之前的Blog【SpringBoot学习笔记 一】SpringBoot基本概念和项目初始化里我提到，SpringBoot初始化的项目结构包含如下四部分，也就是只要包含如下四部分就可以启动一个最简单的SpringBoot
- 一个配置文件：application.properties ，管理相关配置信息，在【SpringBoot学习笔记 二】YAML格式文件配置方式详解中介绍到SpringBoot的YAML配置是如何编写又是如何自动绑定到代码实体中的
- 一个测试类：SpringbootApplicationTests，用来进行项目测试，这个暂时还没有用到，不过也不涉及核心机制
- 一个Maven的坐标文件： pom.xml，项目的相关依赖坐标，在【SpringBoot学习笔记 一】SpringBoot基本概念和项目初始化中介绍到了什么是场景启动器，SpringBoot的依赖是如何管理的
- 程序的主启动类：SpringbootApplication，是程序的启动入口，也是SpringBoot自动化配置的出发点。
我们回顾下原来搭建一个springmvc的hello-word的web项目（xml配置的）我们需要在pom中导入各种依赖，然后各个依赖有可能还会存在版本冲突需要各种排除。当你历尽千辛万苦的把依赖解决了，然后还需要编写web.xml、springmvc.xml配置文件等。我们只想写个hello-word项目而已，却把一大把的时间都花在了配置文件和jar包的依赖上面。大大的影响了我们开发的效率，以及加大了web开发的难度。为了简化这复杂的配置、以及各个版本的冲突依赖关系，SpringBoot就应运而生。我们现在通过idea创建一个springboot项目只要分分钟就解决了，你不需要关心各种配置（基本实现零配置），让你真正的实现了开箱即用，这都要归功于 Spring Boot 的自动化配置，也是SpringBoot的核心原理，搞懂了这个就搞懂了SpringBoot到底是怎么实现的。
Spring Factories 机制
Spring Boot 的自动配置是基于 Spring Factories 机制实现的。Spring Factories 机制是 Spring Boot 中的一种服务发现机制，这种扩展机制与 Java SPI 机制十分相似。Spring Boot 会自动扫描所有 Jar 包类路径下 META-INF/spring.factories 文件，并读取其中的内容，进行实例化，这种机制也是 Spring Boot Starter 的基础
Spring Factories文件格式
spring.factories 文件本质上与 properties 文件相似，其中包含一组或多组键值对（key=vlaue）
- key 的取值为接口的完全限定名
- value 的取值为接口实现类的完全限定名
- 一个接口可以设置多个实现类，不同实现类之间使用
，隔开
以Spring的TestContext框架配置为例：
文件的格式如下：
# Default TestExecutionListeners for the Spring TestContext Framework # org.springframework.test.context.TestExecutionListener = \ org.springframework.test.context.web.ServletTestExecutionListener,\ org.springframework.test.context.support.DirtiesContextBeforeModesTestExecutionListener,\ org.springframework.test.context.event.ApplicationEventsTestExecutionListener,\ org.springframework.test.context.support.DependencyInjectionTestExecutionListener,\ org.springframework.test.context.support.DirtiesContextTestExecutionListener,\ org.springframework.test.context.transaction.TransactionalTestExecutionListener,\ org.springframework.test.context.jdbc.SqlScriptsTestExecutionListener,\ org.springframework.test.context.event.EventPublishingTestExecutionListener # Default ContextCustomizerFactory implementations for the Spring TestContext Framework # org.springframework.test.context.ContextCustomizerFactory = \ org.springframework.test.context.web.socket.MockServerContainerContextCustomizerFactory,\ org.springframework.test.context.support.DynamicPropertiesContextCustomizerFactory
文件中配置的内容过长，为了阅读方便而手动换行时，为了防止内容丢失可以使用
\。
Spring Factories 实现原理
spring-core 包里定义了 SpringFactoriesLoader 类，这个类会扫描所有 Jar 包类路径下的 META-INF/spring.factories 文件，并获取指定接口的配置。
loadFactories方法能够获取指定接口的实现类对象，它调用loadFactoryNames方法
public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader) { Assert.notNull(factoryType, "'factoryType' must not be null"); ClassLoader classLoaderToUse = classLoader; if (classLoader == null) { classLoaderToUse = SpringFactoriesLoader.class.getClassLoader(); } // 调用loadFactoryNames获取接口的实现类 List<String> factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse); if (logger.isTraceEnabled()) { logger.trace("Loaded [" + factoryType.getName() + "] names: " + factoryImplementationNames); } // 遍历 factoryNames 数组，创建实现类的对象 List<T> result = new ArrayList(factoryImplementationNames.size()); Iterator var5 = factoryImplementationNames.iterator(); //排序 while(var5.hasNext()) { String factoryImplementationName = (String)var5.next(); result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse)); } AnnotationAwareOrderComparator.sort(result); return result; }
loadFactoryNames方法能够根据接口获取其实现类类名的集合，它调用loadSpringFactories方法
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) { ClassLoader classLoaderToUse = classLoader; if (classLoader == null) { classLoaderToUse = SpringFactoriesLoader.class.getClassLoader(); } String factoryTypeName = factoryType.getName(); //获取自动配置类 return (List)loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
loadSpringFactories方法能够读取该项目中所有 Jar 包类路径下
META-INF/spring.factories文件的配置内容，并以 Map 集合的形式返回
private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) { Map<String, List<String>> result = (Map)cache.get(classLoader); if (result != null) { return result; } else { HashMap result = new HashMap(); try { //扫描所有 Jar 包类路径下的 META-INF/spring.factories 文件 Enumeration urls = classLoader.getResources("META-INF/spring.factories"); while(urls.hasMoreElements()) { URL url = (URL)urls.nextElement(); UrlResource resource = new UrlResource(url); //将扫描到的 META-INF/spring.factories 文件中内容包装成 properties 对象 Properties properties = PropertiesLoaderUtils.loadProperties(resource); Iterator var6 = properties.entrySet().iterator(); while(var6.hasNext()) { Map.Entry<?, ?> entry = (Map.Entry)var6.next(); //提取 properties 对象中的 key 值 String factoryTypeName = ((String)entry.getKey()).trim(); //提取 proper 对象中的 value 值（多个类的完全限定名使用逗号连接的字符串） // 使用逗号为分隔符转换为数组，数组内每个元素都是配置类的完全限定名 String[] factoryImplementationNames = StringUtils.commaDelimitedListToStringArray((String)entry.getValue()); String[] var10 = factoryImplementationNames; int var11 = factoryImplementationNames.length; //遍历配置类数组，并将数组转换为 list 集合 for(int var12 = 0; var12 < var11; ++var12) { String factoryImplementationName = var10[var12]; ((List)result.computeIfAbsent(factoryTypeName, (key) -> { return new ArrayList(); })).add(factoryImplementationName.trim()); } } } //将 propertise 对象的 key 与由配置类组成的 List 集合一一对应存入名为 result 的 Map 中 result.replaceAll((factoryType, implementations) -> { return (List)implementations.stream().distinct().collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)); }); cache.put(classLoader, result); //返回 result return result; } catch (IOException var14) { throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var14); } } }
SpringBoot自动配置实现原理
Spring Boot 自动化配置就是基于 Spring Factories 机制实现的，我们从注解开始一步一步跟进去：
@SpringBootApplication 注解
了解了Spring-Factories机制，我们再回过头来看看SpringBoot是如何利用该机制实现自动配置的。所有 Spring Boot 项目的主启动程序类上都使用了一个 @SpringBootApplication 注解，该注解是 Spring Boot 中最重要的注解之一 ，也是 Spring Boot 实现自动化配置的关键。
package com.example.springboot; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class SpringbootApplication { public static void main(String[] args) { SpringApplication.run(SpringbootApplication.class, args); } }
@SpringBootApplication 是一个组合元注解，其内容如下：
// // Source code recreated from a .class file by IntelliJ IDEA // (powered by FernFlower decompiler) // package org.springframework.boot.autoconfigure; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import org.springframework.beans.factory.support.BeanNameGenerator; import org.springframework.boot.SpringBootConfiguration; import org.springframework.boot.context.TypeExcludeFilter; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.FilterType; import org.springframework.context.annotation.ComponentScan.Filter; import org.springframework.core.annotation.AliasFor; @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @SpringBootConfiguration @EnableAutoConfiguration @ComponentScan( excludeFilters = {@Filter( type = FilterType.CUSTOM, classes = {TypeExcludeFilter.class} ), @Filter( type = FilterType.CUSTOM, classes = {AutoConfigurationExcludeFilter.class} )} ) //自动扫描并加载符合条件的组件或者bean ， 将这个bean定义加载到IOC容器中 public @interface SpringBootApplication { @AliasFor( annotation = EnableAutoConfiguration.class ) Class<?>[] exclude() default {}; @AliasFor( annotation = EnableAutoConfiguration.class ) String[] excludeName() default {}; @AliasFor( annotation = ComponentScan.class, attribute = "basePackages" ) String[] scanBasePackages() default {}; @AliasFor( annotation = ComponentScan.class, attribute = "basePackageClasses" ) Class<?>[] scanBasePackageClasses() default {}; @AliasFor( annotation = ComponentScan.class, attribute = "nameGenerator" ) Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class; @AliasFor( annotation = Configuration.class ) boolean proxyBeanMethods() default true; }
@EnableAutoConfiguration 注解
如上所述@SpringBootApplication 是一个组合元注解，还有@SpringBootConfiguration 和 @EnableAutoConfiguration，其中 @EnableAutoConfiguration 注解是 SpringBoot 自动化配置的核心所在。@EnableAutoConfiguration 注解用于开启 Spring Boot 的自动配置功能
// // Source code recreated from a .class file by IntelliJ IDEA // (powered by FernFlower decompiler) // package org.springframework.boot.autoconfigure; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import org.springframework.context.annotation.Import; @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @AutoConfigurationPackage @Import({AutoConfigurationImportSelector.class}) public @interface EnableAutoConfiguration { String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; Class<?>[] exclude() default {}; String[] excludeName() default {}; }
它使用 Spring 框架提供的 @Import 注解通过 AutoConfigurationImportSelector类（选择器）给容器中导入自动配置组件
AutoConfigurationImportSelector 类
AutoConfigurationImportSelector 类实现了 DeferredImportSelector 接口，AutoConfigurationImportSelector 中还包含一个静态内部类 AutoConfigurationGroup，它实现了 DeferredImportSelector 接口的内部接口 Group，AutoConfigurationImportSelector 类中包含 3 个主要方法
1 getImportGroup方法主要用于获取实现了 DeferredImportSelector.Group 接口的类
public Class<? extends Group> getImportGroup() { //获取实现了 DeferredImportSelector.Gorup 接口的 AutoConfigurationImportSelector.AutoConfigurationGroup 类 return AutoConfigurationImportSelector.AutoConfigurationGroup.class; }
2 process 方法通过调用 getAutoConfigurationEntry() 方法读取 spring.factories 文件中的内容，获得自动配置类的集合
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) { Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector, () -> { return String.format("Only %s implementations are supported, got %s", AutoConfigurationImportSelector.class.getSimpleName(), deferredImportSelector.getClass().getName()); }); //拿到 META-INF/spring.factories中的EnableAutoConfiguration，并做排除、过滤处理 //AutoConfigurationEntry里有需要引入配置类和排除掉的配置类，最终只要返回需要配置的配置类 AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector)deferredImportSelector).getAutoConfigurationEntry(annotationMetadata); //加入缓存,List<AutoConfigurationEntry>类型 this.autoConfigurationEntries.add(autoConfigurationEntry); Iterator var4 = autoConfigurationEntry.getConfigurations().iterator(); while(var4.hasNext()) { String importClassName = (String)var4.next(); //加入缓存，Map<String, AnnotationMetadata>类型 this.entries.putIfAbsent(importClassName, annotationMetadata); } }
getAutoConfigurationEntry方法通过调用 getCandidateConfigurations() 方法来获取自动配置类的完全限定名，并在经过排除、过滤等处理后，将其缓存到成员变量中
protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) { if (!this.isEnabled(annotationMetadata)) { return EMPTY_ENTRY; } else { //获取注解元数据中的属性设置 AnnotationAttributes attributes = this.getAttributes(annotationMetadata); //获取自动配置类 List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes); //删除list 集合中重复的配置类 configurations = this.removeDuplicates(configurations); //获取导入的配置类 Set<String> exclusions = this.getExclusions(annotationMetadata, attributes); //检查是否还存在排除配置类 this.checkExcludedClasses(configurations, exclusions); //删除排除的配置类 configurations.removeAll(exclusions); //获取过滤器，过滤配置类 configurations = this.getConfigurationClassFilter().filter(configurations); //出发自动化配置导入事件 this.fireAutoConfigurationImportEvents(configurations, exclusions); return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions); } }
在 getCandidateConfigurations方法中，根据 Spring Factories 机制调用 SpringFactoriesLoader 的 loadFactoryNames方法
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader()); Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct."); return configurations; }
根据 EnableAutoConfiguration.class （自动配置接口）获取其实现类（自动配置类）的类名的集合
protected Class<?> getSpringFactoriesLoaderFactoryClass() { return EnableAutoConfiguration.class; }
这个接口和类的映射关系在 spring-boot-autoconfigure-xxx.jar 类路径下的 META-INF/spring.factories 中设置如下：
....... # Auto Configure org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\ org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\ org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\ org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\ org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\ org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\ org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\ org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,\ org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\ org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\ org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\ org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\ org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRestClientAutoConfiguration,\ org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.neo4j.Neo4jReactiveDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.neo4j.Neo4jReactiveRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.r2dbc.R2dbcDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.r2dbc.R2dbcRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\ org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\ org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\ org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\ org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\ org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientAutoConfiguration,\ org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\ org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\ org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\ org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\ org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\ org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\ org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\ org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\ org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\ org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\ org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\ org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\ org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\ org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\ org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\ org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\ org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\ org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\ org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\ org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\ org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\ org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\ org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\ org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\ org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\ org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\ org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\ org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\ org.springframework.boot.autoconfigure.availability.ApplicationAvailabilityAutoConfiguration,\ org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\ org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\ org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\ org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\ org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\ org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\ org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\ org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\ org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\ org.springframework.boot.autoconfigure.neo4j.Neo4jAutoConfiguration,\ org.springframework.boot.autoconfigure.netty.NettyAutoConfiguration,\ org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\ org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\ org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,\ org.springframework.boot.autoconfigure.r2dbc.R2dbcTransactionManagerAutoConfiguration,\ org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\ org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\ org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\ org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\ org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\ org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\ org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\ org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\ org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\ org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\ org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\ org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\ org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\ org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\ org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\ org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\ org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\ org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\ org.springframework.boot.autoconfigure.sql.init.SqlInitializationAutoConfiguration,\ org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\ org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\ org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\ org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\ org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\ org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\ org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\ org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\ org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\ org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\ org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\ org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\ org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\ org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\ org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\ org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\ org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\ org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\ org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\ org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\ org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\ org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\ org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\ org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\ org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration .......
以上配置中，value 取值是由多个 xxxAutoConfiguration （使用逗号分隔）组成，每个 xxxAutoConfiguration 都是一个自动配置类。Spring Boot 启动时，会利用 Spring-Factories 机制，将这些 xxxAutoConfiguration 实例化并作为组件加入到容器中，以实现 Spring Boot 的自动配置
