spring Bean的初始化过程解析

AbstractAutowireCapableBeanFactory#applyMergedBeanDefinitionPostProcessors

使用BeanPostProcessor收集带有注解的方法和属性，方便下面初始化时调用。
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyMergedBeanDefinitionPostProcessors

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class<?> beanType, String beanName) {
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
		if (bp instanceof MergedBeanDefinitionPostProcessor) {
			MergedBeanDefinitionPostProcessor bdp = (MergedBeanDefinitionPostProcessor) bp;
			bdp.postProcessMergedBeanDefinition(mbd, beanType, beanName);
		}
	}
}

这里的BeanPostProcessor主要有以下两个：

• AutowiredAnnotationBeanPostProcessor：负责@Autowired、@Value
• CommonAnnotationBeanPostProcessor：负责@Resource、@PostConstruct、@PreDestroy

AutowiredAnnotationBeanPostProcessor#buildAutowiringMetadata

AutowiredAnnotationBeanPostProcessor负责@Autowired、@Value注解的方法和属性的收集。

org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#buildAutowiringMetadata

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
	// autowiredAnnotationTypes是在构造方法中被初始化的
	// autowiredAnnotationTypes @Autowired @Value
	if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
		return InjectionMetadata.EMPTY;
	}
	List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
	Class<?> targetClass = clazz;
	do {
		final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
		// 查找带有@Autowired注解的属性
		ReflectionUtils.doWithLocalFields(targetClass, field -> {
			MergedAnnotation<?> ann = findAutowiredAnnotation(field);
			if (ann != null) {
				if (Modifier.isStatic(field.getModifiers())) {
					if (logger.isInfoEnabled()) {
						logger.info("Autowired annotation is not supported on static fields: " + field);
					}
					return;
				}
				boolean required = determineRequiredStatus(ann);
				currElements.add(new AutowiredFieldElement(field, required));
			}
		});
		// 查找带有@Autowired注解的方法
		ReflectionUtils.doWithLocalMethods(targetClass, method -> {
			Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
			if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
				return;
			}
			MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
			if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
				if (Modifier.isStatic(method.getModifiers())) {
					if (logger.isInfoEnabled()) {
						logger.info("Autowired annotation is not supported on static methods: " + method);
					}
					return;
				}
				if (method.getParameterCount() == 0) {
					if (logger.isInfoEnabled()) {
						logger.info("Autowired annotation should only be used on methods with parameters: " +
								method);
					}
				}
				boolean required = determineRequiredStatus(ann);
				PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
				currElements.add(new AutowiredMethodElement(method, required, pd));
			}
		});
		elements.addAll(0, currElements);
		targetClass = targetClass.getSuperclass();
	}
	while (targetClass != null && targetClass != Object.class);
	return InjectionMetadata.forElements(elements, clazz);
}

CommonAnnotationBeanPostProcessor#postProcessMergedBeanDefinition

CommonAnnotationBeanPostProcessor负责@Resource、@PostConstruct、@PreDestroy注解的方法和属性的收集，收集过程与AutowiredAnnotationBeanPostProcessor的收集过程类似。

org.springframework.context.annotation.CommonAnnotationBeanPostProcessor#postProcessMergedBeanDefinition

public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
	// 构造方法会注入@PostConstruct、@PreDestroy
	// 父类会查找带有@PostConstruct、@PreDestroy注解的方法
	super.postProcessMergedBeanDefinition(beanDefinition, beanType, beanName);
	// 查找带有@Resource注解的属性和方法
	InjectionMetadata metadata = findResourceMetadata(beanName, beanType, null);
	metadata.checkConfigMembers(beanDefinition);
}

AbstractAutowireCapableBeanFactory#populateBean

调用各个BeanPostProcessor.postProcessProperties对属性进行设置。

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
	... ...
	if (hasInstAwareBpps) {
		if (pvs == null) {
			pvs = mbd.getPropertyValues();
		}
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
			if (bp instanceof InstantiationAwareBeanPostProcessor) {
				InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
				// 调用各个BeanPostProcessor.postProcessProperties对属性进行设置
				/**
				 * AutowiredAnnotationBeanPostProcessor处理@Autowired
				 * CommonAnnotationBeanPostProcessor处理@Resource
				 *
				 * @see AutowiredAnnotationBeanPostProcessor#postProcessProperties(org.springframework.beans.PropertyValues, java.lang.Object, java.lang.String)
				 * @see org.springframework.context.annotation.CommonAnnotationBeanPostProcessor#postProcessProperties(org.springframework.beans.PropertyValues, java.lang.Object, java.lang.String)
				 */
				PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);

	... ...
}

CommonAnnotationBeanPostProcessor#postProcessProperties

对带有@Resource注解的属性进行设置，对带有@Resource注解的方法进行调用。

org.springframework.context.annotation.CommonAnnotationBeanPostProcessor#postProcessProperties

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
	// 上面的方法postProcessMergedBeanDefinition向缓存中设置了@PostConstruct、@PreDestroy注解的方法，@Resource注解的属性和方法
	// 这里会从缓存中取出对这些属性进行设置，方法进行调用
	InjectionMetadata metadata = findResourceMetadata(beanName, bean.getClass(), pvs);
	try {
		/**
		 * @see org.springframework.beans.factory.annotation.InjectionMetadata#inject(java.lang.Object, java.lang.String, org.springframework.beans.PropertyValues)
		 */
		metadata.inject(bean, beanName, pvs);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);
	}
	return pvs;
}

AutowiredAnnotationBeanPostProcessor#postProcessProperties方法也是如此，对带有@Autowired注解的属性进行设置，对带有@Autowired注解的方法进行调用。

InjectionMetadata#inject

org.springframework.beans.factory.annotation.InjectionMetadata#inject

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
	Collection<InjectedElement> checkedElements = this.checkedElements;
	Collection<InjectedElement> elementsToIterate =
			(checkedElements != null ? checkedElements : this.injectedElements);
	if (!elementsToIterate.isEmpty()) {
		for (InjectedElement element : elementsToIterate) {
			/**
			 * CommonAnnotationBeanPostProcessor注入的是InjectionMetadata.InjectedElement对象
			 * @see InjectedElement#inject(java.lang.Object, java.lang.String, org.springframework.beans.PropertyValues)
			 *
			 * AutowiredAnnotationBeanPostProcessor注入的是AutowiredFieldElement和AutowiredMethodElement
			 * @see AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject(java.lang.Object, java.lang.String, org.springframework.beans.PropertyValues)
			 * @see AutowiredAnnotationBeanPostProcessor.AutowiredMethodElement#inject(java.lang.Object, java.lang.String, org.springframework.beans.PropertyValues)
			 */
			element.inject(target, beanName, pvs);
		}
	}
}

InjectedElement的调用主要是反射进行属性的设置和方法的调用，如果属性是引用类型，将会触发beanFactory.getBean()方法从Spring容器中获取对象。

AbstractAutowireCapableBeanFactory#initializeBean

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)

上面的populateBean()完成了对象的初始化，下面将会调用对象的初始化方法完成最后的初始化过程。

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
	if (System.getSecurityManager() != null) {
		AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
			invokeAwareMethods(beanName, bean);
			return null;
		}, getAccessControlContext());
	}
	else {
		// 调用aware方法
		// BeanNameAware/BeanClassLoaderAware/BeanFactoryAware对应的setXxx方法
		invokeAwareMethods(beanName, bean);
	}
	Object wrappedBean = bean;
	if (mbd == null || !mbd.isSynthetic()) {
		// 处理EnvironmentAware/EmbeddedValueResolverAware/ResourceLoaderAware/
		// ApplicationEventPublisherAware/MessageSourceAware/ApplicationContextAware
		// 处理@PostConstruct
		wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
	}
	try {
		// InitializingBean.afterPropertiesSet()
		// 调用init-method
		invokeInitMethods(beanName, wrappedBean, mbd);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(
				(mbd != null ? mbd.getResourceDescription() : null),
				beanName, "Invocation of init method failed", ex);
	}
	if (mbd == null || !mbd.isSynthetic()) {
		// AOP代理入口
		wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
	}
	return wrappedBean;
}

AbstractAutowireCapableBeanFactory#invokeAwareMethods

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeAwareMethods

这里只完成了BeanNameAware、BeanClassLoaderAware、BeanFactoryAware接口的初始化方法调用，还有部分aware接口将通过下面的BeanPostProcessor完成调用。

private void invokeAwareMethods(String beanName, Object bean) {
	if (bean instanceof Aware) {
		if (bean instanceof BeanNameAware) {
			((BeanNameAware) bean).setBeanName(beanName);
		}
		if (bean instanceof BeanClassLoaderAware) {
			ClassLoader bcl = getBeanClassLoader();
			if (bcl != null) {
				((BeanClassLoaderAware) bean).setBeanClassLoader(bcl);
			}
		}
		if (bean instanceof BeanFactoryAware) {
			((BeanFactoryAware) bean).setBeanFactory(AbstractAutowireCapableBeanFactory.this);
		}
	}
}

AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization

public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName)
		throws BeansException {
	Object result = existingBean;
	for (BeanPostProcessor processor : getBeanPostProcessors()) {
		/**
		 * ApplicationContextAwareProcessor调用EnvironmentAware/EmbeddedValueResolverAware/ResourceLoaderAware/
		 * ApplicationEventPublisherAware/MessageSourceAware/ApplicationContextAware
		 *
		 * ImportAwareBeanPostProcessor调用ImportAware（Bean不仅需要实现ImportAware接口，还要有@Import注解）
		 * InitDestroyAnnotationBeanPostProcessor 处理@PostConstruct
		 *
		 * @see org.springframework.context.support.ApplicationContextAwareProcessor#postProcessBeforeInitialization(java.lang.Object, java.lang.String)
		 * @see org.springframework.context.annotation.ConfigurationClassPostProcessor.ImportAwareBeanPostProcessor#postProcessBeforeInitialization(java.lang.Object, java.lang.String)
		 * @see org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization(java.lang.Object, java.lang.String)
		 */
		Object current = processor.postProcessBeforeInitialization(result, beanName);
		if (current == null) {
			return result;
		}
		result = current;
	}
	return result;
}

ApplicationContextAwareProcessor#postProcessBeforeInitialization

ApplicationContextAwareProcessor负责EnvironmentAware、EmbeddedValueResolverAware、ResourceLoaderAware、ApplicationEventPublisherAware、MessageSourceAware、ApplicationContextAware接口的调用。

org.springframework.context.support.ApplicationContextAwareProcessor#postProcessBeforeInitialization

public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
	if (!(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
			bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
			bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)){
		return bean;
	}
	AccessControlContext acc = null;
	if (System.getSecurityManager() != null) {
		acc = this.applicationContext.getBeanFactory().getAccessControlContext();
	}
	if (acc != null) {
		AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
			invokeAwareInterfaces(bean);
			return null;
		}, acc);
	}
	else {
		invokeAwareInterfaces(bean);
	}
	return bean;
}
private void invokeAwareInterfaces(Object bean) {
	if (bean instanceof EnvironmentAware) {
		((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
	}
	if (bean instanceof EmbeddedValueResolverAware) {
		((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
	}
	if (bean instanceof ResourceLoaderAware) {
		((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
	}
	if (bean instanceof ApplicationEventPublisherAware) {
		((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
	}
	if (bean instanceof MessageSourceAware) {
		((MessageSourceAware) bean).setMessageSource(this.applicationContext);
	}
	if (bean instanceof ApplicationContextAware) {
		((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
	}
}

ImportAwareBeanPostProcessor#postProcessBeforeInitialization

ImportAwareBeanPostProcessor主要负责ImportAware接口的调用。

ImportAware的具体使用如下：

ImportAwareBean.java

package com.morris.spring.entity.imports;

import org.springframework.context.annotation.ImportAware;
import org.springframework.core.type.AnnotationMetadata;

public class ImportAwareBean implements ImportAware {
	@Override
	public void setImportMetadata(AnnotationMetadata importMetadata) {
		// 在这里可以拿到注入ImportAwareBean的类ImportConfig上的注解@Transactional
		System.out.println(importMetadata.getAnnotationTypes());
	}
}

ImportConfig.java

package com.morris.spring.config;

import com.morris.spring.entity.imports.ImportAwareBean;
import org.springframework.context.annotation.Import;
import org.springframework.transaction.annotation.Transactional;
@Transactional
@Import(ImportAwareBean.class)
public class ImportConfig {
}

org.springframework.context.annotation.ConfigurationClassPostProcessor.ImportAwareBeanPostProcessor#postProcessBeforeInitialization

public Object postProcessBeforeInitialization(Object bean, String beanName) {
	if (bean instanceof ImportAware) {
		ImportRegistry ir = this.beanFactory.getBean(IMPORT_REGISTRY_BEAN_NAME, ImportRegistry.class);
		// bean必须通过@Import注解注入进来，importingClass才会有值
		AnnotationMetadata importingClass = ir.getImportingClassFor(ClassUtils.getUserClass(bean).getName());
		if (importingClass != null) {
			((ImportAware) bean).setImportMetadata(importingClass);
		}
	}
	return bean;
}

InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization

public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
	// LifecycleMetadata缓存数据从何来？
	// 前面在调用CommonAnnotationBeanPostProcessor#postProcessMergedBeanDefinition收集@Resource注解时，同时会调用
	// 父类InitDestroyAnnotationBeanPostProcessor#postProcessMergedBeanDefinition收集@PostConstruct注解

	// 负责@PostConstruct方法的调用
	LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
	try {
		metadata.invokeInitMethods(bean, beanName);
	}
	catch (InvocationTargetException ex) {
		throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
	}
	catch (Throwable ex) {
		throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
	}
	return bean;
}

LifecycleMetadata缓存数据从何来？前面在调用CommonAnnotationBeanPostProcessor#postProcessMergedBeanDefinition收集@Resource注解时，同时会调用
父类InitDestroyAnnotationBeanPostProcessor#postProcessMergedBeanDefinition收集@PostConstruct注解。

而PostConstruct注解是在CommonAnnotationBeanPostProcessor的构造方法中指定的。

public CommonAnnotationBeanPostProcessor() {
	setOrder(Ordered.LOWEST_PRECEDENCE - 3);
	setInitAnnotationType(PostConstruct.class);
	setDestroyAnnotationType(PreDestroy.class);
	ignoreResourceType("javax.xml.ws.WebServiceContext");
}

AbstractAutowireCapableBeanFactory#invokeInitMethods

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#invokeInitMethods

protected void invokeInitMethods(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
		throws Throwable {
	boolean isInitializingBean = (bean instanceof InitializingBean);
	if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
		if (logger.isTraceEnabled()) {
			logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
		}
		if (System.getSecurityManager() != null) {
			try {
				AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
					((InitializingBean) bean).afterPropertiesSet();
					return null;
				}, getAccessControlContext());
			}
			catch (PrivilegedActionException pae) {
				throw pae.getException();
			}
		}
		else {
			// 调用InitializingBean.afterPropertiesSet()
			((InitializingBean) bean).afterPropertiesSet();
		}
	}
	if (mbd != null && bean.getClass() != NullBean.class) {
		String initMethodName = mbd.getInitMethodName();
		if (StringUtils.hasLength(initMethodName) &&
				!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
				!mbd.isExternallyManagedInitMethod(initMethodName)) {
			// 调用init-method
			invokeCustomInitMethod(beanName, bean, mbd);
		}
	}
}

@PostConstruct、afterPropertiesSet、init-method

@PostConstruct、InitializingBean.afterPropertiesSet、init-method这三者要实现的功能都是一致的，都是在bean实例化并初始化完成之后进行调用。

具体使用如下：

InitBean.java

package com.morris.spring.entity;

import org.springframework.beans.factory.InitializingBean;
import javax.annotation.PostConstruct;
public class InitBean implements InitializingBean {
	@PostConstruct
	public void postConstruct() {
		System.out.println("PostConstruct");
	}
	@Override
	public void afterPropertiesSet() throws Exception {
		System.out.println("afterPropertiesSet");
	public void init(){
		System.out.println("init");
}

InitDemo.java

package com.morris.spring.demo.annotation;

import com.morris.spring.entity.InitBean;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class InitDemo {
	@Bean(initMethod = "init")
	public InitBean initBean() {
		return new InitBean();
	}
	public static void main(String[] args) {
		new AnnotationConfigApplicationContext(InitDemo.class);
}

运行结果如下：

PostConstruct
afterPropertiesSet
init

运行结果与源码分析的一致，先执行PostConstruct，再执行afterPropertiesSet，最后initMethod。

注意这三种初始化方法都不能带有参数。

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