There are many discussions around here about this, but none seem to really answer the question.
I'm currently considering using the Symfony 2 service container. The more I look at it though, the more it seems like I could just do the same thing with a service factory. Consider the following example:
services.yml
services:
my_mailer:
class: Acme\Mailer
arguments: [sendmail]
newsletter_manager:
class: Acme\Newsletter\NewsletterManager
arguments: [#my_mailer]
Now to get a newsletter manager, I do something like:
// this has been configured with the above yml file
$serviceContainer = new ServiceContainer();
$newsletterManager = $serviceContainer->get("newsletter_manager");
However, consider the following factory style code:
class ServiceContainer
{
public function getMyMailer()
{
return new Acme\Mailer("sendmail");
}
public function getNewsletterManager()
{
return new Acme\Newsletter\NewsletterManager($this->getMyMailer());
}
}
And then using it:
$serviceContainer = new ServiceContainer();
$newsletterManager = $serviceContainer->getNewsletterManager();
Is there something I'm missing here? Because I don't really see the advantage to using a DI container if a factory can do all of that for me.
I have heard using factories causes your code to "depend on the factory". I either don't understand this argument or the person arguing against factories is confused. Only the top level class (the composition root) would have a reference to the factory, just like it would be the only one with the reference to the DI container.
Is there something I'm missing here? Because I don't really see the
advantage to using a DI container if a factory can do all of that for
me.
Yes - you are using your DI container using the Service Locator (anti-)pattern instead of injecting your dependencies into your classes. Your class should not have to reference your DI container at all.
Instead there should be just a single aggregate root where you create the container and resolve all dependencies. Once you have resolved your top level class instance all of its dependencies (and the dependencies of the dependencies and so on) are then resolved using the IoC container using the Hollywood Principle - when class instances are created all of its dependencies are passed in, the instance never asks for a dependency itself.
Related
I am new to dependency injection pattern. I love the idea, but struggle to apply it to my case. I have a singleton object, let’s call it X, which I need often in many parts of my program, in many different classes, sometimes deep in the call stack. Usually I would implement this as a globally available singleton. How is this implemented within the DI pattern, specifically with .NET Core DI container? I understand I need to register X with the DI container as a singleton, but how then I get access to it? DI will instantiate classes with constructors which will take reference to X, that’s great – but I need X deep within the call hierarchy, within my own objects which .NET Core or DI container know nothing about, in objects that were created using new rather than instantiated by the DI container.
I guess my question is – how does global singleton pattern aligns/implemented by/replaced by/avoided with the DI pattern?
Well, "new is glue" (Link). That means if you have new'ed an instance, it is glued to your implementation. You cannot easily exchange it with a different implementation, for example a mock for testing. Like gluing together Lego bricks.
I you want to use proper dependency injection (using a container/framework or not) you need to structure your program in a way that you don't glue your components together, but instead inject them.
Every class is basically at hierarchy level 1 then. You need an instance of your logger? You inject it. You need an instance of a class that needs a logger? You inject it. You want to test your logging mechanism? Easy, you just inject something that conforms to your logger interface that logs into a list and the at the end of your test you can check your list and see if all the required logs are there. That is something you can automate (in contrast to using your normal logging mechanism and checking the logfiles by hand).
That means in the end, you don't really have a hierarchy, because every class you have just gets their dependencies injected and it will be the container/framework or your controlling code that determines what that means for the order of instantiation of objects.
As far as design patterns go, allow me an observation: even now, you don't need a singleton. Right now in your program, it would work if you had a plain global variable. But I guess you read that global variables are "bad". And design patterns are "good". And since you need a global variable and singleton delivers a global variable, why use the "bad", when you can use the "good" right? Well, the problem is, even with a singleton, the global variable is bad. It's a drawback of the pattern, a toad you have to swallow for the singleton logic to work. In your case, you don't need the singleton logic, but you like the taste of toads. So you created a singleton. Don't do that with design patterns. Read them very carefully and make sure you use them for the intended purpose, not because you like their side-effects or because it feels good to use a design pattern.
Just an idea and maybe I need your thought:
public static class DependencyResolver
{
public static Func<IServiceProvider> GetServiceProvider;
}
Then in Startup:
public void Configure(IApplicationBuilder app, IServiceProvider serviceProvider)
{
DependencyResolver.GetServiceProvider = () => { return serviceProvider; };
}
And now in any deed class:
DependencyResolver.GetServiceProvider().GetService<IService>();
Here's a simplified example of how this would work without a singleton.
This example assumes that your project is built in the following way:
the entry point is main
main creates an instance of class GuiCreator, then calls the method createAndRunGUI()
everything else is handled by that method
So your simplified code looks like this:
// main
// ... (boilerplate)
container = new Container();
gui = new GuiCreator(container.getDatabase(), container.getLogger(), container.getOtherDependency());
gui.createAndRunGUI();
// ... (boilerplate)
// GuiCreator
public class GuiCreator {
private IDatabase db;
private ILogger log;
private IOtherDependency other;
public GuiCreator(IDatabase newdb, ILogger newlog, IOtherDependency newother) {
db = newdb;
log = newlog;
other = newother;
}
public void createAndRunGUI() {
// do stuff
}
}
The Container class is where you actually define which implementations will be used, while the GuiCreator contructor takes interfaces as arguments. Now let's say the implementation of ILogger you choose has itself a dependency, defined by an interface its contructor takes as argument. The Container knows this and resolves it accordingly by instantiating the Logger as new LoggerImplementation(getLoggerDependency());. This goes on for the entire dependency chain.
So in essence:
All classes keep instances of interfaces they depend upon as members.
These members are set in the respective constructor.
The entire dependency chain is thus resolved when the first object is instantiated. Note that there might/should be some lazy loading involved here.
The only places where the container's methods are accessed to create instances are in main and inside the container itself:
Any class used in main receives its dependencies from main's container instance.
Any class not used in main, but rather used only as a dependency, is instantiated by the container and receives its dependencies from within there.
Any class used neither in main nor indirectly as a dependency somewhere below the classes used in main will obviously never be instantiated.
Thus, no class actually needs a reference to the container. In fact, no class needs to know there even is a container in your project. All they know is which interfaces they personally need.
The Container can either be provided by some third party library/framework or you can code it yourself. Typically, it will use some configuration file to determine which implementations are actually supposed to be used for the various interfaces. Third party containers will usually perform some sort of code analysis supported by annotations to "autowire" implementations, so if you go with a ready-made tool, make sure you read up on how that part works because it will generally make your life easier down the road.
I'm having troubles getting the advantage of a IoC (DI) container like Ninject, Unity or whatever. I understand the concepts as follows:
DI: Injecting a dependency into the class that requires it (preferably via constructor injection). I totally see why the less tight coupling is a good thing.
public MyClass{
ISomeService svc;
public MyClass(ISomeService svc){
svc = svc;
}
public doSomething(){
svc.doSomething();
}
}
Service Locator: When a "container" is used directly inside the class that requires a dependancy, to resolve the dependancy. I do get the point that this generates another dependancy and I also see that basically nothing is getting injected.
public MyClass{
public MyClass(){}
public doSomething(){
ServiceLocator.resolve<ISomeService>().doSomething();
}
}
Now, what confuses me is the concept of a "DI container". To me, it looks exactly like a service locator which - as far as I read - should only be used in the entry point / startup method of an application to register and resolve the dependancies and inject them into the constructors of other classes - and not within a concrete class that needs the dependancy (probably for the same reason why Service locators are considered "bad")
What is the purpose of using the container when I could just create the dependancy and pass it to the constructor?
public void main(){
DIContainer.register<ISomeService>(new SomeService());
// ...
var myclass = new MyClass(DIContainer.resolve<ISomeService>());
myclass.doSomething();
}
Does it really make sense to pass all the dependancies to all classes in the application initialization method? There might be 100 dependancies which will be eventually needed (or not) and just because it's considered a good practice you set create them in the init method?
What is the purpose of using the container when I could just create the dependancy and pass it to the constructor?
DI containers are supposed to help you create an object graph quickly. You just tell it which concrete implementations you want to use for which abstractions (the registration phase), and then it can create any objects you want want (resolve phase).
If you create the dependencies and pass them to the constructor (in the application initialization code), then you are actually doing Pure DI.
I would argue that Pure DI is a better approach in many cases. See my article here
Does it really make sense to pass all the dependancies to all classes in the application initialization method? There might be 100 dependancies which will be eventually needed (or not) and just because it's considered a good practice you set create them in the init method?
I would say yes. You should create the object graph when your application starts up. This is called the composition root.
If you need to create objects after your application has started then you should use factories (mainly abstract factories). And such factories will be created with the other objects in the composition roots.
Your classes shouldn't do much in the constructor, this will make the cost of creating all the dependencies at the composition root low.
However, I would say that it is OK to create some types of objects using the new keyword in special cases. Like when the object is a simple Data Transfer Object (DTO)
I've recently been challenged on my view that singletons are only good for logging and configuration. And with Dependency Injection I am now not seeing a reason why you can't use your services or repositories as singletons.
There is no coupling because DI injects a singleton instance through an interface. The only reasonable argument is that your services might have shared state, but if you think about it, services should be stand alone units without any shared state. Yes they do get injected with the repositories but you only have one way that a repository instance is created and passed to the service. Since repositories should never have a shared state I don't see any reasons why it also cannot be a singleton.
So for example a simple service class would look like this:
public class GraphicService : IGraphicService
{
private IGraphicRepository _rep;
public GraphicService(IGraphicRepository rep)
{
_rep = rep;
}
public void CreateGraphic()
{
...
_rep.SaveGraphic(graphic):
}
}
No state is shared in service except repository which also doesn't change or have it's own state.
So the question is, if your services and repositories don't have any state and only passed in, through interface, configuration or anything else that's instantiated the same way they then why wouldn't you have them as singleton?
If you're using the Singleton pattern i.e a static property of a class, then you have the tightly coupling problem.
If you need just a single instance of a class and you're using a DI Container to control its lifetime, then it's not a problem as there are none of the drawbacks. The app doesn't know there is a singleton in place, only the DI Container knows about it.
Bottom line, a single instance of a class is a valid coding requirement, the only issue is how you implement it. The Di Container is the best approach. the Singleton pattern is for quick'n dirty apps where you don't care enough about maintainability and testing.
Some projects use singleton for dependence lookup before dependency injection gets populated. for example iBATIS jpetstore if I'm not mistaken. It's convenient that you can access your dependence gloablly like
public class GraphicService : IGraphicService
{
private IGraphicRepository _rep = GraphicRepository.getInstance();
public void CreateGraphic()
{
...
_rep.SaveGraphic(graphic):
}
}
But this harms testability (for not easy to replace dependence with test doubles) and introduces implicit strong dependence (IGraphicService depends on both the abstraction and the implementation).
Dependeny injection sovles these. I don't see why can't use singleton in your case, but it doesn't add much value when using DI.
I have a base controller from which inherit all my controllers. This base controller has some properties I'd like to inject the using property injection.
My controller registration looks like this
builder.RegisterControllers(Assembly.GetExecutingAssembly()
I don't know how to access the base class and inject the properties.
This should work:
builder.RegisterControllers(typeof(MvcApplication).Assembly).PropertiesAutowired();
Some more info on the autofac website: http://code.google.com/p/autofac/wiki/PropertyInjection
You may want to consider using an Autofac Aggregate Service:
An aggregate service is useful when you need to treat a set of dependencies as one dependency. When a class depends on several constructor-injected services, or have several property-injected services, moving those services into a separate class yields a simpler API.
An example is super- and subclasses where the superclass have one or more constructor-injected dependencies. The subclasses must usually inherit these dependencies, even though they might only be useful to the superclass. With an aggregate service, the superclass constructor parameters can be collapsed into one parameter, reducing the repetitiveness in subclasses. Another important side effect is that subclasses are now insulated against changes in the superclass dependencies, introducing a new dependency in the superclass means only changing the aggregate service definition.
This works for me:
using Autofac;
using Autofac.Integration.Web;
using Autofac.Integration.Web.Mvc;
builder.RegisterType<ExtensibleActionInvoker>().As<IActionInvoker>();
builder.RegisterControllers(Assembly.GetExecutingAssembly()).InjectActionInvoker();
nickvane's answer is correct. Just make sure
builder.RegisterControllers(typeof(MvcApplication).Assembly).PropertiesAutowired();
is after your all dependencies are registered. Or in other words, just right before you build your container.
So final code will look like
.
.
.
builder.RegisterControllers(typeof(MvcApplication).Assembly).PropertiesAutowired();
var container = builder.Build();
Need to call also for MVC:
DependencyResolver.SetResolver(new AutofacDependencyResolver(container));
For Web API:
HttpConfiguration config = GlobalConfiguration.Configuration;
config.DependencyResolver = new AutofacWebApiDependencyResolver(container);
On this AutoFac "Best Practices" page (http://code.google.com/p/autofac/wiki/BestPractices), they say:
Don't Pass the Container Around
Giving components access to the container, or storing it in a public static property, or making functions like Resolve() available on a global 'IoC' class defeats the purpose of using dependency injection. Such designs have more in common with the Service Locator pattern.
If components have a dependency on the container, look at how they're using the container to retrieve services, and add those services to the component's (dependency injected) constructor arguments instead.
So what would be a better way to have one component "dynamically" instantiate another? Their second paragraph doesn't cover the case where the component that "may" need to be created will depend on the state of the system. Or when component A needs to create X number of component B.
To abstract away the instantiation of another component, you can use the Factory pattern:
public interface IComponentBFactory
{
IComponentB CreateComponentB();
}
public class ComponentA : IComponentA
{
private IComponentBFactory _componentBFactory;
public ComponentA(IComponentBFactory componentBFactory)
{
_componentBFactory = componentBFactory;
}
public void Foo()
{
var componentB = _componentBFactory.CreateComponentB();
...
}
}
Then the implementation can be registered with the IoC container.
A container is one way of assembling an object graph, but it certainly isn't the only way. It is an implementation detail. Keeping the objects free of this knowledge decouples them from infrastructure concerns. It also keeps them from having to know which version of a dependency to resolve.
Autofac actually has some special functionality for exactly this scenario - the details are on the wiki here: http://code.google.com/p/autofac/wiki/DelegateFactories.
In essence, if A needs to create multiple instances of B, A can take a dependency on Func<B> and Autofac will generate an implementation that returns new Bs out of the container.
The other suggestions above are of course valid - Autofac's approach has a couple of differences:
It avoids the need for a large number of factory interfaces
B (the product of the factory) can still have dependencies injected by the container
Hope this helps!
Nick
An IoC takes the responsibility for determining which version of a dependency a given object should use. This is useful for doing things like creating chains of objects that implement an interface as well as having a dependency on that interface (similar to a chain of command or decorator pattern).
By passing your container, you are putting the onus on the individual object to get the appropriate dependency, so it has to know how to. With typical IoC usage, the object only needs to declare that it has a dependency, not think about selecting between multiple available implementations of that dependency.
Service Locator patterns are more difficult to test and it certainly is more difficult to control dependencies, which may lead to more coupling in your system than you really want.
If you really want something like lazy instantiation you may still opt for the Service Locator style (it doesn't kill you straight away and if you stick to the container's interface it is not too hard to test with some mocking framework). Bear in mind, though that the instantiation of a class that doesn't do much (or anything) in the constructor is immensely cheap.
The container's I have come to know (not autofac so far) will let you modify what dependencies should be injected into which instance depending on the state of the system such that even those decisions can be externalized into the configuration of the container.
This can provide you plenty of flexibility without resorting to implementing interaction with the container based on some state you access in the instance consuming dependencies.