Creating a collection of beans in Spring using @Configuration

Question

How can I create a collection of beans that will be properly managed by Spring using a class with a @Configuration annotation.

I would like to do something like this:

@Configuration
public Config {
    @Autowired
    private SomeConfiguration config;

    @Bean
    public List<MyBean> myBeans() {
        List<MyBean> beans = new ArrayList<MyBean>();
        for (Device device : config.getDevices()) {
            beans.add(new MyBean(device));
        }
        return beans;
    }
}

But the MyBean instances aren't post processed. So their @Autowired methods are not called, the beans are not registered as mbeans and etc. The list is however accessible so that I can autowire a List of MyBean objects.

I cannot use something like:

@Configuration
public Config {
    @Autowired
    private SomeConfiguration config;

    @Bean
    public MyBean myBean1() { ... }

    @Bean
    public MyBean myBean2() { ... }
}

Since the number of MyBean instances are not known before runtime. The reason I want to do this is because we are controlling a physical machine that have a variable amount of components. And I want to have one bean per component.

I'm currently achieving our goal by using a BeanFactoryPostProcessor like this:

@Component
public class MyBeansFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Autowired
    private SomeConfiguration config;

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeanException {
        for (Device device : config.getDevices()) {
            createAndRegister(BeanDefinitionRegistry) beanFactory, device);
        }
    }

    private void createAndRegister(BeanDefinitionRegistry registry, Device device) {
        register.registerBeanDefinition("device" + device.getId(), BeanDefinitionBuilder.genericBeanDefinition(MyBean.class).addConstructorArgValue(device).getBeanDefinition());
    }
}

But this just feels like a really ugly hack.

Solution 1

In order to inject your MyBean list try @Resource instead of @Autowired. for e.g.

@Resource
public List<MyBean> myBeans

Solution 2

I believe that another option in this case is to use @PostConstruct in the following manner:

@Configuration
public Config {

    @Autowired
    private SomeConfiguration config;

    List<MyBean> beans = new ArrayList<MyBean>();

    @Bean
    public List<MyBean> myBeans() {     

        return beans;
    }

    @PostConstruct
    public void init() {
        for (Device device : config.getDevices()) {
            beans.add(new MyBean(device));
        }   
    }
}

The @PostConstruct annotation is useful for initializing properties. It guarantees that the annotated method will only be called once when the bean is created.

Correct me if I'm wrong

Solution 3

You can use the ConfigurableListableBeanFactory which supports SmartLifecycle so if you register the bean before your app is fully initialized it will call start() for you and other post processing steps.

However - if you call beanFactory.registerSingleton after spring has initialized you will manually need to call start() - on the bright side though you bean is still fully wired into the lifecycle management and spring will call stop() for you when the application context is shutdown.

@Autowired
private ConfigurableListableBeanFactory beanFactory;

@Bean
public List<MyBean> myBeansList() {

    List<MyBean> mylist; // Construct your list dynamically

    while(myCondition) {
        MyBean bean;
        // Manually register each instance with Spring
        beanFactory.registerSingleton("unique-name-for-this-bean",bean);
    }

    // Return your list as a bean so you can still autowire the list of beans
    // but each bean has already been manually added to the context
    return mylist;
}

Solution 4

It's not possible using @Configuration to define more than one bean per method (AFAIK). So you will have to contnue using a BFPP or use ApplicationContect.getAutowireCapableBeanFactory().autowire(object);

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Comments

[sebnukem]

Still no acceptable and good answer on this not so uncommon use case? That's crazy.

[pcmoen]

Thanks this will ensure that all @Autowired methods are called. But it doesn't register the beans with JMX and apply other post processing. I've also tried ApplicationContect.getAutowireCapableBeanFactory().applyBean‌​PostProcessorsBefore‌​Initialization(bean, beanName); ApplicationContect.getAutowireCapableBeanFactory().applyBean‌​PostProcessorsAfterI‌​nitialization(bean, beanName); But since there are no bean definitions they will not perform full post processing.

[Sean Patrick Floyd]

@pcm that makes sense, then I guess you are already doing it the best possible way