@Autowired注解实现原理

在讨论代码细节之前，我们再来了解下基础知识。Spring管理可用于整个应用程序的Java对象bean。他们所在的Spring容器，被称为应用程序上下文。这意味着我们不需要处理他们的生命周期(初始化，销毁)。该任务由此容器来完成。另外，该上下文具有入口点，在Web应用程序中，是dispatcher servlet。容器(也就是该上下文)会在它那里被启动并且所有的bean都会被注入。

说的再清楚点，请看<context:annotation-config />的定义:

<xsd:element name="annotation-config"> <xsd:annotation> <xsd:documentation><![CDATA[ Activates various annotations to be detected in bean classes: Spring's @Required and @Autowired, as well as JSR 250's @PostConstruct, @PreDestroy and @Resource (if available), JAX-WS's @WebServiceRef (if available), EJB 3's @EJB (if available), and JPA's @PersistenceContext and @PersistenceUnit (if available). Alternatively, you may choose to activate the individual BeanPostProcessors for those annotations. Note: This tag does not activate processing of Spring's @Transactional or EJB 3's @TransactionAttribute annotation. Consider the use of the <tx:annotation-driven> tag for that purpose. See javadoc for org.springframework.context.annotation.AnnotationConfigApplicationContext for information on code-based alternatives to bootstrapping annotation-driven support. ]]></xsd:documentation> </xsd:annotation> </xsd:element> 可以看出 : 类内部的注解，如：@Autowired、@Value、@Required、@Resource以及EJB和WebSerivce相关的注解，是容器对Bean对象实例化和依赖注入时，通过容器中注册的Bean后置处理器处理这些注解的。

所以配置了上面这个配置(<context:component-scan>假如有配置这个，那么就可以省略<context:annotation-config />)后，将隐式地向Spring容器注册AutowiredAnnotationBeanPostProcessor、CommonAnnotationBeanPostProcessor、RequiredAnnotationBeanPostProcessor、PersistenceAnnotationBeanPostProcessor以及这4个专门用于处理注解的Bean后置处理器。

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。 源码分析如下:

通过org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor可以实现依赖自动注入。通过这个类来处理@Autowired和@Value这俩Spring注解。它也可以管理JSR-303的@Inject注解(如果可用的话)。在AutowiredAnnotationBeanPostProcessor构造函数中定义要处理的注解:

public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware { ... /** * Create a new AutowiredAnnotationBeanPostProcessor * for Spring's standard {@link Autowired} annotation. * <p>Also supports JSR-330's {@link javax.inject.Inject} annotation, if available. */ @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.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring"); } catch (ClassNotFoundException ex) { // JSR-330 API not available - simply skip. } } ... } 之后，有几种方法来对@Autowired注解进行处理。

第一个，private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz)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。

private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) { LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<>(); Class<?> targetClass = clazz; do { final LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<>(); //分析所有字段 ReflectionUtils.doWithLocalFields(targetClass, field -> { //findAutowiredAnnotation(field)此方法后面会解释 AnnotationAttributes ann = findAutowiredAnnotation(field); if (ann != null) { if (Modifier.isStatic(field.getModifiers())) { if (logger.isWarnEnabled()) { logger.warn("Autowired annotation is not supported on static fields: " + field); } return; } boolean required = determineRequiredStatus(ann); currElements.add(new AutowiredFieldElement(field, required)); } }); //分析所有方法 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.isWarnEnabled()) { logger.warn("Autowired annotation is not supported on static methods: " + method); } return; } if (method.getParameterCount() == 0) { if (logger.isWarnEnabled()) { logger.warn("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); //返回一个InjectionMetadata初始化的对象实例 return new InjectionMetadata(clazz, elements); } ... /** * 'Native' processing method for direct calls with an arbitrary target instance, * resolving all of its fields and methods which are annotated with {@code @Autowired}. * @param bean the target instance to process * @throws BeanCreationException if autowiring failed */ public void processInjection(Object bean) throws BeanCreationException { Class<?> clazz = bean.getClass(); InjectionMetadata metadata = findAutowiringMetadata(clazz.getName(), clazz, null); try { metadata.inject(bean, null, null); } catch (BeanCreationException ex) { throw ex; } catch (Throwable ex) { throw new BeanCreationException( "Injection of autowired dependencies failed for class [" + clazz + "]", ex); } } InjectionMetadata类包含要注入的元素的列表。注入是通过Java的API Reflection (Field set(Object obj, Object value) 或Method invoke(Object obj，Object ... args)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 class InjectionMetadata { ... 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()) { boolean debug = logger.isDebugEnabled(); for (InjectedElement element : elementsToIterate) { if (debug) { logger.debug("Processing injected element of bean '" + beanName + "': " + element); } //看下面静态内部类的方法 element.inject(target, beanName, pvs); } } } ... public static abstract class InjectedElement { protected final Member member; protected final boolean isField; ... /** * 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 { //具体的注入看此处咯 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private AnnotationAttributes findAutowiredAnnotation(AccessibleObject ao)。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) { for (Class<? extends Annotation> type : this.autowiredAnnotationTypes) { AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ao, type); if (attributes != null) { return attributes; } } } return null; } 在上面的文章中，我们看到了Spring中自动注入过程。通过整篇文章可以看到，这种依赖注入是一种便捷易操作方式(可以在字段以及方法上完成)，也促使我们逐渐在抛弃XML配置文件。还增强了代码的可读性。