大家用过spring的肯定都用过AutoWired注解，但是你想过自动注入的原理吗，这次就来说下自动注入是怎么实现的。在之前的spring的ioc容器启动过程中，我们都知道,ioc容器的启动是从AbstractApplicationContext的refresh方法开始的,在ioc容器启动时会初始化加载的BeanPostProcessor,那么BeanPostProcessor是什么呢？BeanPostProcessor就是在bean初始化时操作的后置处理器，那么这和自动注入有什么关系呢？这就要从我们要说的AutowiredAnnotationBeanPostProcessor说起了。先来看下AutowiredAnnotationBeanPostProcessor的类的结构

AutowiredAnnotationBeanPostProcessor

AutowiredAnnotationBeanPostProcessor实现了BeanPostProcessor接口，当 Spring 容器启动时，AutowiredAnnotationBeanPostProcessor 将扫描 Spring 容器中所有 Bean，当发现 Bean 中拥有@Autowired 注解时就找到和其匹配（默认按类型匹配）的 Bean，并注入到对应的地方中去。先来看下buildAutowiringMetadata方法

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
    List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
    Class<?> targetClass = clazz;

    do {
        final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();

        ReflectionUtils.doWithLocalFields(targetClass, field -> {
            AnnotationAttributes 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));
            }
        });

        ReflectionUtils.doWithLocalMethods(targetClass, method -> {
            Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
            if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
                return;
            }
            AnnotationAttributes 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 new InjectionMetadata(clazz, elements);
}

buildAutowiringMetadata方法解析等待自动注入类的所有属性。它通过分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata类的实例来实现。InjectionMetadata类包含要注入的元素的列表。注入是通过Java的 Reflection Field set方法或Method invoke方法完成的。此过程直接在AutowiredAnnotationBeanPostProcessor的方法中调用public void processInjection(Object bean) throws BeanCreationException。它将所有可注入的bean检索为InjectionMetadata实例，并调用它们的inject()方法。

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) {
            if (logger.isTraceEnabled()) {
                logger.trace("Processing injected element of bean '" + beanName + "': " + element);
            }
            element.inject(target, beanName, pvs);
        }
    }
}

InjectedElement的inject方法

protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
            throws Throwable {

        if (this.isField) {
            Field field = (Field) this.member;
            ReflectionUtils.makeAccessible(field);
            field.set(target, getResourceToInject(target, requestingBeanName));
        }
        else {
            if (checkPropertySkipping(pvs)) {
                return;
            }
            try {
                Method method = (Method) this.member;
                ReflectionUtils.makeAccessible(method);
                method.invoke(target, getResourceToInject(target, requestingBeanName));
            }
            catch (InvocationTargetException ex) {
                throw ex.getTargetException();
            }
        }
    }

AutowiredAnnotationBeanPostProcessor类中的另一个重要方法是findAutowiredAnnotation。它通过分析属于一个字段或一个方法的所有注解来查找@Autowired注解。如果未找到@Autowired注解，则返回null，字段或方法也就视为不可注入。

@Nullable
private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao) {
    if (ao.getAnnotations().length > 0) {  // autowiring annotations have to be local
        for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) {
            AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type);
            if (attributes != null) {
                return attributes;
            }
        }
    }
    return null;
}

至此，spring的自动注入就完成了。
AutowiredAnnotationBeanPostProcessor的分析就到这里了。