spring中自动注入注解的实现方式

目录

• 类结构
• 获取注解
• 注入
• Field 注入
• Method 注入
• 总结

在前面的文章中，我们介绍过，基于注解的包扫描模式下，会默认注册一系列的后置处理器，其中，就包含一个 AutowiredAnnotationBeanPostProcessor，这个处理器默认就会处理 @Autowired 和 @Value 注解。

类结构

从图中可知，这是一个 MergedBeanDefinitionPostProcessor 实现类，所以会在每一个实例对象创建时，当实例化结束，还未提前暴露时，对实例对象进行处理。

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

获取注解

private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
	// 确定缓存 key
	String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
	// 从缓存获取 InjectionMetadata
	InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
	// metadata 为 null，python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);
				}
				// 创建 metadata
				metadata = buildAutowiringMetadata(clazz);
				// 放入缓存
				this.injectionMetadataCache.put(cacheKey, metadata);
			}
		}
	}
	return metadata;
}
private InjectionMetadata buildAutowiringMetadata(Class<?> clazz) {
	// autowiredAnnotationTypes 两种类型 @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<>();
		
		// 处理 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;
				}
				// 默认 true
				boolean required = determineRequiredStatus(ann);
				currElements.add(new AutowiredFieldElement(field, required));
			}
		});

		// 处理 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) {
		// 获取指定 type 对应的 TypeMappedAnnotation，存在，将对应的 TypeMappedAnnotation 返回
		MergedAnnotation<?> annotation = annotations.get(type);
		if (annotation.isPresent()) {
			return annotation;
		}
	}
	return null;
}

可以看到，获取当前类及其父类中 Field 或 Method 上定义的注解，此时封装为 TypeMappedAnnotations，接着获取指定注入注解类型对应的 TypeMappedAnnotation，存在，返回 TypeMappedAnnotation，从中获取注解属性 "required" 对应的值，默认 true，之后将 Field 或 Method 封装成 InjectionMetadata.InjectedElement，加入 elements 集合，最后包装成 InjectionMetadata，放入 AutowiredAnnotationBeanPostProcessor 中 injectionMetadataCache 缓存。

有一点要注意，就是 Method 注入时，虽然获取了 pd，但测试中 pd 为 null，也不影响注入，由此也说明注入的方法不一定必须是 setter 或 getter 方法。

注入

实例化完 bean 之后，进行属性填充，执行 AbstractAutowireCapableBeanFactory#populateBean，此时会http://www.devze.com调用 InstantiationAwareBeanPostProcessor#postProcessProperties，AutowiredAnnotationBeanPostProcessor 实现了这个方法。

@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android面已经介绍过了，此时会直接从缓存获取到 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) {
		OqInsfUNM	element.inject(target, beanName, pvs);
		}
	}
}

遍历，对每个 InjectedElement，调用 inject 方法。从上面介绍可知，InjectedElement 共两种，分别是针对 Field 的 AutowiredFieldElement，以及针对 Method 的 AutowiredMethodElement。下面分别介绍。

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");
	// 创建了一个 SimpleTypeConverter 作为 typeConverter
	TypeConverter typeConverter = beanFactory.getTypeConverter();
	Object value;
	try {
		// 解析依赖，对于基本类型，解析后经过转换返回包装类型
		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;
}

逻辑比较简单，解析出注入的值，接着反射注入值。

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 {
		// 解析参数
		arguments = resolveMethodArguments(method, bean, beanName);
	}
	if (arguments != null) {
		try {
			// 反射调用方法，注入属性
			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();
	// 解析每一个参数
	for (int i = 0; i < arguments.length; i++) {
		// 封装 MethodParameter
		MethodParameter methodParam = new MethodParameter(method, i);
		// 封装 DependencyDescriptor
		DependencyDescriptor currDesc = new DependencyDescriptor(methodParam, this.required);
		currDesc.setContainingClass(bean.getClass());
		descriptors[i] = currDesc;
		try {
			// 解析依赖，required 默认 true，遇见无法解析的直接抛出异常
			Object arg = beanFactory.resolveDependency(currDesc, beanName, autowiredBeans, typeConverter);
			if (arg == null && !this.required) {
				arguments = null;
				break;
			}
			// 构造参数
			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.cachedMethodArgumentOqInsfUNMs = null;
			}
			this.cached = true;
		}
	}
	return arguments;
}

可以看到，不管是 Field 注入还是 Method 注入，都是先封装出一个 DependencyDescriptor，接着执行 beanFactory.resolveDependency 进行依赖的解析，不同点在于 Field 解析完就返回了，而 Method 解析完是为了构造出参数数组。

之后，不管是 Field 注入，还是 Method 注入，都是利用反射完成注入。这也说明，Method 注入时，只要能正常的封装出参数数组，就能完成属性或字段注入。

总结

以上为个人经验，希望能给大家一个参考，也希望大家多多支持编程客栈(www.devze.com)。