Dynamic Qualifier in Spring Boot

1. Overview

In this article, we will learn about the dynamic qualifier in Spring Boot.

Autowiring by type may lead to multiple candidates, so it is often necessary to have more control over the selection process.

One way to achieve this is with Spring’s @Primary annotation. @Primary informs Spring framework to give preference to a particular bean when multiple beans are candidates of autowiring.

@Primary is an effective way to use autowiring by type with several instances when you can determine one primary candidate. Alternatively, you can use Spring’s @Qualifier annotation to control the selection process using the bean name.

You could have encountered scenarios where you had to decide the bean to use at runtime.

2. Dynamic Qualifier in Spring Boot

Let’s see a real world example where dynamic qualifier would be significant.

Consider the following interface Vehicle.

public interface Vehicle
{
   String getNumber();
}

Both the Car and Bike classes implement Vehicle interface. We have specified the name for each bean using the @Qualifier annotation.

@Service
@Qualifier("car_vehicle")
public class Car implements Vehicle
{
    @Override
    public String getType() {
        return "car_vehicle";
    }
}


@Service
@Qualifier("bike_vehicle")
public class Bike implements Vehicle
{

    @Override
    public String getType() {
        return "bike_vehicle";
    }

}

The following VehicleController handles the HTTP GET requests /getVehicle from the client.

Based on the path variable type, we had to either use the bean Car or Bike. Here, we are using the application context to get the bean using its name.

@RestController("/")
public class VehicleController {

    @Autowired
    private ApplicationContext applicationContext;

    @GetMapping("/getVehicle/{type}")
    public String getVehicle(@PathVariable("type")
                                   String type) {

        Vehicle vehicle = applicationContext.getBean(type, Vehicle.class);
        return vehicle.getType();
    }
}

If you hit the URL http://localhost:8080/getVehicle/bike_vehicle, the getVehicle method returns the value bike_vehicle correctly.

2.1. Use BeanFactory.

The BeanFactory is the root interface for accessing the Spring bean container and contains utility methods to get specific beans. It is a central registry of application components and centralizes configuration of them.

Since BeanFactory is a Spring bean, you can autowire and access directly in your class:

@Autowired
private BeanFactory beanFactory;

You can use the getBean method of the BeanFactory to get the required bean rather than using the application context.

@RestController("/")
public class VehicleController {

    @Autowired
    private BeanFactory beanFactory;


    @GetMapping("/getVehicle/{type}")
    public String getVehicle(@PathVariable("type")
                                   String type) {

        Vehicle vehicle = beanFactory.getBean(type, Vehicle.class);
        return vehicle.getType();
    }
}

If you re-run and hit the URL http://localhost:8080/getVehicle/bike_vehicle, the getVehicle method returns the value bike_vehicle correctly.

2.2. Autowire all beans into a map

Since Spring 4.3, you can collect all the bean implementations of a particular type into a Map comprising Bean name as key and Bean instance as value. You can later determine which one to use at runtime.

For example, the Spring collects all the beans (Car and Bike) of the Vehicle interface in the vehicleMap.

@Autowired
private Map<String, Vehicle> vehicleMap;

At runtime, we will use the bean name passed as the path variable to get the required bean from the Map.

@RestController("/")
public class VehicleController {

    @Autowired
    private Map<String, Vehicle> vehicleList;


    @GetMapping("/getVehicle/{type}")
    public String getVehicle(@PathVariable("type")
                                   String type) {

        Vehicle vehicle = vehicleList.get(type);
        return vehicle.getNumber();
    }
}

3. Conclusion

To sum up, we have learned the dynamic qualifier in Spring boot.