大家用過spring的肯定都用過AutoWired注解，但是你想過自動注入的原理嗎，這次就來說下自動注入是怎么實現(xiàn)的。在之前的spring的ioc容器啟動過程中，我們都知道,ioc容器的啟動是從AbstractApplicationContext的refresh方法開始的,在ioc容器啟動時會初始化加載的BeanPostProcessor,那么BeanPostProcessor是什么呢？BeanPostProcessor就是在bean初始化時操作的后置處理器，那么這和自動注入有什么關(guān)系呢？這就要從我們要說的AutowiredAnnotationBeanPostProcessor說起了。先來看下AutowiredAnnotationBeanPostProcessor的類的結(jié)構(gòu)

AutowiredAnnotationBeanPostProcessor

AutowiredAnnotationBeanPostProcessor實現(xiàn)了BeanPostProcessor接口，當(dāng) Spring 容器啟動時，AutowiredAnnotationBeanPostProcessor 將掃描 Spring 容器中所有 Bean，當(dāng)發(fā)現(xiàn) Bean 中擁有@Autowired 注解時就找到和其匹配（默認(rèn)按類型匹配）的 Bean，并注入到對應(yīng)的地方中去。先來看下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方法解析等待自動注入類的所有屬性。它通過分析所有字段和方法并初始化org.springframework.beans.factory.annotation.InjectionMetadata類的實例來實現(xiàn)。InjectionMetadata類包含要注入的元素的列表。注入是通過Java的 Reflection Field set方法或Method invoke方法完成的。此過程直接在AutowiredAnnotationBeanPostProcessor的方法中調(diào)用public void processInjection(Object bean) throws BeanCreationException。它將所有可注入的bean檢索為InjectionMetadata實例，并調(diào)用它們的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類中的另一個重要方法是findAutowiredAnnotation。它通過分析屬于一個字段或一個方法的所有注解來查找@Autowired注解。如果未找到@Autowired注解，則返回null，字段或方法也就視為不可注入。

@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的自動注入就完成了。
AutowiredAnnotationBeanPostProcessor的分析就到這里了。