If it was created by the class that is injecting it into the object that uses it then isn't it just moving object creation one step up the stack? And wouldn't this mean that all objects needed by lower level classes would need to be passed through each object one at a time until it reached the object that needed it?
All objects and their dependencies could be set up at the very beginning, but wouldn't this dent performance as objects will be hanging around until needed?
Yes, it is moving the object instantiation up the stack. But it is moving it up the stack to a place where you can make a better decision as to which implementation to actually use. If I want to replace my data access layer with a stubbed version to do performance testing of the business logic, I can without changing a single line of the business logic code.
There are may ways to inject your dependencies. In my case, I use constructor injection everywhere. Using this method, if a lower level class needs a dependency, it just puts the interface for that dependency in its constructor. No need to pass from a class higher up in the stack. If you need the same instance in both classes, then you should look at the lifestyle/scope of when registering your dependency into the container so that both classes happen to get passed the same instance.
Some DI implementations use lazy loading to instantiate their objects. (i.e. it's not until the object is attempted to be used that it is actually instantiated) Some do not. Also, you would need quite the large dependency graph to make a dent in performance. Keep your constructors simple and fast (a good practice anyway) and this will not be a problem, I assure you. And DI containers are smart about releasing objects that are no longer in use (again, pay special attention to lifestyle/scope).
I hope this helps.
Related
I am bit new to dependency injection & came across subjected question while working.
Lets say I have a class 'Employee' which has one method this method say 'Promote' gets conditionally called that too on rarest scenario.
'Promote' method uses an object of 'ValueAddition', now is it best practice to have this objected injected via constructor & user global object or should I
resolve the dependency in method itself?
What is recommended best practice? or any pointer on the life time of resolved dependency would be helpful.
In general you should always look to inject dependencies rather than use service location (ie, resolving a dependency directly from the inversion of control container). There's a great, pretty well known blog article here called "Service Locator is an Anti-Pattern" that can help clarify the reasons.
You may run into other issues as you expand that seem to point you to using service location. In general, you really should design around those issues instead of falling back to service location.
I only need the object in one method. Consider moving the functionality of the method to a different object. For example, maybe the Promote() method should be on an IEmployeePromoter where you resolve that and it takes the special object. Then you'd call Promote(Employee) and pass in the employee rather than changing the dependencies for an Employee. Sometimes your inversion of control container will have a way to generate a factory (like Func<T> or Lazy<T>) to help you defer resolution until you actually need it.
I have way too many dependencies. Usually this means your object is doing too much. The single responsibility principle can help you refactor your objects to be a more manageable size. Smaller objects generally need fewer dependencies because they have far fewer concerns to manage.
There's a good amount of documentation and books out there on the pattern of dependency injection. It might be good to widen the scope of your search to looking at the patterns and practices in general when not applied to a particular framework (or even language) and it can help inform you of best practices in your own code.
When should I use property injection?
Should I use by default constructor injection if instance creation in fully controlled?
Am I right that using a constructor injection I write container-agnostic code?
When should I use property injection?
You should use property injection in case the dependency is truly optional, when you have a Local Default, or when your object graph contains a cyclic dependency.
Property Injection however causes Temporal Coupling and when writing Line of Business applications, your dependencies should never be optional: you should instead apply the Null Object pattern.
Neither should you use a Local Default, which is:
"A default implementation of an abstraction that's defined in the same assembly as the consumer." [DIPP&P, Section 4.2.2]
Local Defaults should not be used in Line of Business applications, because it complicates testing, hides the dependency, and makes it easy to forget to configure the dependency.
Neither should object graphs have cyclic dependencies. This is an indication of a problem in your application design.
Should I use by default constructor injection if instance creation in fully controlled?
Yes. Constructor injection is the best way. It makes it very easy to see which dependencies a class has, makes it possible to make dependencies required, and prevents Temporal Coupling.
Am I right that using a constructor injection I write container-agnostic code?
This is correct. Constructor injection allows you to delay the decision of which DI library to use, and whether at all you use a DI library.
For a more detailed explanation of the above, and much more, read the book Dependency Injection Principles, Practices, and Pattern (DIPP&P) by Mark Seemann and myself.
The usefulness of property injection is so limited, that while working on the book, Mark and I even discussed labeling Property Injection an anti-pattern. In the end we felt we couldn't make a case that was as strong as, for instance, Ambient Context, which we did decide to describe as an anti-pattern in that edition. The case for Ambient Context was crystal clear, while being much muddier for Property Injection. But this is why, however, we added many warning notes to the Property Injection section (4.4), because we feel strongly Property Injection is not a good solution for the majority of cases.
There are several problems, though, that Property Injection seems to solve at first, such as the problem of Constructor Over-Injection (where a constructor contains many dependencies). Constructor Over-Injection, however, is almost always caused by design deficits, such as:
Class having too many responsibilities
Lack of natural clusters
Choosing inheritance over composition
Application of Cross-Cutting Concerns through base classes instead of Decorators or Interceptors
The accepted answer argues in favor of constructor injection and takes a rather critical stance toward property injection. As such, it does not put focus on addressing the problems that property injection actually solves, if used correctly. Therefore I want to take the opportunity to address some of these points and also to provide some counter arguments to the accepted answer.
When should I use property injection?
Imagine you have a project with 100+ controllers and all those controllers are extending a custom base controller (parent-service). In such a situation, where a service is extended by several child-services, employing constructor injection is a burden: For every constructor you create, you need to relay arguments to your parent-service's constructor. If you decide to extend your parent service's constructor signature you will also be forced to extend the signatures of all child-services' constructors.
To make this example more vivid, say you start your project with a base controller having a parameterless constructor.
Now after a month you decide you want a logger service in your base controller. → Not only will you have to change the base controller constructor's signature but also those of your 100+ child controllers.
Now again after a month you need access to a user service in your base controller → Again, you'll have to change the constructor signature of your 100+ child controllers.
you get the point...
With property injection you can easily circumvent this whole inconvenience by simply adding the necessary properties to your parent service and letting your DI-mechanism handle the injection via reflection. As a side effect, this also greatly reduces the risk of merge conflicts (since the files touched is reduced to a minimum).
So far, I have been talking mostly about controllers but this example counts for any situation in which you have a service hierarchy – the deeper or broader this hierarchy gets, the greater the burden of constructor injection. However, avoiding service-hierarchies altogether may not always be a reasonable choice in a project.
One could say the decision between property and constructor injection is really a decision between a pragmatism and OOP "purism".
From a "purist" OOP perspective, the rule is (as stated in the accepted answer) to initialize all required fields of a class via its constructor in order to avoid granting any access to the newly created instance in an "unfinished" state (which may result in an exception being later thrown).
With reference to this rule, an OOP-purist has a valid point in saying that property injection (temporarily) leaves your services in an "unfinished" state (the timespan between when your constructor has returned and the moment your property is injected) and that this increases the risk that your application may break.
However, when talking about services managed by an IoC/DI container, this risk is practically reduced to zero if you consider that your DI-mechanism is responsible for resolving the dependency graph and wiring everything up before any user-action or api-request actually makes it into your system or needs being processed. For example, at the time a controller's action is invoked you can be sure that your services were properly wired up and injected into your controller's properties (given of course, that you configured them correctly in advance).
Also the argument that it is only possible with constructor injection to make your dependencies "required" is rather weak in a world where you are not responsible for manually injecting services into your classes but delegate this task to your IoC mechanism. Even worse, you may get a false sense of security because you stated via constructor that a ServiceX requires ServiceY – but if you forgot to register your ServiceY with your DI-mechanism, you merely get null injected into your ServiceX's constructor.
Another "argument" against property injection is that it becomes harder for your fellow programmers to distinguish between properties that are managed by the DI mechanism and those which are simply non-DI-related. However, in this case you can just use a marker attribute to "opt-in" for DI or add short comments above your properties to clear things up, when the case is not clear. Also, in a service-class, it is rather unusual to have properties referring to other services that are not supposed to be manged by your DI mechanism.
Finally, as for saying that constructor injection makes unit testing easier (since you know what dependencies are required by a class), I would simply argue, that with property injection you will soon enough notice that you forgot to include a dependency when your tests begin to fail due to a certain service being undefined.
Should I use by default constructor injection if instance creation in fully controlled?
With all the above being said, I think I can answer your second question with: Not necessarily.
It depends on your project's size, the kind of service-hierarchy you employ, how often your parent-services' dependencies change and how much time and resources you are willing to invest in managing parameters and passing arguments up the service-hierarchy.
Am I right that using a constructor injection I write container-agnostic code?
Yes! – Under the premise that you are not injecting the container itself... which you shouldn't! ;)
All the above being said, here are some quotes from Martin Fowler's great discussion on dependency injection directly addressing the question of constructor vs setter/property injection, and I can fully subscribe to the last quote :)
If you have multiple constructors and inheritance, then things can get particularly awkward. In order to initialize everything you have to provide constructors to forward to each superclass constructor, while also adding you own arguments. This can lead to an even bigger explosion of constructors.
Despite the disadvantages my preference is to start with constructor injection, but be ready to switch to setter injection as soon as the problems I've outlined above start to become a problem.
This issue has led to a lot of debate between the various teams who provide dependency injectors as part of their frameworks. However it seems that most people who build these frameworks have realized that it's important to support both mechanisms, even if there's a preference for one of them.
A final remark: If you, for some reason, want to switch back from property injection to constructor injection, no problem, you can always add a constructor with the parameters to be injected and assign the properties via your constructor – dead-simple.
A best practice in DI I've read in a few places is not to inject object B just to get at object C, but to inject C instead.
But if a single method from C is all that is required, would you just inject that method instead of C?
If so, what about if a few methods from C were required? Is there a point that it's just more convenient to pass in the full object and live with the fact that you're getting stuff you have no interest in?
Or does that point indicate that maybe class C has too many varied responsibilities and needs to be extracted into multiple smaller classes, the objects of which can then be injected without as much baggage?
Don't be afraid to state the obvious, this is all new to me.
If the dependency has (many) more methods than you care about, it's a pretty good sign that it's a Header Interface that violates the Interface Segregation Principle.
If you have control over the interface, I'd suggest splitting it up into several smaller Role Interfaces. You can still have one concrete class implementing more than one Role Interface if that makes more sense for your specific implementation.
If you don't control the design of the dependency, I'd tend towards injecting the whole interface, as it still represents a cohesive collection of behavior (even if we don't agree with the design choice of the original designer). You might need more of that behavior later on.
There are a lot of questions out there about whether singletons are "bad," and what patterns to use instead. They're generally focused on the singleton design pattern, which involves retrieving the singleton instance from a static method on the class. This is not one of those questions.
Ever since I really "discovered" dependency injection several months back, I've been driving its adoption in our team, removing static and singleton patterns from our code over time, and using constructor-based injection wherever possible. We've adopted conventions so that we don't have to keep adding explicit bindings to our DI modules. We even use the DI framework to provide logger instances, so that we can automatically tell each logger which class it's in without additional code. Now that I have a single place where I can control how various types are bound, it's really easy to determine what the life cycle of a particular category of classes (utility classes, repositories, etc.) should be.
My initial thinking was that there would probably be some advantage to binding classes as singletons if I expected them to be used fairly often. It just means there's a lot less newing going on, especially when the object you're creating ends up having a big dependency tree. Pretty much the only non-static fields on any of these classes are those values getting injected into the constructor, so there's relatively little memory overhead to keeping an instance around when it's not being used.
Then I read "Singleton I love you, but you're bringing me down" on www.codingwithoutcomments.com, and it made me wonder whether I had the right idea. After all, Ninject compiles object-creation functions by default, so there's very little reflection overhead involved in creating additional instances of these objects. Because we're keeping business logic out of the constructor, creating new instances is a really lightweight operation. And the objects don't have a bunch of non-static fields, so there isn't a lot of memory overhead involved in creating new instances, either.
So at this point, I'm beginning to think that it probably won't matter much either way. Are there additional considerations I haven't taken into account? Has anyone actually experienced a significant improvement in performance by changing the life cycles of certain types of objects? Is following the DI pattern the only real important thing, or are there other reasons that using a single instance of an object can be inherently "bad?"
I am using singleton instances to cache expensive to create objects (like nhibernate session factory) or for objects I want to share across the application.
If in uncertainty I would rather have the object be recreated every time it is used and mark it as singleton or "per thread"/"per request"/"per whatever" only if you really need to.
Scattering singletons all over the place to potentially save some newing up is premature optimization and can lead to really nasty bugs because you can't be sure if the object is new or shared across multiple objects/instances.
Even if the object has no state at the moment and seems save to share, someone can refactor later and add state to it which then may be unintentionally shared between different objects/instances.
I am not sure if memory management is the big issue unless you are creating A LOT of objects. I found using singleton from my IoC container helpful when dealing with something like caching, where creating the caching object and making a connection to the caching servers is expensive during creation so we create the caching object once and reuse it.
A single instance can be harmful however cause it can create a bottleneck in performance as you have a single instance serving multiple requests in the case of services.
It shouldn't matter much for memory usage, but you'll get increased modularity, easier testability with mock objects, and the aesthetic ugliness of having to write out dependencies explicity will encourage programmers to make each class more orthogonal and independent. It's easier to gloss over dependencies when your class is calling globals, which a singleton basically is, and it can make your life harder later.
As noted in Gary's answer, the whole point of DI is testability (you can easily mock out all references to a class since they are all listed in the constructor), not runtime performance.
You want to do TDD (test-drive development), right?
I've had a certain feeling these last couple of days that dependency-injection should really be called "I can't make up my mind"-pattern. I know this might sound silly, but really it's about the reasoning behind why I should use Dependency Injection (DI). Often it is said that I should use DI, to achieve a higher level of loose-coupling, and I get that part. But really... how often do I change my database, once my choice has fallen on MS SQL or MySQL .. Very rarely right?
Does anyone have some very compelling reasons why DI is the way to go?
Two words, unit testing.
One of the most compelling reasons for DI is to allow easier unit testing without having to hit a database and worry about setting up 'test' data.
DI is very useful for decoupling your system. If all you're using it for is to decouple the database implementation from the rest of your application, then either your application is pretty simple or you need to do a lot more analysis on the problem domain and discover what components within your problem domain are the most likely to change and the components within your system that have a large amount of coupling.
DI is most useful when you're aiming for code reuse, versatility and robustness to changes in your problem domain.
How relevant it is to your project depends upon the expected lifespan of your code. Depending on the type of work you're doing zero reuse from one project to the next for the majority of code you're writing might actually be quite acceptable.
An example for use the use of DI is in creating an application that can be deployed for several clients using DI to inject customisations for the client, which could also be described as the GOF Strategy pattern. Many of the GOF patterns can be facilitated with the use of a DI framework.
DI is more relevant to Enterprise application development in which you have a large amount of code, complicated business requirements and an expectation (or hope) that the system will be maintained for many years or decades.
Even if you don't change the structure of your program during development phases you will find out you need to access several subsystems from different parts of your program. With DI each of your classes just needs to ask for services and you're free of having to provide all the wiring manually.
This really helps me on concentrating on the interaction of things in the software design and not on "who needs to carry what around because someone else needs it later".
Additionally it also just saves a LOT of work writing boilerplate code. Do I need a singleton? I just configure a class to be one. Can I test with such a "singleton"? Yes, I still can (since I just CONFIGURED it to exist only once, but the test can instantiate an alternative implementation).
But, by the way before I was using DI I didn't really understand its worth, but trying it was a real eye-opener to me: My designs are a lot more object-oriented as they have been before.
By the way, with the current application I DON'T unit-test (bad, bad me) but I STILL couldn't live with DI anymore. It is so much easier moving things around and keeping classes small and simple.
While I semi-agree with you with the DB example, one of the large things that I found helpful to use DI is to help me test the layer I build on top of the database.
Here's an example...
You have your database.
You have your code that accesses the database and returns objects
You have business domain objects that take the previous item's objects and do some logic with them.
If you merge the data access with your business domain logic, your domain objects can become difficult to test. DI allows you to inject your own data access objects into your domain so that you don't depend on the database for testing or possibly demonstrations (ran a demo where some data was pulled in from xml instead of a database).
Abstracting 3rd party components and frameworks like this would also help you.
Aside from the testing example, there's a few places where DI can be used through a Design by Contract approach. You may find it appropriate to create a processing engine of sorts that calls methods of the objects you're injecting into it. While it may not truly "process it" it runs the methods that have different implementation in each object you provide.
I saw an example of this where the every business domain object had a "Save" function that the was called after it was injected into the processor. The processor modified the component with configuration information and Save handled the object's primary state. In essence, DI supplemented the polymorphic method implementation of the objects that conformed to the Interface.
Dependency Injection gives you the ability to test specific units of code in isolation.
Say I have a class Foo for example that takes an instance of a class Bar in its constructor. One of the methods on Foo might check that a Property value of Bar is one which allows some other processing of Bar to take place.
public class Foo
{
private Bar _bar;
public Foo(Bar bar)
{
_bar = bar;
}
public bool IsPropertyOfBarValid()
{
return _bar.SomeProperty == PropertyEnum.ValidProperty;
}
}
Now let's say that Bar is instantiated and it's Properties are set to data from some datasource in it's constructor. How might I go about testing the IsPropertyOfBarValid() method of Foo (ignoring the fact that this is an incredibly simple example)? Well, Foo is dependent on the instance of Bar passed in to the constructor, which in turn is dependent on the data from the datasource that it's properties are set to. What we would like to do is have some way of isolating Foo from the resources it depends upon so that we can test it in isolation
This is where Dependency Injection comes in. What we want is to have some way of faking an instance of Bar passed to Foo such that we can control the properties set on this fake Bar and achieve what we set out to do, test that the implementation of IsPropertyOfBarValid() does what we expect it to do, i.e. return true when Bar.SomeProperty == PropertyEnum.ValidProperty and false for any other value.
There are two types of fake object, Mocks and Stubs. Stubs provide input for the application under test so that the test can be performed on something else. Mocks on the other hand provide input to the test to decide on pass\fail.
Martin Fowler has a great article on the difference between Mocks and Stubs
I think that DI is worth using when you have many services/components whose implementations must be selected at runtime based on external configuration. (Note that such configuration can take the form of an XML file or a combination of code annotations and separate classes; choose what is more convenient.)
Otherwise, I would simply use a ServiceLocator, which is much "lighter" and easier to understand than a whole DI framework.
For unit testing, I prefer to use a mocking API that can mock objects on demand, instead of requiring them to be "injected" into the tested unit from a test. For Java, one such library is my own, JMockit.
Aside from loose coupling, testing of any type is achieved with much greater ease thanks to DI. You can put replace an existing dependency of a class under test with a mock, a dummy or even another version. If a class is created with its dependencies directly instantiated it can often be difficult or even impossible to "stub" them out if required.
I just understood tonight.
For me, dependancy injection is a method for instantiate objects which require a lot of parameters to work in a specific context.
When should you use dependancy injection?
You can use dependancy injection if you instanciate in a static way an object. For example, if you use a class which can convert objects into XML file or JSON file and if you need only the XML file. You will have to instanciate the object and configure a lot of thing if you don't use dependancy injection.
When should you not use depandancy injection?
If an object is instanciated with request parameters (after a submission form), you should not use depandancy injection because the object is not instanciated in a static way.