I'm learning Seam and JPA/Hibernate and while I could find some examples on how to build a DAO class with Hibernate I'm a bit confused on how to do the same thing with Seam (or even if that is at all necessary).
I do know that seam manages the transactions using its conversations so I don't (?) have to worry about committing / rolling back the operations manually.
What I still don't get is how to extend EntityHome and EntityList objects beyond the ones generated by seam-gen to create DAOs that would provide me with the fine grained operations / joins I need in my application.
Am I missing something ?
I do know that seam manages the
transactions using its conversations
so I don't (?) have to worry about
committing / rolling back the
operations manually.
Yes, you dont need to worry about it, if there is an exception, seam will automatically do a rollback. The same thing for commit when there is no exception. I think you can control this manually too with seam annotations.
The DAO pattern is created when you need to separate the persistencie tier from business tier. EntityHome and EntityList are exactly the persistence tier. You dont need to create a dao.
The best path for whom are starting with seam is study the example that come with seam package .. see examples like dvdstore and booking. they are quite helpfull
Regards,
The other useful thing to is EntityQuery or HibernateEntityQuery. You specify your queries in XML and then can reference them as Seam components throughout your application. Although I use this much liked NamedQuery in JPA, I don't think this is a standard practice.
<framework:entity-query name="User_findByEmailAddress" ejbql="SELECT u FROM User u">
<framework:restriction>
<value>u.emailAddress = #{emailAddress}</value>
</framework:restriction>
</framework:entity-query>
Then in your Java code you can do:
#In
private EntityQuery<User> User_findByEmailAddress;
...
Contexts.getEventContext().set("emailAddress", emailAddress);
User user = User_findByEmailAddress.getSingleResult();
If you want to use this in your xhtml page, you can also use it there with built-in support for pagination.
Walter
Related
the Topic already says one of the key roles regarding ORM
Don't run your own ORM Implementation
But, I have a situation here where I'm not sure how to get our Requirements implemented properly.
To give you a bit of background, currently we are using Spring Data JPA with Hibernate as JPA Implementation and all is fine so far.
But we have separate fields which we want to "manage" automatically, a bit similar to Auditing Annotations from Hibernate (#CreatedBy, #ModifiedBy, ...).
In our case this is e.g. a specific "instance" the entity belongs to.
Our Application is rather a Framework than an App, so other Developers frequently add Entities and we want to keep it simple and intuitive.
But, we do not only want to set it automatically on storage but also add it as condition for most "simple and frequent" queries (see my related question here Inject further query conditions / inject automatic entity field values when using Spring Data JPA Repositories).
Thus, I thought about building a simple Layer on top of the EntityManager and its Criteria API to support at least simple Queries like
findById(xx)
findByStringAttribute(String attribute, String value)
findByIntegerAttribute(int attribute, String value)
...
I'm not sure if this is too broad of a question but, what are your thoughts on that? Is this a reasonable idea or should I skip that idea?
I just started a simple Java testproject which manages some entities using Hibernate and provides a REST interface to manipulate these objects and provide some additional business logic. The REST interface is created using RESTEasy and Jetty.
Everything works fine so far, but I have the feeling that I'm actually writing too much boilerplate code. As I don't have much experience in these Java frameworks I'm just wondering if anyone could give me a hint on how to improve the situation.
Creting Hibernate Sessions per Request
Well, as far as I understood I have to create a Hibernate session per request and at the end I have to close it. So currently all of my service methods look like this:
Session session = HibernateUtil.getInstance().getSessionFactory().openSession();
...
...
...
session.close();
Is there any way to remove these two lines in order to somehow do this automatically?
Currently my service is registered as a RestEASY singleton. Will changing to a RESTeasy ressource and creating the session in the constructor solve the problem? I think it will solve the problem of creating the session. But wherer to close it?
In C++ this can easily done be creating a scoped object which closes the session at the end. But in Java?
Whenever such a REST request is made I have to check for a valid session (the user has to login previously). Is a ServletFilter the right way to do this?
General: Are there any other patterns or frameworks I should consider using? I mean I want to keep it as simple as possible and especially as I dont have that much experience I dont want to end up using Spring or whatever heavyweight framework. Seems that I'm used to the simplicity of Python and Django but for this little project I have to use Java.
THanks so far!
Hibernate's current recommended approach for managing Sessions is detailed on this wiki page. In particular, I think you need to read the last paragraph: This is all very difficult, can't this be done easier?
In the end, you do need to tell the persistence layer that "I'm about to do something" (which usually also gets the Session to do it with) and "I'm done doing it". You can do it with annotations, or JTA transactions, but that information still has to be communicated!
Inject SessionFactory to your Data Access Object and use sessionFactory.getCurrentSession() to access Hibernate Session object.
you can make use of any of the Factory classes available to implement this..
Then your code should look like this..
sessionFactory.getCurrentSession().save(newInstance);
You should try writing a Filter that does this. Spring's OpenSessionInViewFilter is a good place to start if you need an example.
What would you suggest as a good and practical but simple pattern for a solution with:
HTML + JSP (as a view/presentation)
Servlets (controller, request, session-handling)
EJB (persistence, businesslogic)
MySQL DB
And is it necessary to use an own layer of DAO for persistence? I use JPA to persist objects to my DB.
Should I withdraw business logic from my EJB? All online sources tell me different things and confuses me...
I would definitely put the business logic in Stateless Session Beans. Stateless session beans are nice as they nicely capture the transaction boundaries. And it decouples the View layer from the persistence layer.
Take care that the methods of the SSB correspond to small business goals the user wants to achieve.
Another point is that you must be sure that the data you return has all data in the object tree and that you do not rely on lazy loading to get the rest, because this causes all kind of problems.
Stay as far away as possible from Stateful Session Beans : they are bad news and are a broken concept in the context of a web application.
For long running things, consider using Message Driven Beans which you trigger by sending a JMS message. These are a nice way to do background processing which frees the business logic faster, keeps transactions shorter and returns control to the end user faster.
What would you suggest as a good and practical but simple pattern for a solution with JSP/Servlets + EJB + MySQL
Use the MVC framework of your choice, Stateless Session Beans for the business logic and transaction management (prefer local interfaces if you don't need remoting), Entities for persistence.
Inject your EJBs wherever possible (if you are using Java EE 6, this means anywhere and you can also skip the interface).
And is it necessary to use an own layer of DAO for persistence? I use JPA to persist objects to my DB.
Some might say yes, I say no in most cases. The EntityManager already implements the Domain Store pattern, there is no need to shield it behind a DAO for simple needs.
You might want to read the following resources for more opinions on this:
Has JPA Killed the DAO?,
JPA/EJB3 killed the DAO,
and the more recent DAOs Aren't Dead - But They Either Collapsed Or Disappeared
Should I withdraw business logic from my EJB?
I wouldn't. Put your business logic in your (Stateless) Session Beans. EJB3 are POJOs, they are easily testable, there is no need to delegate the business logic to another layer.
Anno 2012 I would not recommend going for Servlets and JSP as the presentation layer. This was all the rage in 2002, but that's a decade ago.
Today Java EE has an excellent MVC framework build in called JSF. You are much better off using this instead. You most likely want to fetch some Widgets from the component library PrimeFaces, as all the standard ones are a bit basic. A utility library like OmniFaces is a great addition as well.
Regarding the DAOs; don't go as far as directly using the entity manager in (JSF) backing beans, but if you are already using Service classes (transactional boundaries) for your business logic , using a DAO as well might be overkill.
There are still some small advantages of a DAO, like a dao.findByName(...) looks a little clearer than loading a named query, setting a parameter, and getting a (single) result, but the cost is that you have to maintain a seperate DAO for each Entity, probably in addition to some Service.
I'm hesitating between two designs of a database project using Hibernate.
Design #1.
(1) Create a general data provider interface, including a set of DAO interfaces and general data container classes. It hides the underneath implementation. A data provider implementation could access data in database, or an XML file, or a service, or something else. The user of a data provider does not to know about it.
(2) Create a database library with Hibernate. This library implements the data provider interface in (1).
The bad thing about Design #1 is that in order to hide the implementation details, I need to create two sets of data container classes. One in the general data provider interface - let's call them DPI-Objects, the other set is used in the database library, exclusively for entity/attribute mapping in Hibernate - let's call them H-Objects. In the DAO implementation, I need to read data from database to create H-Objects (via Hibernate) and then convert H-Objects into DPI-Objects.
Design #2.
Do not create a general data provider interface. Expose H-Objects directly to components that use the database lib. So the user of the database library needs to be aware of Hibernate.
I like design #1 more, but I don't want to create two sets of data container classes. Is that the right way to hide H-Objects and other Hibernate implementation details from the user who uses the database-based data provider?
Are there any drawbacks of Design #2? I will not implement other data provider in the new future, so should I just forget about the data provider interface and use Design #2?
What do you think about this? Thanks for your time!
Hibernate Domain objects are simple POJO so you won't have to create separate DPI-objects, H-Object themselves can be used directly. In DAO you can control whether they come from hibernate or anything else.
I highly recommend reading Chapter 4 "Hitting the database" of Spring in Action, 3rd edition, even if you aren't using Spring in your application. Although my second recommendation would be to use Spring :-)
The DAO pattern is a great way to keep database and ORM logic isolated in the DAO implementation, and you only need one set of entity objects. You can make that happen without Spring, it just takes more work managing your sessions and transactions.
If I understand your post, this is sort of a middle-ground between Design 1 and Design 2. The H-Objects (the entities that Hibernates loads and persists) don't need any Hibernate specific code in them at all. That makes them perfectly acceptable to be used as your DPI-Objects.
I've had arguments with folks in the past who complain that the use of JPA or Hibernate Annotations exposes Hibernate specifics through the DAO interface. I personally take a more pragmatic view, since annotations are just metadata, and don't directly affect the operation of your entity classes.
If you do feel that the annotations expose too much, then you can go old school and use Hibernate Mappings instead. Then your H-Objects are 100% Hibernate free :-)
I recommend design #2. Simply construct domain objects, and let hibernate look after them. Don't write separate classes that are persisted.
Hibernate tries to hide most of the persistence business from you. You may need to add a few small annotations to your entities to help it along. But certainly don't make separate classes.
You may need some very small DAO classes. For example, if you have a Person entity, it would be fairly common practice to have a PersonDAO object that saves a person. Having said that, the code inside the DAO will be very simple, so for a really small project, it may not be worth it. For a large project, it's probably worth keeping your persistence code separate from your business logic, in case you want to use a different persistence technology later.
I'm asking this question given my chosen development frameworks of JPA (Hibernate implementation of), Spring, and <insert MVC framework here - Struts 1, Struts 2, Spring MVC, Stripes...>.
I've been thinking a bit about relationships in my entity layer - for example I have an order entity that has many order lines. I've set up my app so that it eagerly loads the order lines for every order. Do you think this is a lazy way to get around the lazy initialization problems that I would come across if I was to set the fetch strategy to false?
The way I see it, I have the following alternatives when retrieving entities and their associations:
Use the Open Session In View pattern to create the session on each request and commit the transaction before returning the response.
Implement a DTO (Data Transfer Object) layer such that every DAO query I execute returns the correctly initialized DTO for my purposes. I don't really like this option much because in my experience I've found that it creates a lot of boilerplate copying code and becomes messy to maintain.
Don't map any associations in JPA so that every query I execute returns only the entities I'm interested in - this will probably require me to have DTOs anyway and will be a pain to maintain and I think defeats the purpose of having an ORM in the first place.
Eagerly fetch all (or most associations) - in the example above, always fetch all order lines when I retrieve an order.
So my question is, when and under what circumstances would you use which of these options? Do you always stick with one way of doing it?
I would ask a colleague but I think that if I even mentioned the term 'Open Session in View' I would be greeted with blank stares :( What I'm really looking for here is some advice from a senior or very experienced developer.
Thanks guys!
Open Session in View has some problems.
For example, if the transaction fails, you might know it too late at commit time, once you are nearly done rendering your page (possibly the response already commited, so you can't change the page !) ... If you had know that error before, you would have followed a different flow and ended up rendering a different page...
Other example, reading data on-demand might turn to many "N+1 select" problems, that kill your performance.
Many projects use the following path:
Maintain transactions at the business layer ; load at that point everything you are supposed to need.
Presentation layer runs the risk of LazyExceptions : each is considered a programming error, caught during tests, and corrected by loading more data in the business layer (you have the opportunity to do it efficiently, avoiding "N+1 select" problems).
To avoid creating extra classes for DTOs, you can load the data inside the entity objects themselves. This is the whole point of the POJO approach (uses by modern data-access layers, and even integration technologies like Spring).
I've successfully solved all my lazy initialization problems with Open Session In View -pattern (ie. the Spring implementation). The technologies I used were the exact same as you have.
Using this pattern allows me to fully map the entity relationships and not worry about fetching child entities in the dao. Mostly. In 90% of the cases the pattern solves the lazy initialization needs in the view. In some cases you'll have to "manually" initialize relationships. These cases were rare and always involved very very complex mappings in my case.
When using Open Entity Manager In View pattern it's important to define the entity relationships and especially propagation and transactional settings correctly. If these are not configured properly, there will be errors related to closed sessions when some entity is lazily initialized in the view and it fails due to the session having been closed already.
I definately would go with option 1. Option 2 might be needed sometimes, but I see absolutely no reason to use option 3. Option 4 is also a no no. Eagerly fetching everything kills the performance of any view that needs to list just a few properties of some parent entities (orders in tis case).
N+1 Selects
During development there will be N+1 selects as a result of initializing some relationships in the view. But this is not a reason to discard the pattern. Just fix these problems as they arise and before delivering the code to production. It's as easy to fix these problems with OEMIV pattern as it's with any other pattern: add the proper dao or service methods, fix the controller to call a different finder method, maybe add a view to the database etc.
I have successfully used the Open-Session-in-View pattern on a project. However, I recently read in "Spring In Practice" of an interesting potential problem with non-repeatable reads if you manage your transactions at a lower layer while keeping the Hibernate session open in the view layer.
We managed most of our transactions in the service layer, but kept the hibernate session open in the view layer. This meant that lazy reads in the view were resulting in separate read transactions.
We managed our transactions in our service layer to minimize transaction duration. For instance, some of our service calls resulted in both a database transaction and a web service call to an external service. We did not want our transaction to be open while waiting for a web service call to respond.
As our system never went into production, I am not sure if there were any real problems with it, but I suspect that there was the potential for the view to attempt to lazily load an object that has been deleted by someone else.
There are some benefits of DTO approach though. You have to think beforehand what information you need. In some cases this will prevent you from generating n+1 select statements. It helps also to see where to use eager fetching and/or optimized views.
I'll also throw my weight behind the Open-Session-in-View pattern, having been in the exact same boat before.
I work with Stripes without spring, and have created a manual filter before that tends to work well. Coding transaction logic on the backend turns messy really quick as you've mentioned. Eagerly fetching everything becomes TERRIBLE as you map more and more objects to each other.
One thing I want to add that you may not have come across is Stripersist and Stripernate - Stripersist being the more JPA flavor - auto-hydration filters that take a lot of the work off your shoulders.
With Stripersist you can say things like /appContextRoot/actions/view/3 and it will auto-hydrate the JPA Entity on the ActionBean with id of 3 before the event is executed.
Stripersist is in the stripes-stuff package on sourceforge. I now use this for all new projects, as it's clean and easily supports multiple datasources if necessary.
Does the Order and Order Lines compose a high volume of data? Do they take part in online processes where real-time response is required? If so, you might consider not using eager fetching - it does make a huge diference in performance. If the amount of data is small, there is no problem in eager fetching.
About using DTOs, it might be a viable implementation.
If your business layer is used internally by your own application (i.e a small web app and its business logic) it'd probably be best to use your own entities in your view with open session in view pattern since it's simpler.
If your entities are used by many applications (i.e a backend application providing a service in your corporation) it'd be interesting to use DTOs since you would not expose your model to your clients. Exposing it could mean you would have a harder time refactoring your model since it could mean breaking contracts with your clients. A DTO would make that easier since you have another layer of
abstraction. This can be a bit strange since EJB3 would theorically eliminate the need of DTOs.