I am creating a web application with JPA. I have configured Hibernate with connection pool c3p0.
In my case it is not possible to inject EntityManager using #PersistenceContext annotation.
In Java EE documentation they say it is thread-safe to use EntityManagerFactory instance to concurrently create EntityManager instances.
So I am using one static EntityManagerFactory instance for my persistence unit in my web app and create EntityManagers using it.
But they say that EntityManagers cannot be used concurrently (not thread-safe).
So according to this I create an EntityManager instance separately per each servlet request, use it in same thread and then dispose it.
Can I do it this way ?
Yes, and by the way - this is exactly what #PersistenceContext will do. It will:
Create EntityManager once #Transactional is invoked (or in case OpenEntityManagerInViewFilter is set up - when the filter is invoked)
Put it in ThreadLocal variables. So that any code within this thread has access to it.
Once the execution is out of #Transactional method (or out of the filter) - it will close the EntityManager
Related
In Tapestry there are (mainly) two ways to obtain an EntityManager instance:
By injecting the EntityManager directly:
#Inject
#PersistenceContext(unitName = MyPersistenceUnit)
private EntityManager entityManager;
By using the EntityManagerManager (which is also injected):
EntityManager entityManager = entityManagerManager.getEntityManager(MyPersistenceUnit);
The EntityManagerManager states in its javadoc that it manages one EntityManager per thread:
Manages EntityManagers for the current thread. An
EntityManager is created as needed and closed at the end
of each request.
The implementation of this service is per-thread.
Does this also apply for injecting the EntityManager directly? So for example, if I start a transaction in one thread, like that:
entityManager.getTransaction().begin();
will the transaction of the EntityManager in another thread be inactive - i.e. will entityManager.getTransaction().isActive() be false for other threads, where the EntityManager also was injected?
More generally I am curious about what should be used. What are the differences / advantages of one over the other?
Both approaches are equal, you can use any with same guarantees:
EntityManager is always a per-thread instance, nothing is shared between threads. Each EntityManager is discarded/closed at the end of each request (when PerthreadManager#cleanup() method is called).
EntityManagerManager is used as a source in EntityManagerObjectProvider which produces instances of EntityManager so that you could simply write #Inject EntityManager instead of obtaining it from the manager explicitly.
I'm developing an application, and I started with creating my JPA entities (annotation + persistence.xml). Then in my persistence.xml file, I created a connection for a MySql data base (the connection is fine).
The problem is that I just don't know how to persist my entities without creating a "main class".
Do I have to run my main class for every single entity that I'm going to create?
To persist an entity, you need an instance of entityManager. Since you have a Java EE container you can get an instance of entityManager using the annotation #PersistenceContext in some bean
What I mean by some bean ? It's a bean managed by the Java EE container. So, for instance you have to define a bean like this :
#Stateless
public class MyController{
#PersistenceContext
private EntityManager em;
public void persistIt(Object anEntity){
em.persist(anEntity);
}
}
The annotation #Stateless indicate to the container that it must manage this class and take care of providing an instance of the entityManager when needed.
So to answer precisely to your question: it is not mandatory to have another class to persist an entity, as soon as you find a way to get access to an entityManager instance.
But:
it is definitively NOT a best practice.
to take advantages of entityManager injection: you must use another managed bean so that entityManager can be injected by the container.
Additionally, the controller is where you can handle your transactions properly.
Can #PersistenceUnit be used with JTA in JPA? If so, how is it possible?
Per http://tomee.apache.org/jpa-concepts.html:
With <persistence-unit transaction-type="RESOURCE_LOCAL"> [...]
You must use the EntityManagerFactory to get an EntityManager
[...]
An EntityManagerFactory can be injected via the #PersistenceUnit annotation only (not #PersistenceContext)
With <persistence-unit transaction-type="JTA"> [...]
An EntityManager can be injected via the #PersistenceContext annotation only (not #PersistenceUnit)
I have a similar code which uses JTA and #PersistenceUnit at the same time. But sometimes I am having NullPointerException when accesing transaction (defined as #Resource).
Using JTA means that you delegate the work to the container. You can override it by using a UserTransaction. Your quotation contains all the answers you want to know. Using PersistenceUnit to get an EntityManager won't work.
If you are using RESOURCE_LOCAL you are responsible for the transaction itself, by using EntityManager.getTransaction(). An entity manager is created by an EntityManagerFactory. To obtain that factory, you can use PersistenceUnit.
So the simple answer is no, if you rely on container managed entity managers.
As example see http://docs.oracle.com/javaee/6/tutorial/doc/bnbqw.html
Application Managed Entity Managers = RESOURCE_LOCAL can use UserTransaction (which are part of JTA).
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
I am developing a JavaEE6 based web application using JBoss7.
In my application I am injecting the EntityManager in my EJBs as:
class ForumServiceEJB
{
#PersistenceContext(type=EXTENDED)
private EntityManager em;
}
class TopicServiceEJB
{
#PersistenceContext(type=EXTENDED)
private EntityManager em;
}
The problem when I update some data using ForumServiceEJB's EntityManager then the changes are made into DB but TopicServiceEJB's EntityManager is not able to see those changes and the results are always fetched from Cache.
I am using ExtendedPerssisteenceContext as My Entities contain child Entity Collections of Lazy Loading type.
How can I use/Inject EntityManager of type ExtendedPersistenceContext and make different EntityManager in one EJB can still see the changes done by other different EJB EntityManagers?
Somewhere I read EntityManagers should be RequestScoped objects.
public class MyEntityManagerProducers {
#Produces #RequestScoped
public EntityManager createDbEm() {
return Persistence.createEntityManagerFactory("forumDb").
createEntityManager();
}
public void disposeUdEm(#Disposes EntityManager em) {
em.close();
}
Is it the way to go?
I am using ExtendedPerssisteenceContext as My Entities contain child
Entity Collections of Lazy Loading type.
This is not a good reason to use EXTENDED. I would suggest you to make it default, which is TRANSACTION. And it's good to make your EntityManager request-scoped, or method-scoped, in non-enterprise environment or when using application-managed persistence, as this is not a very heavy object to create. Moreover, neither using application-scoped EntityManager is a good idea, as it is not threadsafe.
Having said that, as you are using JBoss, you should let the container handle the lifecycle of EntityManager, in case you are using JTA. Hence, just inject that with everything default
Note:
Only stateful session beans can have a container-managed, extended entity manager.
Links:
http://javanotepad.blogspot.com/2007/08/managing-jpa-entitymanager-lifecycle.html
https://blogs.oracle.com/enterprisetechtips/entry/extended_persistence_context_in_stateful
Suggestions:
Your business method should know whether to load the children or not. But that's the ideal case. A number of times we can't say that and completely depends on the user input -- we can't predict that well. Therefore, there are two solutions available,
make a separate AJAX call to load children
Use the filter called open-session-in-view. I would prefer the former.
Links:
https://community.jboss.org/wiki/OpenSessionInView
http://static.springsource.org/spring/docs/1.2.9/api/org/springframework/orm/hibernate3/support/OpenSessionInViewFilter.html
Why is Hibernate Open Session in View considered a bad practice?
I am using resource local entitymanager with JPA 1 . What is the best practice for creating EntityManager that is
Create do job close entitymanager everytime.(Entitymanager not heavy weight but still boring )
Create a entitymanager for every thread and store it as ThreadLocal variable and use it (a-where should i close entitymanager
b-Any entitystored in a session scoped bean will be a detached entity because entitymanager which entity in its persistense context closed or belong another thread now.)
Store entitymanager in a session bean and use same entitymanager during session(Sessions unawere of each others )
You should be injecting the EntityManager using the #PersistenceContext annotation. Don't worry about where it comes from — that's dependency injection at work.
I create EntityManagers per request and close them in a ServletRequestListener.