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！
