spring中BeanPostProcessor之四：AutowiredAnnotationBeanPostProcessor（01）

在《spring中BeanPostProcessor之二：CommonAnnotationBeanPostProcessor（01）》中分析了CommonAnnotationBeanPostProcessor类中的postProcessMergedBeanDefinition方法的作用，即是对类中的@Resources、@WebServiceRef、@EJB三个注解进行解析并缓存起来，以便后续执行postProcessProperties方法的时候用到缓存的信息。在spring启动过程中还有一个和CommonAnnotationBeanPostProcessor类相似的bean后置处理器，该类就是AutowiredAnnotationBeanPostProcessor，该bean后置处理器的作用是处理@Autowired、@Value、@Inject注解。

一、概述

前面说到AutowiredAnnotationBeanPostProcessor类是解析@Autowired注解的，那么该注解的作用是什么那，是怎么定义的那

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {

    /**
     * Declares whether the annotated dependency is required.
     * <p>Defaults to {@code true}.
     */
    boolean required() default true;

}

上面是该注解的定义，该注解的作用是自动注入，该注解是Spring提供的注解（@Resource、@WebServiceRef、@EJB均是java提供的）。

AutowriedAnnotationBeanPostProcessor要解析@Autowired注解那么按照前面分析的CommonAnnotationBeanPostProcessor和InitDestroyBeanPostProcessor两个类，必须要先有要解析的类型，也就是初始化解析的类型，看AutoworedAnnotationBeanPostProcessor的构造方法，

@SuppressWarnings("unchecked")
    public AutowiredAnnotationBeanPostProcessor() {
        this.autowiredAnnotationTypes.add(Autowired.class);
        this.autowiredAnnotationTypes.add(Value.class);
        try {
            this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
                    ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
            logger.trace("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
        }
        catch (ClassNotFoundException ex) {
            // JSR-330 API not available - simply skip.
        }
    }

上面是其默认的构造方法，可以看到向autowiredAnnotationTypes中加了三个注解：@Autowired、@Value、@Inject，其中@Inject是java中的注解，其余两个均是Spring提供的注解。那么这里其实是说明AutowiredAnnotationBeanPostProcessor类是解析这三个注解的，我们继续往下分析。

二、详述

1、方法概述

下面看AutoWiredAnnotationBeanPostProcessor类中的方法，

上面给出了AutoWiredAnnotationBeanPostProcessor类中的主要的方法，重点看postProcessMergedBeanDefinition和postProcessProperties方法，postProcessPropertyValue方法在前边已经说过该方法已经废弃了，调用的是postProcessProperties方法。

2、postProcessMergedBeanDefinition

该方法的作用是解析类中标记了@AutoWired、@Value、@Inject注解的属性和方法，下面看具体的方法定义，

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

2.1、findAutowiringMetadata

下面看findAutowiringMetadata方法

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.
         //从缓存中获得该类的信息

        InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
        //判断是否需要刷新缓存
        if (InjectionMetadata.needsRefresh(metadata, clazz)) {
            synchronized (this.injectionMetadataCache) {
                metadata = this.injectionMetadataCache.get(cacheKey);
                if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                    if (metadata != null) {
                        metadata.clear(pvs);
                    }
                    //查找目标类clazz中的标识了@AutoWired、@Value、@Inejct注解
                    metadata = buildAutowiringMetadata(clazz);
                    this.injectionMetadataCache.put(cacheKey, metadata);
                }
            }
        }
        return metadata;
    }

从上面的代码中可以看出最终是把标识了@AutoWired、@Value、@Inject注解的方法或字段信息封装为Metadata然后放在了injectionMetadataCache中，重点看buildAutowiringMetadata方法，

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
        /**autowiredAnnotationTypes中得值为
         * @AutoWired org.springframework.beans.factory.annotation.Autowired
         * @Value org.springframework.beans.factory.annotation.Value
         * @Inject javax.inject.Inject
         *
         */

        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<>();

            //判断字段上是否存在autowiredAnnotationTypes中的注解
            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));
                }
            });

            //判断方法上是否存在autowiredAnnotationTypes中的注解
            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);
                    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);
    }

该代码比较多，主要是对目标类中的方法和字段信息进行了检查，判断是否标注了@AutoWired、@Value、@Inject三个注解，最后返回InjectionMetadata.forElements该方法是做什么的那，由于前边生成的对象是elements的对象，看elements对象的类型，

List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();

是一个ArrayList类型，泛型是InjectionMetadata.InejctedElement，下面看该方法，

public static InjectionMetadata forElements(Collection<InjectedElement> elements, Class<?> clazz) {
        return (elements.isEmpty() ? InjectionMetadata.EMPTY : new InjectionMetadata(clazz, elements));
    }

从上面的代码可以看到，最终是new了一个InjectionMetadata的实例。最终buildAutowiringMetadata方法返回的是一个InjectionMetadata的实例。

buildAutowiringMetadata方法执行完以后，在findAutowiringMetadata方法中，把返回的InjectionMetadata实例放入了injectionMetadataCache中，最终返回一个InjectionMetadata实例。

2.2、checkConfigMembers

在postProcessMergedBeanDefinition方法中的第二行代码即调用了checkConfigMembers方法

metadata.checkConfigMembers(beanDefinition);

该方法的定义如下，

public void checkConfigMembers(RootBeanDefinition beanDefinition) {
        Set<InjectedElement> checkedElements = new LinkedHashSet<>(this.injectedElements.size());
        for (InjectedElement element : this.injectedElements) {
            Member member = element.getMember();
            if (!beanDefinition.isExternallyManagedConfigMember(member)) {
                beanDefinition.registerExternallyManagedConfigMember(member);
                checkedElements.add(element);
                if (logger.isTraceEnabled()) {
                    logger.trace("Registered injected element on class [" + this.targetClass.getName() + "]: " + element);
                }
            }
        }
        this.checkedElements = checkedElements;
    }

从上面的代码中可以看出首先new了一个checkedElements，使用的是injectedElements的长度，injectedElements是什么那，怎么赋值的那。在buildAutowiringMetadata方法的最后调用了InectionMetadata.forElements方法，在该方法中最后生成了一个InjectionMetadata对象，传入了elements对象，便是该对象。

checkConfiMembers方法主要是调用了beanDefintion.isExternallyManagedConfigMemgber方法，该方法暂时不讨论。

3、postProcessProperties

上面分析了postProcessMergedBeanDefinition方法后，我们知道该方法的作用就是收集类中的标注了@AutoWired、@Value、@Inject注解的字段或方法，那么postProcessProperties方法便是在进行赋值的时候调用的，

//完成@AutoWired属性的注入
    @Override
    public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
        //再次获取了类中标注了@AutoWired、@Value、@Inject属性的信息
        InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
        try {
            //属性注入
            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;
    }

在该方法中又一次获取了类中标注了@AutoWired、@Value、@Inject属性的信息，重点是下面这行代码，

//属性注入
            metadata.inject(bean, beanName, pvs);

下面看inject方法，

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
        Collection<InjectedElement> checkedElements = this.checkedElements;
        //如果checkedElements为空，则取injectedElements
        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);
                }
                //进行属性注入
                element.inject(target, beanName, pvs);
            }
        }
    }

上面在进行属性注入的时候调用了element.inject方法，该方法在InjectionMetadata.InjectedElement类中，进去该方法看到如下，

/**
         * Either this or {@link #getResourceToInject} needs to be overridden.
         */
        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();
                }
            }
        }

看这方法的注释，意思是需要覆盖该方法，也就是说这里调用的不是上面的inject方法，在回到下面的代码

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
        Collection<InjectedElement> checkedElements = this.checkedElements;
        //如果checkedElements为空，则取injectedElements
        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);
                }
                //进行属性注入
                element.inject(target, beanName, pvs);
            }
        }
    }

遍历elementsToIterate，通过调试我们看到这里的element是下面的类型

再来看AutowiredFieldElement是什么，

AutowiredFieldElement是AutowiredAnnotationBeanPostProcessor中的内部类，继承了InjectionMetadata.InjectedElement类，上面提到了该方法，那么该类中肯定覆盖了inject方法

@Override
        protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
            Field field = (Field) this.member;
            Object value;
            if (this.cached) {
                value = resolvedCachedArgument(beanName, this.cachedFieldValue);
            }
            else {
                DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
                desc.setContainingClass(bean.getClass());
                Set<String> autowiredBeanNames = new LinkedHashSet<>(1);
                Assert.state(beanFactory != null, "No BeanFactory available");
                TypeConverter typeConverter = beanFactory.getTypeConverter();
                try {
                    value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
                }
                catch (BeansException ex) {
                    throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
                }
                synchronized (this) {
                    if (!this.cached) {
                        if (value != null || this.required) {
                            this.cachedFieldValue = desc;
                            registerDependentBeans(beanName, autowiredBeanNames);
                            if (autowiredBeanNames.size() == 1) {
                                String autowiredBeanName = autowiredBeanNames.iterator().next();
                                if (beanFactory.containsBean(autowiredBeanName) &&
                                        beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {
                                    this.cachedFieldValue = new ShortcutDependencyDescriptor(
                                            desc, autowiredBeanName, field.getType());
                                }
                            }
                        }
                        else {
                            this.cachedFieldValue = null;
                        }
                        this.cached = true;
                    }
                }
            }
            if (value != null) {
                ReflectionUtils.makeAccessible(field);
                field.set(bean, value);
            }
        }
    }

此方法的具体执行逻辑暂时不说。

三、使用场景

上面大体分析了AutowiredAnnotationBeanPostProcessor类中的postProcessMergedBeanDefinition和postProcessProperties两个方法，了解了其作用及执行时机，文中遗留的问题后面会继续分析。

有不当之处，欢迎指正，感谢！