What is Dependency Injection

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Possible Duplicate    What is dependency injection        Spring is the framework from where the concept Dependency Injection came to picture   What is purpose of DI   How does it benefit   How is it implemented

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In simple words dependency injection  DI  is the way to remove dependencies or tight coupling between different object  Dependency Injection gives a cohesive behavior to each object    DI is the implementation of IOC principal of Spring which says  Don t call us we will call you   Using dependency injection programmer doesn t need to create object using the new keyword    Objects are once loaded in Spring container and then we reuse them whenever we need them by fetching those objects from Spring container using getBean String beanName  method

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Dependency Injection means a way  actually any-way  for one part of code  e g a class  to have access to dependencies  other parts of code  e g other classes  it depends upon  in a modular way without them being hardcoded  so they can change or be overriden freely  or even be loaded at another time  as needed    and ps   yes it has become an overly-hyped 25  name for a rather simple  concept   my  25 cents

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The best definition I ve found so far is one by James Shore        Dependency Injection  is a 25-dollar   term for a 5-cent concept          Dependency injection means giving an   object its instance variables           There is an article by Martin Fowler that may prove useful  too   Dependency injection is basically providing the objects that an object needs  its dependencies  instead of having it construct them itself  It s a very useful technique for testing  since it allows dependencies to be mocked or stubbed out   Dependencies can be injected into objects by many means  such as constructor injection or setter injection   One can even use specialized dependency injection frameworks  e g  Spring  to do that  but they certainly aren t required  You don t need those frameworks to have dependency injection  Instantiating and passing objects  dependencies  explicitly is just as good an injection as injection by framework

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DI is how real objects actually interact with each other without one object being responsible for the existence of another object  Objects should be treated in equality  They are all objects  No one should act like a creator  This is how you do justice to your objects   Simple example    if you need a physician  you simply go and find  an existing  one  You will not be thinking of creating a physician from scratch to help you  He already exists and he may serve you or other objects  He has the right to exist whether you  a single object  needs him or not because his purpose is to serve one or more objects  Who decided his existence is Almighty God  not natural selection  Therefore  one advantage of DI is to avoid creating useless redundant objects living without a purpose during the lifetime of your universe  i e  application

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I would propose a slightly different  short and precise definition of what Dependency Injection is  focusing on the primary goal  not on the technical means  following along from here       Dependency Injection is the process of creating the static  stateless   graph of service objects  where each service is parametrised by its   dependencies    The objects that we create in our applications  regardless if we use Java  C  or other object-oriented language  usually fall into one of two categories  stateless  static and global    service objects     modules   and stateful  dynamic and local    data objects      The module graph - the graph of service objects - is typically created on application startup  This can be done using a container  such as Spring  but can also be done manually  by passing parameters to object constructors  Both ways have their pros and cons  but a framework definitely isn   t necessary to use DI in your application   One requirement is that the services must be parametrised by their dependencies  What this means exactly depends on the language and approach taken in a given system  Usually  this takes the form of constructor parameters  but using setters is also an option  This also means that the dependencies of a service are hidden  when invoking a service method  from the users of the service   When to use  I would say whenever the application is large enough that encapsulating logic into separate modules  with a dependency graph between the modules gives a gain in readability and explorability of the code

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Start here   Also   see A-beginners-guide-to-Dependency-Injection   Obsolete   Elsewhere on SO    what-is-dependency-injection dependency-injection-vs-factory-pattern when-to-use-dependency-injection difference-between-dependency-injection-and-inversion-of-control

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The whole point of Dependency Injection  DI  is to keep application source code clean and stable    clean of dependency initialization code stable regardless of dependency used   Practically  every design pattern separates concerns to make future changes affect minimum files   The specific domain of DI is delegation of dependency configuration and initialization   Example  DI with shell script  If you occasionally work outside of Java  recall how source is often used in many scripting languages  Shell  Tcl  etc   or even import in Python misused for this purpose    Consider simple dependent sh script      bin sh   Dependent touch          one txt   two txt  archive files  one txt   two txt    The script is dependent  it won t execute successfully on its own  archive files is not defined    You define archive files in archive files zip sh implementation script  using zip in this case       bin sh   Dependency function archive files       zip files zip          Instead of source-ing implementation script directly in the dependent one  you use an injector sh  container  which wraps both  components       bin sh    Injector source   archive files zip sh source   dependent sh   The archive files dependency has just been injected into dependent script   You could have injected dependency which implements archive files using tar or xz   Example  removing DI  If dependent sh script used dependencies directly  the approach would be called dependency lookup  which is opposite to dependency injection       bin sh   Dependent    dependency look-up source   archive files zip sh  touch          one txt   two txt  archive files  one txt   two txt    Now the problem is that dependent  component  has to perform initialization itself   The  component  s source code is neither clean nor stable because every changes in initialization of dependencies requires new release for  components  s source code file as well   Last words  DI is not as largely emphasized and popularized as in Java frameworks   But it s a generic approach to split concerns of    application development  single source code release lifecycle  application deployment  multiple target environments with independent lifecycles    Using configuration only with dependency lookup does not help as number of configuration parameters may change per dependency  e g  new authentication type  as well as number of supported types of dependencies  e g  new database type

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Before going to the technical description first visualize it with a real-life example because you will find a lot of technical stuff to learn dependency injection but the majority of the people can t get the core concept of it  In the first picture   assume that you have a car factory with a lot of units  A car is actually built in the assembly unit but it needs engine  seats as well as wheels  So an assembly unit is dependent on these all units and they are the dependencies of the factory  You can feel that now it is too complicated to maintain all of the tasks in this factory because along with the main task  assembling a car in the Assembly unit  you have to also focus on other units  It is now very costly to maintain and the factory building is huge so it takes your extra bucks for rent  Now  look at the second picture  If you find some provider companies that will provide you with the wheel  seat  and engine for cheaper than your self-production cost then now you don t need to make them in your factory  You can rent a smaller building now just for your assembly unit which will lessen your maintenance task and reduce your extra rental cost  Now you can also focus only on your main task  Car assembly   Now we can say that all the dependencies for assembling a car are injected on the factory from the providers  It is an example of a real-life Dependency Injection  DI   Now in the technical word  dependency injection is a technique whereby one object  or static method  supplies the dependencies of another object  So  transferring the task of creating the object to someone else and directly using the dependency is called dependency injection  This will help you now to learn DI with a technical explanation  This will show when to use DI and when you should not

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Dependency Injection is a type of implementation of the  Inversion of Control  principle on which is based Frameworks building   Frameworks as stated in  Design Pattern  of GoF are classes that implement the main control flow logic raising the developer to do that  in this way Frameworks realize the inversion of control principle   A way to implement as a technique  and not as class hierarchy  this IoC principle it is just Dependency Injection   DI consists mainly into delegate the mapping of classes instances and type reference to that instances  to an external  entity   an object  static class  component  framework  etc      Classes instances are the  dependencies   the external binding of the calling component with the class instance through the reference it is the  injection    Obviously you can implement this technique in many way as you want from OOP point of view  see for example constructor injection  setter injection  interface injection   Delegating a third party to carry out the task of match a ref to an object it is very useful when you want to completely separate a component that needs some services from the same services implementation    In this way  when designing components  you can focus exclusively on their architecture and their specific logic  trusting on interfaces for collaborating with other objects without worry about any type of implementation changes of objects services used  also if the same object you are using will be totally replaced  obviously respecting the interface

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The popular answers are unhelpful  because they define dependency injection in a way that isn t useful  Let s agree that by  dependency  we mean some pre-existing other object that our object X needs  But we don t say we re doing  dependency injection  when we say   foo   Foo- gt new  bar     We just call that passing parameters into the constructor  We ve been doing that regularly ever since constructors were invented    Dependency injection  is considered a type of  inversion of control   which means that some logic is taken out of the caller  That isn t the case when the caller passes in parameters  so if that were DI  DI would not imply inversion of control   DI means there is an intermediate level between the caller and the constructor which manages dependencies  A Makefile is a simple example of dependency injection  The  caller  is the person typing  make bar  on the command line  and the  constructor  is the compiler  The Makefile specifies that bar depends on foo  and it does a  gcc -c foo cpp  gcc -c bar cpp   before doing a  gcc foo o bar o -o bar   The person typing  make bar  doesn t need to know that bar depends on foo  The dependency was injected between  make bar  and gcc   The main purpose of the intermediate level is not just to pass in the dependencies to the constructor  but to list all the dependencies in just one place  and to hide them from the coder  not to make the coder provide them    Usually the intermediate level provides factories for the constructed objects  which must provide a role that each requested object type must satisfy  That s because by having an intermediate level that hides the details of construction  you ve already incurred the abstraction penalty imposed by factories  so you might as well use factories

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Very short   What is the purpose of DI  With dependency injection  objects don t define their dependencies themselves  the dependencies are injected to them as needed   How does it benefit   The objects don t need to know where and how to get their dependencies  which results in loose coupling between objects  which makes them a lot easier to test   How is it implemented   Usually a container manages the lifecycle of objects and their dependencies based on a configuration file or annotations

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Let s imagine that you want to go fishing    Without dependency injection  you need to take care of everything yourself  You need to find a boat  to buy a fishing rod  to look for bait  etc  It s possible  of course  but it puts a lot of responsibility on you  In software terms  it means that you have to perform a lookup for all these things  With dependency injection  someone else takes care of all the preparation and makes the required equipment available to you  You will receive   be injected   the boat  the fishing rod and the bait - all ready to use

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Try taking a look at  http   martinfowler com articles injection html

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Example  we have 2 class Client and Service  Client will use Service  public class Service       public void doSomeThingInService                              Without Dependency Injection  Way 1       public class Client       public void doSomeThingInClient             Service service   new Service            service doSomeThingInService              Way 2   public class Client       Service service   new Service        public void doSomeThingInClient             service doSomeThingInService              Way 3    public class Client       Service service      public Client             service   new Service              public void doSomeThingInClient             service doSomeThingInService              1  2  3  Using  Client client   new Client    client doSomeThingInService      Advantages   Simple   Disadvantages   Hard for test Client class When we change Service constructor  we need to change code in all place create Service object   Use Dependency Injection  Way 1  Constructor injection  public class Client       Service service       Client Service service            this service   service                Example Client has 2 dependency         Client Service service  IDatabas database              this service   service            this database   database                public void doSomeThingInClient             service doSomeThingInService              Using   Client client   new Client new Service        Client client   new Client new Service    new SqliteDatabase     client doSomeThingInClient      Way 2  Setter injection  public class Client       Service service       public void setService Service service            this service   service             public void doSomeThingInClient             service doSomeThingInService              Using   Client client   new Client    client setService new Service     client doSomeThingInClient      Way 3  Interface injection  Check https   en wikipedia org wiki Dependency injection        Now  this code is already follow Dependency Injection and it is easier for test Client class  However  we still use new Service   many time and it is not good when change Service constructor  To prevent it  we can use DI injector like 1  Simple manual Injector  public class Injector       public static Service provideService            return new Service               public static IDatabase provideDatatBase            return new SqliteDatabase              public static ObjectA provideObjectA            return new ObjectA provideService                  Using  Service service   Injector provideService      2  Use library  For Android dagger2  Advantages   Make test easier When you change the Service  you only need to change it in Injector class If you use use Constructor Injection  when you look at constructor of Client  you will see how many dependency of Client class   Disadvantages    If you use use Constructor Injection  the Service object is created when Client created  sometime we use function in Client class without use Service so created Service is wasted   Dependency Injection definition  https   en wikipedia org wiki Dependency injection     A dependency is an object that can be used  Service    An injection is the passing of a dependency  Service  to a dependent object  Client  that would use it

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Dependency Injection is a practice where objects are designed in a manner where they receive instances of the objects from other pieces of code  instead of constructing them internally  This means that any object implementing the interface which is required by the object can be substituted in without changing the code  which simplifies testing  and improves decoupling   For example  consider these clases     public class PersonService     public void addManager  Person employee  Person newManager             public void removeManager  Person employee  Person oldManager             public Group getGroupByManager  Person manager              public class GroupMembershipService       public void addPersonToGroup  Person person  Group group             public void removePersonFromGroup  Person person  Group group                In this example  the implementation of PersonService  addManager and PersonService  removeManager would need an instance of the GroupMembershipService in order to do its work  Without Dependency Injection  the traditional way of doing this would be to instantiate a new GroupMembershipService in the constructor of PersonService and use that instance attribute in both functions  However  if the constructor of GroupMembershipService has multiple things it requires  or worse yet  there are some initialization  setters  that need to be called on the GroupMembershipService  the code grows rather quickly  and the PersonService now depends not only on the GroupMembershipService but also everything else that GroupMembershipService depends on  Furthermore  the linkage to GroupMembershipService is hardcoded into the PersonService which means that you can t  dummy up  a GroupMembershipService for testing purposes  or to use a strategy pattern in different parts of your application      With Dependency Injection  instead of instantiating the GroupMembershipService within your PersonService  you d either pass it in to the PersonService constructor  or else add a Property  getter and setter  to set a local instance of it  This means that your PersonService no longer has to worry about how to create a GroupMembershipService  it just accepts the ones it s given  and works with them   This also means that anything which is a subclass of GroupMembershipService  or implements the GroupMembershipService interface can be  injected  into the PersonService  and the PersonService doesn t need to know about the change

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http   en wikipedia org wiki Dependency injection

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from Book Apress Spring Persistence with Hibernate Oct 2010     The purpose of dependency injection is to decouple the work of   resolving external software components from your application business   logic Without dependency injection  the details of how a component   accesses required services can get muddled in with the component   s   code  This not only increases the potential for errors  adds code   bloat  and magnifies maintenance complexities  it couples components   together more closely  making it difficult to modify dependencies when   refactoring or testing

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Doesn t  dependency injection  just mean using parameterized constructors and public setters   James Shore s article shows the following examples for comparison      Constructor without dependency injection         public class Example      private DatabaseThingie myDatabase      public Example          myDatabase   new DatabaseThingie             public void doStuff                   myDatabase getData                            Constructor with dependency injection         public class Example      private DatabaseThingie myDatabase      public Example DatabaseThingie useThisDatabaseInstead         myDatabase   useThisDatabaseInstead          public void doStuff                   myDatabase getData

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To make Dependency Injection concept simple to understand  Let s take an example of switch button to toggle on off  a bulb   Without Dependency Injection  Switch needs to know beforehand which bulb I am connected to  hard-coded dependency   So   Switch -  PermanentBulb   switch is directly connected to permanent bulb  testing not possible easily    Switch    PermanentBulb   new Bulb    PermanentBulb Toggle        With Dependency Injection  Switch only knows I need to turn on off whichever Bulb is passed to me  So   Switch -  Bulb1 OR Bulb2 OR NightBulb  injected dependency     Switch AnyBulb     pass it whichever bulb you like AnyBulb Toggle        Modifying James Example for Switch and Bulb   public class SwitchTest      TestToggleBulb          MockBulb mockbulb   new MockBulb             MockBulb is a subclass of Bulb  so we can          inject  it here       Switch switch   new Switch mockBulb         switch ToggleBulb         mockBulb AssertToggleWasCalled             public class Switch      private Bulb myBulb      public Switch          myBulb   new Bulb             public Switch Bulb useThisBulbInstead         myBulb   useThisBulbInstead           public void ToggleBulb                   myBulb Toggle

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we can achieve a Dependency injection to know it   class Injector     constructor         this dependencies           this register    key  value    gt          this dependencies key    value               resolve    args        let func   null      let deps   null      let scope   null      const self   this      if  typeof args 0       string           func   args 1         deps   args 0  replace    g      split             scope   args 2               else         func   args 0         deps   func toString   match   function s          s            m  1  replace    g      split             scope   args 1                   return     args    gt          func apply scope        deps map dep   gt  self dependencies dep   amp  amp  dep         self dependencies dep    args shift                   injector   new Injector     injector register  module1        gt    console log  hello       injector register  module2        gt    console log  world        var doSomething1   injector resolve function  module1  module2  other      module1      module2      console log other       doSomething1  Other     console log  --------    var doSomething2   injector resolve  module1 module2    function  a  b  c      a      b      console log c       doSomething2  Other      above is a implementation by javascript

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The best analogy I can think of is the surgeon and his assistant s  in an operation theater  where the surgeon is the main person and his assistant who provides the various surgical components when he needs it so that the surgeon can concentrate on the one thing he does best  surgery   Without the assistant the surgeon has to get the components himself every time he needs one   DI for short  is a technique to remove a common additional responsibility  burden  on components to fetch the dependent components  by providing them to it   DI brings you closer to the Single Responsibility  SR  principle  like the surgeon who can concentrate on surgery   When to use DI   I would recommend using DI in almost all production projects   small big   particularly in ever changing business environments     Why   Because you want your code to be easily testable  mockable etc so that you can quickly test your changes and push it to the market  Besides why would you not when you there are lots of awesome free tools frameworks to support you in your journey to a codebase where you have more control

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Dependency injection is one possible solution to what could generally be termed the  Dependency Obfuscation  requirement  Dependency Obfuscation is a method of taking the  obvious  nature out of the process of providing a dependency to a class that requires it and therefore obfuscating  in some way  the provision of said dependency to said class  This is not necessarily a bad thing  In fact  by obfuscating the manner by which a dependency is provided to a class then something outside the class is responsible for creating the dependency which means  in various scenarios  a different implementation of the dependency can be supplied to the class without making any changes to the class  This is great for switching between production and testing modes  eg   using a  mock  service dependency    Unfortunately the bad part is that some people have assumed you need a specialized framework to do dependency obfuscation and that you are somehow a  lesser  programmer if you choose not to use a particular framework to do it  Another  extremely disturbing myth  believed by many  is that dependency injection is the only way of achieving dependency obfuscation  This is demonstrably and historically and obviously 100  wrong but you will have trouble convincing some people that there are alternatives to dependency injection for your dependency obfuscation requirements   Programmers have understood the dependency obfuscation requirement for years and many alternative solutions have evolved both before and after dependency injection was conceived  There are Factory patterns but there are also many options using ThreadLocal where no injection to a particular instance is needed - the dependency is effectively injected into the thread which has the benefit of making the object available  via convenience static getter methods  to any class that requires it without having to add annotations to the classes that require it and set up intricate XML  glue  to make it happen  When your dependencies are required for persistence  JPA JDO or whatever  it allows you to achieve  tranaparent persistence  much easier and with domain model and business model classes made up purely of POJOs  i e  no framework specific locked in annotations

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What is dependency Injection   Dependency Injection DI  means to decouple the objects which are dependent on each other  Say object A is dependent on Object B so the idea is to decouple these object from each other  We don   t need to hard code the object using new keyword rather sharing dependencies to objects at runtime in spite of compile time  If we talk about   How Dependency Injection works in Spring   We don   t need to hard code the object using new keyword rather define the bean dependency in the configuration file  The spring container will be responsible for hooking up all   Inversion of Control  IOC   IOC is a general concept and it can be expressed in many different ways and Dependency Injection is one concrete example of IOC   Two types of Dependency Injection    Constructor Injection Setter Injection   1     Constructor-based dependency injection   Constructor-based DI is accomplished when the container invokes a class constructor with a number of arguments  each representing a dependency on other class   public class Triangle    private String type   public String getType        return type      public Triangle String type       constructor injection     this type type        lt bean id triangle  class   com test dependencyInjection Triangle  gt           lt constructor-arg value  20   gt     lt  bean gt    2     Setter-based dependency injection   Setter-based DI is accomplished by the container calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean   public class Triangle    private String type    public String getType        return type       public void setType String type              setter injection     this type   type           lt  -- setter injection -- gt    lt bean id  triangle  class  com test dependencyInjection Triangle  gt           lt property name  type  value  equivialteral   gt    NOTE  It is a good rule of thumb to use constructor arguments for mandatory dependencies and setters for optional dependencies  Note that the if we use annotation based than  Required annotation on a setter can be used to make setters as a required dependencies

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What is the purpose of DI    The purpose of Dependency Injection is to reduce coupling in your application to make it more flexible and easier to test      How does it benefit    Objects don t have hard coded dependencies  If you need to change the implementation of a dependency  all you have to do is Inject a different type of Object      How does it implemented    There are various methods of Dependency Injection  Check out the Wikipedia article to see examples of each  Once you understand those  you can start investigating the various Dependency Injection frameworks

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This is the most simple explanation about Dependency Injection and Dependency Injection Container I have ever seen   Without Dependency Injection   Application needs Foo  e g  a controller   so  Application creates Foo Application calls Foo   Foo needs Bar  e g  a service   so  Foo creates Bar Foo calls Bar   Bar needs Bim  a service  a repository        so  Bar creates Bim Bar does something     With Dependency Injection   Application needs Foo  which needs Bar  which needs Bim  so  Application creates Bim Application creates Bar and gives it Bim Application creates Foo and gives it Bar Application calls Foo   Foo calls Bar   Bar does something     Using a Dependency Injection Container   Application needs Foo so  Application gets Foo from the Container  so    Container creates Bim Container creates Bar and gives it Bim Container creates Foo and gives it Bar  Application calls Foo   Foo calls Bar   Bar does something     Dependency Injection and dependency Injection Containers are different things    Dependency Injection is a method for writing better code a DI Container is a tool to help injecting dependencies   You don t need a container to do dependency injection  However a container can help you

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Dependency Injection  DI  is one from Design Patterns  which uses the basic feature of OOP - the relationship in one object with another object  While inheritance inherits one object to do more complex and specific another object  relationship or association simply creates a pointer to another object from one object using attribute  The power of DI is in combination with other features of OOP as are interfaces and hiding code  Suppose  we have a customer  subscriber  in the library  which can borrow only one book for simplicity   Interface of book   package com deepam hidden   public interface BookInterface    public BookInterface setHeight int height   public BookInterface setPages int pages      public int getHeight    public int getPages       public String toString        Next we can have many kind of books  one of type is fiction   package com deepam hidden   public class FictionBook implements BookInterface   int height   0     height in cm int pages   0     number of pages      constructor    public FictionBook            TODO Auto-generated constructor stub     Override public FictionBook setHeight int height      this height   height    return this      Override public FictionBook setPages int pages      this pages   pages    return this            Override public int getHeight            TODO Auto-generated method stub     return height      Override public int getPages            TODO Auto-generated method stub     return pages      Override public String toString        return   height      height           pages      pages         Now subscriber can have association to the book   package com deepam hidden   import java lang reflect Constructor  import java lang reflect InvocationTargetException   public class Subscriber   BookInterface book       constructor   public Subscriber            TODO Auto-generated constructor stub       injection I public void setBook BookInterface book        this book   book        injection II public BookInterface setBook String bookName        try           Class lt   gt  cl   Class forName bookName           Constructor lt   gt  constructor   cl getConstructor       use it for parameters in constructor         BookInterface book    BookInterface  constructor newInstance              book    BookInterface  Class forName bookName  newInstance          catch  InstantiationException e            e printStackTrace          catch  IllegalAccessException e            e printStackTrace          catch  ClassNotFoundException e            e printStackTrace          catch  NoSuchMethodException e            e printStackTrace          catch  SecurityException e            e printStackTrace          catch  IllegalArgumentException e            e printStackTrace          catch  InvocationTargetException e            e printStackTrace              return book     public BookInterface getBook       return book     public static void main String   args            All the three classes can be hidden for it s own implementation  Now we can use this code for DI   package com deepam implement   import com deepam hidden Subscriber  import com deepam hidden FictionBook   public class CallHiddenImplBook    public CallHiddenImplBook            TODO Auto-generated constructor stub    public void doIt         Subscriber ab   new Subscriber            injection I     FictionBook bookI   new FictionBook        bookI setHeight 30      cm     bookI setPages 250       ab setBook bookI      inject     System out println  injection I     ab getBook   toString             injection II     FictionBook bookII     FictionBook  ab setBook  com deepam hidden FictionBook    setHeight 5  setPages 108      inject and set     System out println  injection II     ab getBook   toString              public static void main String   args        CallHiddenImplBook kh   new CallHiddenImplBook        kh doIt          There are many different ways how to use dependency injection  It is possible to combine it with Singleton  etc   but still in basic it is only association realized by creating attribute of object type inside another object  The usefulness is only and only in feature  that code  which we should write again and again is always prepared and done for us forward  This is why DI so closely binded with Inversion of Control  IoC  which means  that our program passes control another running module  which does injections of beans to our code   Each object  which can be injected can be signed or considered as a Bean   For example in Spring it is done by creating and initialization ApplicationContext container  which does this work for us  We simply in our code create the Context and invoke initialization the beans  In that moment injection has been done automatically

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I think since everyone has written for DI  let me ask a few questions     When you have a configuration of DI where all the actual implementations not interfaces  that are going to be injected into a class  for e g services to a controller  why is that not some sort of hard-coding   What if I want to change the object at runtime  For example  my config already says when I instantiate MyController  inject for FileLogger as ILogger  But I might want to inject DatabaseLogger   Every time I want to change what objects my AClass needs  I need to now look into two places - The class itself and the configuration file  How does that make life easier  If Aproperty of AClass is not injected  is it harder to mock it out   Going back to the first question  If using new object   is bad  how come we inject the implementation and not the interface  I think a lot of you are saying we re in fact injecting the interface but the configuration makes you specify the implementation of that interface   not at runtime    it is hardcoded during compile time    This is based on the answer  Adam N posted   Why does PersonService no longer have to worry about GroupMembershipService  You just mentioned GroupMembership has multiple things objects properties  it depends on  If GMService was required in PService  you d have it as a property  You can mock that out regardless of whether you injected it or not  The only time I d like it to be injected is if GMService had more specific child classes  which you wouldn t know until runtime  Then you d want to inject the subclass  Or if you wanted to use that as either singleton or prototype  To be honest  the configuration file has everything hardcoded as far as what subclass for a type  interface  it is going to inject during compile time     EDIT   A nice comment by Jose Maria Arranz on DI  DI increases cohesion by removing any need to determine the direction of dependency and write any glue code   False  The direction of dependencies is in XML form or as annotations  your dependencies are written as XML code and annotations  XML and annotations ARE source code   DI reduces coupling by making all of your components modular  i e  replaceable  and have well-defined interfaces to each other   False  You do not need a DI framework to build a modular code based on interfaces   About replaceable  with a very simple  properties archive and Class forName you can define which classes can change  If ANY class of your code can be changed  Java is not for you  use an scripting language   By the way  annotations cannot be changed without recompiling   In my opinion there is one only reason for DI frameworks  boiler plate reduction  With a well done factory system you can do the same  more controlled and more predictable as your preferred DI framework  DI frameworks promise code reduction  XML and annotations are source code too   The problem is this boiler plate reduction is just real in very very simple cases  one instance-per class and similar   sometimes in the real world picking the appropriated service object is not as easy as mapping a class to a singleton object

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I know there are already many answers  but I found this very helpful  http   tutorials jenkov com dependency-injection index html   No Dependency   public class MyDao      protected DataSource dataSource   new DataSourceImpl       driver    url    user    password         data access methods      public Person readPerson int primaryKey                 Dependency   public class MyDao      protected DataSource dataSource   null     public MyDao String driver  String url  String user  String password        this dataSource   new DataSourceImpl driver  url  user  password            data access methods      public Person readPerson int primaryKey            Notice how the DataSourceImpl instantiation is moved into a constructor  The constructor takes four parameters which are the four values needed by the DataSourceImpl  Though the MyDao class still depends on these four values  it no longer satisfies these dependencies itself  They are provided by whatever class creating a MyDao instance

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What is Dependency Injection  DI    As others have said  Dependency Injection DI  removes the responsibility of direct creation  and management of the lifespan  of other object instances upon which our class of interest  consumer class  is dependent  in the UML sense   These instances are instead passed to our consumer class  typically as constructor parameters or via property setters  the management of the dependency object instancing and passing to the consumer class is usually performed by an Inversion of Control  IoC  container  but that s another topic    DI  DIP and SOLID  Specifically  in the paradigm of Robert C Martin s SOLID principles of Object Oriented Design  DI is one of the possible implementations of the Dependency Inversion Principle  DIP   The DIP is the D of the SOLID mantra  - other DIP implementations include the Service Locator  and Plugin patterns   The objective of the DIP is to decouple tight  concrete dependencies between classes  and instead  to loosen the coupling by means of an abstraction  which can be achieved via an interface  abstract class or pure virtual class  depending on the language and approach used   Without the DIP  our code  I ve called this  consuming class   is directly coupled to a concrete dependency and is also often burdened with the responsibility of knowing how to obtain  and manage  an instance of this dependency  i e  conceptually    I need to create use a Foo and invoke method  GetBar       Whereas after application of the DIP  the requirement is loosened  and the concern of obtaining and managing the lifespan of the Foo dependency has been removed    I need to invoke something which offers  GetBar       Why use DIP  and DI    Decoupling dependencies between classes in this way allows for easy substitution of these dependency classes with other implementations which also fulfil the prerequisites of the abstraction  e g  the dependency can be switched with another implementation of the same interface   Moreover  as others have mentioned  possibly the most common reason to decouple classes via the DIP is to allow a consuming class to be tested in isolation  as these same dependencies can now be stubbed and or mocked   One consequence of DI is that the lifespan management of dependency object instances is no longer controlled by a consuming class  as the dependency object is now passed into the consuming class  via constructor or setter injection    This can be viewed in different ways    If lifespan control of dependencies by the consuming class needs to be retained  control can be re-established by injecting an  abstract  factory for creating the dependency class instances  into the consumer class  The consumer will be able to obtain instances via a Create on the factory as needed  and dispose of these instances once complete  Or  lifespan control of dependency instances can be relinquished to an IoC container  more about this below     When to use DI    Where there likely will be a need to substitute a dependency for an equivalent implementation   Any time where you will need to unit test the methods of a class in isolation of its dependencies   Where uncertainty of the lifespan of a dependency may warrant experimentation  e g  Hey  MyDepClass is thread safe - what if we make it a singleton and inject the same instance into all consumers     Example  Here s a simple C  implementation  Given the below Consuming class   public class MyLogger      public void LogRecord string somethingToLog             Console WriteLine   0 HH mm ss  -  1    DateTime Now  somethingToLog            Although seemingly innocuous  it has two static dependencies on two other classes  System DateTime and System Console  which not only limit the logging output options  logging to console will be worthless if no one is watching   but worse  it is difficult to automatically test given the dependency on a non-deterministic system clock   We can however apply DIP to this class  by abstracting out the the concern of timestamping as a dependency  and coupling MyLogger only to a simple interface   public interface IClock       DateTime Now   get        We can also loosen the dependency on Console to an abstraction  such as a TextWriter  Dependency Injection is typically implemented as either constructor injection  passing an abstraction to a dependency as a parameter to the constructor of a consuming class  or Setter Injection  passing the dependency via a setXyz   setter or a  Net Property with  set   defined   Constructor Injection is preferred  as this guarantees the class will be in a correct state after construction  and allows the internal dependency fields to be marked as readonly  C   or final  Java   So using constructor injection on the above example  this leaves us with   public class MyLogger   ILogger    Others will depend on our logger        private readonly TextWriter  output      private readonly IClock  clock          Dependencies are injected through the constructor     public MyLogger TextWriter stream  IClock clock                 output   stream           clock   clock             public void LogRecord string somethingToLog                   We can now use our dependencies through the abstraction             and without knowledge of the lifespans of the dependencies          output Write   0 yyyy-MM-dd HH mm ss  -  1     clock Now  somethingToLog              A concrete Clock needs to be provided  which of course could revert to DateTime Now  and the two dependencies need to be provided by an IoC container via constructor injection   An automated Unit Test can be built  which definitively proves that our logger is working correctly  as we now have control over the dependencies - the time  and we can spy on the written output    Test  public void LoggingMustRecordAllInformationAndStampTheTime            Arrange     var mockClock   new Mock lt IClock gt         mockClock Setup c   gt  c Now  Returns new DateTime 2015  4  11  12  31  45        var fakeConsole   new StringWriter            Act     new MyLogger fakeConsole  mockClock Object           LogRecord  Foo            Assert     Assert AreEqual  2015-04-11 12 31 45 - Foo   fakeConsole ToString         Next Steps  Dependency injection is invariably associated with an Inversion of Control container IoC   to inject  provide  the concrete dependency instances  and to manage lifespan instances  During the configuration   bootstrapping process  IoC containers allow the following to be defined    mapping between each abstraction and the configured concrete implementation  e g   any time a consumer requests an IBar  return a ConcreteBar instance   policies can be set up for the lifespan management of each dependency  e g  to create a new object for each consumer instance  to share a singleton dependency instance across all consumers  to share the same dependency instance only across the same thread  etc  In  Net  IoC containers are aware of protocols such as IDisposable and will take on the responsibility of Disposing dependencies in line with the configured lifespan management    Typically  once IoC containers have been configured   bootstrapped  they operate seamlessly in the background allowing the coder to focus on the code at hand rather than worrying about dependencies      The key to DI-friendly code is to avoid static coupling of classes  and not to use new   for the creation of Dependencies   As per above example  decoupling of dependencies does require some design effort  and for the developer  there is a paradigm shift needed to break the habit of newing dependencies directly  and instead trusting the container to manage dependencies    But the benefits are many  especially in the ability to thoroughly test your class of interest   Note   The creation   mapping   projection  via new       of POCO   POJO   Serialization DTOs   Entity Graphs   Anonymous JSON projections et al - i e   Data only  classes or records - used or returned from methods are not regarded as Dependencies  in the UML sense  and not subject to DI  Using new to project these is just fine

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It means that objects should only have as many dependencies as is needed to do their job and the dependencies should be few  Furthermore  an object   s dependencies should be on interfaces and not on    concrete    objects  when possible   A concrete object is any object created with the keyword new   Loose coupling promotes greater reusability  easier maintainability  and allows you to easily provide    mock    objects in place of expensive services   The    Dependency Injection     DI  is also known as    Inversion of Control     IoC   can be used as a technique for encouraging this loose coupling   There are two primary approaches to implementing DI    Constructor injection  Setter injection   Constructor injection  It   s the technique of passing objects dependencies to its constructor   Note that the constructor accepts an interface and not concrete object  Also  note that an exception is thrown if the orderDao parameter is null  This emphasizes the importance of receiving a valid dependency  Constructor Injection is  in my opinion  the preferred mechanism for giving an object its dependencies  It is clear to the developer while invoking the object which dependencies need to be given to the    Person    object for proper execution   Setter Injection  But consider the following example    Suppose you have a class with ten methods that have no dependencies  but you   re adding a new method that does have a dependency on IDAO  You could change the constructor to use Constructor Injection  but this may force you to changes to all constructor calls all over the place  Alternatively  you could just add a new constructor that takes the dependency  but then how does a developer easily know when to use one constructor over the other  Finally  if the dependency is very expensive to create  why should it be created and passed to the constructor when it may only be used rarely     Setter Injection    is another DI technique that can be used in situations such as this   Setter Injection does not force dependencies to be passed to the constructor  Instead  the dependencies are set onto public properties exposed by the object in need  As implied previously  the primary motivators for doing this include    Supporting dependency injection without having to modify the constructor of a legacy class  Allowing expensive resources or services to be created as late as possible and only when needed    Here is the example of how the above code would look like   public class Person       public Person           public IDAO Address           set   addressdao   value            get               if  addressdao    null                throw new MemberAccessException  addressdao                                   has not been initialized                return addressdao                       public Address GetAddress                   code that uses the addressdao object           to fetch address details from the datasource                   Should not be called directly         use the public property instead     private IDAO addressdao

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Dependency Injection is passing dependency to other objects or framework  dependency injector    Dependency injection makes testing easier  The injection can be done through constructor    SomeClass   has its constructor as following     public SomeClass         myObject   Factory getObject        Problem  If myObject involves complex tasks such as disk access or network access  it is hard to do unit test on SomeClass    Programmers have to mock myObject and might intercept the factory call   Alternative solution     Passing myObject in as an argument to the constructor     public SomeClass  MyClass myObject        this myObject   myObject      myObject can be passed directly which makes testing easier     One common alternative is defining a do-nothing constructor  Dependency injection can be done through setters   h t  MikeVella    Martin Fowler documents a third alternative  h t  MarcDix   where classes explicitly implement an interface for the dependencies programmers wish injected    It is harder to isolate components in unit testing without dependency injection    In 2013  when I wrote this answer  this was a major theme on the Google Testing Blog  It remains the biggest advantage to me  as programmers not always need the extra flexibility in their run-time design  for instance  for service locator or similar patterns   Programmers often need to isolate the classes during testing

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DI allows us to swap out components  improve testability and ensure that components are loosely coupled  DI allows to resolve dependencies at run time using DI containers such as Windsor Castle  Unity  Spring net  MEF which allows applications to be extensible

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Dependency Injection is the practice to make decoupled component agnostic from some of their dependencies  this follow the SOLID guideline that say     Dependency inversion principle  one should  depend upon abstractions    not concretions    The better implementation of Dependency Injection is the Composition Root design pattern  as it s allow your components to be decoupled from the dependency injection container   I recommand this great article on Composition Root  http   blog ploeh dk 2011 07 28 CompositionRoot  written by Mark Seemann  here is essential points from this article      A Composition Root is a  preferably  unique location in an application   where modules are composed together            Only applications should have Composition Roots  Libraries and   frameworks shouldn t            A DI Container should only be referenced from the Composition Root    All other modules should have no reference to the container    The documentation of Di-Ninja  a dependency injection framework  is a very good example to demonstrate how works the principles of Composition Root and Dependency Injection  https   github com di-ninja di-ninja As I know  is the only one DiC in javascript that implement the Composition-Root design pattern

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Dependency Injection for 5 year olds   When you go and get things out of the refrigerator for yourself  you can cause problems  You might leave the door open  you might get something Mommy or Daddy doesn t want you to have  You might be even looking for something we don t even have or which has expired   What you should be doing is stating a need   I need something to drink with lunch   and then we will make sure you have something when you sit down to eat

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I found this funny example in terms of loose coupling  Source  Understanding dependency injection Any application is composed of many objects that collaborate with each other to perform some useful stuff  Traditionally each object is responsible for obtaining its own references to the dependent objects  dependencies  it collaborate with  This leads to highly coupled classes and hard-to-test code  For example  consider a Car object  A Car depends on wheels  engine  fuel  battery  etc  to run  Traditionally we define the brand of such dependent objects along with the definition of the Car object  Without Dependency Injection  DI   class Car    private Wheel wh   new NepaliRubberWheel      private Battery bt   new ExcideBattery         The rest    Here  the Car object is responsible for creating the dependent objects  What if we want to change the type of its dependent object - say Wheel - after the initial NepaliRubberWheel   punctures  We need to recreate the Car object with its new dependency say ChineseRubberWheel    but only the Car manufacturer can do that  Then what does the Dependency Injection do for us     When using dependency injection  objects are given their dependencies at run time rather than compile time  car manufacturing time   So that we can now change the Wheel whenever we want  Here  the dependency  wheel  can be injected into Car at run time  After using dependency injection  Here  we are injecting the dependencies  Wheel and Battery  at runtime  Hence the term   Dependency Injection  We normally rely on DI frameworks such as Spring  Guice  Weld to create the dependencies and inject where needed  class Car    private Wheel wh     Inject an Instance of Wheel  dependency of car  at runtime   private Battery bt     Inject an Instance of Battery  dependency of car  at runtime   Car Wheel wh Battery bt          this wh   wh        this bt   bt          Or we can have setters   void setWheel Wheel wh          this wh   wh         The advantages are   decoupling the creation of object  in other word  separate usage from the creation of object  ability to replace dependencies  eg  Wheel  Battery  without changing the class that uses it Car  promotes  quot Code to interface not to implementation quot  principle ability to create and use mock dependency during test  if we want to use a Mock of Wheel during test instead of a real instance   we can create Mock Wheel object and let DI framework inject to Car

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Let s try simple example with Car and Engine classes  any car need an engine to go anywhere  at least for now  So below how code will look without dependency injection     public class Car       public Car                 GasEngine engine   new GasEngine            engine Start             public class GasEngine       public void Start                 Console WriteLine  I use gas as my fuel               And to instantiate the Car class we will use next code     Car car   new Car      The issue with this code that we tightly coupled to GasEngine and if we decide to change it to ElectricityEngine then we will need to rewrite Car class  And the bigger the application the more issues and headache we will have to add and use new type of engine    In other words with this approach is that our high level Car class is dependent on the lower level GasEngine class which violate Dependency Inversion Principle DIP  from SOLID  DIP suggests that we should depend on abstractions  not concrete classes  So to satisfy this we introduce IEngine interface and rewrite code like below         public interface IEngine               void Start               public class GasEngine   IEngine               public void Start                         Console WriteLine  I use gas as my fuel                          public class ElectricityEngine   IEngine               public void Start                         Console WriteLine  I am electrocar                         public class Car               private readonly IEngine  engine          public Car IEngine engine                         engine   engine                     public void Run                          engine Start                      Now our Car class is dependent on only the IEngine interface  not a specific implementation of engine   Now  the only trick is how do we create an instance of the Car and give it an actual concrete Engine class like GasEngine or ElectricityEngine  That s where Dependency Injection comes in         Car gasCar   new Car new GasEngine        gasCar Run       Car electroCar   new Car new ElectricityEngine        electroCar Run      Here we basically inject pass  our dependency Engine instance  to Car constructor  So now our classes have loose coupling between objects and their dependencies  and we can easily add new types of engines without changing the Car class   The main benefit of the Dependency Injection that classes are more loosely coupled  because they do not have hard-coded dependencies  This follows the Dependency Inversion Principle  which was mentioned above  Instead of referencing specific implementations  classes request abstractions  usually interfaces  which are provided to them when the class is constructed      So in the end Dependency injection is just a technique for   achieving loose coupling between objects and their dependencies    Rather than directly instantiating dependencies that class needs in   order to perform its actions  dependencies are provided to the class    most often  via constructor injection    Also when we have many dependencies it is very good practice to use Inversion of Control IoC  containers which we can tell which interfaces should be mapped to which concrete implementations for all our dependencies and we can have it resolve those dependencies for us when it constructs our object  For example  we could specify in the mapping for the IoC container that the IEngine dependency should be mapped to the GasEngine class and when we ask the IoC container for an instance of our Car class  it will automatically construct our Car class with a GasEngine dependency passed in    UPDATE  Watched course about EF Core from Julie Lerman recently and also liked her short definition about DI      Dependency injection is a pattern to allow your application to inject   objects on the fly to classes that need them  without forcing those   classes to be responsible for those objects  It allows your code to be   more loosely coupled  and Entity Framework Core plugs in to this same   system of services

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Any nontrivial application is made up of two or more classes that collaborate with each other to perform some business logic  Traditionally  each object is responsible for obtaining its own references to the objects it collaborates with  its dependencies   When applying DI  the objects are given their dependencies at creation time by some external entity that coordinates each object in the system  In other words  dependencies are injected into objects   For further details please see enter link description here

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From the Book   Well-Grounded Java Developer  Vital techniques of Java 7 and polyglot programming     DI is a particular form of IoC  whereby the process of finding your dependencies is   outside the direct control of your currently executing code

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Dependency Injection  DI  is part of Dependency Inversion Principle  DIP  practice  which is also called Inversion of Control  IoC   Basically you need to do DIP because you want to make your code more modular and unit testable  instead of just one monolithic system  So you start identifying parts of the code that can be separated from the class and abstracted away  Now the implementation of the abstraction need to be injected from outside of the class  Normally this can be done via constructor  So you create a constructor that accepts the abstraction as a parameter  and this is called dependency injection  via constructor   For more explanation about DIP  DI  and IoC container you can read Here

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From Christoffer Noring  Pablo Deeleman s book    Learning Angular - Second Edition       As our applications grow and evolves  each one of our code entities will internally require instances of other objects  which are better known as dependencies in the world of software engineering  The action of passing such dependencies to the dependent client is known as injection  and it also entails the participation of another code entity  named the injector  The injector will take responsibility for instantiating and bootstrapping the required dependencies so they are ready for use from the very moment they are successfully injected in the client  This is very important since the client knows nothing about how to instantiate its own dependencies and is only aware of the interface they implement in order to use them    From  Anton Moiseev  book    Angular Development with Typescript  Second Edition          In short  DI helps you write code in a loosely coupled way and makes your code more testable and reusable

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Dependency injection is the heart of the concept related with Spring Framework While creating the framework of any project spring may perform a vital role and here dependency injection come in pitcher   Actually Suppose in java you created two different classes as class A and class B  and whatever the function are available in class B you want to use in class A  So at that time dependency injection can be used  where you can crate object of one class in other in the same way you can inject an entire class in another class to make it accessible  by this way dependency can be overcome   DEPENDENCY INJECTION IS SIMPLY GLUING TWO CLASSES AND AT THE SAME TIME KEEPING THEM SEPARATE

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All the above answers are good  my aim is to explain the concept in a simple way so that anyone without a programming knowledge can also understand concept Dependency injection is one of the design pattern that help us to create complex systems in a simpler manner  We can see a wide variety of application of this pattern in our day to day life  Some of the examples are Tape recorder  VCD  CD Drive etc   The above image is an image of Reel-to-reel portable tape recorder  mid-20th century  Source  The primary intention of a tape recorder machine is to record or playback sound  While designing a system it require a reel to record or playback sound or music  There are two possibilities for designing this system  we can place the reel inside the machine we can provide a hook for the reel where it can be placed   If we use the first one we need to open the machine to change the reel  if we opt for the second one  that is placing a hook for reel  we are getting an added benefit of playing any music by changing the reel  and also reducing the function only to playing whatever in the reel  Like wise dependency injection is the  process of externalizing the dependencies to focus only on the specific functionality of the component so that independent components can be coupled together to form a complex system  The main benefits we achieved by using dependency injection   High cohesion and loose coupling  Externalizing dependency and looking only on responsibility  Making things as components and to combine to form a large systems with high capabilities  It helps to develop high quality components since they are independently developed they are properly tested  It helps to replace the component with another if one fails   Now a days these concept forms the basis of well known frameworks in programming world  The Spring Angular etc are the well-known software frameworks built on the top of this concept Dependency injection is a pattern used to create instances of objects that other objects rely upon without knowing at compile time which class will be used to provide that functionality or simply the way of injecting properties to an object is called dependency injection  Example for Dependency injection 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 off the instantiation for us 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       You can also read Difference between Inversion of Control  amp  Dependency Injection

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The accepted answer is a good one - but I would like to add to this that DI is very much like the classic avoiding of hardcoded constants in the code     When you use some constant like a database name you d quickly move it from the inside of the code to some config file and pass a variable containing that value to the place where it is needed   The reason to do that is that these constants usually change more frequently than the rest of the code   For example if you d like to test the code in a test database     DI is analogous to this in the world of Object Oriented programming   The values there instead of constant literals are whole objects - but the reason to move the code creating them out from the class code is similar - the objects change more frequently then the code that uses them   One important case where such a change is needed is tests

[java] What is Dependency Injection?

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