How can I select Spring bean instance at runtime

Question

Based on parameters passed to a method, I need to select from one of many Spring beans that are implementations of the same class, but configured with different parameters.

E.g. if user A invokes the method, I need to call dooFoo() on bean A, but if it's user B then I need to call the very same method, only on bean B.

Is there a 'Springier' way of doing this other than sticking all the beans in a map, and deriving a key from the parameters passed to my method?

Answer 1

We face that issue in our project, and we solve it through a Factory-Like class. The client class -the one that needed the bean at runtime- had an instance of the factory, that was injected through Spring:

@Component
public class ImTheClient{

    @Autowired
    private ImTheFactory factory;

    public void doSomething(
            Parameters parameters) throws Exception{        
        IWantThis theInstance = factory.getInstance(parameters);        

    }

}

So, the IWantThis instance depends on the runtime value of the parameters parameter. The Factory implementation goes like this:

@Component
public class ImTheFactoryImpl implements
        ImTheFactory {

    @Autowired
    private IWantThisBadly anInstance;
    @Autowired
    private IAlsoWantThis anotherInstance;

    @Override
    public IWantThis getInstance(Parameters parameters) {
        if (parameters.equals(Parameters.THIS)) {
            return anInstance;
        }

        if (parameters.equals(Parameters.THAT)) {
            return anotherInstance;
        }

        return null;
    }
}

So, the factory instance holds reference to both of the posible values of the IWantThis class, being IWantThisBadly and IAlsoWantThis both implementations of IWantThis.

Answer 2

Seems like do you want a ServiceLocator using the application context as registry.

See ServiceLocatorFactoryBean support class for creating ServiceLocators mapping keys to bean names without coupling client code to Spring.

Other option is to use a naming convention or annotation based configuration.

for example, assuming that you annotate Services with @ExampleAnnotation("someId"), you can use something like the following Service Locator to retrieve them.

public class AnnotationServiceLocator implements ServiceLocator {

    @Autowired
    private ApplicationContext context;
    private Map<String, Service> services;

    public Service getService(String id) {
        checkServices();
        return services.get(id);
    }

    private void checkServices() {
        if (services == null) {
            services = new HashMap<String, Service>();
            Map<String, Object> beans = context.getBeansWithAnnotation(ExampleAnnotation.class);
            for (Object bean : beans.values()) {
                ExampleAnnotation ann = bean.getClass().getAnnotation(ExampleAnnotation.class);
                services.put(ann.value(), (Service) bean);
            }
        }
    }   
}