Based on this question, THE way to initialize new object with runtime parameters when using ioc-container is to create Abstract Factory.
In my example, I have this class:
internal sealed class AssetsDownloadingProcess
{
private readonly IBackgroundWorker _backgroundWorker;
private readonly IAssetsStorage _assetsStorage;
private readonly Parameters _parameters;
public AssetsDownloadingProcess(IBackgroundWorker backgroundWorker,
IAssetsStorage assetsStorage, Parameters parameters)
{
_parameters = parameters.Clone();
_backgroundWorker = backgroundWorker;
_assetsStorage = assetsStorage;
}
}
And a factory to construct it:
internal sealed class AssetsDownloadingProcessFactory
{
private readonly IBackgroundWorker _backgroundWorker;
private readonly IAssetsStorage _assetsStorage;
public AssetsDownloadingProcessFactory(IBackgroundWorker backgroundWorker,
IAssetsStorage assetsStorage)
{
_backgroundWorker = backgroundWorker;
_assetsStorage = assetsStorage;
}
public AssetsDownloadingProcess CreateProcess(
AssetsDownloadingProcess.Parameters parameters)
{
return new AssetsDownloadingProcess(
_backgroundWorker, _assetsStorage, parameters);
}
}
as you can see, AssetsDownloadingProcess does not implement any interface and will never be replaced with another class. Therefore this factory is nothing more than useless piece of code. It could be completely omitted in favour of AssetsDownloadingProcessFactory constructor. However, then I can't use dependency injection to constructor.
I would like to use benefits of Injection from my IoC Container without the hassle of creating factory and creating useless code. What is the correct way to do this? Am I missing something or using DI wrong?
In general, you should prevent using runtime data to create and initialize your application components, as described here. The mere fact that runtime data is passed in through the constructor forces you to create a Factory.
The solution given by the article to the problem of injecting runtime data into components is to let runtime data flow through method calls on an initialized object graph by either:
pass runtime data through method calls of the API, or
retrieve runtime data from specific abstractions that allow resolving runtime data.
When runtime data is not used during object graph construction, the component can be created using DI inside the Composition Root and you problem therefore goes away.
Doing so however is not always feasible, and when it isn't, an Abstract Factory is the solution.
Since Object Composition should solely take place in the application's Composition Root however, this means that your Abstract Factory must be an Abstraction. Only this way can you prevent that the construction of your AssetsDownloadingProcess component takes place inside the Composition Root.
The way to do this is to:
Define an abstraction, i.e. IAssetsDownloadingProcessFactory in the application layer where the consumers of this factory live.
Create an implementation of this abstraction inside the Composition Root.
Not using an abstraction means that consumers will take a dependency on the concrete AssetsDownloadingProcessFactory class (which is a Dependency Inversion Principle violation) and it pulls the object composition out of the Composition Root. This will cause object composition to be scattered throughout the application, which hinders maintainability.
I'm increasingly finding myself mixing runtime parameters and implicit construction injection and it smells bad to me.
Example - I have a base class describing a Filter, and various inherited types for specific filters (tag, category, date, author, etc etc)
var filter = StructureMap.ObjectFactory
.With("caption").EqualTo("Posts filtered by tag:")
.With("parameters").EqualTo(parameters)
.With("displayInSummary").EqualTo(true)
.GetInstance<TagListFilter>();
The reason I do this is because in the constructor I have an interface using which I wish StructureMap to inject a concrete class (IArticleConfigurator):
public TagListFilter(string caption, IDictionary<string,string> parameters, bool displayInSummary, IArticleConfigurator configurator)
:base(caption, parameters,displayInSummary, configurator)
But it just occured to me that I replaced a simple constructor, albeit with a concrete class instead of interface, with, essentially, the same thing but using DI to inject 1 concrete type. I'm doing this because currently our configs are in a xml file but will be moved to a CMS, so seemed like a good idea to use an interface.
It seems wrong and not in the spirit of DI.
Should I use a factory to generate my various filters? If so, can I still leverage DI to get a concrete instance of my IArticleConfigurator?
You shouldn't pass parameters explicitly from one dependency to another or, at least, you should minimize their number. One big disadvantage of resolving instances with parameters is that you specify parameter names as string literals - which make you code very fragile in changes of constructor signature.
One example I might think of (note that I have no clue regarding your domain and responsibilities of entities) is to inject provider or, as you already said, factory. For example, create something like ITagListFilterConfigurationProvider (you should change the name as you want to, I just trying to give motivation). You might create very abstract provider like IFilterConfigurationProvider with three methods as below, if you have same parameters for filters:
interface ITagListFilterConfigurationProvider
{
string Caption { get; }
IDictionary<string,string> GetParameters();
bool IsDisplayInSummary { get; }
}
Now you constructor will look like:
public TagListFilter(ITagListFilterConfigurationProvider configurationProvider, IArticleConfigurator configurator)
All you need is to implement it as you already did (because you are passing concrete parameters to constructor) and extract this behaviour to provider. What is left - is to register concrete provider with StructureMap and resolve filter without passing any concrete parameters
var filter = StructureMap.ObjectFactory.GetInstance<TagListFilter>();
I'm trying to understand dependency injections (DI), and once again I failed. It just seems silly. My code is never a mess; I hardly write virtual functions and interfaces (although I do once in a blue moon) and all my configuration is magically serialized into a class using json.net (sometimes using an XML serializer).
I don't quite understand what problem it solves. It looks like a way to say: "hi. When you run into this function, return an object that is of this type and uses these parameters/data."
But... why would I ever use that? Note I have never needed to use object as well, but I understand what that is for.
What are some real situations in either building a website or desktop application where one would use DI? I can come up with cases easily for why someone may want to use interfaces/virtual functions in a game, but it's extremely rare (rare enough that I can't remember a single instance) to use that in non-game code.
First, I want to explain an assumption that I make for this answer. It is not always true, but quite often:
Interfaces are adjectives; classes are nouns.
(Actually, there are interfaces that are nouns as well, but I want to generalize here.)
So, e.g. an interface may be something such as IDisposable, IEnumerable or IPrintable. A class is an actual implementation of one or more of these interfaces: List or Map may both be implementations of IEnumerable.
To get the point: Often your classes depend on each other. E.g. you could have a Database class which accesses your database (hah, surprise! ;-)), but you also want this class to do logging about accessing the database. Suppose you have another class Logger, then Database has a dependency to Logger.
So far, so good.
You can model this dependency inside your Database class with the following line:
var logger = new Logger();
and everything is fine. It is fine up to the day when you realize that you need a bunch of loggers: Sometimes you want to log to the console, sometimes to the file system, sometimes using TCP/IP and a remote logging server, and so on ...
And of course you do NOT want to change all your code (meanwhile you have gazillions of it) and replace all lines
var logger = new Logger();
by:
var logger = new TcpLogger();
First, this is no fun. Second, this is error-prone. Third, this is stupid, repetitive work for a trained monkey. So what do you do?
Obviously it's a quite good idea to introduce an interface ICanLog (or similar) that is implemented by all the various loggers. So step 1 in your code is that you do:
ICanLog logger = new Logger();
Now the type inference doesn't change type any more, you always have one single interface to develop against. The next step is that you do not want to have new Logger() over and over again. So you put the reliability to create new instances to a single, central factory class, and you get code such as:
ICanLog logger = LoggerFactory.Create();
The factory itself decides what kind of logger to create. Your code doesn't care any longer, and if you want to change the type of logger being used, you change it once: Inside the factory.
Now, of course, you can generalize this factory, and make it work for any type:
ICanLog logger = TypeFactory.Create<ICanLog>();
Somewhere this TypeFactory needs configuration data which actual class to instantiate when a specific interface type is requested, so you need a mapping. Of course you can do this mapping inside your code, but then a type change means recompiling. But you could also put this mapping inside an XML file, e.g.. This allows you to change the actually used class even after compile time (!), that means dynamically, without recompiling!
To give you a useful example for this: Think of a software that does not log normally, but when your customer calls and asks for help because he has a problem, all you send to him is an updated XML config file, and now he has logging enabled, and your support can use the log files to help your customer.
And now, when you replace names a little bit, you end up with a simple implementation of a Service Locator, which is one of two patterns for Inversion of Control (since you invert control over who decides what exact class to instantiate).
All in all this reduces dependencies in your code, but now all your code has a dependency to the central, single service locator.
Dependency injection is now the next step in this line: Just get rid of this single dependency to the service locator: Instead of various classes asking the service locator for an implementation for a specific interface, you - once again - revert control over who instantiates what.
With dependency injection, your Database class now has a constructor that requires a parameter of type ICanLog:
public Database(ICanLog logger) { ... }
Now your database always has a logger to use, but it does not know any more where this logger comes from.
And this is where a DI framework comes into play: You configure your mappings once again, and then ask your DI framework to instantiate your application for you. As the Application class requires an ICanPersistData implementation, an instance of Database is injected - but for that it must first create an instance of the kind of logger which is configured for ICanLog. And so on ...
So, to cut a long story short: Dependency injection is one of two ways of how to remove dependencies in your code. It is very useful for configuration changes after compile-time, and it is a great thing for unit testing (as it makes it very easy to inject stubs and / or mocks).
In practice, there are things you can not do without a service locator (e.g., if you do not know in advance how many instances you do need of a specific interface: A DI framework always injects only one instance per parameter, but you can call a service locator inside a loop, of course), hence most often each DI framework also provides a service locator.
But basically, that's it.
P.S.: What I described here is a technique called constructor injection, there is also property injection where not constructor parameters, but properties are being used for defining and resolving dependencies. Think of property injection as an optional dependency, and of constructor injection as mandatory dependencies. But discussion on this is beyond the scope of this question.
I think a lot of times people get confused about the difference between dependency injection and a dependency injection framework (or a container as it is often called).
Dependency injection is a very simple concept. Instead of this code:
public class A {
private B b;
public A() {
this.b = new B(); // A *depends on* B
}
public void DoSomeStuff() {
// Do something with B here
}
}
public static void Main(string[] args) {
A a = new A();
a.DoSomeStuff();
}
you write code like this:
public class A {
private B b;
public A(B b) { // A now takes its dependencies as arguments
this.b = b; // look ma, no "new"!
}
public void DoSomeStuff() {
// Do something with B here
}
}
public static void Main(string[] args) {
B b = new B(); // B is constructed here instead
A a = new A(b);
a.DoSomeStuff();
}
And that's it. Seriously. This gives you a ton of advantages. Two important ones are the ability to control functionality from a central place (the Main() function) instead of spreading it throughout your program, and the ability to more easily test each class in isolation (because you can pass mocks or other faked objects into its constructor instead of a real value).
The drawback, of course, is that you now have one mega-function that knows about all the classes used by your program. That's what DI frameworks can help with. But if you're having trouble understanding why this approach is valuable, I'd recommend starting with manual dependency injection first, so you can better appreciate what the various frameworks out there can do for you.
As the other answers stated, dependency injection is a way to create your dependencies outside of the class that uses it. You inject them from the outside, and take control about their creation away from the inside of your class. This is also why dependency injection is a realization of the Inversion of control (IoC) principle.
IoC is the principle, where DI is the pattern. The reason that you might "need more than one logger" is never actually met, as far as my experience goes, but the actually reason is, that you really need it, whenever you test something. An example:
My Feature:
When I look at an offer, I want to mark that I looked at it automatically, so that I don't forget to do so.
You might test this like this:
[Test]
public void ShouldUpdateTimeStamp
{
// Arrange
var formdata = { . . . }
// System under Test
var weasel = new OfferWeasel();
// Act
var offer = weasel.Create(formdata)
// Assert
offer.LastUpdated.Should().Be(new DateTime(2013,01,13,13,01,0,0));
}
So somewhere in the OfferWeasel, it builds you an offer Object like this:
public class OfferWeasel
{
public Offer Create(Formdata formdata)
{
var offer = new Offer();
offer.LastUpdated = DateTime.Now;
return offer;
}
}
The problem here is, that this test will most likely always fail, since the date that is being set will differ from the date being asserted, even if you just put DateTime.Now in the test code it might be off by a couple of milliseconds and will therefore always fail. A better solution now would be to create an interface for this, that allows you to control what time will be set:
public interface IGotTheTime
{
DateTime Now {get;}
}
public class CannedTime : IGotTheTime
{
public DateTime Now {get; set;}
}
public class ActualTime : IGotTheTime
{
public DateTime Now {get { return DateTime.Now; }}
}
public class OfferWeasel
{
private readonly IGotTheTime _time;
public OfferWeasel(IGotTheTime time)
{
_time = time;
}
public Offer Create(Formdata formdata)
{
var offer = new Offer();
offer.LastUpdated = _time.Now;
return offer;
}
}
The Interface is the abstraction. One is the REAL thing, and the other one allows you to fake some time where it is needed. The test can then be changed like this:
[Test]
public void ShouldUpdateTimeStamp
{
// Arrange
var date = new DateTime(2013, 01, 13, 13, 01, 0, 0);
var formdata = { . . . }
var time = new CannedTime { Now = date };
// System under test
var weasel= new OfferWeasel(time);
// Act
var offer = weasel.Create(formdata)
// Assert
offer.LastUpdated.Should().Be(date);
}
Like this, you applied the "inversion of control" principle, by injecting a dependency (getting the current time). The main reason to do this is for easier isolated unit testing, there are other ways of doing it. For example, an interface and a class here is unnecessary since in C# functions can be passed around as variables, so instead of an interface you could use a Func<DateTime> to achieve the same. Or, if you take a dynamic approach, you just pass any object that has the equivalent method (duck typing), and you don't need an interface at all.
You will hardly ever need more than one logger. Nonetheless, dependency injection is essential for statically typed code such as Java or C#.
And...
It should also be noted that an object can only properly fulfill its purpose at runtime, if all its dependencies are available, so there is not much use in setting up property injection. In my opinion, all dependencies should be satisfied when the constructor is being called, so constructor-injection is the thing to go with.
I think the classic answer is to create a more decoupled application, which has no knowledge of which implementation will be used during runtime.
For example, we're a central payment provider, working with many payment providers around the world. However, when a request is made, I have no idea which payment processor I'm going to call. I could program one class with a ton of switch cases, such as:
class PaymentProcessor{
private String type;
public PaymentProcessor(String type){
this.type = type;
}
public void authorize(){
if (type.equals(Consts.PAYPAL)){
// Do this;
}
else if(type.equals(Consts.OTHER_PROCESSOR)){
// Do that;
}
}
}
Now imagine that now you'll need to maintain all this code in a single class because it's not decoupled properly, you can imagine that for every new processor you'll support, you'll need to create a new if // switch case for every method, this only gets more complicated, however, by using Dependency Injection (or Inversion of Control - as it's sometimes called, meaning that whoever controls the running of the program is known only at runtime, and not complication), you could achieve something very neat and maintainable.
class PaypalProcessor implements PaymentProcessor{
public void authorize(){
// Do PayPal authorization
}
}
class OtherProcessor implements PaymentProcessor{
public void authorize(){
// Do other processor authorization
}
}
class PaymentFactory{
public static PaymentProcessor create(String type){
switch(type){
case Consts.PAYPAL;
return new PaypalProcessor();
case Consts.OTHER_PROCESSOR;
return new OtherProcessor();
}
}
}
interface PaymentProcessor{
void authorize();
}
** The code won't compile, I know :)
The main reason to use DI is that you want to put the responsibility of the knowledge of the implementation where the knowledge is there. The idea of DI is very much inline with encapsulation and design by interface.
If the front end asks from the back end for some data, then is it unimportant for the front end how the back end resolves that question. That is up to the requesthandler.
That is already common in OOP for a long time. Many times creating code pieces like:
I_Dosomething x = new Impl_Dosomething();
The drawback is that the implementation class is still hardcoded, hence has the front end the knowledge which implementation is used. DI takes the design by interface one step further, that the only thing the front end needs to know is the knowledge of the interface.
In between the DYI and DI is the pattern of a service locator, because the front end has to provide a key (present in the registry of the service locator) to lets its request become resolved.
Service locator example:
I_Dosomething x = ServiceLocator.returnDoing(String pKey);
DI example:
I_Dosomething x = DIContainer.returnThat();
One of the requirements of DI is that the container must be able to find out which class is the implementation of which interface. Hence does a DI container require strongly typed design and only one implementation for each interface at the same time. If you need more implementations of an interface at the same time (like a calculator), you need the service locator or factory design pattern.
D(b)I: Dependency Injection and Design by Interface.
This restriction is not a very big practical problem though. The benefit of using D(b)I is that it serves communication between the client and the provider. An interface is a perspective on an object or a set of behaviours. The latter is crucial here.
I prefer the administration of service contracts together with D(b)I in coding. They should go together. The use of D(b)I as a technical solution without organizational administration of service contracts is not very beneficial in my point of view, because DI is then just an extra layer of encapsulation. But when you can use it together with organizational administration you can really make use of the organizing principle D(b)I offers.
It can help you in the long run to structure communication with the client and other technical departments in topics as testing, versioning and the development of alternatives. When you have an implicit interface as in a hardcoded class, then is it much less communicable over time then when you make it explicit using D(b)I. It all boils down to maintenance, which is over time and not at a time. :-)
Quite frankly, I believe people use these Dependency Injection libraries/frameworks because they just know how to do things in runtime, as opposed to load time. All this crazy machinery can be substituted by setting your CLASSPATH environment variable (or other language equivalent, like PYTHONPATH, LD_LIBRARY_PATH) to point to your alternative implementations (all with the same name) of a particular class. So in the accepted answer you'd just leave your code like
var logger = new Logger() //sane, simple code
And the appropriate logger will be instantiated because the JVM (or whatever other runtime or .so loader you have) would fetch it from the class configured via the environment variable mentioned above.
No need to make everything an interface, no need to have the insanity of spawning broken objects to have stuff injected into them, no need to have insane constructors with every piece of internal machinery exposed to the world. Just use the native functionality of whatever language you're using instead of coming up with dialects that won't work in any other project.
P.S.: This is also true for testing/mocking. You can very well just set your environment to load the appropriate mock class, in load time, and skip the mocking framework madness.
There's something I just don't get about guice: According to what I've read so far, I'm supposed to use the Injector only in my bootstrapping class (in a standalone application this would typically be in the main() method), like in the example below (taken from the guice documentation):
public static void main(String[] args) {
/*
* Guice.createInjector() takes your Modules, and returns a new Injector
* instance. Most applications will call this method exactly once, in their
* main() method.
*/
Injector injector = Guice.createInjector(new BillingModule());
/*
* Now that we've got the injector, we can build objects.
*/
RealBillingService billingService = injector.getInstance(RealBillingService.class);
...
}
But what if not all Objects I ever need can be created during startup? Maybe I want to respond to some user interaction when the application is running? Don't I have to keep my injector around somewhere (e.g. as a static variable) and then call injector.getInstance(SomeInterface.class) when I need to create a new object?
Of course spreading calls to Injector.getInstance() all over the place seems not to be desirable.
What am I getting wrong here?
Yes, you basically only should use the Injector to create get the instance for the root-object. The rest of the application shouldn't touch the Guice-Container. As you've noticed, you still need to create some objects when required. There are different approaches for doing that, each suitable for different needs.
Inject a Provider
Provider is a interface from Guice. It allows you to request a new instance of a object. That object will be created using Guice. For example.
class MyService{
private Provider<Transaction> transactionProvider;
public MainGui(Provider<Transaction> transactionProvider){
this.transactionProvider = transactionProvider;
}
public void actionStarted(){
Transaction transaction = transactionProvider.get();
}
Build a Factory
Often you need some kind of factory. This factory uses some injected services and some parameters and creates a new object for you. Then you use this factory for new instances. Then you inject that factory and use it. There also help for this with the AssistedInject-extension
I think with these two possibilities you rarely need to use the Guice-Injector itself. However sometimes is still appropriate to use the injector itself. Then you can inject the Injector to a component.
To extend on the answer Gamlor posted, you need to also differentiate between the object types you are using.
For services, injection is the correct solution, however, don't try to always make data objects (which are generally the leafs in your object graph) injectable. There may be situations where that is the correct solution, but injecting a Provider<List> is probably not a good idea. A colleague of mine ended up do that, it made the code base very confusing after a while. We just finished cleaning it all out and the Guice modules are much more specific now.
In the abstract, I think the general idea is that if responding to user events is part of the capabilities of your application, then, well...
BillingService billingService = injector.getInstance(BillingService.class);
billingService.respondToUserEvent( event );
I guess that might be a little abstract, but the basic idea is that you get from Guice your top-level application class. Judging from your question, I guess that maybe BillingService isn't your top-level class?
I'm a bit of a DI newbie, so forgive me if this is the wrong approach or a silly question.
Let's say I have a form which creates/updates an order, and I know it's going to need to retrieve a list of products and customers to display. I want to pass in the Order object that it's editing, but I also want to inject the ProductsService and CustomersService as dependencies.
So I will want my IoC container (whichever one I go with) to supply the services, but it'll be up to the calling code to supply the Order object to edit.
Should I declare the constructor as taking the Order object as the first parameter and the ProductsService and CustomersService after that, eg:
public OrderForm(Order order, ProductsService prodsSvc, CustomersService custsSvc)
... or should the dependencies come first and the Order object last, eg:
public OrderForm(ProductsService prodsSvc, CustomersService custsSvc, Order order)
Does it matter? Does it depend on which IoC container I use? Or is there a "better" way?
Matt, you shouldn't mix normal parameters with dependencies. Since your object will be created in the internals of IoC container, how are you going to specify necessary arguments?
Mixing dependency and normal arguments will make logic of your program more complicated.
In this case it would be better to declare dependency properties (i.e. remove dependencies from constructor) or initialize order field after IoC constructed OrderForm and resolved it's dependencies (i.e. remove normal parameters from constructor).
Also you can declare all of your parameters, including order as dependencies.
I disagree with #aku's answer.
I think what you're doing is fine and there are also other ways to do it that are no more or less right. For instance, one may question whether this object should be depending on services in the first place.
Regardless of DI, I feel it is helpful to clarify in your mind at least the kind of state each object holds, such as the real state (Order), derived state (if any), and dependencies (services):
http://tech.puredanger.com/2007/09/18/spelunking/
On any constructor or method, I prefer the real data to be passed first and dependencies or external stuff to be passed last. So in your example I'd prefer the first.
I feel a bit uneasy about allowing an instance of OrderForm to be instantiated without the required reference to an Order instance. One reason might be that this would prevent me from doing upfront checking for null orders. Any further thoughts?
I suppose I could take some comfort in knowing that OrderForm objects will only be instantiated by a Factory method that ensures the Order property is set after making the call to the IoC framework.
I am just a bit late to the party, but I would suggest using a factory in this case: the factory constructor would take the required *Service dependencies which the DI system would resolve and inject, while its Build() method would accept any additional state parameter - like Order. The factory would be, of course, registered itself in the DI system and would play just nicely with it.
public class OrderFormFactory
{
private readonly ProductsService _prodsSvc;
private readonly CustomersService _custsSvc;
public OrderFormFactory(ProductsService prodsSvc, CustomersService custsSvc)
{
_prodsService = prodsService ?? throw new ArgumentNullException(nameof(prodsService));
_custsSvc = custsSvc ?? throw new ArgumentNullException(nameof(custsSvc));
}
public OrderForm Build(Order order)
{
// TODO: Any additional logic
return new OrderForm(_prodsService, _custsSvc, order);
}
}