Spring annotation @Resource source code analysis

CommonAnnotationBeanPostProcessor is used in Spring to process common annotations in JavaEE5 (mainly EJB-related annotations) and JAX-WS-related annotations in Java6. It can process annotations related to Bean life cycle events such as @PostConstruct and @PreDestroy. The core is to process @Resourceannotations, as well as JAX-WS related annotations.

1. Trigger method

• After each bean is instantiated, the Spring container calls the postProcessMergedBeanDefinitionmethod of the post-processor CommonAnnotationBeanPostProcessor to find out whether the bean has the @Resource annotation.
• When Spring is instantiating each bean, it calls populateBeanthe attribute injection, that is, calls the postProcessPropertyValuesmethod to find out whether the bean has the @Resource annotation.

2. Constructor

//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");
	}

3. Injection

//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;
	}

Continue to follow and see metadata.inject(bean, beanName, pvs)how

//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);
		}
	}
	}

The AutowiredAnnotationBeanPostProcessordifference here is that the method called by AutowiredAnnotationBeanPostProcessor element.inject(target, beanName, pvs)is implemented by itself, as shown in the figure:

The original method is CommonAnnotationBeanPostProcessorcalled , as follows:element.inject(target, beanName, pvs)

//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();
		}
	}
	}

Here we focus on getResourceToInject(target, requestingBeanName)the method. The implementation of this method is to specifically obtain the value in @Resource. We can see that in the CommonAnnotationBeanPostProcessor class, there is an implementation of this method:

//CommonAnnotationBeanPostProcessor.java

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

lazyLookupIs a member variable of the CommonAnnotationBeanPostProcessor internal class ResourceElement, indicating whether lazy loading, the default is false. Let's first look at the non-lazy loading process, namely 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 code:

//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;
	}

The logic here is relatively simple:

• First determine whether @Resource is queried by name or type. If it is a type, call the dependency parser to find it from the Spring container according to the type (this is the same as the code of @Autowired: Spring annotation @Autowired source code analysis )
• If it is by name, the method of BeanFactory is called directly, and it getBean()is looked up from the Spring container according to BeanName
• Finally, due to dependency injection, it is necessary to re-register the Bean's dependencies

总结

• The @Resource annotation can be injected either by name or by type.

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