SpringBoot面试之SpringBoot自动装配原理

SpringBoot自动装配原理

背景

最近因为各种原因，我又重新加入到了找工作的大军当中。昨天在面试的时候与面试官聊到我们项目都是基于SpringBoot开发的，然后面试官就顺口问了句："SpringBoot项目会引入许多的starter，比如：mybatis-plus-boot-starter等，它是怎么完成加载的？又是怎么完成按需加载的呢？"。当时听到这个问题有点懵，稳了一秒后反应过来，这不就是在问SpringBoot自动装配的原理吗？

引出问题

什么是SpringBoot自动装配

SpringBoot 定义了一套接口规范，这套规范规定：SpringBoot 在启动时会扫描外部引用 jar 包中的META-INF/spring.factories文件，将文件中配置的类型信息加载到 Spring 容器，并执行类中定义的各种操作。对于外部 jar 来说，只需要按照 SpringBoot 定义的标准，就能将自己的功能装置进 SpringBoot。

简单来说，SpringBoot在启动时会扫描META-INF/spring.factories文件，把配置文件中的的相关配置类信息加载到Spring容器中，交给容器管理。

SpringBoot自动装配的原理

核心注解@SpringBootApplication

@Target({ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(
            excludeFilters = {@ComponentScan.Filter(
                    type = FilterType.CUSTOM,
                    classes = {TypeExcludeFilter.class}
            ), @ComponentScan.Filter(
                    type = FilterType.CUSTOM,
                    classes = {AutoConfigurationExcludeFilter.class}
            )}
    )
    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;
    }

我们可以看到@SpringBootApplication注解下面包含了三个注解：

@SpringBootConfiguration：声明是启动类一个配置类

@EnableAutoConfiguration ：启用SpringBoot的自动装配机制

@ComponentScan ：扫描被@Component (@Service,@Controller)注解的 bean，注解默认会扫描启动类所在的包下所有的类 ，可以自定义不扫描某些 bean。如下图所示，容器中将排除TypeExcludeFilter和AutoConfigurationExcludeFilter

@EnableAutoConfiguration注解是实现自动装配的核心注解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
// 将main包下所有的组件注册Spring容器中
@AutoConfigurationPackage
// 加载自动装配的类
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
​
    Class<?>[] exclude() default {};
​
    String[] excludeName() default {};
}

@Import注解注入了一个类AutoConfigurationImportSelector.class，实现自动装配的核心代码就在AutoConfigurationImportSelector这个文件中。

AutoConfigurationImportSelector加载自动装配的类

AutoConfigurationImportSelector类的源码

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    public AutoConfigurationImportSelector() {
    }
​
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }
}

AutoConfigurationImportSelector是实现了DeferredImportSelector接口，我们看下DeferredImportSelector接口源码

public interface DeferredImportSelector extends ImportSelector {
    @Nullable
    default Class<? extends DeferredImportSelector.Group> getImportGroup() {
        return null;
    }
​
    public interface Group {
        void process(AnnotationMetadata metadata, DeferredImportSelector selector);
​
        Iterable<DeferredImportSelector.Group.Entry> selectImports();
​
        public static class Entry {
            private final AnnotationMetadata metadata;
            private final String importClassName;
​
            public Entry(AnnotationMetadata metadata, String importClassName) {
                this.metadata = metadata;
                this.importClassName = importClassName;
            }
​
           .........
        }
    }
}
​

DeferredImportSelector接口又继承了ImportSelector类，里面有一个核心的方法selectImports，获取自动注入类的集合。

public interface ImportSelector {
    String[] selectImports(AnnotationMetadata importingClassMetadata);
​
    @Nullable
    default Predicate<String> getExclusionFilter() {
        return null;
    }
}

从源码可以看出，ImportSelector类中的selectImports方法，是获取所有的需要被注册的bean的类的全名的数组集合。

接下来我们分析一下AutoConfigurationImportSelector类的ImportSelector方法

public String[] selectImports(AnnotationMetadata annotationMetadata) {
        // 是否开启自动装配
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            // 获取所有需要装配的bean
            AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }

getAutoConfigurationEntry方法我们需要重点关注：

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
    // 再次判断自动装配是否开启
    if (!this.isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    } else {
        // 获取 @EnableAutoConfiguration注解的 exclude 和 excludeName 属性
        AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
        // 获取该依赖下的 spring.factories配置文件的内容，所有的自动装配的类名
        List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
        // 删除重复的数据，去重的作用
        configurations = this.removeDuplicates(configurations);
        // 获取spring.autoconfigure.exclude 配置文件的内容并与exclude 和 excludeName 属性合并到一个集合中
        Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
        // 检验configurations中需要排除的内容
        this.checkExcludedClasses(configurations, exclusions);
        // 移除configurations中需要排除的内容
        configurations.removeAll(exclusions);
        // 再次对configurations中的内容进行过滤
        configurations = this.getConfigurationClassFilter().filter(configurations);
        // 关闭spring监听器中的自动装配事件
        this.fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }
}

这里我们重点关注一下getCandidateConfigurations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;
  }

SpringFactoriesLoader#loadFactoryNames方法传入EnableAutoConfiguration类和ClassLoader类加载器

    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        ClassLoader classLoaderToUse = classLoader;
        if (classLoader == null) {
            classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
        }
​
        String factoryTypeName = factoryType.getName();
        return (List)loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
    }

loadSpringFactories方法里是吧spring.factories里的相关配置类加载到容器中

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
        Map<String, List<String>> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }
​
        result = new HashMap<>();
        try {
            // 加载"META-INF/spring.factories"配置文件
            Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                // 将配置信息封装成properties对象
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    String factoryTypeName = ((String) entry.getKey()).trim();
                    String[] factoryImplementationNames =
                            StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
                    for (String factoryImplementationName : factoryImplementationNames) {
                        result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
                                .add(factoryImplementationName.trim());
                    }
                }
            }
​
            // Replace all lists with unmodifiable lists containing unique elements
            result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
                    .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
            // 将结果添加到本地缓存中
            cache.put(classLoader, result);
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
        return result;
    }

下面是spring.factores文件

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
​
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer
​
# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.autoconfigure.integration.IntegrationPropertiesEnvironmentPostProcessor
​
# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener
​
# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition
​
# 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.ReactiveMultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebSessionIdResolverAutoConfiguration,\
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
​
# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.data.redis.RedisUrlSyntaxFailureAnalyzer,\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jooq.NoDslContextBeanFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.MissingR2dbcPoolDependencyFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.MultipleConnectionPoolConfigurationsFailureAnalzyer,\
org.springframework.boot.autoconfigure.r2dbc.NoConnectionFactoryBeanFailureAnalyzer,\
org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer
​
# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider
​
# DataSource initializer detectors
org.springframework.boot.sql.init.dependency.DatabaseInitializerDetector=\
org.springframework.boot.autoconfigure.flyway.FlywayMigrationInitializerDatabaseInitializerDetector
​
# Depends on database initialization detectors
org.springframework.boot.sql.init.dependency.DependsOnDatabaseInitializationDetector=\
org.springframework.boot.autoconfigure.batch.JobRepositoryDependsOnDatabaseInitializationDetector,\
org.springframework.boot.autoconfigure.quartz.SchedulerDependsOnDatabaseInitializationDetector,\
org.springframework.boot.autoconfigure.session.JdbcIndexedSessionRepositoryDependsOnDatabaseInitializationDetector
​

读取到spring.factories中的信息之后返回，然后在对读取到的内容进行去重，过滤等操作。

总结

1. @SpringBootApplication注解下有三个注解，分别是@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan，自动装配的核心就是 @EnableAutoConfiguration注解
2. @EnableAutoConfiguration 注解下面通过@Import注解注入了一个ImportSelector的子类AutoConfigurationImportSelector类，里面实现了selectImports方法。然后调用getAutoConfigurationEntry 方法，然后是getCandidateConfigurations 方法发，然后是loadFactoryNames 方法，最后是loadSpringFactories方法，加载到META-INF/spring.factories文件中的自动配置类信息。
3. 拿到自动配置类的信息之后，然后根据@EnableAutoConfiguration注解的 exclude 和 excludeName属性过滤 spring.factories 配置文件的内容。后面还会对配置信息进行去重，过滤等操作，获取最终的配置信息集合。
4. 拿到过滤后的配置类信息之后，把这些类注册到Spring容器中，交给Spring容器进行管理。