Spring註解@Resource源碼分析

CommonAnnotationBeanPostProcessor是Spring中用於處理JavaEE5中經常使用註解(主要是EJB相關的註解)和Java6中關於JAX-WS相關的註解，能夠處理@PostConstruct、@PreDestroy等Bean生命週期相關事件的註解，該後置處理最核心的是處理@Resource註解，同時還能夠處理JAX-WS相關的註解。

1、觸發方式

• Spring容器在每一個Bean實例化以後，調用後置處理器CommonAnnotationBeanPostProcessor的postProcessMergedBeanDefinition方法，查找該Bean是否有@Resource註解。
• Spring在每一個Bean實例化的時候，調用populateBean進行屬性注入的時候，即調用postProcessPropertyValues方法，查找該Bean是否有@Resource註解。

2、構造函數

//CommonAnnotationBeanPostProcessor.java

//構造方法
public CommonAnnotationBeanPostProcessor() {
	setOrder(Ordered.LOWEST_PRECEDENCE - 3);
	//設置初始的註解類型爲@PostConstruct
	setInitAnnotationType(PostConstruct.class);
	//設置銷燬的註解爲@ PreDestroy
	setDestroyAnnotationType(PreDestroy.class);
	//當使用@Resource註解時，忽略JAX-WS的資源類型
	ignoreResourceType("javax.xml.ws.WebServiceContext");
	}

3、注入

//CommonAnnotationBeanPostProcessor.java

public PropertyValues postProcessPropertyValues(
		PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {

	//獲取@Resource註解中配置的屬性值元數據
	InjectionMetadata metadata = findResourceMetadata(beanName, bean.getClass(), pvs);
	try {
		//注入屬性值，與AutowiredAnnotationBeanPostProcessor中處理相同
		metadata.inject(bean, beanName, pvs);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);
	}
	return pvs;
	}

繼續追蹤，看metadata.inject(bean, beanName, pvs)方法

//InjectionMetadata.java

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
	Collection<InjectedElement> checkedElements = this.checkedElements;
	//要注入的字段集合
	Collection<InjectedElement> elementsToIterate =
			(checkedElements != null ? checkedElements : this.injectedElements);
	if (!elementsToIterate.isEmpty()) {
		boolean debug = logger.isDebugEnabled();
		//遍歷 注入
		for (InjectedElement element : elementsToIterate) {
			if (debug) {
				logger.debug("Processing injected element of bean '" + beanName + "': " + element);
			}
			element.inject(target, beanName, pvs);
		}
	}
	}

這裏和AutowiredAnnotationBeanPostProcessor不一樣的是，AutowiredAnnotationBeanPostProcessor調用的element.inject(target, beanName, pvs)方法是本身實現的，如圖：

而CommonAnnotationBeanPostProcessor調用的element.inject(target, beanName, pvs)是原始方法，以下：

//InjectionMetadata.java

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

	if (this.isField) {
		Field field = (Field) this.member;
		//強吻訪問
		ReflectionUtils.makeAccessible(field);
		//給字段賦值，即屬性注入
		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();
		}
	}
	}

這裏重點看getResourceToInject(target, requestingBeanName)方法，該方法的實現是具體獲取@Resource中的值的。 咱們能夠看到在CommonAnnotationBeanPostProcessor類中，對該方法有實現：

//CommonAnnotationBeanPostProcessor.java

protected Object getResourceToInject(Object target, @Nullable String requestingBeanName) {
	return (this.lazyLookup ? buildLazyResourceProxy(this,requestingBeanName) :
		getResource(this, requestingBeanName));
	}

lazyLookup是CommonAnnotationBeanPostProcessor內部類ResourceElement的一個成員變量，表示是否懶加載，默認是false。 咱們先來看下非懶加載的流程，即getResource(this, requestingBeanName):

//CommonAnnotationBeanPostProcessor.java

//根據給定名稱或者類型獲取資源對象
	protected Object getResource(LookupElement element, @Nullable String requestingBeanName) throws BeansException {
	//若是註解對象元素的mappedName屬性不爲空
	if (StringUtils.hasLength(element.mappedName)) {
		//根據JNDI名稱和類型去Spring的JNDI容器中獲取Bean
		return this.jndiFactory.getBean(element.mappedName, element.lookupType);
	}
	//若是該後置處理器的alwaysUseJndiLookup屬性值爲true
	if (this.alwaysUseJndiLookup) {
		//從Spring的JNDI容器中查找指定JDNI名稱和類型的Bean
		return this.jndiFactory.getBean(element.name, element.lookupType);
	}
	if (this.resourceFactory == null) {
		throw new NoSuchBeanDefinitionException(element.lookupType,
				"No resource factory configured - specify the 'resourceFactory' property");
	}
	//使用autowiring自動依賴注入裝配，經過給定的名稱和類型從資源容器獲取Bean對象
	//通常狀況下，都是走這一步
	return autowireResource(this.resourceFactory, element, requestingBeanName);
	}

autowireResource代碼：

//CommonAnnotationBeanPostProcessor.java

protected Object autowireResource(BeanFactory factory, LookupElement element, @Nullable String requestingBeanName)
		throws BeansException {

	Object resource;
	Set<String> autowiredBeanNames;
	String name = element.name;

	if (this.fallbackToDefaultTypeMatch && element.isDefaultName &&
			factory instanceof AutowireCapableBeanFactory && !factory.containsBean(name)) {
		autowiredBeanNames = new LinkedHashSet<>();
		//根據類型從Spring容器中查找資源
		//調用依賴解析器，跟@Autowired是一樣的代碼
		resource = ((AutowireCapableBeanFactory) factory).resolveDependency(
				element.getDependencyDescriptor(), requestingBeanName, autowiredBeanNames, null);
		if (resource == null) {
			throw new NoSuchBeanDefinitionException(element.getLookupType(), "No resolvable resource object");
		}
	}
	//根據名稱從Spring容器中查找資源
	else {
		resource = factory.getBean(name, element.lookupType);
		autowiredBeanNames = Collections.singleton(name);
	}

	//註冊Bean的依賴關係
	if (factory instanceof ConfigurableBeanFactory) {
		ConfigurableBeanFactory beanFactory = (ConfigurableBeanFactory) factory;
		for (String autowiredBeanName : autowiredBeanNames) {
			if (requestingBeanName != null && beanFactory.containsBean(autowiredBeanName)) {
				beanFactory.registerDependentBean(autowiredBeanName, requestingBeanName);
			}
		}
	}

	return resource;
	}

這裏的邏輯比較簡單：

• 首先判斷@Resource是按名稱來查詢仍是類型，若是是類型，則調用依賴解析器根據類型從Spring容器中查找（這裏和@Autowired的代碼同樣：Spring註解@Autowired源碼分析）
• 若是是按名稱，則直接調用BeanFactory的getBean()方法，根據BeanName從Spring容器中查找
• 最後因爲發生了依賴注入，須要重新註冊Bean的依賴關係

總結

• @Resource註解既能夠按照名稱來注入，也能夠按類型來注入。