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spring中自動(dòng)注入注解的實(shí)現(xiàn)方式

更新時(shí)間：2024年09月19日 09:18:15 作者：潭影空人心

在Spring框架中,AutowiredAnnotationBeanPostProcessor負(fù)責(zé)處理@Autowired和@Value注解,實(shí)現(xiàn)依賴注入,首先通過(guò)TypeMappedAnnotations獲取注解,并根據(jù)注解屬性構(gòu)建InjectionMetadata,存入緩存

在前面的文章中，我們介紹過(guò)，基于注解的包掃描模式下，會(huì)默認(rèn)注冊(cè)一系列的后置處理器，其中，就包含一個(gè) AutowiredAnnotationBeanPostProcessor，這個(gè)處理器默認(rèn)就會(huì)處理 @Autowired 和 @Value 注解。

類結(jié)構(gòu)

從圖中可知，這是一個(gè) MergedBeanDefinitionPostProcessor 實(shí)現(xiàn)類，所以會(huì)在每一個(gè)實(shí)例對(duì)象創(chuàng)建時(shí)，當(dāng)實(shí)例化結(jié)束，還未提前暴露時(shí)，對(duì)實(shí)例對(duì)象進(jìn)行處理。

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

獲取注解

private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
	// 確定緩存 key
	String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
	// 從緩存獲取 InjectionMetadata
	InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
	// metadata 為 null，表示需要刷新
	if (InjectionMetadata.needsRefresh(metadata, clazz)) {
		synchronized (this.injectionMetadataCache) {
			metadata = this.injectionMetadataCache.get(cacheKey);
			if (InjectionMetadata.needsRefresh(metadata, clazz)) {
				if (metadata != null) {
					metadata.clear(pvs);
				}
				// 創(chuàng)建 metadata
				metadata = buildAutowiringMetadata(clazz);
				// 放入緩存
				this.injectionMetadataCache.put(cacheKey, metadata);
			}
		}
	}
	return metadata;
}
private InjectionMetadata buildAutowiringMetadata(Class<?> clazz) {
	// autowiredAnnotationTypes 兩種類型 @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<>();
		
		// 處理 Field 上注解
		ReflectionUtils.doWithLocalFields(targetClass, field -> {
			MergedAnnotation<?> ann = findAutowiredAnnotation(field);
			if (ann != null) {
				// static 不支持注入
				if (Modifier.isStatic(field.getModifiers())) {
					if (logger.isInfoEnabled()) {
						logger.info("Autowired annotation is not supported on static fields: " + field);
					}
					return;
				}
				// 默認(rèn) true
				boolean required = determineRequiredStatus(ann);
				currElements.add(new AutowiredFieldElement(field, required));
			}
		});

		// 處理 Method 上注解
		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);
                // Method 比 Field 多了 PropertyDescriptor
				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);
}

@Nullable
private MergedAnnotation<?> findAutowiredAnnotation(AccessibleObject ao) {
	// TypeMappedAnnotations
	MergedAnnotations annotations = MergedAnnotations.from(ao);
	for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {
		// 獲取指定 type 對(duì)應(yīng)的 TypeMappedAnnotation，存在，將對(duì)應(yīng)的 TypeMappedAnnotation 返回
		MergedAnnotation<?> annotation = annotations.get(type);
		if (annotation.isPresent()) {
			return annotation;
		}
	}
	return null;
}

可以看到，獲取當(dāng)前類及其父類中 Field 或 Method 上定義的注解，此時(shí)封裝為 TypeMappedAnnotations，接著獲取指定注入注解類型對(duì)應(yīng)的 TypeMappedAnnotation，存在，返回 TypeMappedAnnotation，從中獲取注解屬性 "required" 對(duì)應(yīng)的值，默認(rèn) true，之后將 Field 或 Method 封裝成 InjectionMetadata.InjectedElement，加入 elements 集合，最后包裝成 InjectionMetadata，放入 AutowiredAnnotationBeanPostProcessor 中 injectionMetadataCache 緩存。

有一點(diǎn)要注意，就是 Method 注入時(shí)，雖然獲取了 pd，但測(cè)試中 pd 為 null，也不影響注入，由此也說(shuō)明注入的方法不一定必須是 setter 或 getter 方法。

注入

實(shí)例化完 bean 之后，進(jìn)行屬性填充，執(zhí)行 AbstractAutowireCapableBeanFactory#populateBean，此時(shí)會(huì)調(diào)用 InstantiationAwareBeanPostProcessor#postProcessProperties，AutowiredAnnotationBeanPostProcessor 實(shí)現(xiàn)了這個(gè)方法。

@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;
}

findAutowiringMetadata 前面已經(jīng)介紹過(guò)了，此時(shí)會(huì)直接從緩存獲取到 InjectionMetadata。

// InjectionMetadata
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) {
			element.inject(target, beanName, pvs);
		}
	}
}

遍歷，對(duì)每個(gè) InjectedElement，調(diào)用 inject 方法。從上面介紹可知，InjectedElement 共兩種，分別是針對(duì) Field 的 AutowiredFieldElement，以及針對(duì) Method 的 AutowiredMethodElement。下面分別介紹。

Field 注入

@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
	Field field = (Field) this.member;
	Object value;
	// false
	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) {
        // 反射注入值
		ReflectionUtils.makeAccessible(field);
		field.set(bean, value);
	}
}
@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");
	// 創(chuàng)建了一個(gè) SimpleTypeConverter 作為 typeConverter
	TypeConverter typeConverter = beanFactory.getTypeConverter();
	Object value;
	try {
		// 解析依賴，對(duì)于基本類型，解析后經(jīng)過(guò)轉(zhuǎn)換返回包裝類型
		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;
}

邏輯比較簡(jiǎn)單，解析出注入的值，接著反射注入值。

Method 注入

@Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
	if (checkPropertySkipping(pvs)) {
		return;
	}
	Method method = (Method) this.member;
	Object[] arguments;
	if (this.cached) {
		try {
			arguments = resolveCachedArguments(beanName);
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Unexpected removal of target bean for cached argument -> re-resolve
			arguments = resolveMethodArguments(method, bean, beanName);
		}
	}
	else {
		// 解析參數(shù)
		arguments = resolveMethodArguments(method, bean, beanName);
	}
	if (arguments != null) {
		try {
			// 反射調(diào)用方法，注入屬性
			ReflectionUtils.makeAccessible(method);
			method.invoke(bean, arguments);
		}
		catch (InvocationTargetException ex) {
			throw ex.getTargetException();
		}
	}
}
@Nullable
private Object[] resolveMethodArguments(Method method, Object bean, @Nullable String beanName) {
	int argumentCount = method.getParameterCount();
	Object[] arguments = new Object[argumentCount];
	DependencyDescriptor[] descriptors = new DependencyDescriptor[argumentCount];
	Set<String> autowiredBeans = new LinkedHashSet<>(argumentCount);
	Assert.state(beanFactory != null, "No BeanFactory available");
	TypeConverter typeConverter = beanFactory.getTypeConverter();
	// 解析每一個(gè)參數(shù)
	for (int i = 0; i < arguments.length; i++) {
		// 封裝 MethodParameter
		MethodParameter methodParam = new MethodParameter(method, i);
		// 封裝 DependencyDescriptor
		DependencyDescriptor currDesc = new DependencyDescriptor(methodParam, this.required);
		currDesc.setContainingClass(bean.getClass());
		descriptors[i] = currDesc;
		try {
			// 解析依賴，required 默認(rèn) true，遇見(jiàn)無(wú)法解析的直接拋出異常
			Object arg = beanFactory.resolveDependency(currDesc, beanName, autowiredBeans, typeConverter);
			if (arg == null && !this.required) {
				arguments = null;
				break;
			}
			// 構(gòu)造參數(shù)
			arguments[i] = arg;
		}
		catch (BeansException ex) {
			throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(methodParam), ex);
		}
	}
	synchronized (this) {
		if (!this.cached) {
			if (arguments != null) {
				DependencyDescriptor[] cachedMethodArguments = Arrays.copyOf(descriptors, arguments.length);
				registerDependentBeans(beanName, autowiredBeans);
				if (autowiredBeans.size() == argumentCount) {
					Iterator<String> it = autowiredBeans.iterator();
					Class<?>[] paramTypes = method.getParameterTypes();
					for (int i = 0; i < paramTypes.length; i++) {
						String autowiredBeanName = it.next();
						if (beanFactory.containsBean(autowiredBeanName) &&
								beanFactory.isTypeMatch(autowiredBeanName, paramTypes[i])) {
							cachedMethodArguments[i] = new ShortcutDependencyDescriptor(
									descriptors[i], autowiredBeanName, paramTypes[i]);
						}
					}
				}
				this.cachedMethodArguments = cachedMethodArguments;
			}
			else {
				this.cachedMethodArguments = null;
			}
			this.cached = true;
		}
	}
	return arguments;
}

可以看到，不管是 Field 注入還是 Method 注入，都是先封裝出一個(gè) DependencyDescriptor，接著執(zhí)行 beanFactory.resolveDependency 進(jìn)行依賴的解析，不同點(diǎn)在于 Field 解析完就返回了，而 Method 解析完是為了構(gòu)造出參數(shù)數(shù)組。

之后，不管是 Field 注入，還是 Method 注入，都是利用反射完成注入。這也說(shuō)明，Method 注入時(shí)，只要能正常的封裝出參數(shù)數(shù)組，就能完成屬性或字段注入。