spring源码-依赖注入@Autowired

依赖注入的实现由后置处理器AutowiredAnnotationBeanPostProcessor实现

类图

实现了InstantiationAwareBeanPostProcessor ，MergedBeanDefinitionPostProcessor接口，如果对这两个接口不熟悉的可以阅读上篇文章后置处理器，实现这两个接口就可以介入到Bean的实例化前后和bean的元数据信息解析

依赖注入过程：

第一步、解析注解信息

入口方法：

postProcessMergedBeanDefinition该方法的作用其实比较简单，就是为了缓存下后续需要用到数据，findAutowiringMetadata 这个方法是有缓存的
处理的，所以重复调用的话是直接从缓存获取数据，加快注入的效率。这个方法其实就是去查找被 Autowired、Value 注解的所有属性和方法，当然包
括了目标类的所有父类都会去查找，所以查找的过程其实是个递归的过程。找到所有的成员后，会创建即实例化 
InjectionMetadata 对象并缓存到injectionMetadataCache 。

核心代码：

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


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());
   // 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);
            }
            //解析所有@Autowired @Value注解的方法和变量
            metadata = buildAutowiringMetadata(clazz);
            //缓存起来
            this.injectionMetadataCache.put(cacheKey, metadata);
         }
      }
   }
   return metadata;
}


private InjectionMetadata buildAutowiringMetadata(final Class clazz) {
   if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
      return InjectionMetadata.EMPTY;
   }

   List elements = new ArrayList<>();
   Class targetClass = clazz;

   do {
      final List currElements = new ArrayList<>();
      //解析带Autowired和@Value的字段
      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));
         }
      });
        //解析带Autowired和@Value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);
}

第二步、注入

入口方法

postProcessProperties
该方法在bean实例化后被调用，属性值进行修改，如果postProcessAfterInstantiation方法返回false，该方法可能不会被调用。可以在该方法内对属性值进行修改

核心代码

@Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    //findAutowiringMetadata方法已经在上一步被执行了一次了，并且将注解的元信息已经放到了缓存里，所以这里就直接从缓存中拿到了。 
    //拿到注解元数据后，就开始执行inject方法了。
   InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
   try {
       //依赖注入的核心逻辑
      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;
}

 拿到注解元数据后，就开始执行inject方法进行注入

 public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
   Collection checkedElements = this.checkedElements;
   Collection elementsToIterate =
         (checkedElements != null ? checkedElements : this.injectedElements);
   if (!elementsToIterate.isEmpty()) {
       //遍历所有元数据信息
      for (InjectedElement element : elementsToIterate) {
         if (logger.isTraceEnabled()) {
            logger.trace("Processing injected element of bean '" + beanName + "': " + element);
         }
         //执行依赖注入逻辑
         element.inject(target, beanName, pvs);
      }
   }
}

核心在DefaultListableBeanFactory#doResolveDependency

// 此处autowiredBeanNames是在inject的一个空的Set
// autowired表示最终可以注入进去的bean名称们（因为可能是会有多个符合条件的）
	public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
...
		@Nullable
		public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
										  @Nullable Set autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {

// 记录注入点，其实使用的ThreadLocal~
			InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
			try {
// 只有ShortcutDependencyDescriptor实现了resolveShortcut方法，其实就是根据
// 实现也就一句话：beanFactory.getBean(this.shortcut, this.requiredType)
// ShortcutDependencyDescriptor实在inject完成后创建的，就是有缓存的效果~~~
				Object shortcut = descriptor.resolveShortcut(this);
				if (shortcut != null) {
					return shortcut;
				}

// interface com.fsx.dependency.AInterface
				Class type = descriptor.getDependencyType();

// 拿到@Value注解的value值（是个字符串）  若没有标注@Value  显然就不用那啥了
// 从此处其实也可以看出，@Value注解的优先级对于找到bean来说还是蛮高的
				Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);


// ============ 这部分主要是解析@Value注解
// 解析它的占位符，解析它的SpEL表达式
// 相关处理类曹靠BeanExpressionResolver和StandardBeanExpressionResolver  StandardEvaluationContext等
// 因为关于@Value的文章里详细解过，此处一笔带过~~~
				if (value != null) {
					if (value instanceof String) {
						String strVal = resolveEmbeddedValue((String) value);
						BeanDefinition bd = (beanName != null && containsBean(beanName) ?
								getMergedBeanDefinition(beanName) : null);
						value = evaluateBeanDefinitionString(strVal, bd);
					}
					TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
					try {
						return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
					} catch (UnsupportedOperationException ex) {
// A custom TypeConverter which does not support TypeDescriptor resolution...
						return (descriptor.getField() != null ?
								converter.convertIfNecessary(value, type, descriptor.getField()) :
								converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
					}
				}
// ============


// 此处处理的是多值注入的情况，比如注入Stream