In my recent training sessions on the (core) Spring Framework, I was asked, "If there was one thing that a (Java) Spring developer should know, what should that be?" That question caught me off guard. Yes, the (core) Spring Framework does cover a lot of areas (e.g. beans, configuration, aspect-oriented programming, transactions). And it was difficult for me to point out just one thing. I ended up mentioning everything that we covered in our (3 day) training course.
As I gave that question more thought, I began to think about the most important one. I ended up thinking of how Spring uses aspects to add behavior to managed objects (usually called beans) as the most important. This is how the Spring Framework supports transactions, security, scope, Java-based configuration, among others. And I'm sharing my thoughts here in this post.
ORM and Lazy Loading Exceptions
Most developers who use some form of ORM have encountered an exception that signifies that child entities could not be loaded (e.g. LazyInitializationException).
Some developers who have encountered this would use an "open session in view" (OSIV) pattern to keep the session open and prevent this exception from happening. But I find this to be an overkill. Worse, some developers consider the "open session in view" pattern to be the only solution. A possible underlying cause for this misconception could be that the developer is probably not armed with the knowledge of using the Spring Framework effectively to keep the ORM session open longer.
In the case of JPA, the "open entity manager in view" pattern will create an entity manager at the beginning of the request, bind it to the request thread, and close it when the response is completed.
So, if not the OSIV pattern, what would be a better solution?
The short answer is to use the Spring Framework to keep the session open for the duration that you need it (e.g. @Transactional). Keep on reading as I'll provide a longer answer.
Services and Repositories
In a layered architecture, a typical design pattern is to define a domain or application service (usually defined as an interface) to provide business functionality (e.g. start using a shopping cart, adding items to that shopping cart, searching for products). Domain and application service implementations would typically delegate the retrieval/persistence of domain entities to repositories.
| Presentation Layer |
| Business Layer |
| Data Access (or Infrastructure) Layer |
Repositories (or data access objects) are also defined as interfaces to retrieve/persist domain entities (i.e. provide ORM and CRUD access). Naturally, repository implementations use ORM libraries (e.g. JPA/Hibernate, myBATIS) to retrieve and persist domain entities. With this, it uses the ORM framework's classes to connect to the persistent store, retrieve/persist the entity, and close the connection (called session in Hibernate). There's no problem of lazy loading failures at this point.
The problem of lazy loading failures occur when the service retrieves a domain entity using the repository, and wants to load child entities (after the repository method has returned). By the time the repository returns the domain entity, the ORM session gets closed. Because of this, attempts to access/load child entities in the domain service cause an exception.
The code snippets below illustrate how a lazy loading exception can occur when the child items of an order entity is lazily loaded after being returned by the repository.
@Entity
public class Order {
@OneToMany // defaults to FetchType.LAZY
private List<OrderItem> items;
…
public List<OrderItem> getItems() {…}
}
public class SomeApplicationServiceImpl implements SomeApplicationService {
private OrderRepository orderRepository;
…
@Override
public void method1(…) {
…
order = orderRepository.findById(...);
order.getItems(); // <-- Lazy loading exception occurs!
…
}
…
}
public class OrderRepositoryImpl implements OrderRepository {
@PersistenceContext
private EntityManager em;
…
@Override
public Order findById(...) {...}
…
}
The repository implementation explicitly uses JPA for its ORM (as illustrated with the use of an EntityManager).
At this point, some developers may opt to use eager fetch to prevent the lazy initialization exception. Telling the ORM to eagerly fetch the child items of an order entity will work. But sometimes, we don't need to load the child items. And eagerly loading this might be unnecessary overhead. It would be great to only load it when we need it.
To prevent the lazy initialization exception (and not be forced to eagerly fetch), we'll need to keep the ORM session open until the calling service method returns. In Spring, it can be as simple as annotating the service method as @Transactional to keep the session open. I find that this approach is better than using "open session in view" pattern (or being forced to use eager fetching), since it keeps the session open only for the duration that we intend it to be.
public class SomeApplicationServiceImpl implements SomeApplicationService {
private OrderRepository orderRepository;
…
@Override
@Transactional // <-- open the session (if it's not yet open)
public void method1(…) {
…
order = orderRepository.findById(...);
order.getItems(); // <-- Lazy loading exception should not happen
…
}
…
}
Domain Entities in the Presentation Layer
Even after keeping the ORM session open in the service layer (beyond the repository implementation objects), the lazy initialization exception can still occur when we expose the domain entities to the presentation layer. Again, because of this, some developers prefer the OSIV approach, since it will also prevent lazy initialization exceptions in the presentation layer.
But why would you want to expose domain entities in the presentation layer?
From experience, I've worked with teams who prefer to expose domain entities in the presentation layer. This usually leads to anemic domain model, since presentation layer frameworks need a way to bind input values to the object. This forces domain entities to have getter and setter methods, and a zero-arguments constructor. Having getters and setters will make invariants difficult to enforce. For simple domains, this is workable. But for more complex domains, a richer domain model would be preferred, as it would be easier to enforce invariants.
In a richer domain model, the objects that represent the presentation layer input/output values are actually data transfer objects (DTOs). They represent inputs (or commands) that are carried out in the domain layer. With this in mind, I prefer to use DTOs and maintain a richer domain model. Thus, I don't really run into lazy initialization exceptions in the presentation layer.
Aspects to add behavior to managed objects
Spring intercepts calls to these @Transactional annotated methods to ensure that an ORM session is open.
Transactions (or simply keeping an ORM session open) are not the only behavior provided using aspects. There's security, scope, Java-based configuration, and others. Knowing that the Spring Framework uses aspects to add behavior is one of the key reasons why we let Spring manage the POJOs that we develop.
Conclusion
There you go. That for me is the one most important thing that a Spring Framework developer should know when using the core. Now that I've given my opinion as to what is the one most important thing, how about you? What do you think is the one most important thing to know when tackling Core Spring. Cheers!
