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By definition, encapsulation describes the idea of bundling data and methods that work on that data within one unit, like a class in Java. This concept is also often used to hide the internal representation, or state of an object from the outside. This is called information hiding.
In object-oriented programming (OOP) software design, dependency injection (DI) is the process of supplying a resource that a given piece of code requires. The required resource, which is often a component of the application itself, is called a dependency.
Constructor injection should be the main way that you do dependency injection. It's simple: A class needs something and thus asks for it before it can even be constructed. By using the guard pattern, you can use the class with confidence, knowing that the field variable storing that dependency will be a valid instance.
More specifically, dependency injection is effective in these situations: You need to inject configuration data into one or more components. You need to inject the same dependency into multiple components. You need to inject different implementations of the same dependency.
There is another way of looking at this issue that you might find interesting.
When we use IoC/dependency injection, we're not using OOP concepts. Admittedly we're using an OO language as the 'host', but the ideas behind IoC come from component-oriented software engineering, not OO.
Component software is all about managing dependencies - an example in common use is .NET's Assembly mechanism. Each assembly publishes the list of assemblies that it references, and this makes it much easier to pull together (and validate) the pieces needed for a running application.
By applying similar techniques in our OO programs via IoC, we aim to make programs easier to configure and maintain. Publishing dependencies (as constructor parameters or whatever) is a key part of this. Encapsulation doesn't really apply, as in the component/service oriented world, there is no 'implementation type' for details to leak from.
Unfortunately our languages don't currently segregate the fine-grained, object-oriented concepts from the coarser-grained component-oriented ones, so this is a distinction that you have to hold in your mind only :)
It's a good question - but at some point, encapsulation in its purest form needs to be violated if the object is ever to have its dependency fulfilled. Some provider of the dependency must know both that the object in question requires a Foo, and the provider has to have a way of providing the Foo to the object.
Classically this latter case is handled as you say, through constructor arguments or setter methods. However, this is not necessarily true - I know that the latest versions of the Spring DI framework in Java, for example, let you annotate private fields (e.g. with @Autowired) and the dependency will be set via reflection without you needing to expose the dependency through any of the classes public methods/constructors. This might be the kind of solution you were looking for.
That said, I don't think that constructor injection is much of a problem, either. I've always felt that objects should be fully valid after construction, such that anything they need in order to perform their role (i.e. be in a valid state) should be supplied through the constructor anyway. If you have an object that requires a collaborator to work, it seems fine to me that the constructor publically advertises this requirement and ensures it is fulfilled when a new instance of the class is created.
Ideally when dealing with objects, you interact with them through an interface anyway, and the more you do this (and have dependencies wired through DI), the less you actually have to deal with constructors yourself. In the ideal situation, your code doesn't deal with or even ever create concrete instances of classes; so it just gets given an IFoo through DI, without worrying about what the constructor of FooImpl indicates it needs to do its job, and in fact without even being aware of FooImpl's existance. From this point of view, the encapsulation is perfect.
This is an opinion of course, but to my mind DI doesn't necessarily violate encapsulation and in fact can help it by centralising all of the necessary knowledge of internals into one place. Not only is this a good thing in itself, but even better this place is outside your own codebase, so none of the code you write needs to know about classes' dependencies.
This exposes the dependency being injected and violates the OOP principle of encapsulation.
Well, frankly speaking, everything violates encapsulation. :) It's a kind of a tender principle that must be treated well.
So, what violates encapsulation?
Inheritance does.
"Because inheritance exposes a subclass to details of its parent's implementation, it's often said that 'inheritance breaks encapsulation'". (Gang of Four 1995:19)
Aspect-oriented programming does. For example, you register onMethodCall() callback and that gives you a great opportunity to inject code to the normal method evaluation, adding strange side-effects etc.
Friend declaration in C++ does.
Class extention in Ruby does. Just redefine a string method somewhere after a string class was fully defined.
Well, a lot of stuff does.
Encapsulation is a good and important principle. But not the only one.
switch (principle)
{
case encapsulation:
if (there_is_a_reason)
break!
}
Yes, DI violates encapsulation (also known as "information hiding").
But the real problem comes when developers use it as an excuse to violate the KISS (Keep It Short and Simple) and YAGNI (You Ain't Gonna Need It) principles.
Personally, I prefer simple and effective solutions. I mostly use the "new" operator to instantiate stateful dependencies whenever and wherever they are needed. It is simple, well encapsulated, easy to understand, and easy to test. So, why not?
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