Where is the @Autowired annotation supposed to go - on the property or the method?

Question

Which is more correct?

This (with the @Autowired annotation on the method)?

@Controller public class MyController {     private MyDao myDao;      @Autowired     public MyController(MyDao myDao)     {         this.myDao = myDao;     } 

This (with the @Autowired annotation on the property)?

@Controller public class MyController {     @Autowired     private MyDao myDao;      public MyController(MyDao myDao)     {         this.myDao = myDao;     } 

Where is the @Autowired annotation supposed to go?

Answer 1

According to the Javadoc for Autowired, the annotation can be used on "a constructor, field, setter method or config method". See the full documentation for more details.

I personally prefer your first option (constructor injection), because the myDao field can be marked as final:

@Controller public class MyControllear {     private final MyDao myDao;      @Autowired     public MyController(MyDao myDao) {       this.myDao = myDao;     } 

Constructor injection also allows you to test the class in a unit test without code that depends on Spring.

The second option would be better written as:

@Controller public class MyControllear {     @Autowired     private MyDao myDao;      MyController() {     } 

With field injection, Spring will create the object, then update the fields marked for injection.

One option you didn't mention was putting @Autowired on a setter method (setter injection):

@Controller public class MyControllear {     private MyDao myDao;      MyController() {     }      @Autowired     public void setMyDao(MyDao myDao) {       this.myDao = myDao;     } 

You do not have to choose one or another. You can use field injection for some dependencies and constructor injection for others for the same object.