Spring Boot 中 @EnableXXX 注解的驱动逻辑
作者 | 温安适
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,
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 {
@Override
public void init() {
//省略其他代码
registerBeanDefinitionParser("annotation-config",
new AnnotationConfigBeanDefinitionParser());
}
}复制代码
** 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 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 context
SingletonBeanRegistry 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 class
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set candidates = new LinkedHashSet<>(configCandidates);
Set alreadyParsed = new HashSet<>(configCandidates.size());
do {
parser.parse(candidates);
parser.validate();
Set configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// 读ConfigurationClass的信息，创建BeanDefinition
if (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 oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
Set 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 first
processMemberClasses(configClass, sourceClass);
}
// Process any @PropertySource annotations
for (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 annotations
Set 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 immediately
Set scannedBeanDefinitions =
this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
// Check the set of scanned definitions for any further config classes and parse recursively if needed
for (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 annotations
processImports(configClass, sourceClass, getImports(sourceClass), true);
// Process any @ImportResource annotations
AnnotationAttributes importResource =
AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
if (importResource != null) {
String[] resources = importResource.getStringArray("locations");
Class readerClass = importResource.getClass("reader");
for (String resource : resources) {
String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
configClass.addImportedResource(resolvedResource, readerClass);
}
}
// Process individual @Bean methods
Set beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
// Process default methods on interfaces
processInterfaces(configClass, sourceClass);
// Process superclass, if any
if (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 recurse
return sourceClass.getSuperClass();
}
}
// No superclass -> processing is complete
return 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
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。
推荐阅读：
SpringBoot打造自己的博客项目，含完整项目代码