I am a little curious about project setup.
I have a project which I am trying to update.
So I peppered some of my concrete classes with the #Named("") annotation.
The class actually extends an abstract class which implements an interface.
I want to inject the named class on a class that is not included in that jar.
What are the considerations that I need to keep in mind when doing that?
Your question is very vague, so don't expect this answer to stun you with details.
The class actually extends an abstract class which implements an interface.
This shouldn't be a problem - the final bean instance will have the the type of all the classes and interfaces in the hierarchy. Nothing to worry about.
I want to inject the named class on a class that is not included in that jar.
Firstly, you can only inject beans into another beans. That means your other class (which is not in the same jar) has to become a bean. For this you enable the CDI in the other archive and annotate the class with desired scope.
Secondly, the other jar, has to have a dependency on the one with #Named beans so that the CDI container can see those beans.
Last but not least, make sure you use adequate scopes on beans. The scope belongs to certain context and each has a different lifecycle. So for instance #ApplicationScoped beans live from app start-up till shutdown, while #RequestScoped are only active during a HTTP request.
Related
Back in the middle age, my company created his own IoC container and since then our application is running with it. But the day has come for us to switch to CDI/Weld. Instead of switching the whole thing to CDI (we honeslty can't...), we would like to do this bits by bits starting with our front-end JSF.
The idea is to leave some of the bean resolution to Weld (e.g controller) and some to our old IoC container (e.g services)
For instance:
#Named
#RequestSCoped
ControllerA {
#Inject
private ServiceB service:
}
ControllerA should be managed by Weld, and ServiceB should remain in our old IoC container. Though, Weld should know to resolve it.
So far, our best clue is most probably to use cdi extensions and play with lifecycle events. Any example, feedback or advice is appreciated.
First, welcome to CDI! It's a great framework and a joy to use, especially in combination with JSF. I recommend reading Pro CDI in JavaEE8: https://link.springer.com/book/10.1007/978-1-4842-4363-3 for a deep dive into this which will help explain my answers below
There's a couple of routes you should choose. The simplest would be to use a #Produces annotation in a class that delegates to the older IOC container. See here for an example: https://www.baeldung.com/java-ee-cdi
The really slick, but slightly more complicated way, would be to create a CDI Portable Extension the can delegate resolution to your older container. This is very powerful and gives you hooks into several parts of the CDI container. For reference, see here: https://docs.jboss.org/weld/reference/latest/en-US/html/extend.html#extend
In either scenario, you're going to have to understand the CDI lifecycle (everything is a proxy that delegates to a managed instance) thoroughly and how that interacts with your existing IOC container or you could face memory leaks.
Good luck!
I'm a little confused by the mixed use of JSF2+Spring+EJB3 or any combination of those. I know one of the Spring principal characteristics is dependency injection, but with JSF managed beans I can use #ManagedBean and #ManagedProperty anotations and I get dependency injection functionality. With EJB3 I'm even more confused about when to use it along with JSF or if there is even a reason to use it.
So, in what kind of situation would it be a good idea to use Spring+JSF2 or EJB3+JSF2?
Until now I have created just some small web applications using only JSF2 and never needed to use Spring or EJB3. However, I'm seeing in a lot of places that people are working with all this stuff together.
First of all, Spring and EJB(+JTA) are competing technologies and usually not to be used together in the same application. Choose the one or the other. Spring or EJB(+JTA). I won't tell you which to choose, I will only tell you a bit of history and the facts so that you can easier make the decision.
Main problem they're trying to solve is providing a business service layer API with automatic transaction management. Imagine that you need to fire multiple SQL queries to perform a single business task (e.g. placing an order), and one of them failed, then you would of course like that everything is rolled back, so that the DB is kept in the same state as it was before, as if completely nothing happened. If you didn't make use of transactions, then the DB would be left in an invalid state because the first bunch of the queries actually succeeded.
If you're familiar with basic JDBC, then you should know that this can be achieved by turning off autocommit on the connection, then firing those queries in sequence, then performing commit() in the very same try in whose catch (SQLException) a rollback() is performed. This is however quite tedious to implement everytime.
With Spring and EJB(+JTA), a single (stateless) business service method call counts by default transparently as a single full transaction. This way you don't need to worry about transaction management at all. You do not need to manually create EntityManagerFactory, nor explicitly call em.getTransaction().begin() and such as you would do when you're tight-coupling business service logic into a JSF backing bean class and/or are using RESOURCE_LOCAL instead of JTA in JPA. You could for example have just the following EJB class utilizing JPA:
#Stateless
public class OrderService {
#PersistenceContext
private EntityManager em;
#EJB
private ProductService productService;
public void placeOrder(Order newOrder) {
for (Product orderedproduct : newOrder.getProducts()) {
productService.updateQuantity(orderedproduct);
}
em.persist(newOrder);
}
}
If you have a #EJB private OrderService orderService; in your JSF backing bean and invoke the orderService.placeOrder(newOrder); in the action method, then a single full transaction will be performed. If for example one of the updateQuantity() calls or the persist() call failed with an exception, then it will rollback any so far executed updateQuantity() calls, and leave the DB in a clean and crisp state. Of course, you could catch that exception in your JSF backing bean and display a faces message or so.
Noted should be that "Spring" is a quite large framework which not only competes EJB, but also CDI and JPA. Previously, during the dark J2EE ages, when EJB 2.x was extremely terrible to implement (the above EJB 3.x OrderService example would in EJB 2.x require at least 5 times more code and some XML code). Spring offered a much better alternative which required less Java code (but still many XML code). J2EE/EJB2 learned the lessons from Spring and came with Java EE 5 which offers new EJB3 API which is even more slick than Spring and required no XML at all.
Spring also offers IoC/DI (inversion of control; dependency injection) out the box. This was during the J2EE era configured by XML which can go quite overboard. Nowadays Spring also uses annotations, but still some XML is required. Since Java EE 6, after having learned the lessons from Spring, CDI is offered out the box to provide the same DI functionality, but then without any need for XML. With Spring DI #Component/#Autowired and CDI #Named/#Inject you can achieve the same as JSF does with #ManagedBean/#ManagedProperty, but Spring DI and CDI offers many more advantages around it: you can for example write interceptors to pre-process or post-process managed bean creation/destroy or a managed bean method call, you can create custom scopes, producers and consumers, you can inject an instance of narrower scope in an instance of broader scope, etc.
Spring also offers MVC which essentially competes JSF. It makes no sense to mix JSF with Spring MVC. Further Spring also offers Data which is essentially an extra abstraction layer over JPA, further minimizing DAO boilerplate (but which essentially doesn't represent the business service layer as whole).
See also:
What exactly is Java EE?
JSF Controller, Service and DAO
#Stateless beans versus #Stateful beans
There's no real easy answer here as Spring is many things.
On a really high level, Spring competes with Java EE, meaning you would use either one of them as a full stack framework.
On a finer grained level, the Spring IoC container and Spring Beans compete with the combination of CDI & EJB in Java EE.
As for the web layer, Spring MVC competes with JSF. Some Spring xyzTemplate competes with the JPA interfaces (both can use eg Hibernate as the implementation of those).
It's possible to mix and match; eg use CDI & EJB beans with Spring MVC, OR use Spring Beans with JSF.
You will normally not use 2 directly competing techs together. Spring beans + CDI + EJB in the same app, or Spring MVC + JSF is silly.
This question directly follows from another question of mine here. The last paragraph of the answer to that question mentions CDI technology.
Can you explain what is the purpose of that technology. Is there any relation of that technology with JSF and EJB? I have been through this question but I'm not clear on what is the purpose of that technology yet. I specifically want to know where it fits with other Java EE technologies like EJB and JSF.
Adding to the correct answer of Bozho:
CDI is a superset of JSF's managed beans, and JSF will eventually deprecate their own native managed bean system. In JSF 2.2 steps have already been taken for this.
CDI is not a superset of EJB beans, but they complement each other. CDI does not only provide more advanced DI to EJB beans, but is also capable of giving EJB beans a scope (mostly used for stateful beans). EJB on its turn provides transactional and security services among others , which CDI does not offer. Like JSF managed beans, EJB beans will eventually be merged into the CDI component model. Marina Vatkina (EJB spec lead) among others has strongly hinted at this.
Further reading:
How do CDI and EJB compare? interact?
CDI, when to break out the EJBs
First, CDI is a dependency-injection standard framework. It defines ways for objects to obtain their dependencies not through instantiation (private FooDao dao = new FooDao()) but via a container which creates and manages instances. You can read more about dependency injection (google/wiki).
The standard defines how that works with both JSF and EJB.
your JSF managed beans can be defined via CDI, so that you can inject services into the managed beans, and so that CDI manages the lifecycle of JSF managed beans
EJB can also benefit from the advanced dependency injection of CDI (previously they had their own, feature-poor DI).
In short, CDI binds all components in JavaEE in a way spring does with all of its components, but CDI is a standard.
Is there a difference using a singleton class and an application scoped managed bean to hold application data?
I need to lookup certain JNDI ressources such as remote bean interfaces and therefore I wrote myself a singleton to cache my references and only allow single references. (ServiceLocator)
I opened my website in two different browsers and that singleton got only initialized once. So I assume its application scope?
Any other benefits of a application scope managed bean then being able to access its properties in jsf?
Singletons are not unit testable nor abstractable nor extendable. Singletons are also unnecessarily complex to reliably create and maintain if your sole purpose is to have application scoped data (at least, if you really want a fullworthy singleton for it for some reason -most starters don't even exactly understand what a singleton is supposed to be).
"Just create one" like an application scoped managed bean is much simpler to develop, test and maintain. JSF as framework will guarantee that only one instance will be created and reused during web application's lifetime.
See also:
Singleton vs Just Create one
How to choose the right bean scope?
Given: simple JSF webapp (no Seam), having JSF beans calling few EJB's which in turn load and persist JPA entities. What I want to is to use #Singleton annotation for ejb's and inject EntityManager instead of EntityManagerFactory:
#Singleton
public class MyEJB {
#PersistenceContext(unitName = PERSISTENCE_UNIT_NAME)
protected EntityManager em; // not EntityManagerFactory
}
Spec says that #Singleton is thread-safe, supports concurrency and transaction attributes which (from my pov) makes it safe for calling from JSF beans. I expect also performance benefits because of EntityManager not being recreated for each call and it's internal caching abilities.
My main concern here is create/update operations on JPA entities in the situation when I have several singletons and, as a result, the same count of long-living EntityManagers.
What happens if one singleton updates an JPA instance and how these
changes are populated to other singletons?
As I'm not able to close entity manager, do I need to flush it upon
each entity update?
Would it be better if these few singletons will share the same entity
manager?
I saw only few examples of such design. Why? Are there any serious
drawbacks?
Many thanks in advance!
I expect also performance benefits because of EntityManager not being recreated for each call and it's internal caching abilities.
You might save some memory using singletons, but using it everywhere in your app could make it actually slower, because as there's only one EJB to serve all the concurrent requests by various users of your app, the container locks access to the EJB and when it's busy serving a request it cannot be serving another request. However this can be alleviated to some degree using lock types (i.e. #Lock(WRITE) and #Lock(READ)).
Singletons are useful for times when you want to execute a piece of code periodically using EJB timers, or to update a cache periodically, etc.
What happens if one singleton updates an JPA instance and how these changes are populated to other singletons?
Shouldn't be any different to the way non-singleton EJBs behave.
As I'm not able to close entity manager, do I need to flush it upon each entity update?
If you use CMT, no. At the end of each transaction everything will be flushed automatically.
Would it be better if these few singletons will share the same entity manager?
Looks like premature optimization to me. Just let the container inject the EM for you.
I saw only few examples of such design. Why? Are there any serious drawbacks?
Already explained.
There is one thing I want to mention regarding changing LockType of Singleton EJBs. While in general it sounds like a good idea, you should remember that resources such as EntityManger are NOT thread-safe, so appropriate concurrent access control should be provided. You can annotate methods that access non-thread-safe resources with #Lock(WRITE), but if almost all interface methods of your Singleton EJB access such resources, you will have almost the same situation as with fully write locked one. Alternative is to use Bean Concurrency Management with manual fine-grained synchronization, but it's also questionable decision.
Because of this in general I prefer Stateless EJBs over Singleton and use latter in specific cases.