简单点说就是Spring Boot根据依赖中的jar包，自动选择实例化某些配置，配置类必须有@Configuration注解。
说白了，还是实例化对象，只是实例化的是非main类之外的包。
另外，我们也可以按照自动装配的规范自己定义装配的类。
二、@EnableAutoConfiguration和 @Configuration 的区别@Configuration：表示作用的类是个配置类。我们平时也会写配置类，比如我们系统中的DataSourceConfig类，但是由于这个类是在Starter对应的子目录下，会自动加载，所以@EnableAutoConfiguration就作用不到了。
@EnableAutoConfiguration：是一个加载Starter目录包之外的需要Spring自动生成bean对象（是否需要的依据是"meta-INF/spring.factories"中org.springframework.boot.autoconfigure.EnableAutoConfiguration后面是有能找到那个bean）的带有@Configuration注解的类。一般就是对各种引入的spring-boot-starter依赖包指定的（spring.factories）类进行实例化。
三、@EnableAutoConfiguration案例1、Kafka自动装配
我们会发现，当我们在application.properties中配置spring.kafka.xxx，就可以在程序中直接使用了，说明Kafka已经被实例化了，问题来了，是什么时候实例化的？其实就是@EnableAutoConfiguration注解自动实例化的。实现类：KafkaAutoConfiguration
代码简单解读如下：
2、JMX自动装配
当我们引入Spring Boot Actuator相关包之后就能自动采集到Spring boot应用的相关运行指标了，怎么做到的？
其实这些指标都是通过JMX标准采集的，而Spring boot的自动装配也包含了JMX，即SpringApplicationAdminJmxAutoConfiguration。
四、spring-boot-starter与@EnableAutoConfiguration的关系spring-boot-starter：自动导入组件对应的maven包，目的主要是简单开发人员手动去配置maven各种依赖包，不仅麻烦，碰到版本匹配问题还很容易出错。
@EnableAutoConfiguration：需要依赖已经加载好的包进行bean实例化。但不一定需要依赖spring-boot-starter，自动手动把包加载也可以
五、关于实例化在Jfinal中我们需要通过Plugin的规范去生成对象到上下文中。
而在Spring boot中，我们可以通过@Configuration以及@Service，@Component等注解生成对象到IOC容器中。
未必所有实例化都是通过@EnableAutoConfiguration，@Configuration以及@Service，@Component等注解，也可以通过其他方式（说白了怎么加载类的问题，因为实例化最终就是找到类，然后通过反射生成对象而已）。
其他实例化方式其实可以从meta-INF/spring.factories中看出来，里面不仅有org.springframework.boot.autoconfigure.EnableAutoConfiguration，还有org.springframework.context.ApplicationContextInitializer（初始化的时候实例化），org.springframework.context.ApplicationListener（初始化监听的时候实例化），org.springframework.boot.diagnostics.FailureAnalyzer（应用启动失败分析类）等等。
但是对于这些实例化方式有个共同点，都需要在meta-INF/spring.factories中配置。
六、默认自动装配的类# 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.cloud.CloudAutoConfiguration, org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration, 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.CassandraRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration, org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration, org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration, org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration, org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration, org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration, org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration, org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration, 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.jest.JestAutoConfiguration, org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration, 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.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.flyway.FlywayAutoConfiguration, org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration, org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration, org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration, org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration, 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.mobile.DeviceResolverAutoConfiguration, org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration, org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration, org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration, org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration, org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration, org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration, org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration, org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration, org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration, org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration, org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration, org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration, org.springframework.boot.autoconfigure.session.SessionAutoConfiguration, org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration, org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration, org.springframework.boot.autoconfigure.social.linkedInAutoConfiguration, org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration, org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration, 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.DispatcherServletAutoConfiguration, org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration, org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration, org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration, org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration, org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration, org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration, org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration, org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration, org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration, org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration, org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration
参考：
Spring Boot @EnableAutoConfiguration和 @Configuration的区别
