[java] How does autowiring work in Spring?

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();

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.

Keep in mind that you must enable the @Autowired annotation by adding element <context:annotation-config/> 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

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.

Depends on whether you want the annotations route or the bean XML definition route.

Say you had the beans defined in your applicationContext.xml:

<beans ...>

    <bean id="userService" class="com.foo.UserServiceImpl"/>

    <bean id="fooController" class="com.foo.FooController"/>

</beans>

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.

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:

<bean id="englishGreeting" class="com.bean.EnglishGreeting">
   <property name="greeting" ref="greeting"/>
</bean>

<bean id="greeting" class="com.bean.Greeting">
   <property name="message" value="Hello World"/>
</bean>

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:

<bean id="englishGreeting" class="com.bean.EnglishGreeting"></bean>

<bean id="greeting" class="com.bean.Greeting">
   <property name="message" value="Hello World"/>
</bean>

If still have some doubt then go through below live demo

How does @Autowired work internally ?

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;

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.

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;
    }
}

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.

@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(););
    }
}