Inversion of Control vs Dependency Injection

Question

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

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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

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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  amp  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

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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  amp  annotation based configuration where we define bean object and pass the dependent object with Constructor or setter injection style

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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

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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

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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  amp  DIP are two disjoint sets and DIP is the super set of DI  service locator and some other patterns

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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   amp name   print   Please enter your DOB     scan   amp dob      More print and scan statements  lt Do Something Interesting gt     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  lt typos gt  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 lt typos gt  CheckSpelling string Text       Class EnglishSpellChecker   ISpellChecker         public override Arraylist lt typos gt  CheckSpelling string Text                    All Magic goes here               Class FrenchSpellChecker   ISpellChecker         public override Arraylist lt typos gt  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  lt typos gt  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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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IoC is a generic term meaning that rather than having the application call the implementations provided by a library  also know as toolkit   a framework calls the implementations provided by the application  DI is a form of IoC  where implementations are passed into an object through constructors setters service lookups  which the object will  depend  on in order to behave correctly  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 which implementations to use in metadata files  e g  XML  which are less invasive   With some you can do IoC that would normally be impossible like inject an implementation at pointcuts  See also this Martin Fowler s article

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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

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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

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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  amp  Dependency Injection

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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

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Let s begin with D of SOLID and look at DI and IoC from Scott Millett s book  quot Professional ASP NET Design Patterns quot    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

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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

[dependency-injection] Inversion of Control vs Dependency Injection

Examples related to dependency-injection

Examples related to inversion-of-control