My understanding is that a facade is used as an alternative to dependency injection. Please correct if I'm mistaken. What is not clear is when one should use one or the other.
What are the advantages/disadvantages of each approach? How should I determine when to use one or the other?
Lastly, why not use both? I can create a facade that references an interface. It seems Sentry 2 is written this way. Is there a best practice?
FACADES
Facades are not an alternative to dependency injection.
Laravel Facade is an implementation of the Service Locator Pattern, creating a clean and beautiful way of accessing objects:
MyClass::doSomething();
This is the PHP syntax for a static methods, but Laravel changes the game and make them non-static behind the scenes, giving you a beautiful, enjoyable and testable way of writing your applications.
DEPENDENCY INJECTION
Dependency Injection is, basically, a way of passing parameters to your constructors and methods while automatically instatiating them.
class MyClass {
private $property;
public function __construct(MyOtherClass $property)
{
/// Here you can use the magic of Dependency Injection
$this->property = $property
/// $property already is an object of MyOtherClass
}
}
A better construction of it would be using Interfaces on your Dependency Injected constructors:
class MyClass {
private $property;
public function __construct(MyInterface $property)
{
/// Here you can use the magic of Dependency Injection
$this->property = $property
/// $property will receive an object of a concrete class that implements MyInterface
/// This class should be defined in Laravel elsewhere, but this is a way of also make
/// your application easy to maintain, because you can swap implementations of your interfaces
/// easily
}
}
But note that in Laravel you can inject classes and interfaces the same way. To inject interfaces you just have to tell it wich one will be this way:
App::bind('MyInterface', 'MyOtherClass');
This will tell Laravel that every time one of your methods needs an instance of MyInterface it should give it one of MyOtherClass.
What happens here is that this constuctor has a "dependency": MyOtherClass, which will be automatically injected by Laravel using the IoC container. So, when you create an instance of MyClass, Laravel automatically will create an instance of MyOtherClass and put it in the variable $class.
Dependency Injection is just an odd jargon developers created to do something as simple as "automatic generation of parameters".
WHEN TO USE ONE OR THE OTHER?
As you can see, they are completely different things, so you won't ever need to decide between them, but you will have to decide where go to with one or the other in different parts of your application.
Use Facades to ease the way you write your code. For example: it's a good practice to create packages for your application modules, so, to create Facades for those packages is also a way to make them seem like a Laravel public class and accessing them using the static syntax.
Use Dependency Injection every time your class needs to use data or processing from another class. It will make your code testable, because you will be able to "inject" a mock of those dependencies into your class and you will be also exercising the single responsibility principle (take a look at the SOLID principles).
Facades, as noted, are intended to simplify a potentially complicated interface.
Facades are still testable
Laravel's implementation goes a step further and allows you to define the base-class that the Facade "points" to.
This gives a developer the ability to "mock" a Facade - by switching the base-class out with a mock object.
In that sense, you can use them and still have testable code. This is where some confusion lies within the PHP community.
DI is often cited as making your code testable - they make mocking class dependencies easy. (Sidenote: Interfaces and DI have other important reasons for existing!)
Facades, on the other hand, are often cited as making testing harder because you can't "simply inject a mock object" into whatever code you're testing. However, as noted, you can in fact "mock" them.
Facade vs DI
This is where people get confused regarding whether Facades are an alternative to DI or not.
In a sense, they both add a dependency to your class - You can either use DI to add a dependency or you can use a Facade directly - FacadeName::method($param);. (Hopefully you are not instantiating any class directly within another :D ).
This does not make Facades an alternative to DI, but instead, within Laravel, does create a situation where you may decide to add class dependencies one of 2 ways - either using DI or by using a Facade. (You can, of course, use other ways. These "2 ways" are just the most-often used "testable way").
Laravel's Facades are an implementation of the Service Locator pattern, not the Facade pattern.
In my opinion you should avoid service locator within your domain, opting to only use it in your service and web transport layers.
http://martinfowler.com/articles/injection.html#UsingAServiceLocator
I think that in terms of laravel Facades help you keep you code simple and still testable since you can mock facades however might be a bit harder to tell a controllers dependencies if you use facades since they are probably all over the place in your code.
With dependency injection you need to write a bit more code since you need to deal with creating interfaces and services to handle the depenancies however Its a lot more clear later on what a controller depends on since these are clearly mentioned in the controller constructor.
I guess it's a matter of deciding which method you prefer using
According to the paper written by Martin Fowler, inversion of control is the principle where the control flow of a program is inverted: instead of the programmer controlling the flow of a program, the external sources (framework, services, other components) take control of it. It's like we plug something into something else. He mentioned an example about EJB 2.0:
For example the Session Bean interface
defines ejbRemove, ejbPassivate
(stored to secondary storage), and
ejbActivate (restored from passive
state). You don't get to control when
these methods are called, just what
they do. The container calls us, we
don't call it.
This leads to the difference between framework and library:
Inversion of Control is a key part of
what makes a framework different to a
library. A library is essentially a
set of functions that you can call,
these days usually organized into
classes. Each call does some work and
returns control to the client.
I think, the point of view that DI is IOC, means the dependency of an object is inverted: instead of it controlling its own dependencies, life cycle... something else does it for you. But, as you told me about DI by hands, DI is not necessarily IOC. We can still have DI and no IOC.
However, in this paper (from the pococapsule, another IOC Framework for C/C++), it suggests that because of IOC and DI, the IOC containers and DI frameworks are far more superior to J2EE, since J2EE mixes the framework code into the components, thus not making it Plain Old Java/C++ Object (POJO/POCO).
Inversion of Control Containers other than the Dependency Injection pattern (Archive link)
Additional reading to understand what's the problem with old Component-Based Development Framework, which leads to the second paper above: Why and what of Inversion of Control (Archive link)
My Question: What exactly is IOC and DI? I am confused. Based on pococapsule, IOC is something more significant than just inversion of the control between objects or programmers and frameworks.
The Inversion-of-Control (IoC) pattern, is about providing any kind of callback, which "implements" and/or controls reaction, instead of acting ourselves directly (in other words, inversion and/or redirecting control to the external handler/controller).
For example, rather than having the application call the implementations provided by a library (also known as toolkit), the library and/or framework calls the implementations provided by the application.
The Dependency-Injection (DI) pattern is a more specific version of IoC pattern, where implementations are passed into an object through constructors/setters/service lookups, which the object will "depend" on in order to behave correctly.
Every DI implementation can be considered IoC, but one should not call it IoC, because implementing Dependency-Injection is harder than callback (Don't lower your product's worth by using the general term "IoC" instead).
IoC without using DI, for example, would be the Template pattern because the implementation can only be changed through sub-classing.
DI frameworks are designed to make use of DI, and can define interfaces (or Annotations in Java) to make it easy to pass in the implementations.
IoC containers are DI frameworks that can work outside of the programming language. In some you can configure in metadata files (e.g. XML), the implementations to be used, which are less invasive. With some you can do IoC that would normally be impossible, like injecting an implementation at pointcuts.
See also this Martin Fowler's article.
In short, IoC is a much broader term that includes, but is not limited to, DI
The term Inversion of Control (IoC) originally meant any sort of programming style where an overall
framework or run-time controlled the program flow
Before DI had a name, people started to refer to frameworks that manage Dependencies as Inversion
of Control Containers, and soon, the meaning of IoC gradually drifted towards that particular meaning: Inversion of Control over Dependencies.
Inversion of Control (IoC) means that objects do not create other objects on which they rely to do their work. Instead, they get the objects that they need from an outside source (for example, an xml configuration file).
Dependency Injection (DI) means that this is done without the object intervention, usually by a framework component that passes constructor parameters and set properties.
source
IoC (Inversion of Control) :- It’s a generic term and implemented in several ways (events, delegates etc).
DI (Dependency Injection) :- DI is a sub-type of IoC and is implemented by constructor injection, setter injection or Interface injection.
But, Spring supports only the following two types :
Setter Injection
Setter-based DI is realized by calling setter methods on the user’s beans after invoking a no-argument constructor or no-argument static factory method to instantiate their bean.
Constructor Injection
Constructor-based DI is realized by invoking a constructor with a number of arguments, each representing a collaborator.Using this we can validate that the injected beans are not null and fail fast(fail on compile time and not on run-time), so while starting application itself we get NullPointerException: bean does not exist. Constructor injection is Best practice to inject dependencies.
DI is a subset of IoC
IoC means that objects do not create other objects on which they rely to do their work. Instead, they get the objects that they need from an outside service (for example, xml file or single app service). 2 implementations of IoC, I use, are DI and ServiceLocator.
DI means the IoC principle of getting dependent object is done without using concrete objects but abstractions (interfaces). This makes all components chain testable, cause higher level component doesn't depend on lower level component, only from the interface. Mocks implement these interfaces.
Here are some other techniques to achieve IoC.
IOC (Inversion Of Control): Giving control to the container to get an instance of the object is called Inversion of Control, means instead of you are creating an object using the new operator, let the container do that for you.
DI (Dependency Injection): Way of injecting properties to an object is called Dependency Injection.
We have three types of Dependency Injection:
Constructor Injection
Setter/Getter Injection
Interface Injection
Spring supports only Constructor Injection and Setter/Getter Injection.
Since all the answers emphasize on theory I would like to demonstrate with an example first approach:
Suppose we are building an application which contains a feature to send SMS confirmation messages once the order has been shipped.
We will have two classes, one is responsible for sending the SMS (SMSService), and another responsible for capturing user inputs (UIHandler), our code will look as below:
public class SMSService
{
public void SendSMS(string mobileNumber, string body)
{
SendSMSUsingGateway(mobileNumber, body);
}
private void SendSMSUsingGateway(string mobileNumber, string body)
{
/*implementation for sending SMS using gateway*/
}
}
public class UIHandler
{
public void SendConfirmationMsg(string mobileNumber)
{
SMSService _SMSService = new SMSService();
_SMSService.SendSMS(mobileNumber, "Your order has been shipped successfully!");
}
}
Above implementation is not wrong but there are few issues:
-) Suppose On development environment, you want to save SMSs sent to a text file instead of using SMS gateway, to achieve this; we will end up changing the concrete implementation of (SMSService) with another implementation, we are losing flexibility and forced to rewrite the code in this case.
-) We’ll end up mixing responsibilities of classes, our (UIHandler) should never know about the concrete implementation of (SMSService), this should be done outside the classes using “Interfaces”. When this is implemented, it will give us the ability to change the behavior of the system by swapping the (SMSService) used with another mock service which implements the same interface, this service will save SMSs to a text file instead of sending to mobileNumber.
To fix the above issues we use Interfaces which will be implemented by our (SMSService) and the new (MockSMSService), basically the new Interface (ISMSService) will expose the same behaviors of both services as the code below:
public interface ISMSService
{
void SendSMS(string phoneNumber, string body);
}
Then we will change our (SMSService) implementation to implement the (ISMSService) interface:
public class SMSService : ISMSService
{
public void SendSMS(string mobileNumber, string body)
{
SendSMSUsingGateway(mobileNumber, body);
}
private void SendSMSUsingGateway(string mobileNumber, string body)
{
/*implementation for sending SMS using gateway*/
Console.WriteLine("Sending SMS using gateway to mobile:
{0}. SMS body: {1}", mobileNumber, body);
}
}
Now we will be able to create new mock up service (MockSMSService) with totally different implementation using the same interface:
public class MockSMSService :ISMSService
{
public void SendSMS(string phoneNumber, string body)
{
SaveSMSToFile(phoneNumber,body);
}
private void SaveSMSToFile(string mobileNumber, string body)
{
/*implementation for saving SMS to a file*/
Console.WriteLine("Mocking SMS using file to mobile:
{0}. SMS body: {1}", mobileNumber, body);
}
}
At this point, we can change the code in (UIHandler) to use the concrete implementation of the service (MockSMSService) easily as below:
public class UIHandler
{
public void SendConfirmationMsg(string mobileNumber)
{
ISMSService _SMSService = new MockSMSService();
_SMSService.SendSMS(mobileNumber, "Your order has been shipped successfully!");
}
}
We have achieved a lot of flexibility and implemented separation of concerns in our code, but still we need to do a change on the code base to switch between the two SMS Services. So we need to implement Dependency Injection.
To achieve this, we need to implement a change to our (UIHandler) class constructor to pass the dependency through it, by doing this, the code which uses the (UIHandler) can determine which concrete implementation of (ISMSService) to use:
public class UIHandler
{
private readonly ISMSService _SMSService;
public UIHandler(ISMSService SMSService)
{
_SMSService = SMSService;
}
public void SendConfirmationMsg(string mobileNumber)
{
_SMSService.SendSMS(mobileNumber, "Your order has been shipped successfully!");
}
}
Now the UI form which will talk with class (UIHandler) is responsible to pass which implementation of interface (ISMSService) to consume. This means we have inverted the control, the (UIHandler) is no longer responsible to decide which implementation to use, the calling code does. We have implemented the Inversion of Control principle which DI is one type of it.
The UI form code will be as below:
class Program
{
static void Main(string[] args)
{
ISMSService _SMSService = new MockSMSService(); // dependency
UIHandler _UIHandler = new UIHandler(_SMSService);
_UIHandler.SendConfirmationMsg("96279544480");
Console.ReadLine();
}
}
Rather than contrast DI and IoC directly, it may be helpful to start from the beginning: every non-trivial application depends on other pieces of code.
So I am writing a class, MyClass, and I need to call a method of YourService... somehow I need to acquire an instance of YourService. The simplest, most straightforward way is to instantiate it myself.
YourService service = new YourServiceImpl();
Direct instantiation is the traditional (procedural) way to acquire a dependency. But it has a number of drawbacks, including tight coupling of MyClass to YourServiceImpl, making my code difficult to change and difficult to test. MyClass doesn't care what the implementation of YourService looks like, so MyClass doesn't want to be responsible for instantiating it.
I'd prefer to invert that responsibility from MyClass to something outside MyClass. The simplest way to do that is just to move the instantiation call (new YourServiceImpl();) into some other class. I might name this other class a Locator, or a Factory, or any other name; but the point is that MyClass is no longer responsible for YourServiceImpl. I've inverted that dependency. Great.
Problem is, MyClass is still responsible for making the call to the Locator/Factory/Whatever. Since all I've done to invert the dependency is insert a middleman, now I'm coupled to the middleman (even if I'm not coupled to the concrete objects the middleman gives me).
I don't really care where my dependencies come from, so I'd prefer not to be responsible for making the call(s) to retrieve them. Inverting the dependency itself wasn't quite enough. I want to invert control of the whole process.
What I need is a totally separate piece of code that MyClass plugs into (call it a framework). Then the only responsibility I'm left with is to declare my dependency on YourService. The framework can take care of figuring out where and when and how to get an instance, and just give MyClass what it needs. And the best part is that MyClass doesn't need to know about the framework. The framework can be in control of this dependency wiring process. Now I've inverted control (on top of inverting dependencies).
There are different ways of connecting MyClass into a framework. Injection is one such mechanism whereby I simply declare a field or parameter that I expect a framework to provide, typically when it instantiates MyClass.
I think the hierarchy of relationships among all these concepts is slightly more complex than what other diagrams in this thread are showing; but the basic idea is that it is a hierarchical relationship. I think this syncs up with DIP in the wild.
But the spring documentation says they are same.
http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#beans-introduction
In the first line "IoC is also known as dependency injection (DI)".
IoC - Inversion of control is generic term, independent of language, it is actually not create the objects but describe in which fashion object is being created.
DI - Dependency Injection is concrete term, in which we provide dependencies of the object at run time by using different injection techniques viz. Setter Injection, Constructor Injection or by Interface Injection.
Inversion of control is a design paradigm with the goal of giving more control to the targeted components of your application, the ones getting the work done.
Dependency injection is a pattern used to create instances of objects that other objects rely on without knowing at compile time which class will be used to provide that functionality.
There are several basic techniques to implement inversion of control. These are:
Using a factory pattern
Using a service locator pattern
Using a dependency injection of any given below type:
1). A constructor injection
2). A setter injection
3). An interface injection
Inversion of Control is a generic design principle of software architecture that assists in creating reusable, modular software frameworks that are easy to maintain.
It is a design principle in which the Flow of Control is "received" from the generic-written library or reusable code.
To understand it better, lets see how we used to code in our earlier days of coding. In procedural/traditional languages, the business logic generally controls the flow of the application and "Calls" the generic or reusable code/functions. For example, in a simple Console application, my flow of control is controlled by my program's instructions, that may include the calls to some general reusable functions.
print ("Please enter your name:");
scan (&name);
print ("Please enter your DOB:");
scan (&dob);
//More print and scan statements
<Do Something Interesting>
//Call a Library function to find the age (common code)
print Age
In Contrast, with IoC, the Frameworks are the reusable code that "Calls" the business logic.
For example, in a windows based system, a framework will already be available to create UI elements like buttons, menus, windows and dialog boxes. When I write the business logic of my application, it would be framework's events that will call my business logic code (when an event is fired) and NOT the opposite.
Although, the framework's code is not aware of my business logic, it will still know how to call my code. This is achieved using events/delegates, callbacks etc. Here the Control of flow is "Inverted".
So, instead of depending the flow of control on statically bound objects, the flow depends upon the overall object graph and the relations between different objects.
Dependency Injection is a design pattern that implements IoC principle for resolving dependencies of objects.
In simpler words, when you are trying to write code, you will be creating and using different classes. One class (Class A) may use other classes (Class B and/or D). So, Class B and D are dependencies of class A.
A simple analogy will be a class Car. A car might depend on other classes like Engine, Tyres and more.
Dependency Injection suggests that instead of the Dependent classes (Class Car here) creating its dependencies (Class Engine and class Tyre), class should be injected with the concrete instance of the dependency.
Lets understand with a more practical example. Consider that you are writing your own TextEditor. Among other things, you can have a spellchecker that provides the user with a facility to check the typos in his text. A simple implementation of such a code can be:
Class TextEditor
{
//Lot of rocket science to create the Editor goes here
EnglishSpellChecker objSpellCheck;
String text;
public void TextEditor()
{
objSpellCheck = new EnglishSpellChecker();
}
public ArrayList <typos> CheckSpellings()
{
//return Typos;
}
}
At first sight, all looks rosy. The user will write some text. The developer will capture the text and call the CheckSpellings function and will find a list of Typos that he will show to the User.
Everything seems to work great until one fine day when one user starts writing French in the Editor.
To provide the support for more languages, we need to have more SpellCheckers. Probably French, German, Spanish etc.
Here, we have created a tightly-coupled code with "English"SpellChecker being tightly coupled with our TextEditor class, which means our TextEditor class is dependent on the EnglishSpellChecker or in other words EnglishSpellCheker is the dependency for TextEditor. We need to remove this dependency. Further, Our Text Editor needs a way to hold the concrete reference of any Spell Checker based on developer's discretion at run time.
So, as we saw in the introduction of DI, it suggests that the class should be injected with its dependencies. So, it should be the calling code's responsibility to inject all the dependencies to the called class/code. So we can restructure our code as
interface ISpellChecker
{
Arraylist<typos> CheckSpelling(string Text);
}
Class EnglishSpellChecker : ISpellChecker
{
public override Arraylist<typos> CheckSpelling(string Text)
{
//All Magic goes here.
}
}
Class FrenchSpellChecker : ISpellChecker
{
public override Arraylist<typos> CheckSpelling(string Text)
{
//All Magic goes here.
}
}
In our example, the TextEditor class should receive the concrete instance of ISpellChecker type.
Now, the dependency can be injected in Constructor, a Public Property or a method.
Lets try to change our class using Constructor DI. The changed TextEditor class will look something like:
Class TextEditor
{
ISpellChecker objSpellChecker;
string Text;
public void TextEditor(ISpellChecker objSC)
{
objSpellChecker = objSC;
}
public ArrayList <typos> CheckSpellings()
{
return objSpellChecker.CheckSpelling();
}
}
So that the calling code, while creating the text editor can inject the appropriate SpellChecker Type to the instance of the TextEditor.
You can read the complete article here
DI and IOC are two design pattern that mainly focusing on providing loose coupling between components, or simply a way in which we decouple the conventional dependency relationships between object so that the objects are not tight to each other.
With following examples, I am trying to explain both these concepts.
Previously we are writing code like this
Public MyClass{
DependentClass dependentObject
/*
At somewhere in our code we need to instantiate
the object with new operator inorder to use it or perform some method.
*/
dependentObject= new DependentClass();
dependentObject.someMethod();
}
With Dependency injection, the dependency injector will take care of the instantiation of objects
Public MyClass{
/* Dependency injector will instantiate object*/
DependentClass dependentObject
/*
At somewhere in our code we perform some method.
The process of instantiation will be handled by the dependency injector
*/
dependentObject.someMethod();
}
The above process of giving the control to some other (for example the container) for the instantiation and injection can be termed as Inversion of Control and the process in which the IOC container inject the dependency for us can be termed as dependency injection.
IOC is the principle where the control flow of a program is inverted: instead of the programmer controlling the flow of a program, program controls the flow by reducing the overhead to the programmer.and the process used by the program to inject dependency is termed as DI
The two concepts work together providing us with a way to write much more flexible, reusable, and encapsulated code, which make them as important concepts in designing object-oriented solutions.
Also Recommend to read.
What is dependency injection?
You can also check one of my similar answer here
Difference between Inversion of Control & Dependency Injection
IOC(Inversion Of Control): Giving control to the container to get instance of object is called Inversion of Control. It means instead of you are creating object using new operator, let the container do that for you.
DI(Dependency Injection): Passing the required parameters(properties) from XML to an object(in POJO CLASS) is called Dependency injection.
IOC indicates that an external classes managing the classes of an application,and external classes means a container manages the dependency between class of application.
basic concept of IOC is that programmer don't need to create your objects but describe how they should be created.
The main tasks performed by IoC container are:
to instantiate the application class. to configure the object. to assemble the dependencies between the objects.
DI is the process of providing the dependencies of an object at run time by using setter injection or constructor injection.
IOC - DIP - DI
Inversion of Control (IOC)
Dependency Inversion Principle (DIP)
Dependency Injection (DI)
1- IOC: abstract principle describing an aspect of some software architecture designs in which the flow of control of a system is inverted in comparison to procedural programming.
2-DIP: is Object Oriented Programming(OOP) principle(D of SOLID).
3-DI: is a software design pattern that implements inversion of control and allows a program design to follow the dependency inversion principle.
IOC & DIP are two disjoint sets and DIP is the super set of DI, service locator and some other patterns
IOC (Inversion of Control) is basically design pattern concept of removing dependencies and decoupling them to making the flow non-linear , and let the container / or another entity manage the provisioning of dependencies. It actually follow Hollywood principal “Don’t call us we will call you”.
So summarizing the differences.
Inversion of control :- It’s a generic term to decouple the dependencies and delegate their provisioning , and this can be implemented in several ways (events, delegates etc).
Dependency injection :- DI is a subtype of IOC and is implemented by constructor injection, setter injection or method injection.
The following article describe this very neatly.
https://www.codeproject.com/Articles/592372/Dependency-Injection-DI-vs-Inversion-of-Control-IO
I think the idea can be demonstrated clearly without getting into Object Oriented weeds, which seem to muddle the idea.
// dependency injection
function doSomething(dependency) {
// do something with your dependency
}
// in contrast to creating your dependencies yourself
function doSomething() {
dependency = getDependencySomehow()
}
// inversion of control
application = makeApp(authenticate, handleRequest, sendResponse)
application.run(getRequest())
// in contrast to direct control or a "library" style
application = makeApp()
request = application.getRequest()
if (application.authenticate(request.creds)) {
response = application.handleRequest(request)
application.sendResponse(response)
}
If you tilt your head and squint your eyes, you'll see that DI is a particular implementation of IoC with specific concerns. Instead of injecting models and behaviors into an application framework or higher-order operation, you are injecting variables into a function or object.
Let's begin with D of SOLID and look at DI and IoC from Scott Millett's book "Professional ASP.NET Design Patterns":
Dependency Inversion Principle (DIP)
The DIP is all about isolating your classes from concrete
implementations and having them depend on abstract classes or
interfaces. It promotes the mantra of coding to an interface rather
than an implementation, which increases flexibility within a system by
ensuring you are not tightly coupled to one implementation.
Dependency Injection (DI) and Inversion of Control (IoC)
Closely linked to the DIP are the DI principle and the IoC principle. DI is the act of supplying a low level or dependent class via a
constructor, method, or property. Used in conjunction with DI, these
dependent classes can be inverted to interfaces or abstract classes
that will lead to loosely coupled systems that are highly testable and
easy to change.
In IoC, a system’s flow of control is inverted
compared to procedural programming. An example of this is an IoC
container, whose purpose is to inject services into client code
without having the client code specifying the concrete implementation.
The control in this instance that is being inverted is the act of the
client obtaining the service.
Millett,C (2010). Professional ASP.NET Design Patterns. Wiley Publishing. 7-8.
DIP vs DI vs IoC
[Dependency Inversion Principle(DIP)] is a part of SOLID[About] which ask you to use abstraction instead of realizations
Dependency Injection(DI) - use Aggregation instead of Composition[About] In this case external object is responsible for logic inside. Which allows you to have more dynamic and testable approach
class A {
B b
//injecting B via constructor
init(b: B) {
self.b = b
}
}
Inversion of Control(IoC) very high level definition which is more about control flow. The best example is Inversion of Control(IoC) Container or Framework[About]. For example GUI which is Framework where you don't have a control, everything which you can do is just implement Framework's interface which will be called when some action is happend in the Framework. So control is shifted from your application into the Framework being used
DIP + DI
class A {
IB ib
init(ib: IB) {
self.ib = ib
}
}
Also you can achieve it using:
[Factory Method]
[Service Locator]
[IoC-container(framework)]
More complex example
Dependency rule in multi layer/module structure
Pseudocode:
interface InterfaceInputPort {
func input()
}
interface InterfaceOutputPort {
func output()
}
class A: InterfaceOutputPort {
let inputPort = B(outputPort: self)
func output() {
print("output")
}
}
class B: InterfaceInputPort {
let outputPort: InterfaceOutputPort
init(outputPort: InterfaceOutputPort) {
self.outputPort = outputPort
}
func input() {
print("input")
}
}
//ICO , DI ,10 years back , this was they way:
public class AuditDAOImpl implements Audit{
//dependency
AuditDAO auditDAO = null;
//Control of the AuditDAO is with AuditDAOImpl because its creating the object
public AuditDAOImpl () {
this.auditDAO = new AuditDAO ();
}
}
Now with Spring 3,4 or latest its like below
public class AuditDAOImpl implements Audit{
//dependency
//Now control is shifted to Spring. Container find the object and provide it.
#Autowired
AuditDAO auditDAO = null;
}
Overall the control is inverted from old concept of coupled code to the frameworks like Spring which makes the object available. So that's IOC as far as I know and Dependency injection as you know when we inject the dependent object into another object using Constructor or setters . Inject basically means passing it as an argument. In spring we have XML & annotation based configuration where we define bean object and pass the dependent object with Constructor or setter injection style.
I found best example on Dzone.com which is really helpfull to understand the real different between IOC and DI
“IoC is when you have someone else create objects for you.” So instead of writing "new " keyword (For example, MyCode c=new MyCode())in your code, the object is created by someone else. This ‘someone else’ is normally referred to as an IoC container. It means we handover the rrsponsibility (control )to the container to get instance of object is called Inversion of Control.,
means instead of you are creating object using new operator, let the container do that for you.
DI(Dependency Injection): Way of injecting properties to an object is
called
Dependency injection.
We have three types of Dependency injection
1) Constructor Injection
2) Setter/Getter Injection
3) Interface Injection
Spring will support only Constructor Injection and Setter/Getter Injection.
Read full article IOC and Read Full article DI
1) DI is Child->obj depends on parent-obj. The verb depends is important.
2) IOC is Child->obj perform under a platform. where platform could be school, college, dance class. Here perform is an activity with different implication under any platform provider.
practical example:
`
//DI
child.getSchool();
//IOC
child.perform()// is a stub implemented by dance-school
child.flourish()// is a stub implemented by dance-school/school/
`
-AB
As for this question, I'd say the wiki has already provided detailed and easy-understanding explanations. I will just quote the most significant here.
Implementation of IoC
In object-oriented programming, there are several basic techniques to
implement inversion of control. These are:
Using a service locator pattern Using dependency injection, for
example Constructor injection Parameter injection Setter injection
Interface injection;
Using a contextualized lookup;
Using template method design pattern;
Using strategy design pattern
As for Dependency Injection
dependency injection is a technique whereby one object (or static
method) supplies the dependencies of another object. A dependency is
an object that can be used (a service). An injection is the passing of
a dependency to a dependent object (a client) that would use it.
IoC concept was initially heard during the procedural programming era. Therefore from a historical context IoC talked about inversion of the ownership of control-flow i.e. who owns the responsibility to invoke the functions in the desired order - whether it's the functions themselves or should you invert it to some external entity.
However once the OOP emerged, people began to talk about IoC in OOP context where applications are concerned with object creation and their relationships as well, apart from the control-flow. Such applications wanted to invert the ownership of object-creation (rather than control-flow) and required a container which is responsible for object creation, object life-cycle & injecting dependencies of the application objects thereby eliminating application objects from creating other concrete object.
In that sense DI is not the same as IoC, since it's not about control-flow, however it's a kind of Io*, i.e. Inversion of ownership of object-creation.
What is wrong in my way of explainning DI and IoC?
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.