一、自動配置原理
1. SpringBoot啟動的時候加載主配置類，開啟了自動配置功能 @EnableAutoConfiguration
2. @EnableAutoConfiguration 作用
利用EnableAutoConfigurationImportSelector給容器中導入一些組件？
可以查看selectImports()方法的內容：
//獲取候選的配置 List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
將類路徑下 META-INF/spring.factories 里面配置的所有EnableAutoConfiguration的值加入到了容器中：
# 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
每一個這樣的 xxxAutoConfiguration類都是容器中的一個組件，都加入到容器中；用他們來做自動配置
3. 每一個自動配置類都會進行自動配置功能
4. 以HttpEncodingAutoConfiguration（Http編碼自動配置）為例解釋自動配置原理
@Configuration //表示這是一個配置類，以前編寫的配置文件一樣，也可以給容器中添加組件
@EnableConfigurationProperties(HttpProperties.class) //啟動指定類的ConfigurationProperties功能；將配置文件中對應的值和HttpProperties綁定起來；
並把HttpEncodingProperties加入到ioc容器中
@ConditionalOnWebApplication //Spring底層@Conditional注解（Spring注解版），根據不同的條件，如果滿足指定的條件，整個配置類里面的配置就會生效；
判斷當前應用是否是web應用，如果是，當前配置類生效
@ConditionalOnClass(CharacterEncodingFilter.class) //判斷當前項目有沒有這個類CharacterEncodingFilter；SpringMVC中進行亂碼解決的過濾器；
@ConditionalOnProperty(prefix = "spring.http.encoding", value = "enabled", matchIfMissing = true) //判斷配置文件中是否存在某個配置 spring.http.encoding.enabled；如果不存在，判斷也是成立的
//即使我們配置文件中不配置pring.http.encoding.enabled=true，也是默認生效的；
public class HttpEncodingAutoConfiguration {
//它已經和SpringBoot的配置文件映射了
private final HttpEncodingProperties properties;
//只有一個有參構造器的情況下，參數的值就會從容器中拿
public HttpEncodingAutoConfiguration(HttpEncodingProperties properties) {
this.properties = properties;
}
@Bean //給容器中添加一個組件，這個組件的某些值需要從properties中獲取
@ConditionalOnMissingBean(CharacterEncodingFilter.class) //判斷容器沒有這個組件？
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;
}
// 省略一些代碼
}
根據當前不同的條件判斷，決定這個配置類是否生效？
一但這個配置類生效，這個配置類就會給容器中添加各種組件；這些組件的屬性是從對應的properties類中獲取的，這些類里面的每一個屬性又是和配置文件綁定的。
5. 所有在配置文件中能配置的屬性都是在xxxxProperties類中封裝着，配置文件能配置什么就可以參照某個功能對應的這個屬性類。
總結：
- SpringBoot啟動會加載大量的自動配置類
- 我們看我們需要的功能有沒有SpringBoot默認寫好的自動配置類
- 我們再來看這個自動配置類中到底配置了哪些組件（只要我們要用的組件有，我們就不需要再來配置了）
- 給容器中自動配置類添加組件的時候，會從properties類中獲取某些屬性。我們就可以在配置文件中指定這些屬性的值。
- xxxxAutoConfigurartion：自動配置類，給容器中添加組件；
- xxxxProperties：封裝配置文件中相關屬性；
二、細節
1. @Conditional派生注解（Spring注解版原生的@Conditional作用）
作用：必須是@Conditional指定的條件成立，才給容器中添加組件，配置配里面的所有內容才生效。
| @Conditional擴展注解 | 作用（判斷是否滿足當前指定條件） |
|---|---|
| @ConditionalOnJava | 系統的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 | 滿足SpEL表達式指定 |
| @ConditionalOnClass | 系統中有指定的類 |
| @ConditionalOnMissingClass | 系統中沒有指定的類 |
| @ConditionalOnSingleCandidate | 容器中只有一個指定的Bean，或者這個Bean是首選Bean |
| @ConditionalOnProperty | 系統中指定的屬性是否有指定的值 |
| @ConditionalOnResource | 類路徑下是否存在指定資源文件 |
| @ConditionalOnWebApplication | 當前是web環境 |
| @ConditionalOnNotWebApplication | 當前不是web環境 |
| @ConditionalOnJndi | JNDI存在指定項 |
自動配置類必須在一定的條件下才能生效。
我們怎么知道哪些自動配置類生效？
我們可以通過啟用 debug=true屬性，來讓控制台打印自動配置報告，這樣我們就可以很方便的知道哪些自動配置類生效：
Positive matches:（自動配置類啟用的）
-----------------
AopAutoConfiguration matched:
- @ConditionalOnProperty (spring.aop.auto=true) matched (OnPropertyCondition)
AopAutoConfiguration.ClassProxyingConfiguration matched:
- @ConditionalOnMissingClass did not find unwanted class 'org.aspectj.weaver.Advice' (OnClassCondition)
- @ConditionalOnProperty (spring.aop.proxy-target-class=true) matched (OnPropertyCondition)
Negative matches:（沒有啟動，沒有匹配成功的自動配置類）
-----------------
ActiveMQAutoConfiguration:
Did not match:
- @ConditionalOnClass did not find required class 'javax.jms.ConnectionFactory' (OnClassCondition)
AopAutoConfiguration.AspectJAutoProxyingConfiguration:
Did not match:
- @ConditionalOnClass did not find required class 'org.aspectj.weaver.Advice' (OnClassCondition)
也可以通過以下方法打印出SpringBoot注入的有那么Bean Name：
@Bean
public CommandLineRunner commandLineRunner(ApplicationContext ctx) {
return new CommandLineRunner() {
@Override
public void run(String... args) throws Exception {
System.out.println("Let's inspect the beans provided by Spring Boot:");
String[] beanNames = ctx.getBeanDefinitionNames();
Arrays.sort(beanNames);
for (String beanName : beanNames) {
System.out.println(beanName);
}
}
};
}
總結：springBoot啟動時，利用@EnableAutoConfiguration加載主配置類，開啟了自動配置功能 ，
將類路徑下 META-INF/spring.factories 里面配置的所有EnableAutoConfiguration的值加入到了容器中：（自動配置類），
再由這些自動配置配區加載各自properties配置文件，所以是
factories ---> properties
參考：https://www.cnblogs.com/myitnews/p/12383746.html