作者 |?溫安適

來源 |?https://juejin.im/post/5efdd689e51d4534af686ca9

工作中經常用到，如下注解：

• @EnableEurekaClient

• @EnableFeignClients

• @EnableCircuitBreaker

• @EnableHystrix

他們都是@Enable開頭，各自實現不同的功能，解析這種@Enable的邏輯是什么呢？

@Enable驅動邏輯

找入口

@Enable的模塊驅動，依賴于@Import實現。

@Import作用是裝載導入類，主要包括@Configuration class,ImportSelector實現類,ImportBeanDefinitionRegistrar實現類。

XML時代，經常是@Import,<context:component-scan>,<context:annotation-config>一起使用。

<context:annotation-config>（注解配置）中大概率有我們需要找的邏輯。

根據 Spring Framework 2.0引入的可擴展的XML編程機制，XML Schema命名空間需要與Handler建立映射關系。

該關系配置在相對于classpath下的/META-INF/spring.handlers中。

查看ContextNamespaceHandler 源碼

public?class?ContextNamespaceHandler?extends?NamespaceHandlerSupport?{@Overridepublic?void?init()?{//省略其他代碼registerBeanDefinitionParser("annotation-config",?new?AnnotationConfigBeanDefinitionParser());} }

** 對應AnnotationConfigBeanDefinitionParser這個就是要找的入口**

找核心類

從AnnotationConfigBeanDefinitionParser的parse方法開始一路向下，找到

AnnotationConfigUtils.registerAnnotationConfigProcessors中注冊了ConfigurationClassPostProcessor。

ConfigurationClassPostProcessor類注釋說明

\1. 用于的引導處理@Configuration類

\2. context:annotation-config/或 context:component-scan/時會注冊

否則需要手工編程

\3. ConfigurationClassPostProcessor第一優先級，保證

@Configuration}類中聲明@Bean，在其他 BeanFactoryPostProcessor執行之前被注冊

擴展

AnnotationConfigApplicationContext中new AnnotationBeanDefinitionReader也調用了?AnnotationConfigUtils .

registerAnnotationConfigProcessors

從類注釋中，可以看出ConfigurationClassPostProcessor就是要找的核心類

找核心方法

查看 ConfigurationClassPostProcessor 的層級關系為

Aware系列注入相應資源，Ordered設置優先級，值得關注的就是

postProcessBeanDefinitionRegistry了。

postProcessBeanDefinitionRegistry其內部有2個方法

postProcessBeanDefinitionRegistry在BeanDefinition注冊之后，BeanFactoryPostProcessor執行之前，修改或重寫BeanDefinition

繼承自BeanFactoryPostProcessor的postProcessBeanFactory，BeanDefinition加載之后，Bean實例化之前，重寫或添加BeanDefinition，修改BeanFactory

瀏覽2個方法，都有processConfigBeanDefinitions，從名稱可以看出是處理配置類Bean定義

?

ConfigurationClassPostProcessor#processConfigBeanDefinitions就是要找的核心方法

梳理流程

public?void?processConfigBeanDefinitions(BeanDefinitionRegistry?registry)?{List<BeanDefinitionHolder>?configCandidates?=?new?ArrayList<>();String[]?candidateNames?=?registry.getBeanDefinitionNames();for?(String?beanName?:?candidateNames)?{BeanDefinition?beanDef?=?registry.getBeanDefinition(beanName);if?(ConfigurationClassUtils.isFullConfigurationClass(beanDef)?||ConfigurationClassUtils.isLiteConfigurationClass(beanDef))?{if?(logger.isDebugEnabled())?{logger.debug("Bean?definition?has?already?been?processed?as?a?configuration?class:?"?+?beanDef);}}else?if?(ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef,?this.metadataReaderFactory))?{configCandidates.add(new?BeanDefinitionHolder(beanDef,?beanName));}}//?沒有找到?@Configuration?classes?立即返回if?(configCandidates.isEmpty())?{return;}//根據@Order?值進行排序configCandidates.sort((bd1,?bd2)?->?{int?i1?=?ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());int?i2?=?ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());return?Integer.compare(i1,?i2);});//通過封閉的應用程序上下文，?檢測任何自定義bean名稱生成策略supplied?through?the?enclosing?application?contextSingletonBeanRegistry?sbr?=?null;if?(registry?instanceof?SingletonBeanRegistry)?{sbr?=?(SingletonBeanRegistry)?registry;if?(!this.localBeanNameGeneratorSet)?{BeanNameGenerator?generator?=?(BeanNameGenerator)?sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);if?(generator?!=?null)?{this.componentScanBeanNameGenerator?=?generator;this.importBeanNameGenerator?=?generator;}}}if?(this.environment?==?null)?{this.environment?=?new?StandardEnvironment();}//?解析@Configuration?classConfigurationClassParser?parser?=?new?ConfigurationClassParser(this.metadataReaderFactory,?this.problemReporter,?this.environment,this.resourceLoader,?this.componentScanBeanNameGenerator,?registry);Set<BeanDefinitionHolder>?candidates?=?new?LinkedHashSet<>(configCandidates);Set<ConfigurationClass>?alreadyParsed?=?new?HashSet<>(configCandidates.size());do?{parser.parse(candidates);parser.validate();Set<ConfigurationClass>?configClasses?=?new?LinkedHashSet<>(parser.getConfigurationClasses());configClasses.removeAll(alreadyParsed);//?讀ConfigurationClass的信息，創建BeanDefinitionif?(this.reader?==?null)?{this.reader?=?new?ConfigurationClassBeanDefinitionReader(registry,?this.sourceExtractor,?this.resourceLoader,?this.environment,this.importBeanNameGenerator,?parser.getImportRegistry());}this.reader.loadBeanDefinitions(configClasses);alreadyParsed.addAll(configClasses);candidates.clear();if?(registry.getBeanDefinitionCount()?>?candidateNames.length)?{String[]?newCandidateNames?=?registry.getBeanDefinitionNames();Set<String>?oldCandidateNames?=?new?HashSet<>(Arrays.asList(candidateNames));Set<String>?alreadyParsedClasses?=?new?HashSet<>();for?(ConfigurationClass?configurationClass?:?alreadyParsed)?{alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());}for?(String?candidateName?:?newCandidateNames)?{if?(!oldCandidateNames.contains(candidateName))?{BeanDefinition?bd?=?registry.getBeanDefinition(candidateName);if?(ConfigurationClassUtils.checkConfigurationClassCandidate(bd,?this.metadataReaderFactory)?&&!alreadyParsedClasses.contains(bd.getBeanClassName()))?{candidates.add(new?BeanDefinitionHolder(bd,?candidateName));}}}candidateNames?=?newCandidateNames;}}while?(!candidates.isEmpty());//?將ImportRegistry注冊為bean以支持importware@Configuration類if?(sbr?!=?null?&&?!sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME))?{sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME,?parser.getImportRegistry());}if?(this.metadataReaderFactory?instanceof?CachingMetadataReaderFactory)?{//?Clear?cache?in?externally?provided?MetadataReaderFactory;?this?is?a?no-op//?for?a?shared?cache?since?it'll?be?cleared?by?the?ApplicationContext.((CachingMetadataReaderFactory)?this.metadataReaderFactory).clearCache();} }復制代碼

ConfigurationClassPostProcessor#processConfigBeanDefinitions核心如下：

根據@Order 值進行排序

解析@Configuration class 為ConfigurationClass對象

讀ConfigurationClass的信息，創建BeanDefinition

將ImportRegistry注冊為bean以支持importware@Configuration類

重點關注解析方法

ConfigurationClassParser#parse方法負責解析@Configuration class 為ConfigurationClass對象

查閱其源碼如下：

ConfigurationClassParser#doProcessConfigurationClass代碼如下：

protected?final?SourceClass?doProcessConfigurationClass(ConfigurationClass?configClass,?SourceClass?sourceClass)throws?IOException?{if?(configClass.getMetadata().isAnnotated(Component.class.getName()))?{//?Recursively?process?any?member?(nested)?classes?firstprocessMemberClasses(configClass,?sourceClass);}//?Process?any?@PropertySource?annotationsfor?(AnnotationAttributes?propertySource?:?AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(),?PropertySources.class,org.springframework.context.annotation.PropertySource.class))?{if?(this.environment?instanceof?ConfigurableEnvironment)?{processPropertySource(propertySource);}else?{logger.info("Ignoring?@PropertySource?annotation?on?["?+?sourceClass.getMetadata().getClassName()?+"].?Reason:?Environment?must?implement?ConfigurableEnvironment");}}//?Process?any?@ComponentScan?annotationsSet<AnnotationAttributes>?componentScans?=?AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(),?ComponentScans.class,?ComponentScan.class);if?(!componentScans.isEmpty()?&&!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(),?ConfigurationPhase.REGISTER_BEAN))?{for?(AnnotationAttributes?componentScan?:?componentScans)?{//?The?config?class?is?annotated?with?@ComponentScan?->?perform?the?scan?immediatelySet<BeanDefinitionHolder>?scannedBeanDefinitions?=this.componentScanParser.parse(componentScan,?sourceClass.getMetadata().getClassName());//?Check?the?set?of?scanned?definitions?for?any?further?config?classes?and?parse?recursively?if?neededfor?(BeanDefinitionHolder?holder?:?scannedBeanDefinitions)?{BeanDefinition?bdCand?=?holder.getBeanDefinition().getOriginatingBeanDefinition();if?(bdCand?==?null)?{bdCand?=?holder.getBeanDefinition();}if?(ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand,?this.metadataReaderFactory))?{parse(bdCand.getBeanClassName(),?holder.getBeanName());}}}}//?Process?any?@Import?annotationsprocessImports(configClass,?sourceClass,?getImports(sourceClass),?true);//?Process?any?@ImportResource?annotationsAnnotationAttributes?importResource?=AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(),?ImportResource.class);if?(importResource?!=?null)?{String[]?resources?=?importResource.getStringArray("locations");Class<??extends?BeanDefinitionReader>?readerClass?=?importResource.getClass("reader");for?(String?resource?:?resources)?{String?resolvedResource?=?this.environment.resolveRequiredPlaceholders(resource);configClass.addImportedResource(resolvedResource,?readerClass);}}//?Process?individual?@Bean?methodsSet<MethodMetadata>?beanMethods?=?retrieveBeanMethodMetadata(sourceClass);for?(MethodMetadata?methodMetadata?:?beanMethods)?{configClass.addBeanMethod(new?BeanMethod(methodMetadata,?configClass));}//?Process?default?methods?on?interfacesprocessInterfaces(configClass,?sourceClass);//?Process?superclass,?if?anyif?(sourceClass.getMetadata().hasSuperClass())?{String?superclass?=?sourceClass.getMetadata().getSuperClassName();if?(superclass?!=?null?&&?!superclass.startsWith("java")?&&!this.knownSuperclasses.containsKey(superclass))?{this.knownSuperclasses.put(superclass,?configClass);//?Superclass?found,?return?its?annotation?metadata?and?recursereturn?sourceClass.getSuperClass();}}//?No?superclass?->?processing?is?completereturn?null; }復制代碼

ConfigurationClassParser#doProcessConfigurationClass(ConfigurationClass,AnnatationMetatdata)將@PropertySource， ??@ComponentScan，?@Import，@ImportResource，@Bean等一起處理了。

看到這里基本邏輯已經理清 了，但是有一個疑問

@Configuration中的@Bean沒有其他特殊處理嗎？

瀏覽代碼解決疑問


從上邊瀏覽的代碼可以看到完全模式，會被AOP增強

那什么是完全模式呢？在ConfigurationClassUtils找到如下方法：

public?class?ConfigurationClassUtils{ //省略其他方法 public?static?boolean?isFullConfigurationCandidate(AnnotationMetadata?metadata)?{return?metadata.isAnnotated(Configuration.class.getName()); } }復制代碼

即?@Configuration class是完全模式，@Component，@Bean是輕量級模式

那AOP增強了作用是什么呢？查看 ConfigurationClassEnhancer 的類注釋如下：

/**

* Enhances {@link

Configuration

} classes by generating a CGLIB subclass which

* interacts with the Spring container to respect bean scoping semantics for

* {@code @Bean} methods. Each such {@code @Bean} method will be overridden in

* the generated subclass, only delegating to the actual {@code @Bean} method

* implementation if the container actually requests the construction of a new

* instance. Otherwise, a call to such an {@code @Bean} method serves as a

* reference back to the container, obtaining the corresponding bean by name.

• ?

* @author Chris Beams

* @author Juergen Hoeller

* @since 3.0

* @see #enhance

* @see

ConfigurationClassPostProcessor

*/

class ?ConfigurationClassEnhancer ?{

大概意思如下：

通過CGLIB增強@Configuration class。

每個@Bean方法會生成子類。

首次被調用時，@Bean方法會被執行用于創建bean實例；

再次被調用時，不會再執行創建bean實例，而是根據bean名稱返回首次該方法被執行時創建的bean實例。

總結

1.ConfigurationClassPostProcessor負責篩選@Component Class、@Configuration Class以及@Bean定義的Bean，**

2.ConfigurationClassParser從候選的Bean定義中解析出ConfigurationClass集合，隨后被3.ConfigurationClassBeanDefinitionReader轉換為BeanDefinition

4.ConfigurationClassParser的解析順序，

@PropertySource->@ComponentScan->@Import->@ImportResource->@Bean->接口的默認方法->處理父類

5.@Configuration class是完全模式，@Component，@Bean是輕量級模式

6.CGLIB增強@Configuration class。每個@Bean方法會生成子類。

首次被調用時，@Bean方法會被執行用于創建bean實例；

再次被調用時，不會再執行創建bean實例，而是根據bean名稱返回首次該方法被執行時創建的bean實例。

總結

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