SpringBoot优先加载指定Bean的实现

1. 背景

SpringBoot 框架在启动时可以自动将托管的 Bean 实例化，一般情况下它的依赖注入特性可以正确处理 Bean 之间的依赖关系，无需手动指定某个 Bean 优先创建实例。但是一些特殊的需求确实需要某个 Bean 优先实例化，要实现这样的需求就要对 Bean 对象的创建顺序有一定了解

2. Bean 对象的创建顺序分析

首先我们要知道，SpringBoot 源码中 Bean 对象的实例化都是从 AbstractApplicationContext#refresh() 方法开始的。这个方法包含了容器中对象创建的主流程，主要分为以下几步：
BeanFactory 对象工厂的获取及内置配置
BeanFactory 对象工厂的后置处理，主要是通过 BeanFactoryPostProcessor 添加、修改注册到容器中的 BeanDefinition，BeanFactoryPostProcessor 的子类实现 BeanDefinitionRegistryPostProcessor在执行顺序上优先级更高
BeanDefinitionRegistryPostProcessor 的来源分为两类，一类是直接 new 创建后添加到容器，这种在执行顺序上优先级更高；另一类是框架内部封装为 BeanDefinition 后通过对象工厂使用反射创建，典型如 ConfigurationClassPostProcessor
对于通过 @Component 等注解托管给容器的类，主要由ConfigurationClassPostProcessor 这个 Bean 工厂后置处理器将其扫描封装为 BeanDefinition 并注册，有兴趣的读者可参考 SpringBoot 注解 @Import 的原理-ConfigurationClassPostProcessor 源码解析
BeanPostProcessor 对象后置处理器的实例化
Bean 对象创建及其 BeanPostProcessor 后置处理器在创建对象时的切面应用，这部分逻辑主要在 AbstractApplicationContext#finishBeanFactoryInitialization() 方法中

 @Override
 public void refresh() throws BeansException, IllegalStateException {
 	synchronized (this.startupShutdownMonitor) {
 		// Prepare this context for refreshing.
 		prepareRefresh();

 		// Tell the subclass to refresh the internal bean factory.
 		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

 		// Prepare the bean factory for use in this context.
 		prepareBeanFactory(beanFactory);

 		try {
 			// Allows post-processing of the bean factory in context subclasses.
 			postProcessBeanFactory(beanFactory);

 			// Invoke factory processors registered as beans in the context.
 			invokeBeanFactoryPostProcessors(beanFactory);

 			// Register bean processors that intercept bean creation.
 			registerBeanPostProcessors(beanFactory);

 			// Initialize message source for this context.
 			initMessageSource();

 			// Initialize event multicaster for this context.
 			initApplicationEventMulticaster();

 			// Initialize other special beans in specific context subclasses.
 			onRefresh();

 			// Check for listener beans and register them.
 			registerListeners();

 			// Instantiate all remaining (non-lazy-init) singletons.
 			finishBeanFactoryInitialization(beanFactory);

 			// Last step: publish corresponding event.
 			finishRefresh();
 		}

 		catch (BeansException ex) {
 			if (logger.isWarnEnabled()) {
 				logger.warn("Exception encountered during context initialization - " +
 						"cancelling refresh attempt: " + ex);
 			}

 			// Destroy already created singletons to avoid dangling resources.
 			destroyBeans();

 			// Reset 'active' flag.
 			cancelRefresh(ex);

 			// Propagate exception to caller.
 			throw ex;
 		}

 		finally {
 			// Reset common introspection caches in Spring's core, since we
 			// might not ever need metadata for singleton beans anymore...
 			resetCommonCaches();
 		}
 	}
}

AbstractApplicationContext#finishBeanFactoryInitialization() 方法的核心是调用 DefaultListableBeanFactory#preInstantiateSingletons() 方法实例化 Bean 对象

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
 	// Initialize conversion service for this context.
 	if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
 			beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
 		beanFactory.setConversionService(
 				beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
 	}

 	// Register a default embedded value resolver if no bean post-processor
 	// (such as a PropertyPlaceholderConfigurer bean) registered any before:
 	// at this point, primarily for resolution in annotation attribute values.
 	if (!beanFactory.hasEmbeddedValueResolver()) {
 		beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
 	}

 	// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
 	String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
 	for (String weaverAwareName : weaverAwareNames) {
 		getBean(weaverAwareName);
 	}

 	// Stop using the temporary ClassLoader for type matching.
 	beanFactory.setTempClassLoader(null);

 	// Allow for caching all bean definition metadata, not expecting further changes.
 	beanFactory.freezeConfiguration();

 	// Instantiate all remaining (non-lazy-init) singletons.
 	beanFactory.preInstantiateSingletons();
}

DefaultListableBeanFactory#preInstantiateSingletons() 方法会遍历容器内部的 beanDefinitionNames列表进行 Bean 实例化，也就说这个列表的顺序就决定了 Bean 的创建顺序，而实际上 beanDefinitionNames列表中的元素是 BeanDefinition 注册到 BeanDefinitionRegistry 时产生的

在本节步骤1第2步中，笔者提到通过 @Component 等注解托管给容器的类主要由 ConfigurationClassPostProcessor 扫描注册，那么要想让指定的 Bean 优先加载，只需要在 ConfigurationClassPostProcessor 扫描之前注册指定 Bean 即可

	@Override
 public void preInstantiateSingletons() throws BeansException {
 	if (logger.isTraceEnabled()) {
 		logger.trace("Pre-instantiating singletons in " + this);
 	}

 	// Iterate over a copy to allow for init methods which in turn register new bean definitions.
 	// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
 	List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

 	// Trigger initialization of all non-lazy singleton beans...
 	for (String beanName : beanNames) {
 		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
 		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
 			if (isFactoryBean(beanName)) {
 				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
 				if (bean instanceof FactoryBean) {
 					final FactoryBean<?> factory = (FactoryBean<?>) bean;
 					boolean isEagerInit;
 					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
 						isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
 										((SmartFactoryBean<?>) factory)::isEagerInit,
 								getAccessControlContext());
 					}
 					else {
 						isEagerInit = (factory instanceof SmartFactoryBean &&
 								((SmartFactoryBean<?>) factory).isEagerInit());
 					}
 					if (isEagerInit) {
 						getBean(beanName);
 					}
 				}
 			}
 			else {
 				getBean(beanName);
 			}
 		}
 	}

 	// Trigger post-initialization callback for all applicable beans...
 	for (String beanName : beanNames) {
 		Object singletonInstance = getSingleton(beanName);
 		if (singletonInstance instanceof SmartInitializingSingleton) {
 			final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
 			if (System.getSecurityManager() != null) {
 				AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
 					smartSingleton.afterSingletonsInstantiated();
 					return null;
 				}, getAccessControlContext());
 			}
 			else {
 				smartSingleton.afterSingletonsInstantiated();
 			}
 		}
 	}
 }

3. 实现方式

经过上一节分析，我们知道只要找到一个切入点，在 ConfigurationClassPostProcessor 扫描注册 Bean 之前注册指定 Bean 到容器中就能实现优先加载。SpringBoot 提供了不少这样的切入点，本文主要涉及如下两个：

ApplicationContextInitializer
ApplicationListener

3.1 实现 ApplicationListener 监听初始化事件

该方式实现的步骤如下：

在 SpringBoot 主类中调用 SpringApplication#addListeners() 方法添加一个 ContextInitializedListener 监听器
ContextInitializedListener 监听 ApplicationContextInitializedEvent事件，在事件触发的时候往容器中注册指定的 BeanDefinitionRegistryPostProcessor 后置处理器
BeanDefinitionRegistryPostProcessor 后置处理器实现类在 BeanDefinitionRegistryPostProcessor#postProcessBeanDefinitionRegistry() 方法中将指定 Bean 注册到容器中，从而实现优先加载

@SpringBootApplication()
public class ApiApplication {

    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(ApiApplication.class);
        application.addListeners(new ContextInitializedListener());
        application.run(args);
    }


    static class ContextInitializedListener implements ApplicationListener<ApplicationContextInitializedEvent>, BeanDefinitionRegistryPostProcessor {

        @Override
        public void onApplicationEvent(ApplicationContextInitializedEvent event) {
            AbstractApplicationContext context = (AbstractApplicationContext) event.getApplicationContext();
            context.addBeanFactoryPostProcessor(this);
        }

        @Override
        public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
            registry.registerBeanDefinition("example", new RootBeanDefinition(ContentDTO.class));
        }

        @Override
        public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

        }
    }
}

3.2 实现 ApplicationContextInitializer

该方式实现的原理与事件监听类似，不再赘述

@SpringBootApplication()
public class ApiApplication {

    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(ApiApplication.class);
        application.addInitializers(new MyApplicationContextInitializer());
        application.run(args);
    }


    static class MyApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext>, BeanDefinitionRegistryPostProcessor {

        @Override
        public void initialize(ConfigurableApplicationContext applicationContext) {
            applicationContext.addBeanFactoryPostProcessor(this);
        }

        @Override
        public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
            registry.registerBeanDefinition("example", new RootBeanDefinition(ContentDTO.class));
        }

        @Override
        public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

        }
    }
}

SpringBoot优先加载指定Bean的实现

目录

1. 背景

2. Bean 对象的创建顺序分析

3. 实现方式

3.1 实现 ApplicationListener 监听初始化事件

3.2 实现 ApplicationContextInitializer