Question

There is a @Bean annotation in Spring 3.0. It allows to define a Spring bean directly in a Java code. While browsing Spring reference I found two different ways of using this annotation – inside class annotated with @Configuration and inside class which doesn’t have this annotation.

This section contains following piece of code:

@Component
public class FactoryMethodComponent {

   @Bean @Qualifier("public")
   public TestBean publicInstance() {
      return new TestBean("publicInstance");
   }

   // omitted irrelevant method
}

And here we could see a very similar piece of code, but now @Configuration is in the place:

@Configuration
public class AppConfig {
   @Bean
   public MyService myService() {
      return new MyServiceImpl();
   }
}

Former section of reference contains following explaination:

The @Bean methods in a Spring component are processed differently than their counterparts inside a Spring @Configuration class. The difference is that @Component classes are not enhanced with CGLIB to intercept the invocation of methods and fields. CGLIB proxying is the means by which invoking methods or fields within @Configuration classes @Bean methods create bean metadata references to collaborating objects. Methods are not invoked with normal Java semantics. In contrast, calling a method or field within a @Component classes @Bean method has standard Java semantics.

But CGLIB is a kind of internal stuff which application developer shouldn’t be aware of (in a ideal world, of course). As I understand in both cases Spring invokes method annotated with @Bean to create Spring bean, in both cases these instances are injected to collaborators.

So my question is what is the difference for me as an application developer between this two cases?

Answer 1

The difference is that with @Configuration you can call one @Bean method from another and get a fully initialized instance, as follows:

public class Foo {
    @Value("Hello, world!")
    public String value;
}

@Configuration
public class Config {
    @Bean
    public Foo createFoo() {
        Foo foo = new Foo();
        System.out.println(foo.value); // Prints null - foo not initialized yet
        return foo;
    }

    @Bean
    public Bar createBar() {
        Foo foo = createFoo();
        System.out.println(foo.value); // Prints Hello, world! - foo have been initialized by the interceptor
        return new Bar(foo);
    }
}