Spring @Bean注解深入分析源码执行过程

本文将通过阅读spring源码，分析@Bean注解导入Bean的原理。

从AnnotationConfigApplicationContext对象的创建讲起，因为在创建他的过程中，spring会先注入一系列的处理器，使用这些处理器解析@Configuration Class进而将@Bean标注的方法转为BeanDefinition注入到容器。

其他的ApplicationContext实现在原理上也是一致的，只是入口不同而已。

AnnotationConfigApplicationContext创建

public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
this();
register(componentClasses);
refresh();
}

做了以下事情：

• 创建AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner

• 注册Configuration Bean Class

• refresh()加载、刷新容器：包含着@Configuration Class解析

创建AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner

• AnnotatedBeanDefinitionReader - 用于编程注册Bean类的方便适配器，ClassPathBeanDefinitionScanner的替代方案，支持使用注解方式显示的注册Bean类。有几个重载的registerBean方法，可以将给定的Bean类注册到spring容器，注册的是AnnotatedGenericBeanDefinition对象，他提供了获取Bean类meta信息的方法。

• ClassPathBeanDefinitionScanner - 从类路径扫描组件并注册到容器

在创建AnnotatedBeanDefinitionReader时，会向容器注册几个注解驱动处理器：

AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);

org.springframework.context.annotation.internalConfigurationAnnotationProcessor: ConfigurationClassPostProcessor

• BeanFactoryPostProcessor实现，用于解析@Configuration类。

• 这个处理器是按优先级排序的，因为在@Configuration类中声明的任何Bean方法都必须在任何其他BeanFactoryPostProcessor执行之前注册其对应的BeanDefinition。

org.springframework.context.annotation.internalAutowiredAnnotationProcessor: AutowiredAnnotationBeanPostProcessor

• BeanPostProcessor implementation that autowires annotated fields, setter methods, and arbitrary config methods. Such members to be injected are detected through annotations: by default, Spring&rsquo;s @Autowired and @Value annotations.

• Also supports JSR-330&rsquo;s @Inject annotation, if available, as a direct alternative to Spring&rsquo;s own @Autowired.

• @Autowired支持处理器。

org.springframework.context.annotation.internalCommonAnnotationProcessor: CommonAnnotationBeanPostProcessor

• BeanPostProcessor implementation that supports common Java annotations out of the box.

• 支持Resource、PostConstruct、PreDestroy等注解。

org.springframework.context.event.internalEventListenerProcessor: EventListenerMethodProcessor

org.springframework.context.event.internalEventListenerFactory: DefaultEventListenerFactory

ConfigurationClassPostProcessor中有支持@Bean注解的逻辑。

注册Configuration Bean Class

register(componentClasses);

调用到AnnotatedBeanDefinitionReader的register方法：

this.reader.register(componentClasses);

AnnotatedBeanDefinitionReader类支持使用注解方式显示的注册Bean类。几个重载的registerBean方法，可以将给定的Bean类注册到spring容器，注册的是AnnotatedGenericBeanDefinition对象，他提供了获取Bean类meta信息的方法：

public void registerBean(Class<?> beanClass) {
doRegisterBean(beanClass, null, null, null, null);
}
private <T> void doRegisterBean(Class<T> beanClass, String name,
Class<? extends Annotation>[] qualifiers, Supplier<T> supplier,
BeanDefinitionCustomizer[] customizers) {
   // 1. 创建AnnotatedGenericBeanDefinition对象，封装StandardAnnotationMetadata用于获取Bean的注解元信息
AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(beanClass);
   // skip判断，暂时不做分析
if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
return;
}
   // 2. scope、primary、lazy判断，获取beanName等
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
abd.setScope(scopeMetadata.getScopeName());
String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
if (qualifiers != null) {
for (Class<? extends Annotation> qualifier : qualifiers) {
if (Primary.class == qualifier) {
abd.setPrimary(true);
} else if (Lazy.class == qualifier) {
abd.setLazyInit(true);
} else {
abd.addQualifier(new AutowireCandidateQualifier(qualifier));
}
}
}
   // 3. 封装 BeanDefinitionHolder注册到容器
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
definitionHolder =
       AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
}

创建AnnotatedGenericBeanDefinition需要稍微注意一下：

AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(beanClass);
// AnnotatedGenericBeanDefinition构造方法
public AnnotatedGenericBeanDefinition(Class<?> beanClass) {
setBeanClass(beanClass);
this.metadata = AnnotationMetadata.introspect(beanClass);
}
// AnnotationMetadata.introspect方法
static AnnotationMetadata introspect(Class<?> type) {
return StandardAnnotationMetadata.from(type);
}
// StandardAnnotationMetadata.from方法
static AnnotationMetadata from(Class<?> introspectedClass) {
return new StandardAnnotationMetadata(introspectedClass, true);
}

以上的代码片段分散在不同的类里面，最终AnnotatedGenericBeanDefinition对象会保存一个StandardAnnotationMetadata对象，用于获取BeanMeta信息。

StandardAnnotationMetadata后文会有专门章节进行介绍。

至此，spring只是将@Configuration Class作为一个AnnotatedBeanDefinition注册到了容器中，@Configuration Class解析工作是在refresh时做的。

@Configuration Class解析

refresh方法

这段代码在AbstractApplicationContext类中，此处只截取了与本文相关部分：

public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
           // 只有web应用的实现类重写了这个方法，此处不展开分析
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
           // 这里开始调用BeanFactory处理器
invokeBeanFactoryPostProcessors(beanFactory);

invokeBeanFactoryPostProcessors

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate
       .invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
}

PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors方法：

• 先调用BeanDefinitionRegistryPostProcessor

• 再调用BeanFactoryPostProcessor

调用BeanFactoryPostProcessor与本文分析的内容关系不大，暂时不展开分析，重点看调用BeanDefinitionRegistryPostProcessor的逻辑。

入口在这里：

invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);

进入到invokeBeanDefinitionRegistryPostProcessors方法：

private static void invokeBeanDefinitionRegistryPostProcessors(
Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors,
       BeanDefinitionRegistry registry) {
for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
postProcessor.postProcessBeanDefinitionRegistry(registry);
}
}

此时，就会调用到ConfigurationClassPostProcessor类的postProcessBeanDefinitionRegistry方法。

ConfigurationClassPostProcessor类

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
// 略
   // Derive further bean definitions from the configuration classes in the registry.
   // 从容器中已有的的@Configuration Class定义进一步解析BeanDefinition
   // 此处不只会解析@Bean注解，其他的比如@Import、@ComponentScan等注解他也会解析
processConfigBeanDefinitions(registry);
}

processConfigBeanDefinitions方法代码比较多，此处只截取相关部分：

// 1. Parse each @Configuration class
ConfigurationClassParser 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());
// 此处是一个do while循环
// 因为解析一遍之后，容器里面可能会有新的被注入的@Configuration Class定义，需要进一步解析
// 比如@Import、@ComponentScan等注解就有可能注入新的@Configuration Class定义
do {
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// 2. Read the model and create bean definitions based on its content
this.reader.loadBeanDefinitions(configClasses);
// ...
} while (!candidates.isEmpty());
// ...

以上代码做了两件事：

• Parse @Configuration class

• 解析ConfigurationClass集注册BeanDefinition

Parse @Configuration class

这个步骤是将容器里面的@Configuration Class Bean定 * 析成ConfigurationClass集，ConfigurationClass封装着@Configuration Class的元信息，包括：

• AnnotationMetadata metadata - 注解元信息

• beanName - bean名字

• Set<BeanMethod> beanMethods - 这个就是这个配置类里面使用@Bean导出的Bean集合

• 以及Import相关信息

入口在这里：

parser.parse(candidates);

parse方法：

public void parse(Set<BeanDefinitionHolder> configCandidates) {
for (BeanDefinitionHolder holder : configCandidates) {
BeanDefinition bd = holder.getBeanDefinition();
try {
if (bd instanceof AnnotatedBeanDefinition) {
               // 进入这个分支
parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
} else if (bd instanceof AbstractBeanDefinition &&
                      ((AbstractBeanDefinition) bd).hasBeanClass()) {
parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
} else {
parse(bd.getBeanClassName(), holder.getBeanName());
}
}
}
this.deferredImportSelectorHandler.process();
}

之后进入processConfigurationClass方法：

protected void processConfigurationClass(
   ConfigurationClass configClass, Predicate<String> filter) throws IOException {
   // skip判断
if (this.conditionEvaluator.shouldSkip(
       configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
return;
}
ConfigurationClass existingClass = this.configurationClasses.get(configClass);
if (existingClass != null) {
if (configClass.isImported()) {
if (existingClass.isImported()) {
existingClass.mergeImportedBy(configClass);
}
// Otherwise ignore new imported config class; existing non-imported class overrides it.
return;
} else {
// Explicit bean definition found, probably replacing an import.
// Let's remove the old one and go with the new one.
this.configurationClasses.remove(configClass);
this.knownSuperclasses.values().removeIf(configClass::equals);
}
}
// Recursively process the configuration class and its superclass hierarchy.
   // 递归从本@Configuration Class将其父类解析ConfigurationClass
SourceClass sourceClass = asSourceClass(configClass, filter);
do {
sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
} while (sourceClass != null);

this.configurationClasses.put(configClass, configClass);
}

doProcessConfigurationClass方法，该方法负责解析@Configuration Class，包括以下内容：

• 递归处理内部类

• 处理@PropertySource注解

• 处理@ComponentScan注解

• 处理@Import注解

• 处理@ImportResource注解

• 处理@Bean注解

• 最后获取以下当前@Configuration Class的父类，如果有，则需要继续解析该父类

此处只截取与@Bean解析相关的代码片段：

// Process individual @Bean methods
Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
   // 封装 BeanMethod添加到ConfigurationClass
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}

retrieveBeanMethodMetadata方法：

// Retrieve the metadata for all @Bean methods
private Set<MethodMetadata> retrieveBeanMethodMetadata(SourceClass sourceClass) {
   // 获取注解元数据
AnnotationMetadata original = sourceClass.getMetadata();
   // 获取被@Bean标注的Method元数据集
Set<MethodMetadata> beanMethods = original.getAnnotatedMethods(Bean.class.getName());
if (beanMethods.size() > 1 && original instanceof StandardAnnotationMetadata) {
// Try reading the class file via ASM for deterministic declaration order...
// Unfortunately, the JVM's standard reflection returns methods in arbitrary
// order, even between different runs of the same application on the same JVM.
       // 此处会将无序的beanMethods集转为有序的beanMethods集，
       // 因为StandardAnnotationMetadata使用的是反射方式获取meta信息，
       // 这个不保证顺序，所以需要将其转为SimpleAnnotationMetadata类型，
       // 他内部使用ClassVisitor通过读取字节码文件，按顺序解析获取meta信息。
       // 后续会有专门的章节介绍StandardAnnotationMetadata和SimpleAnnotationMetadata类
try {
AnnotationMetadata asm = this.metadataReaderFactory.getMetadataReader(
               original.getClassName()).getAnnotationMetadata();
Set<MethodMetadata> asmMethods = asm.getAnnotatedMethods(Bean.class.getName());
if (asmMethods.size() >= beanMethods.size()) {
Set<MethodMetadata> selectedMethods = new LinkedHashSet<>(asmMethods.size());
for (MethodMetadata asmMethod : asmMethods) {
for (MethodMetadata beanMethod : beanMethods) {
if (beanMethod.getMethodName().equals(asmMethod.getMethodName())) {
selectedMethods.add(beanMethod);
break;
}
}
}
if (selectedMethods.size() == beanMethods.size()) {
// All reflection-detected methods found in ASM method set -> proceed
beanMethods = selectedMethods;
}
}
} catch (IOException ex) {
// No worries, let's continue with the reflection metadata we started with...
}
}
return beanMethods;
}

到此，解析BeanMethod和MethodMetadata的流程就结束了，后续的逻辑就是封装 BeanDefinition并将其注册到容器。

解析ConfigurationClass集注册BeanDefinition

将ConfigurationClass集进一步解析，将导出、扫描出的组件封装成BeanDefinition注册到容器：

this.reader.loadBeanDefinitions(configClasses);

loadBeanDefinitionsForConfigurationClass方法：

// Read a particular ConfigurationClass,
// registering bean definitions for the class itself and all of its Bean methods.
private void loadBeanDefinitionsForConfigurationClass(
ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {
   // skip判断
if (trackedConditionEvaluator.shouldSkip(configClass)) {
String beanName = configClass.getBeanName();
if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
this.registry.removeBeanDefinition(beanName);
}
this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
return;
}
if (configClass.isImported()) {
registerBeanDefinitionForImportedConfigurationClass(configClass);
}
   // Read the given BeanMethod,
   // registering bean definitions with the BeanDefinitionRegistry based on its contents.
for (BeanMethod beanMethod : configClass.getBeanMethods()) {
loadBeanDefinitionsForBeanMethod(beanMethod);
}
   // Import相关
loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
}

loadBeanDefinitionsForBeanMethod方法，读取指定的BeanMethod对象，将其封装成BeanDefinition注册到容器：

private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) {
ConfigurationClass configClass = beanMethod.getConfigurationClass();
MethodMetadata metadata = beanMethod.getMetadata();
String methodName = metadata.getMethodName();
// Do we need to mark the bean as skipped by its condition?
if (this.conditionEvaluator.shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN)) {
configClass.skippedBeanMethods.add(methodName);
return;
}
if (configClass.skippedBeanMethods.contains(methodName)) {
return;
}
AnnotationAttributes bean = AnnotationConfigUtils.attributesFor(metadata, Bean.class);
Assert.state(bean != null, "No @Bean annotation attributes");
// Consider name and any aliases
List<String> names = new ArrayList<>(Arrays.asList(bean.getStringArray("name")));
String beanName = (!names.isEmpty() ? names.remove(0) : methodName);
// Register aliases even when overridden
for (String alias : names) {
this.registry.registerAlias(beanName, alias);
}
// Has this effectively been overridden before (e.g. via XML)?
if (isOverriddenByExistingDefinition(beanMethod, beanName)) {
if (beanName.equals(beanMethod.getConfigurationClass().getBeanName())) {
throw new BeanDefinitionStoreException("");
}
return;
}
ConfigurationClassBeanDefinition beanDef = new ConfigurationClassBeanDefinition(
       configClass, metadata, beanName);
beanDef.setSource(this.sourceExtractor.extractSource(metadata, configClass.getResource()));
if (metadata.isStatic()) {
// static @Bean method
if (configClass.getMetadata() instanceof StandardAnnotationMetadata) {
beanDef.setBeanClass(
               ((StandardAnnotationMetadata) configClass.getMetadata()).getIntrospectedClass());
} else {
beanDef.setBeanClassName(configClass.getMetadata().getClassName());
}
beanDef.setUniqueFactoryMethodName(methodName);
} else {
// instance @Bean method
beanDef.setFactoryBeanName(configClass.getBeanName());
beanDef.setUniqueFactoryMethodName(methodName);
}
if (metadata instanceof StandardMethodMetadata) {
beanDef.setResolvedFactoryMethod(((StandardMethodMetadata) metadata).getIntrospectedMethod());
}
beanDef.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_CONSTRUCTOR);
beanDef.setAttribute(org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor.
SKIP_REQUIRED_CHECK_ATTRIBUTE, Boolean.TRUE);
AnnotationConfigUtils.processCommonDefinitionAnnotations(beanDef, metadata);
Autowire autowire = bean.getEnum("autowire");
if (autowire.isAutowire()) {
beanDef.setAutowireMode(autowire.value());
}
boolean autowireCandidate = bean.getBoolean("autowireCandidate");
if (!autowireCandidate) {
beanDef.setAutowireCandidate(false);
}
String initMethodName = bean.getString("initMethod");
if (StringUtils.hasText(initMethodName)) {
beanDef.setInitMethodName(initMethodName);
}
String destroyMethodName = bean.getString("destroyMethod");
beanDef.setDestroyMethodName(destroyMethodName);
// Consider scoping
ScopedProxyMode proxyMode = ScopedProxyMode.NO;
AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(metadata, Scope.class);
if (attributes != null) {
beanDef.setScope(attributes.getString("value"));
proxyMode = attributes.getEnum("proxyMode");
if (proxyMode == ScopedProxyMode.DEFAULT) {
proxyMode = ScopedProxyMode.NO;
}
}
// Replace the original bean definition with the target one, if necessary
BeanDefinition beanDefToRegister = beanDef;
if (proxyMode != ScopedProxyMode.NO) {
BeanDefinitionHolder proxyDef = ScopedProxyCreator.createScopedProxy(
new BeanDefinitionHolder(beanDef, beanName), this.registry,
proxyMode == ScopedProxyMode.TARGET_CLASS);
beanDefToRegister = new ConfigurationClassBeanDefinition(
(RootBeanDefinition) proxyDef.getBeanDefinition(), configClass, metadata, beanName);
}
this.registry.registerBeanDefinition(beanName, beanDefToRegister);
}

至此，@Bean注解的核心原理就分析完成了。后续将简单介绍一下AnnotationMetadata和MethodMetadata这两个接口。

AnnotationMetadata接口

Interface that defines abstract access to the annotations of a specific class, in a form that does not require that class to be loaded yet.

用于获取类的注解元数据。

他继承了ClassMetadata接口，所以也可以获取类的相关信息：比如类名、实现的接口、继承的父类等信息。另外，他还支持获取类的MethodMetadata集，即把类的所有方法解析之后封装成MethodMetadata集。

实现类：

• StandardAnnotationMetadata - Uses standard reflection to introspect a given Class.

• SimpleAnnotationMetadata - ASM based.

StandardAnnotationMetadata类

这个类使用反射方式获取类的注解元数据。

我们在上文介绍创建AnnotatedGenericBeanDefinition的过程中，看到过这个类对象的创建方式：

// AnnotationMetadata.introspect方法
static AnnotationMetadata introspect(Class<?> type) {
return StandardAnnotationMetadata.from(type);
}
// StandardAnnotationMetadata.from方法
static AnnotationMetadata from(Class<?> introspectedClass) {
return new StandardAnnotationMetadata(introspectedClass, true);
}

实际上只是把类封装到里面，实现方法里面使用反射获取对应元数据。

SimpleAnnotationMetadata类

StandardAnnotationMetadata类获取出来的元数据不保证顺序，在需要顺序的场景下不适用。

在Parse @Configuration class流程中有一个步骤是调用retrieveBeanMethodMetadata方法获取所有@Bean标注的方法并封装MethodMetadata集，其中有一步就是使用SimpleAnnotationMetadataReadingVisitor读取字节码文件，读取过程中将类的元数据封装到SimpleAnnotationMetadata对象，从而确保了顺序。

代码片段之前记录过，此处再介绍一下：

// Retrieve the metadata for all @Bean methods
private Set<MethodMetadata> retrieveBeanMethodMetadata(SourceClass sourceClass) {
   // 获取注解元数据
AnnotationMetadata original = sourceClass.getMetadata();
   // 获取被@Bean标注的Method元数据集
Set<MethodMetadata> beanMethods = original.getAnnotatedMethods(Bean.class.getName());
if (beanMethods.size() > 1 && original instanceof StandardAnnotationMetadata) {
// Try reading the class file via ASM for deterministic declaration order...
// Unfortunately, the JVM's standard reflection returns methods in arbitrary
// order, even between different runs of the same application on the same JVM.
       // 此处会将无序的beanMethods集转为有序的beanMethods集，
       // 因为StandardAnnotationMetadata使用的是反射方式获取meta信息，
       // 这个不保证顺序，所以需要将其转为SimpleAnnotationMetadata类型，
       // 他内部使用ClassVisitor通过读取字节码文件，按顺序解析获取meta信息。
       // 后续会有专门的章节介绍StandardAnnotationMetadata和SimpleAnnotationMetadata类
try {
           // 这里getMetadataReader得到的是一个SimpleMetadataReader对象
AnnotationMetadata asm = this.metadataReaderFactory.getMetadataReader(
               original.getClassName()).getAnnotationMetadata();
Set<MethodMetadata> asmMethods = asm.getAnnotatedMethods(Bean.class.getName());
if (asmMethods.size() >= beanMethods.size()) {
Set<MethodMetadata> selectedMethods = new LinkedHashSet<>(asmMethods.size());
for (MethodMetadata asmMethod : asmMethods) {
for (MethodMetadata beanMethod : beanMethods) {
if (beanMethod.getMethodName().equals(asmMethod.getMethodName())) {
selectedMethods.add(beanMethod);
break;
}
}
}
if (selectedMethods.size() == beanMethods.size()) {
// All reflection-detected methods found in ASM method set -> proceed
beanMethods = selectedMethods;
}
}
} catch (IOException ex) {
// No worries, let's continue with the reflection metadata we started with...
}
}
return beanMethods;
}

SimpleMetadataReader类

SimpleMetadataReader(Resource resource, ClassLoader classLoader) throws IOException {
SimpleAnnotationMetadataReadingVisitor visitor =
       new SimpleAnnotationMetadataReadingVisitor(classLoader);
   // 这里使用SimpleAnnotationMetadataReadingVisitor读取字节码文件，封装元数据
getClassReader(resource).accept(visitor, PARSING_OPTIONS);
this.resource = resource;
this.annotationMetadata = visitor.getMetadata();
}
// 读取字节码文件完成之后，封装SimpleAnnotationMetadata对象
public void visitEnd() {
String[] memberClassNames = StringUtils.toStringArray(this.memberClassNames);
MethodMetadata[] annotatedMethods = this.annotatedMethods.toArray(new MethodMetadata[0]);
MergedAnnotations annotations = MergedAnnotations.of(this.annotations);
this.metadata = new SimpleAnnotationMetadata(this.className, this.access,
this.enclosingClassName, this.superClassName, this.independentInnerClass,
this.interfaceNames, memberClassNames, annotatedMethods, annotations);
}

MethodMetadata接口

封装方法的元数据。

实现类：

• StandardMethodMetadata - MethodMetadata implementation that uses standard reflection to introspect a given Method.

• SimpleMethodMetadata - ASM based.

StandardMethodMetadata

使用反射方式获取方法元数据。

SimpleMethodMetadata

基于SimpleAnnotationMetadataReadingVisitor读取的字节码数据，封装方法元数据。

来源：https://blog.csdn.net/xuguofeng2016/article/details/128322849