Spring注解@Resource源码分析

一、触发方式

• Spring容器在每个Bean实例化之后，调用后置处理器CommonAnnotationBeanPostProcessor的postProcessMergedBeanDefinition方法，查找该Bean是否有@Resource注解。
• Spring在每个Bean实例化的时候，调用populateBean进行属性注入的时候，即调用postProcessPropertyValues方法，查找该Bean是否有@Resource注解。

二、构造函数

//CommonAnnotationBeanPostProcessor.java

//构造方法
public CommonAnnotationBeanPostProcessor() {
	setOrder(Ordered.LOWEST_PRECEDENCE - 3);
	//设置初始的注解类型为@PostConstruct
	setInitAnnotationType(PostConstruct.class);
	//设置销毁的注解为@ PreDestroy
	setDestroyAnnotationType(PreDestroy.class);
	//当使用@Resource注解时，忽略JAX-WS的资源类型
	ignoreResourceType("javax.xml.ws.WebServiceContext");
	}

三、注入

//CommonAnnotationBeanPostProcessor.java

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

	//获取@Resource注解中配置的属性值元数据
	InjectionMetadata metadata = findResourceMetadata(beanName, bean.getClass(), pvs);
	try {
		//注入属性值，与AutowiredAnnotationBeanPostProcessor中处理相同
		metadata.inject(bean, beanName, pvs);
	}
	catch (Throwable ex) {
		throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);
	}
	return pvs;
	}

继续追踪，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();
		//遍历 注入
		for (InjectedElement element : elementsToIterate) {
			if (debug) {
				logger.debug("Processing injected element of bean '" + beanName + "': " + element);
			}
			element.inject(target, beanName, pvs);
		}
	}
	}

这里和AutowiredAnnotationBeanPostProcessor不同的是，AutowiredAnnotationBeanPostProcessor调用的element.inject(target, beanName, pvs)方法是自己实现的，如图：

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

//InjectionMetadata.java

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

	if (this.isField) {
		Field field = (Field) this.member;
		//强吻访问
		ReflectionUtils.makeAccessible(field);
		//给字段赋值，即属性注入
		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();
		}
	}
	}

这里重点看getResourceToInject(target, requestingBeanName)方法，该方法的实现是具体获取@Resource中的值的。 我们可以看到在CommonAnnotationBeanPostProcessor类中，对该方法有实现：

//CommonAnnotationBeanPostProcessor.java

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

lazyLookup是CommonAnnotationBeanPostProcessor内部类ResourceElement的一个成员变量，表示是否懒加载，默认是false。 我们先来看下非懒加载的流程，即getResource(this, requestingBeanName):

//CommonAnnotationBeanPostProcessor.java

//根据给定名称或者类型获取资源对象
	protected Object getResource(LookupElement element, @Nullable String requestingBeanName) throws BeansException {
	//如果注解对象元素的mappedName属性不为空
	if (StringUtils.hasLength(element.mappedName)) {
		//根据JNDI名称和类型去Spring的JNDI容器中获取Bean
		return this.jndiFactory.getBean(element.mappedName, element.lookupType);
	}
	//如果该后置处理器的alwaysUseJndiLookup属性值为true
	if (this.alwaysUseJndiLookup) {
		//从Spring的JNDI容器中查找指定JDNI名称和类型的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自动依赖注入装配，通过给定的名称和类型从资源容器获取Bean对象
	//一般情况下，都是走这一步
	return autowireResource(this.resourceFactory, element, requestingBeanName);
	}

autowireResource代码：

//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<>();
		//根据类型从Spring容器中查找资源
		//调用依赖解析器，跟@Autowired是同样的代码
		resource = ((AutowireCapableBeanFactory) factory).resolveDependency(
				element.getDependencyDescriptor(), requestingBeanName, autowiredBeanNames, null);
		if (resource == null) {
			throw new NoSuchBeanDefinitionException(element.getLookupType(), "No resolvable resource object");
		}
	}
	//根据名称从Spring容器中查找资源
	else {
		resource = factory.getBean(name, element.lookupType);
		autowiredBeanNames = Collections.singleton(name);
	}

	//注册Bean的依赖关系
	if (factory instanceof ConfigurableBeanFactory) {
		ConfigurableBeanFactory beanFactory = (ConfigurableBeanFactory) factory;
		for (String autowiredBeanName : autowiredBeanNames) {
			if (requestingBeanName != null && beanFactory.containsBean(autowiredBeanName)) {
				beanFactory.registerDependentBean(autowiredBeanName, requestingBeanName);
			}
		}
	}

	return resource;
	}

这里的逻辑比较简单：

• 首先判断@Resource是按名称来查询还是类型，如果是类型，则调用依赖解析器根据类型从Spring容器中查找（这里和@Autowired的代码一样：Spring注解@Autowired源码分析）
• 如果是按名称，则直接调用BeanFactory的getBean()方法，根据BeanName从Spring容器中查找
• 最后由于发生了依赖注入，需要从新注册Bean的依赖关系

总结

• @Resource注解既可以按照名称来注入，也可以按类型来注入。