深入解析 Spring Framework 中 @Autowired 注解的实现原理

摘要

关于@Autowired注解的作用

@Autowired 注解在Spring中的作用是实现依赖注入(Dependency Injection),它用于自动装配(autowiring)Spring Bean 的依赖关系。具体来说, @Autowired 注解有以下作用:

  1. 自动装配依赖 :通过在类的字段、构造函数、方法参数等地方使用 @Autowired 注解,Spring 容器会自动识别需要注入的依赖,并将适当的 Bean 实例注入到目标组件中。
  2. 减少手动配置 :使用 @Autowired 注解可以减少手动配置依赖关系的工作,因为它会自动发现并管理组件之间的依赖关系,从而降低了配置的复杂性。
  3. 提高可维护性@Autowired 注解明确地标识了类的依赖关系,使代码更易于理解和维护,因为它清晰地表达了组件之间的关联。
  4. 解耦:通过将依赖项的注入交给 Spring 容器处理,实现了松散耦合,使组件更容易替换、扩展和测试,同时降低了组件之间的耦合度。
  5. 支持多种装配模式@Autowired 提供了多种装配模式,包括按类型、按名称、按限定符(qualifier)等方式,以满足不同的装配需求。

Spring Framework与@Autowired

在Spring Framework框架中最重要的概念是IoC和DI,通过这两个特性可以实现对象间的依赖关系由框架管理,构造对象间的依赖关系,将依赖对象自动注入到需要它们的类中,在使用时无需手动创建或查找依赖对象,注入依赖关系主要有以下方法:

  • 通过xml配置注入
  • 通过@Autowired等注解注入
  • 当前Spring Framework推荐的通过构造方法注入

无论何种注入方法,Spring都会获取该Bean配置的元数据(Bean定义和依赖关系),那么接下来我将从源码层面分析@Autowired注入依赖的过程。

Bean创建过程

doCreateBean-创建Bean实例

less 复制代码
	/**
	 * Central method of this class: creates a bean instance,
	 * populates the bean instance, applies post-processors, etc.
	 * @see #doCreateBean
	 */
	@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

    	//省略其他代码,展示主要流程
		try {
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isTraceEnabled()) {
				logger.trace("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
			// A previously detected exception with proper bean creation context already,
			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

创建Bean方法的主要逻辑在doCreateBean中,Spring Framework通过doCreateBean创建指定Bean,在该方法中,其中通过populateBean()遍历对应后置处理器,即:一个被注解标注的类被注入到Spring容器时,首先会创建Bean对象,创建后调用populateBean方法以遍历后置处理器通过后置处理器获取到需要的value,将@Autowired注解中的属性(元数据)赋值到Bean中。

populateBean-遍历后置处理器

less 复制代码
  	// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
    //InstantiationAwareBeanPostProcessors可以为@Autowired注解提供后置处理,
		// state of the bean before properties are set. This can be used, for example,
		// to support styles of field injection.
		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      //遍历所有相关后置处理器,获取需要的value
			for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
				if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
					return;
				}
			}
		}
	}

@Autowired注解所需的后置处理器是:AutowiredAnnotationBeanPostProcessor。接下来Spring容器开始使用该注解的后置处理器去获取对应的属性value,假设我们不知道@Autowired注解对应后置处理器的逻辑,那么根据这个需求来猜测后置处理器中的相关逻辑的方法名:需要带有处理、属性,那么对应的单词就是:Process、Properties,对应找一下,postProcessProperties()便是目标方法。

typescript 复制代码
@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    //根据BeanName获取注入的元数据
    InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
    try {
        //元数据Value注入目标Bean中
        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;
}

获取注入的元数据

arduino 复制代码
//用于缓存Spring解析过的Bean元数据
private final Map<String, InjectionMetadata> injectionMetadataCache = new ConcurrentHashMap<>(256);
kotlin 复制代码
	private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
		// Fall back to class name as cache key, for backwards compatibility with custom callers.
		String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
		// 查询缓存中是否存在对应元数据-Quick check on the concurrent map first, with minimal locking.
		InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
		if (InjectionMetadata.needsRefresh(metadata, clazz)) {
			synchronized (this.injectionMetadataCache) {
				metadata = this.injectionMetadataCache.get(cacheKey);
				if (InjectionMetadata.needsRefresh(metadata, clazz)) {
					if (metadata != null) {
						metadata.clear(pvs);
					}
                    //当缓存中不存在指定Bean的MetaData时,构建MetaData
					metadata = buildAutowiringMetadata(clazz);
					this.injectionMetadataCache.put(cacheKey, metadata);
				}
			}
		}
		return metadata;
	}

如何构造Metadata

Spring Framework通过buildAutowiringMetadata()方法解析注解中的数据。

java 复制代码
//需要解析的注解结合
private final Set<Class<? extends Annotation>> autowiredAnnotationTypes = new LinkedHashSet<>(4);


if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
	return InjectionMetadata.EMPTY;
}
kotlin 复制代码
/**
	 * Create a new {@code AutowiredAnnotationBeanPostProcessor} for Spring's
	 * standard {@link Autowired @Autowired} and {@link Value @Value} annotations.
	 * <p>Also supports JSR-330's {@link javax.inject.Inject @Inject} annotation,
	 * if available.
     * 为 Spring 的标准@Autowired和@Value注释创建一个新的AutowiredAnnotationBeanPostProcessor
	 */
	@SuppressWarnings("unchecked")
	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.
		}
	}

这段代码首先会调用 isCandidateClass 方法判断当前类是否为一个候选类,判断的依据就是 类、属性、方法上是否包含autowiredAnnotationTypes 集合中初始化的值(@Autowired@Value@Inject),当Bean的定义中包含集合中对应类型的注解时,被判定为候选类,再去获取该类对应注解中的元数据。

如果Bean中没有没有指定类型的注解时,返回一个空的元数据注入对象。如果有指定注解,则开始获取注解中的元数据。

获取元数据的方式,是通过反射实现的。以下是通过反射获取类、属性、方法中对应注解的逻辑。

以DruidDataSourceWrapper为例:

通过反射获取指定类的字段中的属性

ini 复制代码
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
		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<>();

            //通过反射获取指定类的字段中的属性
			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));
				}
			});
	}

以DruidDataSource为例

通过反射获取指定类的方法中的属性

ini 复制代码
         //通过反射获取指定类的方法中的属性
			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));
				}
			});

元数据注入

从缓存中获取值

AutowiredAnnotationBeanPostProcessor类中的inject方法用来注入元数据。

该方法会首先从缓存中获取元数据,如果缓存中没有,则执行resolvedCachedArgument解析字段值。

less 复制代码
		@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) {
				ReflectionUtils.makeAccessible(field);
				field.set(bean, value);
			}
		}

解析字段值

ini 复制代码
@Nullable
private ConfigurableListableBeanFactory beanFactory;

@Nullable
		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 {
                //解析核心方法
				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;
		}

DependencyDescriptor:表示和处理Bean之间的依赖关系。

resolveDependency方法是接口BeanFactory接口提供的,DefaultListableBeanFactory是BeanFactory的一个实现类。

resolveDependency方法用于解析和解决依赖关系,该方法的作用是根据给定的 DependencyDescriptor 对象,解析并返回不同类型的依赖对象,当前方法最终会走doResolveDependency

less 复制代码
@Override
@Nullable
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
                                @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

    descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
    if (Optional.class == descriptor.getDependencyType()) {
        return createOptionalDependency(descriptor, requestingBeanName);
    }
    else if (ObjectFactory.class == descriptor.getDependencyType() ||
             ObjectProvider.class == descriptor.getDependencyType()) {
        return new DependencyObjectProvider(descriptor, requestingBeanName);
    }
    else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
        return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
    }
    else {
        Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
            descriptor, requestingBeanName);
        if (result == null) {
            result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
        }
        return result;
    }
}

doResolveDependency方法

typescript 复制代码
			Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
			if (multipleBeans != null) {
				return multipleBeans;
			}

			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
			if (matchingBeans.isEmpty()) {
				if (isRequired(descriptor)) {
					raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
				}
				return null;
			}

			String autowiredBeanName;
			Object instanceCandidate;

			if (matchingBeans.size() > 1) {
				autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
				if (autowiredBeanName == null) {
					if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
						return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
					}
					else {
						// In case of an optional Collection/Map, silently ignore a non-unique case:
						// possibly it was meant to be an empty collection of multiple regular beans
						// (before 4.3 in particular when we didn't even look for collection beans).
						return null;
					}
				}
				instanceCandidate = matchingBeans.get(autowiredBeanName);
			}
			else {
				// We have exactly one match.
				Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
				autowiredBeanName = entry.getKey();
				instanceCandidate = entry.getValue();
			}

			if (autowiredBeanNames != null) {
				autowiredBeanNames.add(autowiredBeanName);
			}
			if (instanceCandidate instanceof Class) {
				instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
			}
			Object result = instanceCandidate;
			if (result instanceof NullBean) {
				if (isRequired(descriptor)) {
					raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
				}
				result = null;
			}
			if (!ClassUtils.isAssignableValue(type, result)) {
				throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
			}
			return result;
		}
		finally {
			ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
		}
  • resolveMultipleBean:见名知意, 解析多个Bean,在注入时,当前类中能有不同类型的Bean,如Bean、数组、集合、Map等,该方法针对不同类型的Bean查找返回。
  • findAutowireCandidates:查找满足条件的Bean,该方法查找出来的Bean可能有一个或多个。

以上,完成了满足条件的候选对象列表并注入。

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