SpringBoot 自动配置原理

1. SpringBoot启动的时候加载主配置类，开启了自动配置功能@EnableAutoConfiguration

 2. @EnableAutoConfiguration的作用：

• 　　利用AutoConfigurationImportSelector给容器中导入一些组件；
• 　　可以参考selectimports()方法的内容；

• List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);获取候选配置

  

  SpringFactoriesLoader.loadFactoryNames();
  扫描所有jar包类路径下 META-INF/spring.factories;
  把扫描到的文件内容包装成properties对象;

  

  将类路径下 META-INF/spring.factories 里面配置的所有EnableAutoConfiguration的值加入到了容器中，如下：
  
  # 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
  
  # 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.cloud.CloudServiceConnectorsAutoConfiguration,
  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.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.ElasticsearchAutoConfiguration,
  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.ReactiveRestClientAutoConfiguration,
  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.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.jest.JestAutoConfiguration,
  org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,
  org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,
  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.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.groovy.template.GroovyTemplateAutoConfiguration,
  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.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.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
  
  # Failure analyzers
  org.springframework.boot.diagnostics.FailureAnalyzer=
  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.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

  每一个 xxxAutoConfiguration类都是容器的一个组件，都是加入到容器中；用他们来做自动配置。

3. 每一个自动配置类进行自动配置功能；

4. 以HttpEncodingAutoConfiguration为例解释自动配置：

@Configuration(proxyBeanMethods = false)　　//标识这是一个配置类，可以给容器添加组件
@EnableConfigurationProperties(HttpProperties.class)　　//启动指定类的ConfigurationProperties功能；
将配置文件中对应的值和HttpEncodingProperties绑定起来;并把HttpEncodingProperties加入到ioc容器中
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)　　//Spring底层@Conditional注解，
根据不同的条件，如果满足指定的条件，整个配置类里面的配置就会生效；这里是判断当前应用是否为web应用，如果是，则当前配置类生效

@ConditionalOnClass(CharacterEncodingFilter.class)　　//判断当前项目有没有这个类，
CharacterEncodingFilter：SpringMVC中进行乱码解决的过滤器
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)　　//判断
配置文件中是否存在某个配置 spring.http.encoding.enabled;如果不存在，判断也是成立的

public class HttpEncodingAutoConfiguration {

    private final HttpProperties.Encoding properties;
　　 //只有一个有参构造器，参数的值会默认从容器中拿
    public HttpEncodingAutoConfiguration(HttpProperties properties) {
        this.properties = properties.getEncoding();
    }

    @Bean　　//给容器中添加一个组件，这个组件的某些值需要从properties中获取
    @ConditionalOnMissingBean
    public CharacterEncodingFilter characterEncodingFilter() {
        CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
        filter.setEncoding(this.properties.getCharset().name());
        filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
        filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
        return filter;
    }