【Spring Boot源码分析】@EnableAutoConfiguration注解（一）@AutoConfigurationImportSelector注解的处理

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<String> 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<String> 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 Set<SourceClass> getImports(SourceClass sourceClass) throws IOException {
		Set<SourceClass> imports = new LinkedHashSet<>();
		Set<SourceClass> visited = new LinkedHashSet<>();
		collectImports(sourceClass, imports, visited);
		return imports;
	}

	private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> 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,
			Collection<SourceClass> importCandidates, 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<SourceClass> 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<DeferredImportSelectorHolder> 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(Set<BeanDefinitionHolder> configCandidates) {
    	// 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 Comparator<DeferredImportSelectorHolder> DEFERRED_IMPORT_COMPARATOR =
			(o1, o2) -> AnnotationAwareOrderComparator.INSTANCE.compare(o1.getImportSelector(), o2.getImportSelector());

	private class DeferredImportSelectorHandler {
    	// ...
    	public void process() {
			List<DeferredImportSelectorHolder> 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.