Is there any difference between these two alternatives ... can they both be used interchangeably?
(A) Creating a custom annotation so #Inject can be used instead of #PersistenceContext within a DAO, as shown in the answer to - how-to-stack-custom-annotation-in-java-with-inject-annotation
(B) Using #Named("yourName") to qualify the Producer, such as the following code sample.
public class Resources {
/**
* EntityManager's persistence context is defined here so the #Inject annotation may be used in referencing classes.
*/
#Produces
#Named("MyEm")
#PersistenceContext(unitName = "jboss.managed")
private EntityManager em;
}
#Stateless
public class FiletracksentHome {
..
#Inject
#Named("MyEm")
private EntityManager entityManager;
..
}
They are interchangable, but you should use (A).
The #Named annotation is primarily used for being able to access the object via expression language (EL), e.g. in a JSF view.
The problem is, that the resolution is done via String, thus being neither type safe, nor usually being automatically covered by refactorings in the IDE.
The CDI specification states that it should not be used for qualifying injection points if not be used to integrate legacy code.
Here's a nice article about this topic.
Related
With JEE 5 / EJB 3.0 life of Java developers became much more easier. Later, influenced by Spring and CDI, similar approaches were also adopted in JEE.
Now, I hope I am doing it right, but just to be sure:
I have several Stateless EJBs, which all query and / or modify the database. An example is
#Stateless
public class AddressDBService {
#PersistenceContext
protected EntityManager em;
Some of the Stateless EJB refer the other services like this:
#Stateless
public class AVeDBService {
#PersistenceContext
protected EntityManager em;
#Inject
private HomeToDealDBService homeToDealDBService;
#Inject
private AddressDBService addressDBservice;
and in the Stateless EJBs I have public methods like the ones below:
#TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
public void saveEntity(Home home) throws EntityExistsException {
this.em.persist(home);
addressDBservice.saveAddress(home.getMainAddress(), home);
}
While I am almost certain this usage is correct and thread-safe (the above services are in turn injected into JSF Managed Beans).
Could somebody confirm that my usage is correct, thread-safe and conforms good practices?
My usage seems to be conform with the following questions:
Is EntityManager really thread-safe?
Stateless EJB with more injected EJBs instances
The "is correct?" question can't be answered without know the goal of the project.
It could works? Yes, you have posted java-ee code that could deploy, but is not enough.
I usually use BCE (Boundary Control Entity) pattern and Domain Driven pattern.
In this pattern we use EJB for business logic services or endpoint (JAX-RS) and all other injections, that are the Control part, are CDI objects.
Entities (JPA) could use cascade to avoid to manually save related entities:
addressDBservice.saveAddress(home.getMainAddress(), home);
can be avoided if you define the entity like this:
#Entity
public class Home {
#ManyToOne(cascade=ALL)
private Address mainAddress;
}
The #TransactionAttribute(TransactionAttributeType.REQUIRES_NEW) annotation usually respond to a specific transactions behavior, is not required, so is correct only if is what you want to do.
I'm trying to inject a bean into an entity listener in a Quarkus-application:
#ApplicationScoped
public class MyEntityListener implements Serializable {
#Inject
MyService service;
#PrePersist
#PreUpdate
public void checkWrite(BaseEntity entity) {
service.check(entity);
}
}
But service is always null. Changing scope to #SessionScoped has no effect.
According to this 2 SO-discussions, this should be possible:
CDI injection in EntityListeners
How can I use an EJB in a EntityListener?
I couldn't find any information about which JPA-version Quarkus is using, but since it is a state-of-the-art-framework I think it is JPA 2.1?
So should this be possible and if yes, what am I doing wrong?
I found a Quarkus-issue addressing this problem: https://github.com/quarkusio/quarkus/issues/6948
Seems like Quarkus is lacking support for this feature and maybe it will be implemented in the future. There's also a workaround described.
I have a common generic DAO in common lib. I want in each module which uses this DAO to initialize with its own persistence UNIT
public abstract class GenericDao implements IGenericDao {
#PersistenceContext(unitName = "XXXX")
private EntityManager entityManager;
and in other module
public class CarDao extends GenericDao{
I have lot of projects are using this generic DAO but each project have its own persistence unit.
Persitence unit are differents following the project where is used the common library
The point is i could not use POO with abstract getEntityManager injected in each micro servicies because in common project we have a history DAO common for all microservicies and for each one i have to retrieve the entityManager injected from the microservice
Am i doing wrong or well? and how set th epersistence unit in each project ? (each project have lot fo DAO and i don't want repet each time CRUD methods)
#PersistenceContext(unitName = "XXXX")
private EntityManager entityManager;
This should be done in each concrete class, the abstract one should implement the concrete operation using
getEntityManager().doSomething(entity)
the getter getEntityManager() being abstract.
Imho this is a design smell, EntityManager is already an abstraction and you have nothing to gain encapsulating it.
[edit]
Regarding the "factory" approach, the way in CDI to dynamically inject resources is using producer methods.
You can so create a method returning an EntityManager instance that will dynamically resolve the EntityManagerFactory according to the persistence unit name (see an example here).
Note that this is a very bad idea as the entityManager scope is usually bound to the transaction one, letting the container inject you the entityManager instance guarantee that the scope will be correctly handled (by the container). The only viable configuration with this approach is when you want an "application managed" entityManager
NB: note that the given example will instantiate a new EntityManageFactory instance for each injection which can be really catastrophic according to the way you use it (the EntityManageFactory should be created once for all the application)
be sure to be aware of EntityManager lifecycle before going further.
Thank you guy for your advices in fact i was totally stupid in my genericDao i simply put
public abstract class GenericDao implements IGenericDao {
#PersistenceContext
private EntityManager entityManager;
As we have only one PersistentUnit it will be automatically injected....
was so easy !
then i can use #PersistentContext in all DAOs or simply and best call getEntityManager from their parent IGenericDao
We use Guice Persist to inject EntityManager in our project.
E.g.
public class MyDao{
#Inject
EntityManager em;
public void someMethod(){
//uses em instance
}
}
But it is unclear for us how injected instance of EntityManager is about to be used.
What type of EntityManager is this? (see e.g.: types of entity managers) Under the hood Guice Persist instantiates it via EntityManagerFactory.createEntityManager() so I'd say it's application-managed entity manager. But in official Wiki they write about seesion-per-transaction strategy, which suggests that EntityManager is (pseudo) transaction-scoped.
Should we invoke close() on it manually? Or Guice will take care of it?
What is the scope of first level cache? Only single transaction (like in transaction-scoped entity managers) or as long as I use the same injected instance of EntityManager (like in application managed entity managers)?
Even though the question is perfectly answered by Piotr I'd like to add some practical advise on how to use guice-persist.
I've been having issues with it which were pretty hard to debug. In my application certain threads would display outdated data and sometimes EntityManager instances were left with old dead database connections. The root cause was to be found in the way I used the #Transactional annotation (I only used them for methods that do updates/inserts/deletes, not for read-only methods). guice-persist stores EntityManager instances in a ThreadLocal as soon as you call get() on an injected Provider<EntityManager> instance (which I did for read-only methods). However, this ThreadLocal is only removed if you also call UnitOfWork.end() (which normally is done by the interceptor if #Transactional is on the method). Not doing so will leave the EM instance in the ThreadLocal so that eventually every thread in your thread pool will have an old EM instance with stale cached entities.
So, as long as you stick to the following simple rules the usage of guice-persist is straight forward:
Always inject Provider<EntityManager> instead of EntityManager directly.
For transaction-scoped semantics: always annotate each method with #Transactional (even the read-only methods). This way the JpaLocalTxnInterceptor will intercept the calls to your annotated methods making sure not only to start and commit transactions but also to set and unset EM instances in the ThreadLocal.
For request-scoped semantics: use the PersistFilter servlet filter that ships with guice-persist. It will call begin() and end() on the UnitOfWork for you before and after the request is done, thereby populating and cleaning up the ThreadLocal.
Hope this helps!
I did some research of the source code of Guice-persist and read through comments under Guice-persist wiki pages and these are the answers that I needed:
1 . Lifecycle management of EntityManager is kind of broken if it's injected via #Inject EntityManager. As stated in one of the comments on the Wiki:
I confirm that inject directly an EntityManager instead of a provider
can be dangerous. If you're not inside a UnitOfWork or a method
annotated with #Transaction, the first injection of an EntityManager
in a thread will create a new EntityManager, never destroy it, and
always use this specific EntityManager for this thread (EM are stored
thread-local). This can lead to terrible issues, like injection of
dead entityManager (connection closed, etc) So my recommendation if to
always inject a Provider, or at least to inject
directly an EntityManager only inside an opened UnitOfWork.
So example in my question isn't the most correct usage. It creates singleton instance of EntityManager (per-thread) and will inject this instance everywhere :-(.
However if I've injected Provider and used it inside #Transactional method then the instance of EntityManager would be per-transaction. So the answer to this question is: if injected and used correctly, the entity manager is transaction-scoped.
2 . If injected and used correctly then I don't need to manualy close entity manager (guice-persist will handle that for me). If used incorrectly, closing manually would be very bad idea (closed instance of EntityManager would be injected in every place when I #Inject EntityManager )
3 . If injected and used correctly then the scope of L1 cache is single transaction. If used incorrectly, the scope of the L1 cache is the lifetime of application (EntityManager is singleton)
1. It depends on you module cofiguration. There are some basic bindings:
JpaPersistanceService
public class JpaPersistanceService implements Provider<EntityManager> {
private EntityManagerFactory factory;
public JpaPersistanceService(EntityManagerFactory factory) {
this.factory = factory;
}
#Override
public EntityManager get() {
return factory.createEntityManager();
}
}
Module binding
EntityManagerFactory factory = Persistence.createEntityManagerFactory(getEnvironment(stage));
bind(EntityManager.class).annotatedWith(Names.named("request")).toProvider(new JpaPersistanceService(factory)).in(RequestScoped.class);
bind(EntityManager.class).annotatedWith(Names.named("session")).toProvider(new JpaPersistanceService(factory)).in(SessionScoped.class);
bind(EntityManager.class).annotatedWith(Names.named("app")).toProvider(new JpaPersistanceService(factory)).asEagerSingleton;
Usage
#Inject #Named("request")
private EntityManager em; //inject a new EntityManager class every request
#Inject #Named("session")
private Provider<EntityManager> emProvider; //inject a new EntityManager class each session
//This is little bit tricky, cuz EntityManager is stored in session. In Stage.PRODUCTION are all injection created eagerly and there is no session at injection time. Session binding should be done in lazy way - inject provider and call emProvider.get() when em is needed;
#Inject #Named("application")
private EntityManager em; //inject singleton
2. Yes, you should or you will use JpaPersistModule [javadoc]
3. Well, this is about JPA configuration in persistence.xml and EntityManager scope
I'm injecting a provider .... but I suspect something is wrong.
When I try to redeploy an application ALWAYS I have to restar the server because the JPA classes are cached.
It happens the following pseudo-bug
https://bugs.eclipse.org/bugs/show_bug.cgi?id=326552
Theoretically by injecting a Provider and getting an instance of EntityManager you should not close anything ....
#Path(value = "/user")
#Stateless
public class UserService {
#Inject
private UserManager manager;
#Path(value = "/create")
#GET
#Produces(value = MediaType.TEXT_PLAIN)
public String doCreate(#QueryParam(value = "name") String name) {
manager.createUser(name);
return "OK";
}
}
here is the user manager impl
public class UserManager {
#PersistenceContext(unitName = "shop")
private EntityManager em;
public void createUser(String name) {
User user = new User();
user.setName(name);
// skip some more initializations
em.persist(user);
}
}
the problem is if i do not mark UserService as #Stateless then the manager field is null
but if i mark #Stateless, i can have the manager field injected, and the application works as i can get the data saved into db
just wondering, what is the reason behind this?
and is this the preferred way to wiring the application?
well, i am thinking to pull out the EntityManager to a producer, so that it can be shared
the problem is if I do not mark UserService as #Stateless then the manager field is null
For injection to occur, the class has to be a managed component such as Enterprise Beans, Servlets, Filters, JSF managed beans, etc or CDI managed bean (this is the new part with Java EE 6, you can make any class a managed bean with CDI).
So, if you don't make your JAX-RS endpoint an EJB, how to enable injection? This is nicely explained in JAX-RS and CDI integration using Glassfish v3:
There are two ways CDI managed beans
are enabled:
instantiated by CDI, life-cycle managed by Jersey. Annotate with
#ManagedBean and optionally annotate
with a Jersey scope annotation.
instantiated and managed by CDI. Annotate with a CDI scope annotation,
like #RequestScoped (no #ManagedBean
is required)
I also suggest checking the resources below.
and is this the preferred way to wiring the application?
I'd say yes. CDI is very nice and... don't you like injection?
well, I am thinking to pull out the EntityManager to a producer, so that it can be shared
Shared between what? And why? In you case, you should use an EntityManager with a lifetime that is scoped to a single transaction (a transaction-scoped persistence context). In other words, don't share it (and don't worry about opening and closing it for each request, this is not an expensive operation).
References
JPA 2.0 Specification
Section 7.6 "Container-managed Persistence Contexts"
Section 7.6.1 "Container-managed Transaction-scoped Persistence Context"
Section 7.6.2 "Container-managed Extended Persistence Context"
Resources
Dependency Injection in Java EE 6 - Part 1
Introducing the Java EE 6 Platform: Part 1
TOTD #124: Using CDI + JPA with JAX-RS and JAX-WS
The #Singleton annotation will help: http://www.mentby.com/paul-sandoz/jax-rs-on-glassfish-31-ejb-injection.html