the container won't do it for you and every time you invoke createEntityManager(), you're creating an EntityManager that is connected to the new PersistenceContext. You might use the CDI for EntityManager's sharing.
Stay tuned for Entity Transaction - JPA and Resource-local, will be posting a detailed discussion on it.
Hope it gives a brief idea about the context. & Feel free to post queries.
Read the second part from here
