Spring annotations - @Configuration to invoke spring bean auto-building

Question

If I declare a class using @Bean and then component scan for the class, spring will instantiate the class by invoking it's constructor and injecting constructor args and injecting any fields marked with @Inject. For simplicity's sake, lets call this spring auto-building.

I dislike component scan and wish to avoid it completely (I don't wish to discuss my reasons for not liking it). I would like to use a @Configuration object instead but would still like to have the auto-building functionality available to me. Is it possible to invoke spring to auto-build my objects instead of explicitly having to pass all the constructor arguments in my @Configuration object?

Lets assume that I have a bean:

public class MyServiceImpl implements MyService {
    public MyServiceImpl(Dependency1 d1, Dependency d2) { ... }
    ....
}

I could define a configuration object like this:

@Configuration
public class MyConfiguration {
    // lets assume d1 and d2 are defined in another @Configuration
    @Inject
    Dependency1 d1; 

    @Inject
    Dependency2 d2;

    @Bean
    public MyService myService() { 
        // I dislike how I have to explicitly call the constructor here
        return new MyServiceImpl(d1, d2);
    }
}

But now, I have explicitly had to call the MyServiceImpl constructor myself so I will have to keep updating this as my constructor changes over time.

I was hoping that I could declare an abstract method so that spring auto-building could take place:

@Configuration
public abstract class MyConfiguration {
    @Bean
    public abstract MyServiceImpl myService();
}

But this doesn't work. Is there a way that I can invoke spring auto-building without using a component scan?

In Google Guice, this can be done via the Binder: https://google-guice.googlecode.com/svn/trunk/javadoc/com/google/inject/Binder.html

In Tapestry IOC, this can be done via the ServiceBinder: http://tapestry.apache.org/ioc-cookbook-basic-services-and-injection.html#IoCCookbook-BasicServicesandInjection-SimpleServices

Update

Based on spod's answer, I was able to achieve what I was after (thanks!). Test case included for anyone that wants to do the same:

import java.util.Date;
import javax.inject.Inject;
import junit.framework.Assert;
import org.junit.Test;
import org.springframework.beans.factory.config.AutowireCapableBeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

public class AutoBuildConfigurationTest {
    @Configuration
    public static class MyConfiguration {
        @Inject
        private AutowireCapableBeanFactory beanFactory;

        @Bean
        public Date date() {
            return new Date(12345);
        }

        @Bean
        public MyService myService() {
            return autoBuild(MyService.class);
        }

        protected <T> T autoBuild(Class<T> type) {
            return type.cast(beanFactory.createBean(type, AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR, true));
        }
    }

    public static class MyService {
        private Date date;

        public MyService(Date date) {
            this.date = date;
        }

        public Date getDate() {
            return date;
        }
    }

    @Test
    public void testAutoBuild() {
        ApplicationContext appContext = new AnnotationConfigApplicationContext(MyConfiguration.class);
        MyService myService = appContext.getBean(MyService.class);
        Assert.assertEquals(12345, myService.getDate().getTime());
    }
}

Stefan Podkowinski Stefan Podkowinski · Accepted Answer · 2012-11-27T12:30:05

The java based container configuration doesnt depend on doing a component scan in any way. Its merely a different approach for the XML based component configuration. With the XML configuration you'd just have to declare your bean with the MyServiceImpl class in case its already @inject annotated. Spring would recognize the annotations and take care of them. If you really want to instanciate MyServiceImpl from a @Configuration java class without calling the constructor yourself, then you'd have to make use of the bean factory (havent tested it, just give it a try):

@Configuration
public class MyConfiguration {

    @Autowired AutowireCapableBeanFactory beanFactory;

    @Bean public MyService myService() { 
        return beanFactory.createBean(MyServiceImpl.class, AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR, true);
    }
}

Spring annotations - @Configuration to invoke spring bean auto-building

1 Answers