How does autowiring work in Spring

Question

I m a little confused as to how the inversion of control  IoC  works in Spring   Say I have a service class called UserServiceImpl that implements UserService interface   How would this be  Autowired   And in my Controllers  how would I instantiate an instance of this service   Would I just do the following   UserService userService   new UserServiceImpl

User · Answer

In simple words Autowiring  wiring links automatically  now comes the question who does this and which kind of wiring  Answer is  Container does this and Secondary type of wiring is supported  primitives need to be done manually   Question  How container know what type of wiring    Answer  We define it as byType byName constructor   Question  Is there are way we do not define type of autowiring    Answer  Yes  it s there by doing one annotation   Autowired   Question  But how system know  I need to pick this type of secondary data    Answer  You will provide that data in you spring xml file or by using sterotype annotations to your class so that container can themselves create the objects for you

User · Answer

You just need to annotate your service class UserServiceImpl with annotation    Service  userService     Spring container will take care of the life cycle of this class as it register as service   Then in your controller you can auto wire  instantiate  it and use its functionality    Autowired UserService userService

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How does  Autowired work internally   Example    class EnglishGreeting      private Greeting greeting       setter and getter    class Greeting      private String message       setter and getter      xml file it will look alike if not using  Autowired    lt bean id  englishGreeting  class  com bean EnglishGreeting  gt      lt property name  greeting  ref  greeting   gt   lt  bean gt    lt bean id  greeting  class  com bean Greeting  gt      lt property name  message  value  Hello World   gt   lt  bean gt    If you are using  Autowired then   class EnglishGreeting       Autowired   so automatically based on the name it will identify the bean and inject     private Greeting greeting       setter and getter      xml file it will look alike if not using  Autowired    lt bean id  englishGreeting  class  com bean EnglishGreeting  gt  lt  bean gt    lt bean id  greeting  class  com bean Greeting  gt      lt property name  message  value  Hello World   gt   lt  bean gt    If still have some doubt then go through below live demo   How does  Autowired work internally

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The whole concept of inversion of control means you are free from a chore to instantiate objects manually and provide all necessary dependencies  When you annotate class with appropriate annotation  e g   Service  Spring will automatically instantiate object for you  If you are not familiar with annotations you can also use XML file instead  However  it s not a bad idea to instantiate classes manually  with the new keyword  in unit tests when you don t want to load the whole spring context

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Autowired is an annotation introduced in Spring 2 5  and it s used only for injection    For example   class A        private int id          With setter and getter method    class B        private String name        Autowired    Here we are injecting instance of Class A into class B so that you can use  a  for accessing A s instance variables and methods      A a          With setter and getter method      public void showDetail             System out println  Value of id form A class    a getId

User · Answer

Keep in mind that you must enable the  Autowired annotation by adding element  lt context annotation-config  gt  into the spring configuration file  This will register the AutowiredAnnotationBeanPostProcessor which takes care the processing of annotation   And then you can autowire your service by using the field injection method   public class YourController     Autowired  private UserService userService        I found this from the post Spring  autowired annotation

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Depends on whether you want the annotations route or the bean XML definition route   Say you had the beans defined in your applicationContext xml    lt beans     gt        lt bean id  userService  class  com foo UserServiceImpl   gt        lt bean id  fooController  class  com foo FooController   gt    lt  beans gt    The autowiring happens when the application starts up  So  in fooController  which for arguments sake wants to use the UserServiceImpl class  you d annotate it as follows   public class FooController           You could also annotate the setUserService method instead of this      Autowired     private UserService userService          rest of class goes here     When it sees  Autowired  Spring will look for a class that matches the property in the applicationContext  and inject it automatically  If you have more than one UserService bean  then you ll have to qualify which one it should use   If you do the following   UserService service   new UserServiceImpl      It will not pick up the  Autowired unless you set it yourself

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Standard way    RestController public class Main       UserService userService       public Main            userService   new UserServiceImpl                GetMapping          public String index            return userService print  Example test              User service interface   public interface UserService       String print String text       UserServiceImpl class   public class UserServiceImpl implements UserService        Override     public String print String text            return text     UserServiceImpl             Output  Example test UserServiceImpl  That is a great example of tight coupled classes  bad design example and there will be problem with testing  PowerMockito is also bad    Now let s take a look at SpringBoot dependency injection  nice example of loose coupling   Interface remains the same   Main class    RestController public class Main       UserService userService        Autowired     public Main UserService userService           this userService   userService              GetMapping          public String index            return userService print  Example test              ServiceUserImpl class    Component public class UserServiceImpl implements UserService        Override     public String print String text            return text     UserServiceImpl             Output  Example test UserServiceImpl  and now it s easy to write test    RunWith MockitoJUnitRunner class  public class MainTest        Mock     UserService userService        Test     public void indexTest             when userService print  Example test    thenReturn  Example test UserServiceImpl             String result   new Main userService  index             assertEquals result   Example test UserServiceImpl              I showed  Autowired annotation on constructor but it can also be used on setter or field

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First  and most important - all Spring beans are managed - they  live  inside a container  called  application context    Second  each application has an entry point to that context  Web applications have a Servlet  JSF uses a el-resolver  etc  Also  there is a place where the application context is bootstrapped and all beans - autowired  In web applications this can be a startup listener   Autowiring happens by placing an instance of one bean into the desired field in an instance of another bean  Both classes should be beans  i e  they should be defined to live in the application context   What is  living  in the application context  This means that the context instantiates the objects  not you  I e  - you never make new UserServiceImpl   - the container finds each injection point and sets an instance there   In your controllers  you just have the following    Controller    Defines that this class is a spring bean  RequestMapping   users   public class SomeController           Tells the application context to inject an instance of UserService here      Autowired     private UserService userService        RequestMapping   login       public void login  RequestParam  username   String username              RequestParam  password   String password                The UserServiceImpl is already injected and you can use it         userService login username  password              A few notes    In your applicationContext xml you should enable the  lt context component-scan gt  so that classes are scanned for the  Controller   Service  etc  annotations  The entry point for a Spring-MVC application is the DispatcherServlet  but it is hidden from you  and hence the direct interaction and bootstrapping of the application context happens behind the scene  UserServiceImpl should also be defined as bean - either using  lt bean id      class      gt  or using the  Service annotation  Since it will be the only implementor of UserService  it will be injected  Apart from the  Autowired annotation  Spring can use XML-configurable autowiring  In that case all fields that have a name or type that matches with an existing bean automatically get a bean injected  In fact  that was the initial idea of autowiring - to have fields injected with dependencies without any configuration  Other annotations like  Inject   Resource can also be used

User · Answer

Spring dependency inject help you to remove coupling from your classes  Instead of creating object like this   UserService userService   new UserServiceImpl      You will be using this after introducing DI    Autowired private UserService userService    For achieving this you need to create a bean of your service in your ServiceConfiguration file  After that you need to import that ServiceConfiguration class to your WebApplicationConfiguration class so that you can autowire that bean into your Controller like this   public class AccController         Autowired     private UserService userService       You can find a java configuration based POC here  example

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There are 3 ways you can create an instance using  Autowired   1   Autowired on Properties  The annotation can be used directly on properties  therefore eliminating the need for getters and setters        Component  userService       public class UserService            public String getName                 return  service name                         Component     public class UserController             Autowired         UserService userService          In the above example  Spring looks for and injects userService when UserController is created   2   Autowired on Setters  The  Autowired annotation can be used on setter methods  In the below example  when the annotation is used on the setter method  the setter method is called with the instance of userService when UserController is created   public class UserController        private UserService userService        Autowired     public void setUserService UserService userService                this userService   userService            3   Autowired on Constructors  The  Autowired annotation can also be used on constructors  In the below example  when the annotation is used on a constructor  an instance of userService is injected as an argument to the constructor when UserController is created   public class UserController        private UserService userService        Autowired     public UserController UserService userService            this userService  userService

[java] How does autowiring work in Spring?

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