Spring 3 - Dynamic Autowiring at runtime based on another object attribute

Question

I am working on a Spring 3.1 MVC application and for one of my scenarios, I had to write two implementations of a DAO. I would like to know how to autowire this in a service

Answer 1

My solution would be as follows:

a method isResponsibleFor in the VehicleDao interface:

interface VehicleDao {
    public boolean isResponsibleFor(Vehicle vehicle);
}

example implementation:

@Repository
class CarDAO implements VehicleDAO {
    public boolean isResponsibleFor(Vehicle vehicle) {
        return "CAR".equals(vehicle.getType());
    }
}

then autowire a list of all VehicleDao-implementations in the VehicleService:

public class VehicleServiceImpl implements VehicleService {

    @Autowired
    private List<VehicleDao> vehicleDaos;

    private VehicleDao daoForVehicle(Vehicle vehicle) {
         foreach(VehicleDao vehicleDao : vehicleDaos) {
              if(vehicleDao.isResponsibleFor(vehicle) {
                   return vehicleDao;
              }
         }

         throw new UnsupportedOperationException("unsupported vehicleType");
    }

    @Transactional
    public void save(Vehicle vehicle) {
         daoForVehicle(vehicle).save(vehicle);
    }
}

This has the advantage that you don't need to modify the service when you are adding a new vehicleType at a later time - you just need to add a new VehicleDao-implementation.

Answer 2

Please one note: previous solution is great and simple, but spring is creating each instance of each bean.

So my solution is to create abstract class VehicleService instead of interface

then in appContext

@Bean
@Primary
VehicleService paymentProviderService()
{
    return ...new (your requested instance by your own)...
}

Answer 3

Since Vehicle is a @Model it is a runtime value, you won't be able to use it for autowiring values.

The vehicle has to be passed to the service methods as an argument.

public interface VehicleService { void doSomething(Vehicle vehicle); }

Assuming both CarDao and TrainDAO are implementing the same VehicleDao Service, other wise it won't make much sense.

So in your VehicleServiceImpl I would recommend you to write a method like getVehicleDao(Vehicle v){} to get the correct instance of the dao.

public class VehicleServiceImpl implements VehicleService {

    @Resource(name="carDAO")
    private VehicleDao carDAO ;


    @Resource(name="trainDAO")
    private VehicleDao trainDAO ;

    private VehicleDao getDao(Vehicle v){
        if(v instanceof Train){
            return trainDao;
        } else if(v instanceof Car) {
            return carDao;
        } else {
            throw new RuntimeException("unsupported vehicle type");
        }
    }

    void doSomething(Vehicle vehicle){
        VehicleDao dao = getDao(vehicle);
        dao.doSomethind(vehicle);
    }
}

Answer 4

There is an cleaner option to accomplish this strategy. We know Spring is smart enough to inject where it sees a bean that it has control over, so when it sees your VehicleDAO with an @Autowire or @Resource annotation, it will correctly inject that concrete implementation.

Keeping that in mind, this will work anywhere you ask Spring to perform dependency injection, using a Map<> or a List<> for instance.

// Injects the map will all VehicleDAO implementations where key = the
// concrete implementation (The default name is the class name, otherwise you 
// can specify the name explicitly).
@Autowired
private Map<String, VehicleDAO> vehicleDAO;

// When you want to use a specific implementaion of VehicleDAO, just call the  
// 'get()' method on the map as in:
...
vehicleDAO.get("carDAO").yourMethod();
...