Currently there are two approaches to resolving dependencies between modularized JavaConfig @Configuration classes. Both work, but have significant downsides:

1) @ExternalBean

    @Configuration
    public abstract class AppConfig {
        @Bean
        public OrderService orderService() {
            return new OrderServiceImpl(orderRepository());
        }
    }

    @ExternalBean
    public abstract OrderRepository orderRepository();

The 'external bean' will need to be provided by some other contributor to the ApplicationContext at runtime. Could be another JavaConfig @Configuration class, could be definitions out of XML, etc.

Downsides here are that the syntax is a little awkward and 'clever'. User is encouraged to use abstract, but it's not immediately clear why, or what happens with that abstract method (user would have to think about CGLIB-based implementation to truly understand it, etc)


2) extend ConfigurationSupport and lookup beans with getBean()

    @Configuration
    public abstract class AppConfig extends ConfigurationSupport {
        @Bean
        public OrderService orderService() {
            return new OrderServiceImpl((OrderRepository) this.getBean("orderRepository");
        }
    }

again, our 'orderRepository' bean will be provided by some other contributor of bean definitions at runtime.

Downsides to this approach include having to subclass ConfigurationSupport, and fragile string-based lookups (can be a little bit better if using the new type-safe getBean() method on ConfigurationSupport, but is still somewhat cumbersome, feels like dependency lookup rather than dependency injection.


The solution:

A more elegant approach to modularizing configurations would be to supply dependencies as parameters to bean creation methods. Applying this to our example above, syntax becomes very intuitive / familiar:

    @Configuration
    public abstract class AppConfig extends ConfigurationSupport {
        @Bean
        public OrderService orderService(OrderRepository orderRepository) {
            return new OrderServiceImpl(orderRepository);
        }
    }