一、概述
@EnableAutoConfiguration注解是Spring Boot中配置自动装载的总开关。本文将从@EnableAutoConfiguration入手，尝试通过源码分析增强对Spring Boot的理解。
所用版本：Spring Boot 2.2.0.M5 + Spring Framework 5.2.0.RC1
1. 功能的实现：（Spring Boot部分）
boot.autoconfigure.EnableAutoConfiguration注解
-> @Import了一个AutoConfigurationImportSelector实例
-> AutoConfigurationImportSelector类（implement
ImportSelector），实现了
selectImports() 方法，用来筛选被@Import的Configuration类（减去exclude等）
2. 接口的调度：（Spring部分）
context.annotation.ConfigurationClassParser类的parse() 方法
-> 调用对应不同BeanDefinition类型的parse() 方法
| -> 调用context.annotation.ConfigurationClassParser.doProcessConfigurationClass()方法处理ConfigurationClass
| -> 调用
processImports()方法来处理所有@Import注解
| -> 遍历每个@Import标签，生成被注入的
ImportSelector子类的实例
| -> 对于普通ImportSelector，调用其
selectImport()方法，筛掉exclude的，再嵌套
processImports()，对需要被@Import的类的@Import注解进行处理
| -> 对于DefferedImportSelector，只加入deferredImportSelectors列表中
-> 对defferedImportImportSelectors调用相应handler的process()方法进行处理
-> 对DefferedImportImportSelector调用
processImports()
3. 接口在框架中的位置：（其中一条路径，由顶向下）
【Spring Boot部分】
boot.SpringApplication.main() 或 ApplicationStarter.main()
boot.SpringApplication.run()
boot.SpringApplication.refreshContext()
boot.SpringApplication.refresh()
boot.web.servlet.context.ServletWebServerApplicationContext.refresh()
【Spring部分】
context.support.AbstractApplicationContext.refresh()
context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors()
context.support.PostPreprocessorRegistrationDelegate.invokeBeanFactoryPostProcessors()
context.annotation.ConfigurationClassPostProcessor.postProcessBeanFactory()
context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions()
context.annotation.ConfigurationClassParser.parse()
（正是上一小结所述接口）
二、源码细节
(SpringBoot) boot.autoconfigure.EnableAutoConfiguration注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class) // 载入selector,识别AutoConfigutaion类并import
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
// @interface的参数,以方法形式声明
Class[] exclude() default {};
String[] excludeName() default {};
}
@EnableAutoConfiguration注解有两个参数和一个属性变量。
注入了一个AutoConfigurationImportSelector类实例，看起来应该是用于筛选AutoConfiguration的Import的。
@AutoConfigurationPackage待以后另行分析。
(SpringBoot) boot.autoconfigure.AutoConfigurationImportSelector类
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
//......
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
// 如果AutoConfiguration没开,返回{}
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
// 将spring-autoconfigure-metadata.properties的键值对配置载入到PropertiesAutoConfigurationMetadata对象中并返回
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
// 基于各种配置计算需要import的configuration和exclusion
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
// 判断AudoConfiguration是否开启
protected boolean isEnabled(AnnotationMetadata metadata) {
if (getClass() == AutoConfigurationImportSelector.class) {
// 如果配置文件中有"spring.boot.enableautoconfiguration",返回该字段的值;否则返回true
return getEnvironment().getProperty(EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class, true);
}
return true;
}
protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
// 获取注解的属性值
AnnotationAttributes attributes = getAttributes(annotationMetadata);
// 从META-INF/spring.factories文件中获取EnableAutoConfiguration所对应的configurations
List configurations = getCandidateConfigurations(annotationMetadata, attributes);
// 去重,List转Set再转List
configurations = removeDuplicates(configurations);
// 从注解的exclude/excludeName属性中获取排除项
Set exclusions = getExclusions(annotationMetadata, attributes);
// 对于不属于AutoConfiguration的exclude报错
checkExcludedClasses(configurations, exclusions);
// 从configurations去除exclusions
configurations.removeAll(exclusions);
// 由所有AutoConfigurationImportFilter类的实例再进行一次筛选,去
configurations = filter(configurations, autoConfigurationMetadata);
// 把AutoConfigurationImportEvent绑定在所有AutoConfigurationImportListener子类实例上
fireAutoConfigurationImportEvents(configurations, exclusions);
// 返回(configurations, exclusions)组
return new AutoConfigurationEntry(configurations, exclusions);
}
// ......
}
可见selectImports()是AutoConfigurationImportSelector的核心函数，其核心功能就是获取spring.factories中EnableAutoConfiguration所对应的Configuration类列表，由@EnableAutoConfiguration注解中的exclude/excludeName参数筛选一遍，再由AutoConfigurationImportFilter类所有实例筛选一遍，得到最终的用于Import的configuration和exclusion。
该函数是被谁调用的呢？在org.springframework.context.annotation.ConfigurationClassParser类中被processImports()调用，而processImports()函数被doProcessConfigurationClass()调用。下面从doProcessConfigurationClass() 看起。
(Spring) context.annotation.ConfigurationClassParser.doProcessConfigurationClass()方法
其中，configClass是一个ConfigurationClass实例，记录了bean name（返回的bean名）和meta data（配置数据）；sourceClass是简单封装后的有注解的类，主要方便对类的注解的使用，初始值是封装过的configClass。
doProcessConfigurationClass() 对ConfigurationClass进行了各种配置，包括process @ComponentScan, process @Bean, process @Import等等。如果该SourceClass有父类，返回父类，否则返回null。
@Nullable
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
throws IOException {
// ......
// Process any @Import annotations
processImports(configClass, sourceClass, getImports(sourceClass), true);
// ......
// 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;
}
其中，processImports()方法的第三个参数中，getImports()方法嵌套的遍历了sourceClass的注解，搜集所有@Import注解的值，即被Import的类名集合。
private SetgetImports(SourceClass sourceClass) throws IOException { Set imports = new LinkedHashSet<>(); Set visited = new LinkedHashSet<>(); collectImports(sourceClass, imports, visited); return imports; } private void collectImports(SourceClass sourceClass, Set imports, Set visited) throws IOException { // 如果是头一次添加 if (visited.add(sourceClass)) { for (SourceClass annotation : sourceClass.getAnnotations()) { String annName = annotation.getMetadata().getClassName(); // 当前注解的类名是否是Import if (!annName.equals(Import.class.getName())) { // 嵌套遍历被Import的类 collectImports(annotation, imports, visited); } } // 增加Import注解的值，即被Import的类名 imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value")); } }
(Spring) context.annotation.ConfigurationClassParser.processImports()方法
processImports() 的第一个参数configClass，是上层函数processConfigurationClass()的唯一参数，即被处理的Configuration类。第二个参数currentSourceClass是configClass的SourceClass类封装。第三个参数是嵌套遍历出的所有需要被Import的类。第四个参数指定是否检查循环import。
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass, CollectionimportCandidates, boolean checkForCircularImports) { // ...... for (SourceClass candidate : importCandidates) { // 如果candidate（即被@Import的类）是ImportSelector的子类 if (candidate.isAssignable(ImportSelector.class)) { // Candidate class is an ImportSelector -> delegate to it to determine imports Class candidateClass = candidate.loadClass(); // 生成candidate class（一个ImportSelector子类）的实例 ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class); // 将ImportSelector子类实例挂载为对应功能的Aware类（用于消息通知？） ParserStrategyUtils.invokeAwareMethods( selector, this.environment, this.resourceLoader, this.registry); // DeferredImportSelector是一种特殊的ImportSelector，这里单独处理 if (selector instanceof DeferredImportSelector) { // 挂到deferredImportSelectors列表上 this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector); } else { // 筛选符合要求的@Import的名字 String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata()); // 转换成名字对应的类的集合 Collection importSourceClasses = asSourceClasses(importClassNames); // 嵌套判断被@Import的类是不是还有@Import注解 processImports(configClass, currentSourceClass, importSourceClasses, false); } } // ...... } // ...... }
(Spring) context.annotation.ConfigurationClassParser.DefferedImportSelectorHandler私有类
deferredImportSelectors已被初始化为ArrayList<>()，因此全部走else分支。
private class DeferredImportSelectorHandler {
@Nullable
private List deferredImportSelectors = new ArrayList<>();
public void handle(ConfigurationClass configClass, DeferredImportSelector importSelector) {
DeferredImportSelectorHolder holder = new DeferredImportSelectorHolder(
configClass, importSelector); // 一个封装后的pair
// deferredImportSelectors被初始化为ArrayList<>()，所以if分支永远不会执行到？
if (this.deferredImportSelectors == null) {
DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
handler.register(holder); // 注册到DeferredImportSelectorGroupingHandler.configurationClasses中
handler.processGroupImports(); // -> processImports()
}
else {
// 所有的DeferredImportSelector类实例都挂到deferredImportSelectors列表上
this.deferredImportSelectors.add(holder);
}
}
// ......
}
那么deferredImportSelectors上挂载的DefferedImportSelector类是何时处理的呢？
doProcessConfigurationClass() 方法被processConfigurationClass() 调用，而processConfigurationClass() 被parse() 调用。可以看到在处理完所有的普通ImportSelector类后，即嵌套载入需要的被Import的类的实例之后，再统一处理DefferedImportSelector类。
(Spring) context.annotation.ConfigurationClassParser.processConfigurationClass() 方法
protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
// ...
// Recursively process the configuration class and its superclass hierarchy.
SourceClass sourceClass = asSourceClass(configClass);
do {
// 如果sourceClass有父类会返回父类，否则返回null
sourceClass = doProcessConfigurationClass(configClass, sourceClass);
}
while (sourceClass != null);
this.configurationClasses.put(configClass, configClass);
}
(Spring) context.annotation.ConfigurationClassParser.parse() 方法
public void parse(SetconfigCandidates) { // 处理所有的ImportSelector类，其中DeferredImportSelector类只挂在deferredImportSelectorHandler列表上不处理，其他均处理，即嵌套遍历被Import的类 for (BeanDefinitionHolder holder : configCandidates) { BeanDefinition bd = holder.getBeanDefinition(); try { if (bd instanceof AnnotatedBeanDefinition) { 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()); } } catch (BeanDefinitionStoreException ex) { throw ex; } catch (Throwable ex) { throw new BeanDefinitionStoreException( "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex); } } // 处理deferredImportSelectorHandler上挂着的DeferredImportSelector类 this.deferredImportSelectorHandler.process(); } protected final void parse(@Nullable String className, String beanName) throws IOException { Assert.notNull(className, "No bean class name for configuration class bean definition"); MetadataReader reader = this.metadataReaderFactory.getMetadataReader(className); processConfigurationClass(new ConfigurationClass(reader, beanName)); } protected final void parse(Class clazz, String beanName) throws IOException { processConfigurationClass(new ConfigurationClass(clazz, beanName)); } protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException { processConfigurationClass(new ConfigurationClass(metadata, beanName)); }
再来看看DeferredImportSelector实例是如何被处理的。
(Spring) context.annotation.ConfigurationClassParser.DefferedImportSelectorHandler私有类
和普通ConfigurationClass一样，DefferedImportSelector最后也是先注册到列表中，再依次嵌套处理，只不过在import前多了一个根据order排序。
// 对@Order(...)进行排序 private static final ComparatorDEFERRED_IMPORT_COMPARATOR = (o1, o2) -> AnnotationAwareOrderComparator.INSTANCE.compare(o1.getImportSelector(), o2.getImportSelector()); private class DeferredImportSelectorHandler { // ... public void process() { List deferredImports = this.deferredImportSelectors; this.deferredImportSelectors = null; try { if (deferredImports != null) { DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler(); deferredImports.sort(DEFERRED_IMPORT_COMPARATOR); // 添加到handler的configurationClasses列表中 deferredImports.forEach(handler::register); // 对handler中每个grouping的每个configClass，调用processImports() handler.processGroupImports(); } } finally { this.deferredImportSelectors = new ArrayList<>(); } } }
DONE.