SpringBoot自动配置原理（源码分析）

Posted 2021-06-29 二木成林

SpringBoot的启动是从@SpringBootApplication注解开始的。

进入到该注解中，发现由重要的三个注解@SpringBootConfiguration、@EnableAutoConfiguration和@CompoentScan注解组成。

其中@SpringBootConfiguration内是使用了一个@Configuration注解，表示当前是一个配置类。

其中@ComponentScan指定扫描哪些组件，属于Spring的注解。

其中@EnableAutoConfiguration注解见名知义，这是一个与自动配置有关的注解，打开它，里面有一个@AutoConfigurationPackage注解，在该注解中指定了默认的包规则。

为什么说@AutoConfigurationPackage注解指定了默认的包规则呢，打开该注解，查看到@Import({Registrar.class})，将Registrar类作为组件引入。

打开Registrar类，在registerBeanDefinitions()方法中的唯一一句代码上打上断点，再用debug模式运行项目，计算(new AutoConfigurationPackages.PackageImports(metadata)).getPackageNames()的值就是我们主启动类所在的包路径，这就是默认的包规则。

在@EnableAutoConfiguration注解类中除了@AutoFigurationPackage外，还有@Import({AutoConfigurationImportSelector.class})，将AutoConfigurationImportSelector类作为Bean组件导入，是整个自动配置的核心。

进入AutoConfigurationImportSelector类的源码中，关注getAutoConfigurationEntry()方法。该方法给容器批量导入一系列组件。调用List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes)获取到所有需要导入到容器中的配置类，在该行代码打上断点，debug运行查看，可以看到导入了127个配置类。

再点击进入到getCandidateConfigurations()方法内

再点击进入到loadFactoryNames()方法内

看到loadSpringFactories()方法，进入

可以查看到该方法利用加载所有组件，从META-INF/spring.factories位置来加载一个文件，默认扫描我们当前系统里面所有META-INF/spring.factories位置的文件。

那么我们就可以在第三方加包中找到spring-boot-autoconfigure-2.3.4.RELEASE.jar包中有META-INF/spring.factories文件

其中spring.factories文件关于自动配置的内容如下，刚好127行，在该文件中写死了spring-boot一启动就要给容器中加载的所有配置类。

# 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.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.Neo4jRepositoriesAutoConfiguration,\\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\\
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcDataAutoConfiguration,\\
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcRepositoriesAutoConfiguration,\\
org.springframework.boot.autoconfigure.data.r2dbc.R2dbcTransactionManagerAutoConfiguration,\\
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.orm.jpa.HibernateJpaAutoConfiguration,\\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\\
org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,\\
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.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

虽然我们127个场景的所有自动配置启动的时候都会默认全部加载，但实际上并没有全部装配，因为所有的XXXAutoConfiguration自动配置类都使用了条件装配注解@Conditional及其子注解，最终会按需配置，而不是配置所有。

比如我们来查看DispatcherServletAutoConfiguration自动配置类，看名字是与springmvc自动配置相关的。

点击进入查看源码，发现一个条件注解@ConditionalOnClass，要求必须有DispatcherServlet类该类的自动化配置才会生效，我们是导入了的，所以没有任何问题。

但没有导入的就不会生效了，比如redis的自动化配置

如果我们要定制自动化配置，只需要查看有哪些属性，然后在application.properties或application.yaml文件中设置相应的属性值即可。

例如我们查看文件上传的自动配置类MultipartAutoConfiguration，发现它里面所有属性配置都在MultipartProperties类中，其他自动配置类的属性也在对应的XXXProperties类中，还有，注意使用的前缀是"spring.servlet.multipart"

点开MultipartProperties类，发现可以配置的属性如下：

我们就可以直接在application.application文件中进行配置了，配置后就会自动生效

配置后我们发现MultipartProperties类中的左侧边栏中多了一些小图标

总结：

• SpringBoot先加载所有的自动配置类xxxAutoConfiguration。
• 但每个自动配置类都装配了条件注解@Conditional，按照条件进行生效，默认都会绑定配置文件指定的值，在xxxProperties类中拿值，而xxxProperties又和application.properties配置文件进行了绑定。
• 生效的自动配置类会给容器中装配很多组件。
• 只要容器中有这些组件，相当于这些功能就有了。
• 如果我们想要定制化配置，可以直接使用@Bean替换底层的组件，也可以看这个组件获取的配置文件什么值就去application.properties中修改。

参考链接：

• 自动配置原理入门
• Spring Boot面试杀手锏————自动配置原理