Spring Boot 中 @EnableXXX 注解的驱动逻辑探讨

作者 | 温安适

来源 | https://juejin.im/post/5efdd689e51d4534af686ca9

工作中经常用到，如下注解：

@EnableEurekaClient
@EnableFeignClients
@EnableCircuitBreaker
@EnableHystrix

他们都是@Enable开头，各自实现不同的功能，解析这种@Enable的逻辑是什么呢？

@Enable驱动逻辑

找入口

@Enable的模块驱动，依赖于@Import实现。

@Import作用是装载导入类，主要包括@Configuration class,ImportSelector实现类,ImportBeanDefinitionRegistrar实现类。

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

<context:annotation-config>（注解配置）中大概率有我们需要找的逻辑。

根据 Spring Framework 2.0引入的可扩展的XML编程机制，XML Schema命名空间需要与Handler建立映射关系。

该关系配置在相对于classpath下的/META-INF/spring.handlers中。

查看ContextNamespaceHandler 源码

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

** 对应AnnotationConfigBeanDefinitionParser这个就是要找的入口**

找核心类

从AnnotationConfigBeanDefinitionParser的parse方法开始一路向下，找到

AnnotationConfigUtils.registerAnnotationConfigProcessors中注册了ConfigurationClassPostProcessor。

ConfigurationClassPostProcessor类注释说明

\1. 用于的引导处理@Configuration类

\2. context:annotation-config/或 context:component-scan/时会注册

否则需要手工编程

\3. ConfigurationClassPostProcessor第一优先级，保证

@Configuration}类中声明@Bean，在其他 BeanFactoryPostProcessor执行之前被注册

扩展

AnnotationConfigApplicationContext中new AnnotationBeanDefinitionReader也调用了 AnnotationConfigUtils .

registerAnnotationConfigProcessors

从类注释中，可以看出ConfigurationClassPostProcessor就是要找的核心类

找核心方法

查看 ConfigurationClassPostProcessor 的层级关系为

Aware系列注入相应资源，Ordered设置优先级，值得关注的就是

postProcessBeanDefinitionRegistry了。

postProcessBeanDefinitionRegistry其内部有2个方法

postProcessBeanDefinitionRegistry在BeanDefinition注册之后，BeanFactoryPostProcessor执行之前，修改或重写BeanDefinition
继承自BeanFactoryPostProcessor的postProcessBeanFactory，BeanDefinition加载之后，Bean实例化之前，重写或添加BeanDefinition，修改BeanFactory

浏览2个方法，都有processConfigBeanDefinitions，从名称可以看出是处理配置类Bean定义

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));}}// 没有找到 @Configuration classes 立即返回if (configCandidates.isEmpty()) {return;}//根据@Order 值进行排序configCandidates.sort((bd1, bd2) -> {int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());return Integer.compare(i1, i2);});//通过封闭的应用程序上下文， 检测任何自定义bean名称生成策略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);// 读ConfigurationClass的信息，创建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());// 将ImportRegistry注册为bean以支持importware@Configuration类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();}
}复制代码

ConfigurationClassPostProcessor#processConfigBeanDefinitions核心如下：

根据@Order 值进行排序
解析@Configuration class 为ConfigurationClass对象
读ConfigurationClass的信息，创建BeanDefinition
将ImportRegistry注册为bean以支持importware@Configuration类

重点关注解析方法

ConfigurationClassParser#parse方法负责解析@Configuration class 为ConfigurationClass对象

查阅其源码如下：

ConfigurationClassParser#doProcessConfigurationClass代码如下：

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;
}复制代码

ConfigurationClassParser#doProcessConfigurationClass(ConfigurationClass,AnnatationMetatdata)将@PropertySource， @ComponentScan， @Import，@ImportResource，@Bean等一起处理了。

看到这里基本逻辑已经理清了，但是有一个疑问

@Configuration中的@Bean没有其他特殊处理吗？

浏览代码解决疑问

![img](data:image/svg+xml;utf8,)

从上边浏览的代码可以看到完全模式，会被AOP增强

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());
}
}复制代码

即 @Configuration class是完全模式，@Component，@Bean是轻量级模式

那AOP增强了作用是什么呢？查看 ConfigurationClassEnhancer 的类注释如下：

/**

* 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：

通过CGLIB增强@Configuration class。

每个@Bean方法会生成子类。

首次被调用时，@Bean方法会被执行用于创建bean实例；

再次被调用时，不会再执行创建bean实例，而是根据bean名称返回首次该方法被执行时创建的bean实例。

总结

1.ConfigurationClassPostProcessor负责筛选@Component Class、@Configuration Class以及@Bean定义的Bean，**

2.ConfigurationClassParser从候选的Bean定义中解析出ConfigurationClass集合，随后被3.ConfigurationClassBeanDefinitionReader转换为BeanDefinition

4.ConfigurationClassParser的解析顺序，

@PropertySource->@ComponentScan->@Import->@ImportResource->@Bean->接口的默认方法->处理父类

5.@Configuration class是完全模式，@Component，@Bean是轻量级模式

6.CGLIB增强@Configuration class。每个@Bean方法会生成子类。

首次被调用时，@Bean方法会被执行用于创建bean实例；

再次被调用时，不会再执行创建bean实例，而是根据bean名称返回首次该方法被执行时创建的bean实例。