I have been looking for a while on the Internet about this but could not find an exact answer. Okay, they say inject an abstraction (interface or something) to a class rather than having it to create the instances it needs, or rather than passing an implementing type.
class Foo
{
private IBar _bar;
public Foo(Ibar bar)
{
_bar = bar;
}
}
Although I don't fully understand how above is reasonably loosely coupled, but my question at this point is about something else. That is, what if a method has to return an instance of something. How, for example, following can be made loosely coupled:
class Foo
{
public IBar GetMeSomething()
{
return new Bar(); // dependency here
}
}
I was just wondering how I resolve the above dependency on Bar? Any experienced programmers please help me understanding this. Or someone may be able to suggest some article comprehensively discussing all such scenarios that eliminate/reduce type inter-dependencies.
When a class need to know "Something" about your program, you can resolve that by passing a instance of another class in your constructor.
If you accept in constructors only interfaces instead of concrete classes you are later able to change implementation easily.
If you have to create Objects at run time you need factories, you class Foo is in your case a IBarFactory (because it instantiate IBars).
Dependencies in constructors via interfaces are easily resolved by any IoC framework (Factories are passed in constructors too), while the implementation of a factory itself is allowed to instantiate objects directly or through a IoC framework because that's the responsability of the class.
Using IoC containers and dependency injection does not make looscoupling happens for magic, you still have to model classes with SOLID principles in mind, but when using SOLID principles IoC containers and DI are of great help.
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 am new to dependency injection and have been doing some reading about it, both on StackOverflow and elsewhere. In practice I am having trouble with using it correctly.
To explain the problem, here's a basic situation where I am not sure how to use DI:
Suppose I have some object that is going to be used in several different classes. However, in order for this object to be usable, it needs certain parameters that I don't have at start-up.
A conceivable way that I can see to do this using DI is to create a blank instance of this object, a method to initialize it with the necessary parameters, and a flag for whether or not it is initialized.
To me, this feels like a hack, because the object shouldn't really exist yet and I am just passing a container around, waiting for the responsible code to initialize it. Is this how it is meant to be done, or am I missing the point?
That is indeed a pretty tough thing to get one's head around when getting started with DI, and something that also isn't easily explained.
Your notion that creating a "blank" object that will be initialized later via a method might be a suboptimal solution is correct - an object should be able to do its work at any time; Initialize() methods are what Mark Seemann calls "temporal coupling" in his book Dependency Injection in .NET. This is an anti-pattern that makes the code using the object dependent on the inner workings of that object and thus breaks encapsulation.
The question is when the required information becomes available, what the "responsible code to initialize it" is, and where it gets the information from - and also how it gets access to the object to initialize it. Ideally, this initializing code would itself be injected into your object, and whenever your object's methods/properties are accessed, it requests initialization from that other dependency.
Also, what happens if the IsInitialized flag returns false? Is that still a valid program state?
In general, as an object in a dependency injected object graph, I should either know all my "configuration" data on creation, or know someone who can give it to me (that someone is another object injected as a dependency).
It might help if you could provide a bit more detail about what kind of parameters the object needs and where they need to come from.
Edit
What you're describing in your comment is pretty much exactly my first encounter with this kind of issue; there's a question somewhere here on SO that I posted back then.
The important thing is to build individual classes (usually, there may be exceptions, but what those are is a matter of experience) in such a way that you assume everything the class needs is present. When the program is running, there need to be other classes then that make sure that assumption will not fail.
Setter injection is something I generally try not to have to avoid said temporal coupling; according to Mark Seemann, setter injection should usually only be used when you already have a good default in place that you just overwrite through the setter. In this case, though, the object would not be able to function properly without that dependency.
This may not be the most elegant way to do this (I usually have the luxury to apply DI in pretty closed code-only environments without having to worry about a UI), but it would work (kind of - it compiles, but is still pseudo code):
public class MainForm
{
private readonly IDataManager _dataManager;
private readonly IConnectionProvider _connectionProvider;
private readonly IConnectionReceiver _connectionReceiver;
public MainForm(IDataManager dataManager, IConnectionProvider connectionProvider, IConnectionReceiver connectionReceiver)
{
this._dataManager = dataManager;
this._connectionProvider = connectionProvider;
this._connectionReceiver = connectionReceiver;
}
public void btnConnect_Click()
{
IConnection connection = this._connectionProvider.GetConnection();
if (null != connection)
{
this._connectionReceiver.SetConnection(connection);
this.SetFormControlsEnabled(true);
}
}
private void SetFormControlsEnabled(bool doEnable)
{
}
}
public interface IConnectionProvider
{
IConnection GetConnection();
}
public interface IConnectionReceiver
{
void SetConnection(IConnection connection);
}
public interface IConnection
{
IConnectionWebService ConnectionWebService { get; }
}
public class ConnectionBridge : IConnection, IConnectionReceiver
{
private IConnection _connection;
#region IConnectionReceiver Members
public void SetConnection(IConnection connection)
{
this._connection = connection;
}
#endregion IConnectionReceiver Members
#region IConnection Members
public IConnectionWebService ConnectionWebService
{
get { return this._connection.ConnectionWebService; }
}
#endregion
}
public interface IConnectionWebService {}
public interface IDataManager { }
public class DataManager : IDataManager
{
public DataManager(IConnection connection)
{
}
}
So, MainForm is the thing that holds it all together. It starts out with its controls disabled, because it knows they need a working IDataManager and that will (by convention) need a connection. When a "connect" button is clicked, the form asks its IConnectionProvider dependency for a connection. It does not care where that connection comes from; the connection provider might display another form to ask for credentials or maybe just read them from a file.
Then the form knows that the connection has to be passed on to the IConnectionReceiver instance, and after that all controls can be enabled. This is not by any DI principle, this is just how we have defined that MainForm works.
The data manager on the other hand has everything it needs from the start - an IConnection instance. That can't do what it's supposed to do at first, but there is other code preventing that from causing problems.
ConnectionBridge is both a decorator for the actual IConnection instance and an adapter decoupling connection acquisition from connection consumption. It does that by employing the Interface Segregation Principle.
As a note on the side, be aware that while Dependency Injection is an important technique, it is only one of several principles you should follow to write what is known as "clean code". The most well known are the SOLID principles (of which DI is one), but there are others like Command-Query-Separation (CQS), "Don't repeat yourself" (DRY) and the Law of Demeter. On top of all that, practice unit testing, precisely Test Driven Development (TDD). These things really make a tremendous difference - but if you're taking up DI of your own accord, you're already off to a good start.
I agree with what GCATNM said and i would like to add that whenever i feel there is an object like this i go and use one of the Factory pattern variants (be it an Abstract Factory, Static Factory, etc ..) and i would inject the factory with the source of the configuration information for this object. So as Marc Seemann also said and i am not quoting: Factories are a great companion of Dependency Injection and you will need them occasionally.
I'm simply looking for advice on the best way I should handle this situation.
Right now I've got several files in a folder called Service. The files contact several functions which do random things of course. Each of these files needs access to the SM Adapter.
My question is, should I implement the ServiceManagerAwareInterface in each of these files OR should I just make a new class which implements the ServiceManagerAwareInterface and just extend my classes on the new class which implements this service?
Both ways work as they should, just not sure which way would be more proper.
If you think that your system will always rely on ZF2, both approaches are equivalent.
Now from an OO design perspective, personally I have a preference for the approach in which you extend your service then implement the ServiceManagerAwareInterface. I would even use an interface for the dependency over the ServiceLocator to protect even more my classes. Why?
Extending your classes does not cost you a lot, same for making your class depending on interfaces.
Let's take this example, Imagine you did not use this approach during a ZF1 project, during which you had probably resolved your dependencies with the Zend_Registry.
Now, let's assume you moved to a ZF2 implementation, how much time you think you'll spend refactoring your code from something like Zend_Registry::get($serviceX) to $this->getServiceManager()->get($serviceX) on your Service layer?
Now Assume you had made the choice of protecting your classes, first by creating your own Service locator interface, as simple as:
public interface MyOwnServiceLocatorInterface{
public function get($service);
}
Under ZF1 you had created an adapter class using the Zend_Registry:
public class MyZF1ServiceLocator implements MyOwnServiceLocatorInterface{
public function get($service){
Zend_Registry::get($service);
}
}
Your Service classes are not coupled to the Zend_Registry, which make the refactoring much more easier.
Now, You decide to move to ZF2 so you'll logically use the ServiceManger. You create then this new Adapter class:
public class MyZF2ServiceLocator implements
ServiceManagerAwareInterface,MyOwnServiceLocatorInterface
{
private $_sm;
public function get($service){
$this->_sm->get($service);
}
public function setServiceManager($serviceManager){
$this->_sm = $serviceManager;
}
}
Again, your Service classes are not coupled to the ZF2 ServiceManger.
Now, how would look like the configuration/registration of you Service layer on the ServiceManager. Well, you'll use your Module::getServiceConfig class for that:
//Module.php
public function getServiceConfig()
{
return array(
'factories'=>array(
'My\ServiceA'=>function($sm){
return new My\ServiceA($sm->get('My\Service\Name\Space\MyZF2ServiceLocator'));
}
//Some other config
)
}
As you can see, no refactoring is needed within your Service classes as we protected them by relying on interface and using adapters. As we used a closure factory, we don't even need to extend our Service classes and implement the ServiceLocatorAwareInterface.
Now, before concluding in my previous example i have to note that I did not treat the case in which my classes are constructed via factories, however, you can check one of my previous answers that address the factory topic but also the importance of loose coupling among an application layers.
you can add initializers to do that. It can reduce repetitive injection in getting the service that pass db adapter. OR, you can set abstract_factories, it will reduce repetitive SM registration. I just posted SM Cheatsheet here, Hope helpful :)
https://samsonasik.wordpress.com/2013/01/02/zend-framework-2-cheat-sheet-service-manager/
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.