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?
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.