Question

Let’s say I have an interface for language change event in my application (it’s based on Vaadin):

public interface ILanguageChangeListener{
    @Subscribe onLanguageChange(LanguageChangeEvent event);
}

And I have many beans that implements this interface annotated with @Component, thus they are available in Spring IoC. I have also an EventBus bean:

<bean id="languageSwitcher" class="com.google.common.eventbus" scope="session" />

Now, after getting an instance of any bean from IoC I have to get also an instance of the languageSwitcher and register the newely created bean in it with:

languageSwitcher.register(myNewBean);

in order to receive this events. Is it possible to somehow tell the IoC that I want to call the register method of the languageSwitcher bean on every new bean that implements the ILanguageChangeListener?

Answer 1

OK, using a BeanPostProcessor, register every bean of your interface:

public class EventBusRegisterBeanPostProcessor implements BeanPostProcessor,
        ApplicationContextAware {

    private ApplicationContext context;

    @Autowired
    private EventBus eventBus; // The only event bus i assume...

    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {

        return bean;
    }

    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {

        if (bean instanceof ILanguageChangeListener) {
            registerToEventBus(bean);
        }

        return bean;
    }

    private void registerToEventBus(Object bean) {
        this.eventBus.register(bean);
    }

    public void setApplicationContext(ApplicationContext applicationContext)
            throws BeansException {
        this.context = applicationContext;
    }

}

Note that if you have many EventBus beans, you should use the ApplicationContext.getBean(String) to get the EventBus you need.

I quote from the javadoc:

In case of a FactoryBean, this callback will be invoked for both the
FactoryBean instance and the objects created by the FactoryBean (as of
Spring 2.0). The post-processor can decide whether to apply to either
the FactoryBean or created objects or both through corresponding bean
instanceof FactoryBean checks.