1. Description

The afterSingletonsInstantiated() method of the SmartInitializingSingleton interface is similar to the callback function after the bean instantiation is executed.
afterSingletonsInstantiated will be executed after the spring container is basically started. At this point all single-column bean s have been initialized. A class that implements the SmartInitializingSingleton interface
You can do some callback operations in afterSingletonsInstantiated.

2. Use of SmartInitializingSingleton in XXL-JOB

xxl-job is a lightweight distributed task scheduling framework. Through a central scheduling platform, multiple executors are scheduled to execute tasks, and the monitoring interface is integrated in the scheduling center, and the interface is simple.

In xxl-job, XxlJobSpringExecutor and XxlJobSimpleExecutor inherit from XxlJobExecutor and also implement the SmartInitializingSingleton interface.
In the afterSingletonsInstantiated() method, XxlJobSpringExecutor completes the method of extracting the @XxlJob annotation for the execution of subsequent tasks. Call the start() method in the parent class method to initialize the netty server.

public class XxlJobSpringExecutor extends XxlJobExecutor implements ApplicationContextAware, SmartInitializingSingleton, DisposableBean {
    private static final Logger logger = LoggerFactory.getLogger(XxlJobSpringExecutor.class);


    // start
    @Override
    public void afterSingletonsInstantiated() {

        // init JobHandler Repository
        /*initJobHandlerRepository(applicationContext);*/

        // init JobHandler Repository (for method)
        initJobHandlerMethodRepository(applicationContext);

        // refresh GlueFactory
        GlueFactory.refreshInstance(1);

        // super start
        try {
            super.start();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
	///Other code omitted
}

The start() method in the parent class: initEmbedServer is used to initialize the netty service.

    // ---------------------- start + stop ----------------------
    public void start() throws Exception {

        // init logpath
        XxlJobFileAppender.initLogPath(logPath);

        // init invoker, admin-client
        initAdminBizList(adminAddresses, accessToken);


        // init JobLogFileCleanThread
        JobLogFileCleanThread.getInstance().start(logRetentionDays);

        // init TriggerCallbackThread
        TriggerCallbackThread.getInstance().start();

        // init executor-server initializes the netty service
        initEmbedServer(address, ip, port, appname, accessToken);
    }

Get the implementation method of the method annotated with the @XxlJob annotation in the container:

    private void initJobHandlerMethodRepository(ApplicationContext applicationContext) {
        if (applicationContext == null) {
            return;
        }
        // init job handler from method
        String[] beanDefinitionNames = applicationContext.getBeanNamesForType(Object.class, false, true);
        for (String beanDefinitionName : beanDefinitionNames) {
            Object bean = applicationContext.getBean(beanDefinitionName);

            Map<Method, XxlJob> annotatedMethods = null;   // referred to : org.springframework.context.event.EventListenerMethodProcessor.processBean
            try {
                annotatedMethods = MethodIntrospector.selectMethods(bean.getClass(),
                        new MethodIntrospector.MetadataLookup<XxlJob>() {
                            @Override
                            public XxlJob inspect(Method method) {
                                return AnnotatedElementUtils.findMergedAnnotation(method, XxlJob.class);
                            }
                        });
            } catch (Throwable ex) {
                logger.error("xxl-job method-jobhandler resolve error for bean[" + beanDefinitionName + "].", ex);
            }
            if (annotatedMethods==null || annotatedMethods.isEmpty()) {
                continue;
            }

            for (Map.Entry<Method, XxlJob> methodXxlJobEntry : annotatedMethods.entrySet()) {
                Method executeMethod = methodXxlJobEntry.getKey();
                XxlJob xxlJob = methodXxlJobEntry.getValue();
                // regist
                registJobHandler(xxlJob, bean, executeMethod);
            }
        }
    }

Similar to other frameworks, just return an instantiated bean in the configuration class decorated with @Configuraton.

@Configuration
public class XxlJobConfig {
    @Bean
    public XxlJobSpringExecutor xxlJobExecutor() {
        logger.info(">>>>>>>>>>> xxl-job config init.");
        XxlJobSpringExecutor xxlJobSpringExecutor = new XxlJobSpringExecutor();
        xxlJobSpringExecutor.setAdminAddresses(adminAddresses);
        xxlJobSpringExecutor.setAppname(appname);
        xxlJobSpringExecutor.setAddress(address);
        xxlJobSpringExecutor.setIp(ip);
        xxlJobSpringExecutor.setPort(port);
        xxlJobSpringExecutor.setAccessToken(accessToken);
        xxlJobSpringExecutor.setLogPath(logPath);
        xxlJobSpringExecutor.setLogRetentionDays(logRetentionDays);

        return xxlJobSpringExecutor;
    }
}

3. Use of SmartInitializingSingleton in spring container

EventListenerMethodProcessor also implements the SmartInitializingSingleton interface, which is mainly used to complete event monitoring by @EventListener annotation.
The processing logic in the afterSingletonsInstantiated() method is similar to that in xxl-job, and it is used to filter the methods modified by the specified annotations in the instance.

public class EventListenerMethodProcessor
		implements SmartInitializingSingleton, ApplicationContextAware, BeanFactoryPostProcessor {

	
	@Override
	public void afterSingletonsInstantiated() {
		ConfigurableListableBeanFactory beanFactory = this.beanFactory;
		Assert.state(this.beanFactory != null, "No ConfigurableListableBeanFactory set");
		String[] beanNames = beanFactory.getBeanNamesForType(Object.class);
		for (String beanName : beanNames) {
			if (!ScopedProxyUtils.isScopedTarget(beanName)) {
				Class<?> type = null;
				try {
					type = AutoProxyUtils.determineTargetClass(beanFactory, beanName);
				}
				catch (Throwable ex) {
					// An unresolvable bean type, probably from a lazy bean - let's ignore it.
					if (logger.isDebugEnabled()) {
						logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
					}
				}
				if (type != null) {
					if (ScopedObject.class.isAssignableFrom(type)) {
						try {
							Class<?> targetClass = AutoProxyUtils.determineTargetClass(
									beanFactory, ScopedProxyUtils.getTargetBeanName(beanName));
							if (targetClass != null) {
								type = targetClass;
							}
						}
						catch (Throwable ex) {
							// An invalid scoped proxy arrangement - let's ignore it.
							if (logger.isDebugEnabled()) {
								logger.debug("Could not resolve target bean for scoped proxy '" + beanName + "'", ex);
							}
						}
					}
					try {
						processBean(beanName, type);
					}
					catch (Throwable ex) {
						throw new BeanInitializationException("Failed to process @EventListener " +
								"annotation on bean with name '" + beanName + "'", ex);
					}
				}
			}
		}
	}

}

write at the end

The above two methods are used to process some custom logic after the single column of the spring container is loaded. EventListenerMethodProcessor is mainly used to filter out @EventListener
The annotated method implements event monitoring through the publish-subscribe mode in subsequent use.

XxlJobSpringExecutor is mainly used to filter out the methods annotated with @XxlJob annotation for starting and stopping subsequent tasks.