Spring Boot 自动配置原理（精髓）

一、自动配置原理（掌握）　

1. SpringBoot启动项目会加载主配置类@SpringBootApplication，开启@EnableAutoConfiguration自动配置功能　　
2. @EnableAutoConfiguration作用：
   1. 通过使用@Import(EnableAutoConfigurationImportSelector.class)给容器加载一些组件。
   2. 通过查看EnableAutoConfigurationImportSelector的父类AutoConfigurationImportSelector的selectImports方法里面的代码：

List<String> configurations = getCandidateConfigurations(annotationMetadata,//获得候选的配置
                    attributes);

　　　　　　3.查看getCandidateConfigurations代码：List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());

 　　　　　　　　3.1 SpringFactoriesLoader.loadFactoryNames扫描类路径下jar包的 META-INF下的spring.factories

　　　　　　　　 3.2 将扫描到的这些内容包装成Properties对象

　　　　　　　　 3.3 从Propertiesz中获取到EnableAutoConfiguration，的值，并将它们添加到容器中。并不是全部都会添加到容器中组件都会生效。

　　3.将类路径下的1.5.19.RELEASEspring-boot-autoconfigure-1.5.19.RELEASE.jarMETA-INFspring.factories里面配置了EnableAutoConfiguration的值都加入容器中。如：

# 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

 　二、XXXAutoConfiguration中解释原理：

　　每一个xxxAutoConfiguration,都是一个组件，都加入到容器中，用它们来做自动配置。

　　以HttpEncodingAutoConfiguration为例

@Configuration　　　　//指定这是一个配置类
@EnableConfigurationProperties(HttpEncodingProperties.class)　　　　//启动指定类ConfigurationProperties功能，将prefix = "spring.http.encoding"

//对应的值和HttpEncodingProperties绑定起来，并将HttpEncodingProperties加入ioc容器中

@ConditionalOnWebApplication　　　　//判断当前应用是否是web应用，底层都是@Conditional
@ConditionalOnClass(CharacterEncodingFilter.class)　　//判断是否存在CharacterEncodingFilter这个类
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true)　　//判断配置文件是否存在这个配置
public class HttpEncodingAutoConfiguration {

   private final HttpEncodingProperties properties;　　//这个和手写配置文件映射了

   public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {//从容器中获取参数的值
      this.properties = properties;
   }

   @Bean　　//给容器添加一个组件，这个组件的某些值需要在properties配置文件中获取
   @ConditionalOnMissingBean(CharacterEncodingFilter.class)　　//判断容器是否有这个组件
   public CharacterEncodingFilter characterEncodingFilter() {
      CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();
      filter.setEncoding(this.properties.getCharset().name());　　//某些值需要在properties中获取

      filter.setForceRequestEncoding(this.properties.shouldForce(Type.REQUEST));
      filter.setForceResponseEncoding(this.properties.shouldForce(Type.RESPONSE));
      return filter;
   }

　　类的所有属性和手写的配置文件绑定：

　　

　　只有在类上的所有条件都满足了。该自动配置类才会生效。

　　一旦配置类生效，这个类就会在ioc容器中添加各种组件，这些组件的属性都是都是在对应手写的properties配置文件中获取的，类的所有属性和properties配置文件绑定了，所有的配置文件中能配置的键值都在XXXProperties类中封装，配置什么就可以参考对应类中属性如：

　　　　

　　　　　　如可以参考键值

　　　　　

 总结：

1. SpringBoot在启动时会加载大量的配置文件
2. 我们可以看我要的功能SpringBoot有没有帮我们加载，加载了那些组件
3. 给IOC容器加载自动配置会从Properties配置文件中获取属性

　　一句话：XXXAutoConfiguration自动配置类

　　　　　　XXXProperties封装的是配置文件对应的属性

怎么知道自动配置类有没有生效？在appliction.properties中加入这句代码>>"debug=true"

　　

　　

@Conditional派生注解（Spring注解版原生的@Conditional作用）

作用：必须是@Conditional指定的条件成立，才给容器中添加组件，配置配里面的所有内容才生效；

@Conditional扩展注解	作用（判断是否满足当前指定条件）
@ConditionalOnJava	系统的java版本是否符合要求
@ConditionalOnBean	容器中存在指定Bean；
@ConditionalOnMissingBean	容器中不存在指定Bean；
@ConditionalOnExpression	满足SpEL表达式指定
@ConditionalOnClass	系统中有指定的类
@ConditionalOnMissingClass	系统中没有指定的类
@ConditionalOnSingleCandidate	容器中只有一个指定的Bean，或者这个Bean是首选Bean
@ConditionalOnProperty	系统中指定的属性是否有指定的值
@ConditionalOnResource	类路径下是否存在指定资源文件
@ConditionalOnWebApplication	当前是web环境
@ConditionalOnNotWebApplication	当前不是web环境
@ConditionalOnJndi	JNDI存在指定项