Using Spring dependency injection it is possible to swap out a dependency at run time by tweaking the XML file.
I don't think this is possible using the Scala cake pattern dependency injection since the dependencies are wired at compile time and to change the dependencies would require re-compilation ?
If so is this an advantage that Spring dependency injection has over using the Scala cake pattern for dependency injection ?
Yes using the cake pattern you will need to recompile. The whole point of the cake pattern is to do compile time dependency injection :)
With spring you will return to "classic" (in java world) runtime dependency injection, which is done through reflection.
Using runtime dependency injection, you might be able to swap out a dependency at runtime either by restarting the whole container or by using specific modules like jrebel to update only part of the context. In exchange you pay the price at application startup as your container has to parse the XML, instanciate all the objects and wire them up. In the case of spring the wiring can be fairly complex requiring multiple passes to complete.
Usually you don't want to swap out dependencies at runtime in production code (there are valid use cases for this but I have rarely encountered them).
On the other hand, the cake pattern is validated at compile time, the wiring is necessarily predetermined (forget about #PostConstruct). Avoiding complex wiring cycles is actually a good thing :) You will also avoid loading and parsing XML files (though in my understanding the current preferred configuration method is using code not XML). Errors in the wiring will appear sooner in cake giving you a quicker feedback.
Last but not least, there are other DI mechanisms available in functionnal programming languages (such as scala) such as using an IO or a Reader Monad. For more about such mechanisms you can start at :
http://blog.originate.com/blog/2013/10/21/reader-monad-for-dependency-injection/
http://fr.slideshare.net/debasishg/dependency-injection-in-scala-beyond-the-cake-pattern
or
Using Reader Monad for Dependency Injection
Related
I am new to spring Boot and i was trying to learn the differences between different dependency injection techniques.
Is there any scenario where dependency injection through field is always prefered over constructor or setter injection?
I read that field injection is always a bad practice.
If its bad then why we still do it?
Short answer: there is no info about that.
Long answer:
It's strange that spring docs don't even mention field injection in any way
DI exists in two major variants, Constructor-based dependency
injection and Setter-based dependency injection.
I haven't found any info about when field injection is preferred over two aforementioned ones, besides the time where you do not need any of the benefits of CBDI and SBDI(for example, easier testing and NPE avoidance) and you want to keep your code short, but even this argument becomes obsolete if you use Lombok. Why does this feature even exist in the first place, IMHO:
1.There was a bad design choice that lead to it's creation and use in production(Spring Framework's first version was published back in 2002 and probably there was no any debate about whether or not field injection is a bad practice at that time)
2.Backward-compatibility stopped from removing this feature because it could break already existing codebase.
I have read that dependency injection is good for testing, in that a class can be tested without its dependencies, but the question comes to my mind if Class A depends on Class B or C or any class, testing Class A independent of some class is yielding a test result of zero, not a failed or past test.
Class A was created to do something and if it is not fed anything whether using new key word or setting up the extra files in Spring, Class A won't do any work.
About the idea of making code modular, readable and maintainable: so business classes became cleaner, but all we did was shift confusion from dirty Java business classes to convoluted XML files and having to delete interfaces used to inject to our loosened objects.
In short, it seems we have to make edits and changes to a file somewhere,right?
Please feel free to put me in my place if my understanding is lacking, just a little irritated with learning Spring because I see the same amount of work just rearranged.
Dependency injection is good for unit testing because you can individually test each method without that method depending on anything else. That way each unit test can test exactly one method.
I would say that if the xml is what’s annoying you check out Spring boot. It’s based on a java configuration so no xml and it simplifies a lot of configuration for you based on your class path. When I first started spring I found the xml very daunting coming from a java background but the annotation based configuration and the auto configuring done by spring boot is extremely helpful for quickly getting applications working.
IMO biggest advantage of using the spring is dependency injection which makes your life easy. For example if you would like to create a new service with three dependencies, then you can create a class very easily using Spring. But without spring, you will end up writing different factory methods which will return you the instances you are looking for. This makes your code very verbose with static method calls. You may want to take a look at the code repositories before spring era.
Again if you would like to use Spring or not is your personal call based on project complexity. But it's other features/advantages cant be overlooked.
And XML files or Java configs are the ways of achieving spring configuration - where you would like to add your business logic is personal flavour. Only thing is you should be consistent all across your project.
I would suggest that you read Martin Fowler's great article on Inversion of Control and Dependency Injection to gain a better understanding of why frameworks like Spring can be really useful to solve a well known set of common dependency injection problems when writing software.
As others have mentioned, there is no obligation to use Spring; and whatever you can do with Spring, you can probably do it by other means like abstract factories, factory methods, or service locators.
If your project is small enough, then you probably wouldn't mind solving the dependency injection issues on your own using some design patterns like those mentioned above. However, depending on the size of your project, many would prefer to use a framework or a library that already packs a bunch of solutions to these recurrent head scratchers.
In regards to the advantages of dependency injection frameworks when doing unit testing is the idea that you don't need to test the dependencies of your class, but only your class.
For example, most likely your application has a layered design. It is very common to have a data access class or a repository that you use to retrieve data from a datasource. Logically, you also have a class where you use that DAO.
Evidently, you already wrote unit testing for your DAO, and therefore, when you're testing your business class (where the DAO is being used) you don't care about testing your DAO again.
Fortunately, since Spring requires some form of dependency injection for your DAO, this means your class must provide a constructor or a setter method through which we can inject that DAO into our business class, right?
Well, then during unit testing of your business class, you can conveniently use those injection points to inject your own fake DAO (i.e. a mock object). That way, you can focus on the testing of your business class and forget about retesting the DAO again.
Now compare this idea with other solutions you may have done on your own:
You inject the dependency directly by instantiating the DAO within your business class.
You use a static factory method within your code to gain access to the DAO.
You use a static method from a service locator within your code to gain access to the DAO.
None of these solutions would make your code easy to test because there is no simple manner to get in the way of choosing exactly what dependency I want injected into my business class while testing it (e.g. how do you change the static factory method to use a fake DAO for testing purposes?).
So, in Spring, using XML configuration or annotations, you can easily have different dependencies being injected into your service object based on a number of conditions. For example, you may have some configurations for testing that evidently would be different than those used in production. And if you have a staging environment, you may even have different XML configurations of dependencies for your application depending on whether it is running in production or integration environments.
This pluggability of dependencies is the key winning factor here in my opinion.
So, as I was saying, my suggestion to you is that you first expand your understanding of what problems Spring core (and in general all dependency injection frameworks) is trying to solve and why it matters, and that will give you a broader perspective and understanding of these problems in a way that you could to determine when it is a good idea to use Spring and when it is not.
What does mean Inversion of Control and Dependency Injection in Spring Framework? and what is difference ? Why in the Spring framework ?
Can any one explain ?
Also suggest the some books to learn Spring framework for beginners ?
I shall write down my simple understanding of this two terms:
For quick understanding just read examples*
Dependency Injection(DI):
Dependency injection generally means passing a dependent object as a parameter to a method, rather than having the method create the dependent object. What it means in practice is that the method does not have a direct dependency on a particular implementation; any implementation that meets the requirements can be passed as a parameter.
With this objects tell thier dependencies.
And spring makes it available. This leads to loosely coupled application development.
Quick Example:EMPLOYEE OBJECT WHEN CREATED,IT WILL AUTOMATICALLY CREATE ADDRESS OBJECT (if address is defines as dependency by Employee object).
Inversion of Control(IoC) Container:
This is common characteristic of frameworks,
IOC manages java objects – from instantiation to destruction through its BeanFactory. -Java components that are instantiated by the IoC container are called beans, and the IoC container manages a bean's scope, lifecycle events, and any AOP features for which it has been configured and coded.
QUICK EXAMPLE:Inversion of Control is about getting freedom, more flexibility, and less dependency. When you are using a desktop computer, you are slaved (or say, controlled). You have to sit before a screen and look at it. Using keyboard to type and using mouse to navigate. And a bad written software can slave you even more. If you replaced your desktop with a laptop, then you somewhat inverted control. You can easily take it and move around. So now you can control where you are with your computer, instead of computer controlling it.
By implementing Inversion of Control, a software/object consumer get more controls/options over the software/objects, instead of being controlled or having less options.
Inversion of control as a design guideline serves the following purposes:
There is a decoupling of the execution of a certain task from implementation.
Every module can focus on what it is designed for.
Modules make no assumptions about what other systems do but rely on their contracts.
Replacing modules has no side effect on other modules I will keep things abstract here, You can visit following links for detail understanding of the topic.
A good read with example
Detailed explanation
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.
As far as I understand, main purpose of dependency injection is to have all dependencies separated declaratively, so that we can easily review and change the dependency structure easily...right?
Then by using dependency annotations spread through out the code, aren't we just going back to non-centralized system (similar to simple new operator), which is harder to tweak?
#Autowired/#Inject annotations usually declare dependencies on interfaces rather than on concrete classes (as in the case of new), thus you still can control which implementations should be injected by controlling which beans are declared in the context. Also, these dependencies can be overriden manually.
#Component-family annotations can be controlled as well, since you can exclude particular classes from component scanning.
The purpose of dependency injection is to decouple the declaration of dependencies from the actual satisfying of those dependencies. How the declaration is done is an orthogonal issue.
#Autowired is a form of dependency declaration. Using #Autowired supports encapsulation. A class' injected dependencies are documented directly in the code instead of in another file.
These types of discussions have tendency to become religious so I'll stear clear of the "main purpose" definition and the semantics of whether this or that pattern is really and truly fulfilled.
I try to look at it like a tool that can offer certain features. For instance, using Spring (DI) is a good way to separate interfaces and implementations. Users of a particular interface need not know how to create the implementation (or where it resides). This is often something good. Using Spring also enables a whole lot of other stuff: AOP and AOP-driven features like transaction handling, scopeing and a whole bunch of pre-built integrations to other frameworks and technologies. Annotations make a lot of this easier and clearer and best of all, I don't have to use them where it's not practical or possible - there is always the option to configure it in XML instead.