Spring中@Autowire，@Value 注解实现原理

本文主要基于SpringBoot-2.3.3.RELEASE, Spring-5.2.8.RELEASE 讲解.
Spring中@Autowire，@Value 注解实现原理，将这两个注解放到一块讲解主要是他们的实现基本一致。
本文涉及注解： @Autowire、@Value、@Qualifier、@Lazy、@Primary、@javax.annotation.Priority

相关类介绍

如下是几个重要的相关类：
org.springframework.beans.factory.config.BeanPostProcessor
org.springframework.beans.factory.support.MergedBeanDefinitionPostProcessor
org.springframework.beans.factory.config.InstantiationAwareBeanPostProcessor
org.s.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor
org.s.beans.factory.annotation.InjectionMetadata.InjectedElement
org.s.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement
org.s.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredMethodElement

BeanPostProcessor

BeanPostProcessor是一个接口类，所有的bean在初始时期都会经过这个后置处理器，主要提供如下两个接口方法，

public interface BeanPostProcessor {
    
    
	@Nullable
	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    
    
		return bean;
	}

	@Nullable
	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
    
    
		return bean;
	}
}

postProcessBeforeInitialization方法在bean初始化方法之前执行.例如bean实现了InitializingBean接口，会在InitializingBean#afterPropertiesSet方法之前执行。（不完全是这样，因为要看创建bean时，某个BeanPostProcessor实现类是否已经注册）
对于BeanPostProcessor#postProcessBeforeInitialization是否在@PostConstruct注解的初始化方法之前执行，这是不确定的，因为@PostConstruct初始化方法的执行是通过InitDestroyAnnotationBeanPostProcessor实现，并且InitDestroyAnnotationBeanPostProcessor通过重写BeanPostProcessor#postProcessBeforeInitialization实现@PostConstruct功能，CommonAnnotationBeanPostProcessor通过继承InitDestroyAnnotationBeanPostProcessor拥有了相同的能力，因此要看BeanPostProcessor 实现类被注册的顺序才能决定是否优先于@PostConstruct注解的方式执行。
postProcessAfterInitialization 方法在bean的初始化方法之后执行。
BeanPostProcessor 的子类接口如下UML图
AbstractAutowireCapableBeanFactory#initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd)源码如下：

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 {
    
    
	    //判断是否实现了Aware接口,执行子类接口方法,
	    //此时只有:BeanNameAware,BeanClassLoaderAware,BeanFactoryAware
		invokeAwareMethods(beanName, bean); 
	}
	Object wrappedBean = bean;
	if (mbd == null || !mbd.isSynthetic()) {
    
    
	    //调用BeanPostProcessor#postProcessBeforeInitialization
	    //ApplicationContextAwareProcessor#postProcessBeforeInitialization
	    //实现EnvironmentAware,..ApplicationContextAware等Aware的回调.
		wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
	}
	try {
    
    
	    //如果bean实现了InitializingBean接口则执行接口的afterPropertiesSet方法
		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()) {
    
    
		wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
	}
	return wrappedBean;
}

MergedBeanDefinitionPostProcessor

@Autowire，@Value 的查找 就是AutowiredAnnotationBeanPostProcessor通过重写该方法实现

public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {
    
    

	void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName);
	
	default void resetBeanDefinition(String beanName) {
    
    
	}
}

BeanDefinition主要封装了bean的类信息，例如是否是单实例（Singleton），初始化方法名，依赖的bean名称等等。

在bean对象被创建之前，如果一个bean的BeanDefinition是一个子类，会先合并父类BeanDefinition，最终返回一个合并后的RootBeanDefinition，MergedBeanDefinitionPostProcessor则是Spring提供给开发的接口，便于对合并后的RootBeanDefinition做自定义修改。在调用AbstractAutowireCapableBeanFactory#doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)时回调MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition方法。

InstantiationAwareBeanPostProcessor

(看注释)
@Autowire，@Value 的赋值 就是AutowiredAnnotationBeanPostProcessor通过重写postProcessProperties方法实现

public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {
    
    
    //创建bean对象实例之前执行
	default Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
    
    
		return null;
	}
    //创建bean对象实例之后执行
	default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {
    
    
		return true;
	}
    //用于给bean对象中的成员属性赋值
    //@Autowire，@Value的赋值就是AutowiredAnnotationBeanPostProcessor通过重写该方法实现
	default PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName)
			throws BeansException {
    
    
		return null;
	}

	@Deprecated   //标记为过时
	default PropertyValues postProcessPropertyValues(
			PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
    
    
		return pvs;
	}
}

AutowiredAnnotationBeanPostProcessor

@Autowire，@Value，@Lookup等注解功能都是由AutowiredAnnotationBeanPostProcessor实现，在本文主要讲解@Autowire，@Value注解功能实现原理。
AutowiredAnnotationBeanPostProcessor的继承关系如下：
在这里插入图片描述
AutowiredAnnotationBeanPostProcessor的构造方法如下：主要定义了@Autowire，@Value两个注解是过滤目标bean的条件，
如果bean中的Field、Method被标记这两个注解，则需要为其注入对应的bean或者属性值。

public AutowiredAnnotationBeanPostProcessor() {
    
    
		this.autowiredAnnotationTypes.add(Autowired.class);
		this.autowiredAnnotationTypes.add(Value.class);
		try {
    
    
			this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
					ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
			logger.trace("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
		}
		catch (ClassNotFoundException ex) {
    
    
			// JSR-330 API not available - simply skip.
		}
	}

在实现@Autowire，@Value功能的过程中，AutowiredAnnotationBeanPostProcessor主要起到两个作用。

重写MergedBeanDefinitionPostProcessor#postProcessMergedBeanDefinition方法，反射遍历beanClass中的所有Field和Method，查找bean中标记有@Autowire，@Value的Field、Method，封装到org.springframework.beans.factory.annotation.InjectionMetadata中。保存在成员变量private final Map<String, InjectionMetadata> injectionMetadataCache = new ConcurrentHashMap<>(256);中。

AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition 源码：

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
    
    
	InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
	metadata.checkConfigMembers(beanDefinition);
}

findAutowiringMetadata方法内部会调用到AnnotationUtils#isCandidateClass(Class<?>, Class<? extends Annotation>)
用于判断某个类是否可能被标记指定的某个注解。源码如下：

public static boolean isCandidateClass(Class<?> clazz, Class<? extends Annotation> annotationType) {
    
    
	return isCandidateClass(clazz, annotationType.getName());
}

public static boolean isCandidateClass(Class<?> clazz, String annotationName) {
    
    
    //如果注解所在包是java开头，任何类上都可能标记这个注解
	if (annotationName.startsWith("java.")) {
    
    
		return true;
	}
	//注解所在包不是java开头的情况
	//可以认为是自定义注解，例如Spring的@Service @Autowired @Value 对Spring来说就是其自定义注解
	//如果类的包是以java开头，则不可能标记自定义的注解
	if (AnnotationsScanner.hasPlainJavaAnnotationsOnly(clazz)) {
    
    
		return false;
	}
	return true;
}

AnnotationsScanner.hasPlainJavaAnnotationsOnly(clazz)

static boolean hasPlainJavaAnnotationsOnly(Class<?> type) {
    
    
	return (type.getName().startsWith("java.") || type == Ordered.class);
}

重写InstantiationAwareBeanPostProcessor#postProcessProperties方法，遍历injectionMetadataCache中的InjectionMetadata，调用org.springframework.beans.factory.annotation.InjectionMetadata#inject方法，为bean的@Autowire和@value属性字段赋值。

AutowiredAnnotationBeanPostProcessor#postProcessProperties 源码：

@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    
    
	InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
	try {
    
    
		metadata.inject(bean, beanName, pvs);
	}
	catch (BeanCreationException ex) {
    
    
		throw ex;
	}
	catch (Throwable ex) {
    
    
		throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
	}
	return pvs;
}

InjectionMetadata里面有个abstract内部类InjectionMetadata.InjectedElement，主要用于给Field或者Method赋值。
AutowiredAnnotationBeanPostProcessor中有两个私有内部类分别继承了InjectedElement。

AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement: 通过反射给Field赋值.
AutowiredAnnotationBeanPostProcessor.AutowiredMethodElement: 通过反射调用setter Method赋值.

AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject源码:

@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
    
    
	Field field = (Field) this.member;
	Object value;
	if (this.cached) {
    
    
		try {
    
    
			value = resolvedCachedArgument(beanName, this.cachedFieldValue);
		}
		catch (NoSuchBeanDefinitionException ex) {
    
    
			// Unexpected removal of target bean for cached argument -> re-resolve
			value = resolveFieldValue(field, bean, beanName);
		}
	}else {
    
    
		value = resolveFieldValue(field, bean, beanName);
	}
	if (value != null) {
    
    
	    //反射赋值
		ReflectionUtils.makeAccessible(field);
		field.set(bean, value);
	}
}

AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#resolveFieldValue源码：

private Object resolveFieldValue(Field field, Object bean, @Nullable String beanName) {
    
    
	DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
	desc.setContainingClass(bean.getClass());
	Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
	Assert.state(beanFactory != null, "No BeanFactory available");
	TypeConverter typeConverter = beanFactory.getTypeConverter();
	Object value;
	try {
    
    
		value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
	} catch (BeansException ex) {
    
    
		throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
	}
	synchronized (this) {
    
    
		if (!this.cached) {
    
    
			Object cachedFieldValue = null;
			if (value != null || this.required) {
    
    
				cachedFieldValue = desc;
				registerDependentBeans(beanName, autowiredBeanNames);
				if (autowiredBeanNames.size() == 1) {
    
    
					String autowiredBeanName = autowiredBeanNames.iterator().next();
					if (beanFactory.containsBean(autowiredBeanName) &&
							beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
    
    
						cachedFieldValue = new ShortcutDependencyDescriptor(
								desc, autowiredBeanName, field.getType());
					}
				}
			}
			this.cachedFieldValue = cachedFieldValue;
			this.cached = true;
		}
	}
	return value;
}

方法内部调用的beanFactory.resolveDependency（默认DefaultListableBeanFactory）整体逻辑大概如下：

判断Filed或者Method上是否有@Lazy注解，如果存在则创建代理，实现懒加载。
不是懒加载，则判断是否存在@Value注解，如果存在则获取value值，最后通过PropertySourcesPropertyResolver#getProperty(java.lang.String, java.lang.Class<T>, boolean)解析属性值（其实就是ConfigurableEnvironment中封装的MutablePropertySources集合），获取到属性值之后还会通过SpringEL进行一次解析。解析之后再由SimpleTypeConverter对象进行一次数据类型转换，转为Filed所需要的类型返回.
如果不存在@Value注解，则是根据@Autowire注入bean, 从DefaultListableBeanFactory中变量所有的beanName，判断对象类型是否和所需的相同，相同则保存beanName到List集合中(一个相同的接口可能出现多个实现类，所以用集合)，
- 遍历beanNames，判断每个bean是否符合要求，如果存在@Qualifier会判断名称是否匹配，匹配则获取对应bean对象。如果没有@Qualifier注解，则获取全部的bean对象，封装到Map中。
- 如果Map中存在多个实例对象，再判断查找带有@Primary注解的实现类，返回对象。同类型实现@Primary只能有一个
- 如果Map中存在多个实例对象，但是没有@Primary注解，再比较是否有@javax.annotation.Priority注解，选择优先高的返回，值越小优先级越高。
- 根据名称匹配
- 不符合以上规则，则抛出异常信息，出现多个bean实现。

Spring中依赖注入Bean的匹配逻辑如下
DefaultListableBeanFactory#determineAutowireCandidate

protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {
    
    
	Class<?> requiredType = descriptor.getDependencyType();
	String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);
	if (primaryCandidate != null) {
    
    
		return primaryCandidate;
	}
	String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);
	if (priorityCandidate != null) {
    
    
		return priorityCandidate;
	}
	// Fallback
	for (Map.Entry<String, Object> entry : candidates.entrySet()) {
    
    
		String candidateName = entry.getKey();
		Object beanInstance = entry.getValue();
		if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||
				matchesBeanName(candidateName, descriptor.getDependencyName())) {
    
    
			return candidateName;
		}
	}
	return null;
}