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 UserServic

Answer 1

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.