@Service
public class A {
  @Autowired
  private B b;

  public A() {
    System.out.println("A construct");
  }

  @PostConstruct
  public void init() {
    initA();
  }

  private void initA() {
    System.out.println("A init");
  }
}

@Service
public class B {
  @Autowired
  private A a;

  public B() {
    System.out.println("B construct");
  }

  @PostConstruct
  public void init() {
    initB();
  }

  private void initB(){
    System.out.println("B init");
  }
}

@Service
public class A {

  private static volatile boolean initialized;

  @Autowired
  private B b;

  public A() {
    System.out.println("A construct");
  }

  @PostConstruct
  public void init() {
    initA();
  }

  public boolean isInitialized() {
    return initialized;
  }

  public void initA() {
    if (!isInitialized()) {
      System.out.println("A init");
    }
    initialized = true;
  }
}

@Service
public class B {

  @Autowired
  private A a;


  public B() {
    System.out.println("B construct");
  }

  @PostConstruct
  public void init() {
    initB();
  }


  private void initB() {
    if (!a.isInitialized()) {
      a.initA();
    }
    System.out.println("B init");
  }

// 代码位置：AbstractAutowireCapableBeanFactory.doCreateBean
// 填充 bean 的各个属性，包括依赖注入
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
  // 调用初始化方法，如果是 InitializingBean 则先调用 afterPropertiesSet 然后调用自定义的init-method 方法
  exposedObject = initializeBean(beanName, exposedObject, mbd);
}

// 代码位置：AbstractBeanFactory.doGetBean
// Guarantee initialization of beans that the current bean depends on. 实例化依赖的 bean
    String[] dependsOn = mbd.getDependsOn();
    if (dependsOn != null) {
      for (String dep : dependsOn) {
        if (isDependent(beanName, dep)) {
          throw new BeanCreationException(mbd.getResourceDescription(),
              beanName, "Circular depends-on relationship between '"
              + beanName + "' and '" + dep + "'");
        }
        registerDependentBean(dep, beanName); // 缓存 bean 依赖的关系
        getBean(dep);
      }
    }

@Component
public class ABeanFactoryPostProcessor implements BeanFactoryPostProcessor {
  @Override
  public void postProcessBeanFactory(ConfigurableListableBeanFactory configurableListableBeanFactory) throws BeansException {
    A.initA();
  }
}

// 代码位置：AbstractApplicationEventMulticaster.ListenerRetriever.getApplicationListeners()
public Collection<ApplicationListener<?>> getApplicationListeners() {
    LinkedList<ApplicationListener<?>> allListeners = new LinkedList<ApplicationListener<?>>();
    for (ApplicationListener<?> listener : this.applicationListeners) {
      allListeners.add(listener);
    }
    if (!this.applicationListenerBeans.isEmpty()) {
      BeanFactory beanFactory = getBeanFactory();
      for (String listenerBeanName : this.applicationListenerBeans) {
        try {
          ApplicationListener<?> listener = beanFactory.getBean(listenerBeanName, ApplicationListener.class);
          if (this.preFiltered || !allListeners.contains(listener)) {
            allListeners.add(listener);
          }
        } catch (NoSuchBeanDefinitionException ex) {
          // Singleton listener instance (without backing bean definition) disappeared -
          // probably in the middle of the destruction phase
        }
      }
    }
    AnnotationAwareOrderComparator.sort(allListeners); // 排序
    return allListeners;
  }

@Component
public class ApplicationListenerA implements ApplicationListener<ApplicationContextEvent>, Ordered {
  @Override
  public void onApplicationEvent(ApplicationContextEvent event) {
    initA();
  }

  @Override
  public int getOrder() {
    return Ordered.HIGHEST_PRECEDENCE; // 比 ApplicationListenerB 优先级高
  }

  public static void initA() {
    System.out.println("A init");
  }
}

@Component
public class ApplicationListenerB implements ApplicationListener<ApplicationContextEvent>, Ordered{
  @Override
  public void onApplicationEvent(ApplicationContextEvent event) {
    initB();
  }

  @Override
  public int getOrder() {
    return Ordered.HIGHEST_PRECEDENCE -1;
  }

  private void initB() {
    System.out.println("B init");
  }
}