One of the definition of ninject in internet is;
"Somewhere in the middle of your application, you're creating a class
inside another class. That means you're creating a dependency.
Dependency Injection is about passing in those dependencies, usually
through the constructor, instead of embedding them."
what i want to learn is, where ever we see a creation of a class inside another class should we use ninject or just we should use in some part of program that we want/need to apply loosely coupling for design purposes because maybe we would like to use different approaches in the future?
Sorry if this is a silly question.
It's a perfectly valid question, and there are no absolute right or wrong answers. Ninject and other IoC frameworks are designed to de-couple dependencies.
So the moment you do this:
public class MyClass1
{
public MyClass1()
{
MyClass2 mc2 = new MyClass2();
}
}
You can categorically say that MyClass1 has a dependency in MyClass2.
For me, my rule is this: do I need to unit test MyClass1, or is it likely I'll need to unit test MyClass1?
If I don't need to unit test it, then I don't find much value in decoupling the two classes.
However, if I do need to unit test MyClass1, then injecting in MyClass2 gives you much better control over your unit tests (and allows you to test MyClass1 in isolation).
You do need to evaluate each case separately though. In the above example, if I need to unit test MyClass1, and MyClass2 is just a basic string formatting class, then I probably wouldn't decouple it. However, if MyClass2 was an email sending class then I would de-couple it. I don't want my unit tests actually sending emails, so I would feed in a fake for my tests instead.
So I don't believe there are any solid rules, but hopefully the above gives you a better idea of when you might decouple, and when you might not decouple.
When coding, I often come across the following pattern:
-A method calls another method (Fine), but the method being called/callee takes parameters, so in the wrapping method, I pass in parameters. Problem is, this dependency carrying can go on and on. How could I avoid this (any sample code appreciated)?
Thanks
Passing a parameter along just because a lower-layer component needs it is a sign of a Leaky Abstraction. It can often be more effective to refactor dependencies to aggregate services and hide each dependency behind an interface.
Cross-cutting concerns (which are often the most common reason to pass along parameters) are best addressed by Decorators.
If you use a DI Container with interception capabilities, you can take advantage of those to implement Decorators very efficiently (some people refer to this as a container's AOP capabilities).
You can use a dependency injection framework. One such is Guice: see http://code.google.com/p/google-guice/
Step 1: Instead of passing everything as separate arguments, group the arguments into a class, let's say X.
Step 2: Add getters to the class X to get the relevant information. The callee should use the getters to get the information instead of relying on parameters.
Step 3: Create an interface class of which class X inherits. Put all the getters in the interface (in C++ this is as pure virtual methods).
Step 4: Make the called methods only depend on the interface.
Refactoring: Introduce Parameter Object
You have a group of parameters that naturally go together?
Replace them with an object.
http://www.refactoring.com/catalog/introduceParameterObject.html
The advantage of the parameter object is that the calls passing them around don't need to change if you add/remove parameters.
(given the context of your answers, I don't think that an IoC library or dependency injection patterns are really what you're after)
Since they cannot be (easily) unit tested, most developers choose to inject objects into Views. Since the views are not (normally) used to construct other views, that is where your DI chain ends. You may have the issue (which I have run into every once in ahwile) where you need to construct objects in the correct order especially when using a DI framework like Unity where an attemt to resolve the object will deadlock. The main thing you need to worry about is circular dependency. In order to do this, read the following article:
Can dependency injection prevent a circular dependency?
I'm refactoring a class and adding a new dependency to it. The class is currently taking its existing dependencies in the constructor. So for consistency, I add the parameter to the constructor.
Of course, there are a few subclasses plus even more for unit tests, so now I am playing the game of going around altering all the constructors to match, and it's taking ages.
It makes me think that using properties with setters is a better way of getting dependencies. I don't think injected dependencies should be part of the interface to constructing an instance of a class. You add a dependency and now all your users (subclasses and anyone instantiating you directly) suddenly know about it. That feels like a break of encapsulation.
This doesn't seem to be the pattern with the existing code here, so I am looking to find out what the general consensus is, pros and cons of constructors versus properties. Is using property setters better?
Well, it depends :-).
If the class cannot do its job without the dependency, then add it to the constructor. The class needs the new dependency, so you want your change to break things. Also, creating a class that is not fully initialized ("two-step construction") is an anti-pattern (IMHO).
If the class can work without the dependency, a setter is fine.
The users of a class are supposed to know about the dependencies of a given class. If I had a class that, for example, connected to a database, and didn't provide a means to inject a persistence layer dependency, a user would never know that a connection to the database would have to be available. However, if I alter the constructor I let the users know that there is a dependency on the persistence layer.
Also, to prevent yourself from having to alter every use of the old constructor, simply apply constructor chaining as a temporary bridge between the old and new constructor.
public class ClassExample
{
public ClassExample(IDependencyOne dependencyOne, IDependencyTwo dependencyTwo)
: this (dependnecyOne, dependencyTwo, new DependnecyThreeConcreteImpl())
{ }
public ClassExample(IDependencyOne dependencyOne, IDependencyTwo dependencyTwo, IDependencyThree dependencyThree)
{
// Set the properties here.
}
}
One of the points of dependency injection is to reveal what dependencies the class has. If the class has too many dependencies, then it may be time for some refactoring to take place: Does every method of the class use all the dependencies? If not, then that's a good starting point to see where the class could be split up.
Of course, putting on the constructor means that you can validate all at once. If you assign things into read-only fields then you have some guarantees about your object's dependencies right from construction time.
It is a real pain adding new dependencies, but at least this way the compiler keeps complaining until it's correct. Which is a good thing, I think.
If you have large number of optional dependencies (which is already a smell) then probably setter injection is the way to go. Constructor injection better reveals your dependencies though.
The general preferred approach is to use constructor injection as much as possible.
Constructor injection exactly states what are the required dependencies for the object to function properly - nothing is more annoying than newing up an object and having it crashing when calling a method on it because some dependency is not set. The object returned by a constructor should be in a working state.
Try to have only one constructor, it keeps the design simple and avoids ambiguity (if not for humans, for the DI container).
You can use property injection when you have what Mark Seemann calls a local default in his book "Dependency Injection in .NET": the dependency is optional because you can provide a fine working implementation but want to allow the caller to specify a different one if needed.
(Former answer below)
I think that constructor injection are better if the injection is mandatory. If this adds too many constructors, consider using factories instead of constructors.
The setter injection is nice if the injection is optional, or if you want to change it halfway trough. I generally don't like setters, but it's a matter of taste.
It's largely a matter of personal taste.
Personally I tend to prefer the setter injection, because I believe it gives you more flexibility in the way that you can substitute implementations at runtime.
Furthermore, constructors with a lot of arguments are not clean in my opinion, and the arguments provided in a constructor should be limited to non-optional arguments.
As long as the classes interface (API) is clear in what it needs to perform its task,
you're good.
I prefer constructor injection because it helps "enforce" a class's dependency requirements. If it's in the c'tor, a consumer has to set the objects to get the app to compile. If you use setter injection they may not know they have a problem until run time - and depending on the object, it might be late in run time.
I still use setter injection from time to time when the injected object maybe needs a bunch of work itself, like initialization.
I personally prefer the Extract and Override "pattern" over injecting dependencies in the constructor, largely for the reason outlined in your question. You can set the properties as virtual and then override the implementation in a derived testable class.
I perfer constructor injection, because this seems most logical. Its like saying my class requires these dependencies to do its job. If its an optional dependency then properties seem reasonable.
I also use property injection for setting things that the container does not have a references to such as an ASP.NET View on a presenter created using the container.
I dont think it breaks encapsulation. The inner workings should remain internal and the dependencies deal with a different concern.
One option that might be worth considering is composing complex multiple-dependencies out of simple single dependencies. That is, define extra classes for compound dependencies. This makes things a little easier WRT constructor injection - fewer parameters per call - while still maintaining the must-supply-all-dependencies-to-instantiate thing.
Of course it makes most sense if there's some kind of logical grouping of dependencies, so the compound is more than an arbitrary aggregate, and it makes most sense if there are multiple dependents for a single compound dependency - but the parameter block "pattern" has been around for a long time, and most of those that I've seen have been pretty arbitrary.
Personally, though, I'm more a fan of using methods/property-setters to specify dependencies, options etc. The call names help describe what is going on. It's a good idea to provide example this-is-how-to-set-it-up snippets, though, and make sure the dependent class does enough error checks. You might want to use a finite state model for the setup.
I recently ran into a situation where I had multiple dependencies in a class, but only one of the dependencies was necessarily going to change in each implementation. Since the data access and error logging dependencies would likely only be changed for testing purposes, I added optional parameters for those dependencies and provided default implementations of those dependencies in my constructor code. In this way, the class maintains its default behavior unless overridden by the consumer of the class.
Using optional parameters can only be accomplished in frameworks that support them, such as .NET 4 (for both C# and VB.NET, though VB.NET has always had them). Of course, you can accomplish similar functionality by simply using a property that can be reassigned by the consumer of your class, but you don't get the advantage of immutability provided by having a private interface object assigned to a parameter of the constructor.
All of this being said, if you are introducing a new dependency that must be provided by every consumer, you're going to have to refactor your constructor and all code that consumers your class. My suggestions above really only apply if you have the luxury of being able to provide a default implementation for all of your current code but still provide the ability to override the default implementation if necessary.
Constructor injection does explicitly reveal the dependencies, making code more readable and less prone to unhandled run-time errors if arguments are checked in the constructor, but it really does come down to personal opinion, and the more you use DI the more you'll tend to sway back and forth one way or the other depending on the project. I personally have issues with code smells like constructors with a long list of arguments, and I feel that the consumer of an object should know the dependencies in order to use the object anyway, so this makes a case for using property injection. I don't like the implicit nature of property injection, but I find it more elegant, resulting in cleaner-looking code. But on the other hand, constructor injection does offer a higher degree of encapsulation, and in my experience I try to avoid default constructors, as they can have an ill effect on the integrity of the encapsulated data if one is not careful.
Choose injection by constructor or by property wisely based on your specific scenario. And don't feel that you have to use DI just because it seems necessary and it will prevent bad design and code smells. Sometimes it's not worth the effort to use a pattern if the effort and complexity outweighs the benefit. Keep it simple.
This is an old post, but if it is needed in future maybe this is of any use:
https://github.com/omegamit6zeichen/prinject
I had a similar idea and came up with this framework. It is probably far from complete, but it is an idea of a framework focusing on property injection
It depends on how you want to implement.
I prefer constructor injection wherever I feel the values that go in to the implementation doesnt change often. Eg: If the compnay stragtegy is go with oracle server, I will configure my datsource values for a bean achiveing connections via constructor injection.
Else, if my app is a product and chances it can connect to any db of the customer , I would implement such db configuration and multi brand implementation through setter injection. I have just taken an example but there are better ways of implementing the scenarios I mentioned above.
When to use Constructor injection?
When we want to make sure that the Object is created with all of its dependencies and to ensure that required dependencies are not null.
When to use Setter injection?
When we are working with optional dependencies that can be assigned reasonable default values within the class. Otherwise, not-null checks must be performed everywhere the code uses the dependency.
Additionally, setter methods make objects of that class open to reconfiguration or re-injection at a later time.
Sources:
Spring documentation ,
Java Revisited
I know this SO question, but it deals with the subject in more general terms.
Should I prefer using partial Mocks over Dependency Injection? My question is based on the following quote from OCMock:
id aMock = [OCMockObject partialMockForObject:anObject]
Creates a mock object that can be used
in the same way as anObject. When a
method that is not stubbed is invoked
it will be forwarded anObject. When a
stubbed method is invoked using a
reference to anObject, rather than the
mock, it will still be handled by the
mock.
This means I could stub my (property-)dependecies away using a partial mock instead of injecting them in the constructor (or via setter injection).
You should design your API so that it makes sense as a general-purpose API, not particularly to support unit testing or dynamic mocks.
The pattern you suggest is simply a variation of the Template Method design pattern, except that the method is a property. If you think that, in general, it makes sense to implement your dependency access as virtual properties, then you can use the technique you describe. This is a well-known unit testing technique called extract and override.
However, I would be vary of doing this for a number of other reasons.
Even if you can override the dependency, the default may drag in references to the 'real', intended depdendency creating a tighter coupling than you may want.
If you forget to extract and override, the default dependency is used, and you may not want that. Sometimes it's better to be explicit about the intended usage.
Say I have a class called PermissionManager which should only exist once for my system and basically fulfills the function of managing various permissions for various actions in my application. Now I have some class in my application which needs to be able to check a certain permission in one of its methods. This class's constructor is currently public, i.e. used by API users.
Until a couple of weeks ago, I would have simply had my class call the following pseudo-code somewhere:
PermissionManager.getInstance().isReadPermissionEnabled(this)
But since I have noticed everyone here hating singletons + this kind of coupling, I was wondering what the better solution would be, since the arguments I have read against singletons seem to make sense (not testable, high coupling, etc.).
So should I actually require API users to pass in a PermissionManager instance in the constructor of the class? Even though I only want a single PermissionManager instance to exist for my application?
Or am I going about this all wrong and should have a non-public constructor and a factory somewhere which passes in the instance of PermissionManager for me?
Additional info Note that when I say "Dependency Injection", I'm talking about the DI Pattern...I am not using any DI framework like Guice or Spring. (...yet)
If you are using a dependency-injection framework, then the common way to handle this is to either pass in a PermissionsManager object in the constructor or to have a property of type PermissionsManager that the framework sets for you.
If this is not feasible, then having users get an instance of this class via factory is a good choice. In this case, the factory passes the PermissionManager in to the constructor when it creates the class. In your application start-up, you would create the single PermissionManager first, then create your factory, passing in the PermissionManager.
You are correct that it is normally unwieldy for the clients of a class to know where to find the correct PermissionManager instance and pass it in (or even to care about the fact that your class uses a PermissionManager).
One compromise solution I've seen is to give your class a property of type PermissionManager. If the property has been set (say, in a unit test), you use that instance, otherwise you use the singleton. Something like:
PermissionManager mManager = null;
public PermissionManager Permissions
{
if (mManager == null)
{
return mManager;
}
return PermissionManager.getInstance();
}
Of course, strictly speaking, your PermissionManager should implement some kind of IPermissionManager interface, and that's what your other class should reference so a dummy implementation can be substituted more easily during testing.
You can indeed start by injecting the PermissionManager. This will make your class more testable.
If this causes problems for the users of that class you can have them use a factory method or an abstract factory. Or you can add a parameterless constructor that for them to call that injects the PermissionManager while your tests use another constructor that you can use to mock the PermissionManager.
Decoupling your classes more makes your classes more flexible but it can also make them harder to use. It depends on the situation what you'll need. If you only have one PermissionManager and have no problem testing the classes that use it then there's no reason to use DI. If you want people to be able to add their own PermissionManager implementation then DI is the way to go.
If you are subscribing to the dependency injection way of doing things, whatever classes need your PermissionManager should have it injected as an object instance. The mechanism that controls its instantiation (to enforce the singleton nature) works at a higher level. If you use a dependency injection framework like Guice, it can do the enforcement work. If you are doing your object wiring by hand, dependency injection favors grouping code that does instantiation (new operator work) away from your business logic.
Either way, though, the classic "capital-S" Singleton is generally seen as an anti-pattern in the context of dependency injection.
These posts have been insightful for me in the past:
Using Dependency Injection to Avoid Singletons
How to Think About the "new" Operator with Respect to Unit Testing
So should I actually require API users to pass in a PermissionManager instance in the constructor of the class? Even though I only want a single PermissionManager instance to exist for my application?
Yes, this is all you need to do. Whether a dependency is a singleton / per request / per thread or a factory method is the responsibility of your container and configuration. In the .net world we would ideally have the dependency on an IPermissionsManager interface to further reduce coupling, I assume this is best practice in Java too.
The singleton pattern is not bad by itself, what makes it ugly is the way it's commonly used, as being the requirement of only wanting a single instance of a certain class, which I think it's a big mistake.
In this case I'd make PermissionManager a static class unless for any reason you need it to be an instanciable type.