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 layer based on another object's attribute.

For example,

    class Vehicle {
        private name;
        private type;
        ..
        ..
        ..
    } 

    @Service
    class VehicleServiceImpl implements VehicleService {

            // There are two implementations to this DAO
            // if Vehicle.type == "CAR", inject CarDAO
            // if Vehicle.type == "TRAIN", inject TrainDAO 
            @Autowired
            private VehicleDAO vehicleDAO ;

    }

    @Repository
    class CarDAO implements VehicleDAO {

    }

    @Repository
    class TrainDAO implements VehicleDAO {

    }

If my Vehicle is a Car, I need to autowire CarDAO and if it's a train, I need to autowire TrainDAO

What is the best way to implement this in spring 3.1.

I was hoping to use either context property place holders or @Qualifier annotation but both these are kind of restricted to lookup based on some property. I am not sure how to do this at runtime based on the property of another object.

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

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();
...