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What is the function of @ Resource annotation in Spring? this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible way.

CommonAnnotationBeanPostProcessor is used in Spring to deal with common annotations in JavaEE5 (mainly EJB-related annotations) and JAX-WS-related annotations in Java6. It can deal with @ PostConstruct, @ PreDestroy and other Bean lifecycle-related events. The core of this post-processing is to deal with @ Resource annotations as well as JAX-WS-related annotations.

I. trigger mode

After each Bean is instantiated, the Spring container calls the postProcessMergedBeanDefinition method of the post processor CommonAnnotationBeanPostProcessor to find out whether the Bean has @ Resource annotations.

When Spring instantiates each Bean and calls populateBean for attribute injection, the postProcessPropertyValues method is called to find out whether the Bean has @ Resource annotations.

Constructor / / CommonAnnotationBeanPostProcessor.java// constructor public CommonAnnotationBeanPostProcessor () {setOrder (Ordered.LOWEST_PRECEDENCE-3); / / set the initial annotation type to @ PostConstruct setInitAnnotationType (PostConstruct.class); / / set the destroyed annotation to @ PreDestroy setDestroyAnnotationType (PreDestroy.class) / / ignore JAX-WS 's resource type ignoreResourceType ("javax.xml.ws.WebServiceContext") when using @ Resource annotation;} III. Inject / / CommonAnnotationBeanPostProcessor.javapublic PropertyValues postProcessPropertyValues (PropertyValues pvs, PropertyDescriptor [] pds, Object bean, String beanName) throws BeansException {/ / get the attribute value metadata configured in @ Resource annotation InjectionMetadata metadata = findResourceMetadata (beanName, bean.getClass (), pvs) Try {/ / injection attribute value, same metadata.inject (bean, beanName, pvs) as in AutowiredAnnotationBeanPostProcessor;} catch (Throwable ex) {throw new BeanCreationException (beanName, "Injection of resource dependencies failed", ex);} return pvs;}

To continue tracking, see the metadata.inject (bean, beanName, pvs) method

/ / InjectionMetadata.javapublic void inject (Object target, @ Nullable String beanName, @ Nullable PropertyValues pvs) throws Throwable {Collection checkedElements = this.checkedElements; / / set of fields to be injected Collection elementsToIterate = (checkedElements! = null? CheckedElements: this.injectedElements); if (! elementsToIterate.isEmpty ()) {boolean debug = logger.isDebugEnabled (); / / ergodic injection for (InjectedElement element: elementsToIterate) {if (debug) {logger.debug ("Processing injected element of bean'" + beanName + "':" + element) } element.inject (target, beanName, pvs);}

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

The element.inject (target, beanName, pvs) called by CommonAnnotationBeanPostProcessor is the original method, as follows:

/ / InjectionMetadata.javaprotected void inject (Object target, @ Nullable String requestingBeanName, @ Nullable PropertyValues pvs) throws Throwable {if (this.isField) {Field field = (Field) this.member; / / forced Kiss to visit ReflectionUtils.makeAccessible (field) / / assign a value to the field, that is, property injection 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 the getResourceToInject (target, requestingBeanName) method, which is implemented by specifically fetching the value in @ Resource. We can see that this method is implemented in the CommonAnnotationBeanPostProcessor class:

/ / CommonAnnotationBeanPostProcessor.javaprotected Object getResourceToInject (Object target, @ Nullable String requestingBeanName) {return (this.lazyLookup? BuildLazyResourceProxy (this,requestingBeanName): getResource (this,requestingBeanName));}

LazyLookup is a member variable of the CommonAnnotationBeanPostProcessor inner class ResourceElement, indicating whether to load lazily. The default is false. Let's first look at the process of non-lazy loading, that is, getResource (this, requestingBeanName):

/ / CommonAnnotationBeanPostProcessor.java// gets the resource object protected Object getResource (LookupElement element, @ Nullable String requestingBeanName) throws BeansException {/ / if the mappedName attribute of the annotation object element is not empty if (StringUtils.hasLength (element.mappedName)) {/ / go to Spring's JNDI container to get Bean return this.jndiFactory.getBean (element.mappedName, element.lookupType) according to the JNDI name and type } / / if the alwaysUseJndiLookup attribute value of the post processor is true if (this.alwaysUseJndiLookup) {/ / find the Bean return this.jndiFactory.getBean (element.name, element.lookupType) of the specified JDNI name and type from the JNDI container of Spring } if (this.resourceFactory = = null) {throw new NoSuchBeanDefinitionException (element.lookupType, "No resource factory configured-specify the 'resourceFactory' property") } / / use autowiring automatic dependency injection assembly to get Bean objects from the resource container with a given name and type / / in general, this is the step return autowireResource (this.resourceFactory, element, requestingBeanName);}

AutowireResource Code:

/ / CommonAnnotationBeanPostProcessor.javaprotected Object autowireResource (BeanFactory factory, LookupElement element, @ Nullable String requestingBeanName) throws BeansException {Object resource; Set autowiredBeanNames; String name = element.name; if (this.fallbackToDefaultTypeMatch & & element.isDefaultName & & factory instanceof AutowireCapableBeanFactory & &! factory.containsBean (name)) {autowiredBeanNames = new LinkedHashSet () / / find resources from the Spring container by type / / call the dependency parser, which is the same code as @ Autowired resource = ((AutowireCapableBeanFactory) factory) .resolveDependency (element.getDependencyDescriptor (), requestingBeanName, autowiredBeanNames, null) If (resource = = null) {throw new NoSuchBeanDefinitionException (element.getLookupType (), "No resolvable resource object");}} / / find the resource else {resource = factory.getBean (name, element.lookupType) from the Spring container by name; autowiredBeanNames = Collections.singleton (name) } / / Registration Bean dependency 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, the dependent parser is called to look it up from the Spring container based on the type (here is the same as the code in @ Autowired: Spring annotation @ Autowired source code analysis)

If it is by name, call the getBean () method of BeanFactory directly and look it up from the Spring container according to BeanName

Finally, due to the occurrence of dependency injection, the dependency relationship of Bean needs to be re-registered.

Summary

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

This is the answer to the question about the role of @ Resource annotation in Spring. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel for more related knowledge.

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