Sorry for potentially a silly question - but is it acceptable with Spring to create a one dedicated service, let's say ServiceFacade, inject 20-30 other services into it and then pass such a ServiceFacade reference as a parameter to different business logic? Will such approach lead to issues within the application?
Yes, it is possible, Spring will correctly handle a bean with 20-30 other dependencies. However it is discouraged from a design point of view. Instead of one ServiceFacade you might have multiple facades, each with manageable number of dependencies, e.g. 5 and a factory returning different facades instances.
Related
Modern Spring recommendations (Spring 4.x onwards) recommend not using field injection (with #Autowired or #Inject), and instead preferring constructor arguments. Several discussions about this here, here and the Spring Framework itself recommends constructor-based DI for mandatory dependencies.
In general I agree perfectly with the rationale, and in fact I haven't used any of the field injection mechanisms for years now.
What is bugging me however is the way we inject Spring Data Repositories. Of course these are mandatory dependencies, the service needs them to work. When one looks at the Spring Data documentation one finds that the reference documentation itself is using #Autowired field injection all over the place, precisely what was recommended as bad practice in the Spring Framework documentation itself.
Of course, it is no problem to have repositories use constructor injection. I have been using this for quite some time. However, sometimes components and services need quite a number of repositories. A service might be interacting with 7 or 8 database tables, and they would need to be in the same transaction.
How does one go about this to have neat code? Using constructor arguments is the safer approach, but leads you to have a huge number of constructor arguments. Setter injection does not seem to fit, these are not optional dependencies. Field injection is supposed to be plain wrong (even though the Spring Data doc uses it).
Is there any other approach one could use? I was tempted to aggregate related repositories in a separate #Component, at least the service constructor gets only one argument with this constructor. It does help a bit, but makes it even harder to interact with a new table (the new repository would need to be added to the aggregate, and the service would need to call the getter for it).
How is it supposed to be done neatly without violating any safety recommendations and adhering to coding standards (few constructor arguments etc.)?
I have a Spring Integration project that has several flows (some where between 10-15). I would like to keep my namespace clean since several flows might have similar sounding components (for ex - several flows might have a channel named fileValidatorChannel). I think I have a couple of different options to keep names from colliding with each other:
A. Preface every component name with the flow that it belongs to. For ex - flowAFileValidatorChannel, flowBFileValidatorChannel, etc
B. Create a context hierarchy where every flow is it's own context and every flow inheriting from a master context where all the common beans/sub-flows are.
What's the better approach? Is there are better way to keep my name space clean?
To be honest your problem isn't clear.
Any Spring Integration component is a bean finally. So, their ids are just to distinguish them from other bean.
Let's imaging if you don't have Spring Integration in your application. So, you would worry about some clean naming strategy for all your beans anyway?
From other side consider to use Spring Integration Flow project:
The goal is to support these, and potentially other semantics while providing better encapsulation and configuration options. Configuration is provided via properties and or referenced bean definitions. Each flow is initialized in a child application context. This allows you to configure multiple instances of the same flow differently.
I'm a newbie to Spring Framework and of course first thing comes to mind about spring is dependency injection. Now i could be wrong since i just started learning about Spring framework (esp. about dependency injection) but i think that dependency injection of beans to said objects is not meant for transaction data. Since the bean definition is for example defined in the spring.xml (a blue print) it is not meant for transactional data but rather for static and small amount of data. I don't see that there's any way to inject thousands of transactional objects into another object using dynamic XML (created during runtime).
So did i get this right? If that is so what's the real benefit of dependency injection?
There are several benefits from using dependency injection containers rather than having components satisfy their own dependencies. Some of these benefits are:
Reduced Dependencies
Reduced Dependency Carrying
More Reusable Code
More Testable Code
More Readable Code
These benefits are explained in more detail here.
You are right, transactional data (eg: data that represents a table row) don't normally be injected declaratively. Spring DI (dependency injection) commonly use to handle collaboration between multiple classes.
Common examples I've seen is along with DAO (data access object) and MVC (model view controller) pattern. In enterprise environment it's common to have a project with dozens or hundreds of database tables -- hence dozens / hundreds of DAO classes which get injected into controller classes.
If you don't use DI, you need to conciously manage which DAO should be created first, and which DAO should be injected into which controller etc. (this is a nightmare)
Code refactoring is (should) be a common thing as well. Business requirement always changes constantly. Without DI one simple refactoring could result in a massive and tricky untangling of 'which class depends on what and where'
Of all the articles I've read about the dependency injection, this is by far the best.
If you are using Spring for DI, I would suggest you read about #primary annotation. Spring makes it even easier to choose the implementation you want(from multiple implementations) for a given service. This article is good.
Dependency Injection and Inversion of Control change the control flow adding to a specific component the responsibility to manage de dependency graph and manage how it will be connected. The result is a great decoupling between dependant and dependency, improving the code maintainability, application reliability, and testability.
All websites state that the Spring core container is the basis for complete Spring framework i.e., it is used across
the all modules like AOP, JDBC module, Web module, etc. As per my understanding, the Spring core container's main purpose is
to inject dependencies, so avoiding the need of factory classes and methods. Is that correct?
Second question: When it is said, Spring core container is the basis for complete Spring framework (e.g., for Spring AOP). As per my understanding, in Spring AOP also, getting the object of classes like
ProxyFactoryBean is achieved by core container. Right?
Thirdly, it is stated that Spring core container avoids the need for programming the use of singletons. How come singleton
classes are avoided by core container?
yep
yep
All beans declared in Spring config files are singleton by default. They are instantiated when your application starts.
First off, your understanding of what you get from Spring is about right. So let's get on to your third question, the interesting one.
The key is it's not that you don't have singletons, it's that they're singletons by configuration. This is a vital difference, as it means you can avoid all the complicated singleton enforcement code (the source of frequent problems) and instead just write exceptionally simple programs that focus on the business end of things. This is particularly important when you are writing a program with non-trivial object lifetimes: for example, in a webapp it makes it very easy to manage the lifespan of objects that hold state associated with a user's session, since if the objects have session scope, they'll be “singleton per user session”. That's enormously easier to work with than many of the alternatives.
The fact that Spring can also help out with transactions is just perfect as transaction handling is distinctly non-trivial, and AOP is the best solution to them in Java that I've seen (other languages have other options open) with Spring supporting a pretty straight-forward way of doing it. Try to do it properly without if you don't believe me. Spring's pretty much wonderful.
I am a big fan of Dependency Injection and the Play Framework, but am having trouble seeing how the two could be exploited together.
There are modules for Spring and Guice, but the way that Play works makes it hard for me to see how DI could be beneficial beyond some quite simple cases.
A good example of this is that Play expects JPA work to be done by static methods associated with the entity in question:
#Entity
Person extends Model {
public static void delete(long id) {
em().find(id).remove();
}
//etc
}
So there is no need for a PersonManager to be injected into controllers in the way it might for a Spring J2EE application. Instead a controller just calls Person.delete(x).
Obviously, DI is beneficial when there are interfaces with external systems, as the concrete implementation can be mocked for testing etc., but I don't see much benefit for a self-contained Play application.
Does anyone have any good examples? Does anyone use it to inject a Manager-style class into Controllers so that a number of operations can be done within the same transaction, for example?
I believe from this sentence you wrote:
"Does anyone have any good examples? Does anyone use it to inject a Manager-style class into Controllers so that a number of operations can be done within the same transaction, for example?"
that before answering the DI question I should note something: transactions are managed automatically by Play. If you check the model documentation you will see that a transaction is automatically created at the beginning of a request, and committed at the end. You can roll it back via JPA or it will be rolled back if an exception is raised.
I mention this because from the wording of your sentence I'm not sure if you are aware of this.
Now, on DI itself, in my (not-so-extensive) experience with DI, I've seen it used mainly to:
Load the ORM (Hibernate) factory/manager
Load Service classes/DAOs into another class to work with them
Sure, there are more scenarios, but these probably cover most of the real usage. Now:
The first one is irrelevant to Play as you get access to your JPA object and transaction automatically
The second one is quite irrelevant too as you mainly work with static methods in controllers. You may have some helper classes that need to be instantiated, and some may even belong to a hierarchy (common interface) so DI would be beneficial. But you could just as well create your won factory class and get rid of the jars of DI.
There is another matter to consider here: I'm not so sure about Guice, but Spring is not only DI, it also provides a lot of extra functionalities which depend on the DI module. So maybe you don't want to use DI in Play, but you want to take advantage of the Spring tools and they will use DI, albeit indirectly (via xml configuration).
The problem in my humble opinion on the static initialization approach of Play! is that it makes testing harder. Once you approach the HTTP vs Object Orientation problem with static members and objects that carries the HTTP message data (request and response) you make a trade of having to create new instances for each request by the ability of make your objects loosely coupled with the rest of your project classes.
One good example of a different design are servlets, it also extends a base class but it approaches the problem with a single instance creation (by default, because there are configurations that enable more instances).
I believe that maybe a mix of the two approaches would be better, having a singleton of each controller would give the same characteristics of a full static class and would allow dependency injection of some kinds of object. But not the objects with request or session scope, once the controller would need to be created every new request. Moreover it would improve testability by inverting the control of dependency injection, thus allowing arbitrary injection points.
Dependencies would be injected by the container or by a test, probably using mocks for the heavy stuff that much likely would already have been tested before.
In my point of view, this static model pushes the developer away from testing controllers because extending FunctionalTest starts the application server, thus paying the price of heavy objects like repositories, services, crawlers, http clients, etc. I don't want to wait a lot of objects to be bootstrapped just to check if some code was executed on the controller, tests should be quick and clear to make developers love them as their programming assistant/guide.
DI is not the ultimate solution to use everywhere... Don't use DI just because you have it in your hands... In play, you don't need DI to develop controllers/models etc... but sometimes it could be a nice design: IMO, you could use it if you have a service with a well know interface but you would like to develop this service outside Play and test it outside play and even test your play project with just a dummy service in order NOT to depend on the full service implementation. Therefore DI can be interesting: you plug the service loosely in play. In fact, this is the original use case for DI afaik...
I just wrote a blog post about setting up a Play Framework application with Google Guice. http://geeks.aretotally.in/dependency-injection-with-play-framework-and-google-guice
I see some benefits, especially when a component of your application requires a different behavior based on a certain context or something like that. But I def believe people should be selective about what goes into a DI context.
It shows again that you should only use dependencies injection if you really have a benefit. If you have complex services it's useful, but in many cases it's not. Read the chapter about models in the play-documentation.
So to give you an example where you can use DI with play. Perhaps you must make a complex calculation, or you create a pdf with a report-engine. There I think DI can be useful, specially for testing. There I think the guice-module and spring-module are useful and can help you.
Niels
As of a year and some change later, Play 2.1 now has support for dependency injection in controllers. Here's their demo project using Spring 3, which lays it out pretty clearly.
Edit: here's another example using Guice and Scala, if that's your poison.