Implement a simple factory pattern with Spring 3 annotations

Question

I was wondering how I could implement the simple factory pattern with Spring 3 annotations. I saw in the documentation that you can create beans that call the factory class and run a factory method. I was wondering if this was possible using annotations only.

I have a controller that currently calls

MyService myService = myServiceFactory.getMyService(test);
result = myService.checkStatus();

MyService is an interface with one method called checkStatus().

My factory class looks like this:

@Component
public class MyServiceFactory {

    public static MyService getMyService(String service) {
        MyService myService;
        
        service = service.toLowerCase();
        
        if (service.equals("one")) {
            myService = new MyServiceOne();
        } else if (service.equals("two")) {
            myService = new MyServiceTwo();
        } else if (service.equals("three")) {
            myService = new MyServiceThree();
        } else {
            myService = new MyServiceDefault();
        }
        
        return myService;
    }
}

MyServiceOne class looks like this :

@Autowired
private LocationService locationService;

public boolean checkStatus() {
      //do stuff
}

When I run this code the locationService variable is always null. I believe this is because I am creating the objects myself inside the factory and autowiring is not taking place. Is there a way to add annotations to make this work correctly?

Thanks

Answer 1

The following worked for me:

The interface consist of you logic methods plus additional identity method:

public interface MyService {
    String getType();
    void checkStatus();
}

Some implementations:

@Component
public class MyServiceOne implements MyService {
    @Override
    public String getType() {
        return "one";
    }

    @Override
    public void checkStatus() {
      // Your code
    }
}

@Component
public class MyServiceTwo implements MyService {
    @Override
    public String getType() {
        return "two";
    }

    @Override
    public void checkStatus() {
      // Your code
    }
}

@Component
public class MyServiceThree implements MyService {
    @Override
    public String getType() {
        return "three";
    }

    @Override
    public void checkStatus() {
      // Your code
    }
}

And the factory itself as following:

@Service
public class MyServiceFactory {

    @Autowired
    private List<MyService> services;

    private static final Map<String, MyService> myServiceCache = new HashMap<>();

    @PostConstruct
    public void initMyServiceCache() {
        for(MyService service : services) {
            myServiceCache.put(service.getType(), service);
        }
    }

    public static MyService getService(String type) {
        MyService service = myServiceCache.get(type);
        if(service == null) throw new RuntimeException("Unknown service type: " + type);
        return service;
    }
}

I've found such implementation easier, cleaner and much more extensible. Adding new MyService is as easy as creating another spring bean implementing same interface without making any changes in other places.

Answer 2

You are right, by creating object manually you are not letting Spring to perform autowiring. Consider managing your services by Spring as well:

@Component
public class MyServiceFactory {

    @Autowired
    private MyServiceOne myServiceOne;

    @Autowired
    private MyServiceTwo myServiceTwo;

    @Autowired
    private MyServiceThree myServiceThree;

    @Autowired
    private MyServiceDefault myServiceDefault;

    public static MyService getMyService(String service) {
        service = service.toLowerCase();

        if (service.equals("one")) {
            return myServiceOne;
        } else if (service.equals("two")) {
            return myServiceTwo;
        } else if (service.equals("three")) {
            return myServiceThree;
        } else {
            return myServiceDefault;
        }
    }
}

But I would consider the overall design to be rather poor. Wouldn't it better to have one general MyService implementation and pass one/two/three string as extra parameter to checkStatus()? What do you want to achieve?

@Component
public class MyServiceAdapter implements MyService {

    @Autowired
    private MyServiceOne myServiceOne;

    @Autowired
    private MyServiceTwo myServiceTwo;

    @Autowired
    private MyServiceThree myServiceThree;

    @Autowired
    private MyServiceDefault myServiceDefault;

    public boolean checkStatus(String service) {
        service = service.toLowerCase();

        if (service.equals("one")) {
            return myServiceOne.checkStatus();
        } else if (service.equals("two")) {
            return myServiceTwo.checkStatus();
        } else if (service.equals("three")) {
            return myServiceThree.checkStatus();
        } else {
            return myServiceDefault.checkStatus();
        }
    }
}

This is still poorly designed because adding new MyService implementation requires MyServiceAdapter modification as well (SRP violation). But this is actually a good starting point (hint: map and Strategy pattern).

Answer 3

Following answer of DruidKuma

Litte refactor of his factory with autowired constructor:

@Service
public class MyServiceFactory {

    private static final Map<String, MyService> myServiceCache = new HashMap<>();

    @Autowired
    private MyServiceFactory(List<MyService> services) {
        for(MyService service : services) {
            myServiceCache.put(service.getType(), service);
        }
    }

    public static MyService getService(String type) {
        MyService service = myServiceCache.get(type);
        if(service == null) throw new RuntimeException("Unknown service type: " + type);
        return service;
    }
}