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一、SpringBoot啟動的時候加載主配置類，開啟了自動配置的功能

@SpringBootApplication public class SpringBoot02Application {public static void main(String[] args) {SpringApplication.run(SpringBoot02Application.class, args);} }

ctrl+右鍵點擊@SpringBootApplication查看到如下內(nèi)容

二、源碼分析之自動配置底層原理

@EnableAutoConfiguration作用：？
1.ctrl+右鍵點擊@EnableAutoConfiguration該注解查看到如下內(nèi)容

@EnableAutoConfiguration:用@AutoConfigurationImportSelector給容器導(dǎo)入一些組件
2.ctrl+右鍵點擊AutoConfigurationImportSelector.class看到以下內(nèi)容

public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return NO_IMPORTS;} else {AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}}

以上就是selectImports方法中的內(nèi)容
其中的AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
我們可以查看調(diào)用該類中的getAutoConfigurationEntry方法中的內(nèi)容進(jìn)行分析，源碼如下所示

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {if (!this.isEnabled(annotationMetadata)) {return EMPTY_ENTRY;} else {AnnotationAttributes attributes = this.getAttributes(annotationMetadata);List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);configurations = this.removeDuplicates(configurations);Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);this.checkExcludedClasses(configurations, exclusions);configurations.removeAll(exclusions);configurations = this.getConfigurationClassFilter().filter(configurations);this.fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);}}

其中的List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes); 它的意思是獲取候選的配置
ctrl+右鍵點擊getCandidateConfigurations追蹤該源碼，可以看到如下內(nèi)容

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");return configurations;}

其中的SpringFactoriesLoader.loadFactoryNames（）作用是：？
ctrl+右鍵點擊loadFactoryNames()查看源碼如下

ClassLoader：類加載器（具體內(nèi)容可以查看jvm相關(guān)內(nèi)容）
分析
根據(jù)上圖我們可以看到70行返回時調(diào)用了loadSpringFactories方法，緊接著在該方法中的81行從類路徑下加載了META-INF/spring.factories資源

所以SpringFactoriesLoader.loadFactoryNames（）作用是：掃描所有jar包類路徑下META-INF/spring.factories

那它掃描該路徑下的META-INF/spring.factories是干什么呢？

我們來查看這幾行核心代碼

Enumeration urls = classLoader.getResources("META-INF/spring.factories");while(urls.hasMoreElements()) {URL url = (URL)urls.nextElement();UrlResource resource = new UrlResource(url);Properties properties = PropertiesLoaderUtils.loadProperties(resource);

通過掃描類路徑下的META-INF/spring.factories獲得url，然后遍歷每一個url，最后把這些url轉(zhuǎn)換為Properties資源文件的形式

List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());

我們上面說了SpringFactoriesLoader.loadFactoryNames（）的作用，現(xiàn)在來看一下里面的參數(shù)第一個參數(shù)this.getSpringFactoriesLoaderFactoryClass()，點進(jìn)去查看源碼

打開導(dǎo)入的第二個spring-boot jar包發(fā)現(xiàn)了上圖返回的EnableAutoConfiguration

所以自此就將META-INF/spring.factories下的每一個xxxAutoConfiguration的類加入到了容器中；讓他們做自動配置（如下圖）

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.Neo4jReactiveDataAutoConfiguration,\ org.springframework.boot.autoconfigure.data.neo4j.Neo4jReactiveRepositoriesAutoConfiguration,\ 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.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.neo4j.Neo4jAutoConfiguration,\ org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\ org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\ org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,\ org.springframework.boot.autoconfigure.r2dbc.R2dbcTransactionManagerAutoConfiguration,\ 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

每一個配置類的功能；
以HttpEncodingAutoConfiguration為例解析自動配置原理
找到該類查看源碼，可以看到幾個注解

@Configuration( proxyBeanMethods = false ) @EnableConfigurationProperties({ServerProperties.class}) @ConditionalOnWebApplication(type = Type.SERVLET ) @ConditionalOnClass({CharacterEncodingFilter.class}) @ConditionalOnProperty(prefix = "server.servlet.encoding",value = {"enabled"},matchIfMissing = true ) public class HttpEncodingAutoConfiguration {

1.@Configuration( //表示這是一個配置類，和自己手動編寫的配置文件一樣，可以給容器添加組件
2.@EnableConfigurationProperties({ServerProperties.class})：啟動指定類的
@ConfigurationProperties功能
ctrl+右鍵點擊ServerProperties查看源碼如下

所有在配置文件中能配置的屬性都是在xxxProperties類中封裝著，能配置什么就可以參照某個功能對應(yīng)的屬性類

3.@ConditionalOnWebApplication：根據(jù)不同的條件，如果滿足指定的條件，整個配置類里面的配置才會生效；判斷該應(yīng)用是否是web應(yīng)用

4.@ConditionalOnClass({CharacterEncodingFilter.class})：判斷當(dāng)前項目有沒有這個類
CharacterEncodingFilter：SpringMVC中在web.xml配置的編碼過濾器

5.@ConditionalOnProperty(
prefix = “server.servlet.encoding”,
value = {“enabled”},
matchIfMissing = true
)
判斷配置文件是否存在某個配置server.servlet.encoding；如果不存在，判斷也成立；即使是我們在配置文件中不配置matchIfMissing = true也是生效的；

總結(jié)：根據(jù)當(dāng)前不同的條件判斷，決定這個配置類是否生效；
如果生效了執(zhí)行以下代碼，例如（僅舉例一個功能）：

@Bean@ConditionalOnMissingBeanpublic CharacterEncodingFilter characterEncodingFilter() {CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();filter.setEncoding(this.properties.getCharset().name());filter.setForceRequestEncoding(this.properties.shouldForce(org.springframework.boot.web.servlet.server.Encoding.Type.REQUEST));filter.setForceResponseEncoding(this.properties.shouldForce(org.springframework.boot.web.servlet.server.Encoding.Type.RESPONSE));return filter;}

@Bean:給容器添加一個組件，這些組件的某些值從properties中獲取

@Bean@ConditionalOnMissingBeanpublic CharacterEncodingFilter characterEncodingFilter() {CharacterEncodingFilter filter = new OrderedCharacterEncodingFilter();filter.setEncoding(this.properties.getCharset().name());filter.setForceRequestEncoding(this.properties.shouldForce(org.springframework.boot.web.servlet.server.Encoding.Type.REQUEST));filter.setForceResponseEncoding(this.properties.shouldForce(org.springframework.boot.web.servlet.server.Encoding.Type.RESPONSE));return filter;}

比如這一句filter.setEncoding(this.properties.getCharset().name());我們可以查看里面的源碼看到該屬性

所以說根據(jù)源碼我們可以在application.properties中定義所需的屬性
比如以下 等等

server.servlet.encoding.enabled=true server.servlet.encoding.charset=UTF-8

根據(jù)當(dāng)前不同的條件判斷，決定這個配置類是否生效
一旦這個配置類生效；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properties類中獲取的，這些類中的每一個屬性是和配置文件綁定的

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1.Spring Boot會自動加載大量的自動配置類
2.我們需要的功能沒有SpringBoot默認(rèn)寫好的自動配置類
3.我們再來看這個自動配置類到底配置了哪些組件；（只要我們有的組件，我們就不需要再來配置了）
4.給容器中添加組件的時候，會從properties類中獲取某個屬性的值，我們就可以再配置文件中指定這些屬性的值
xxxAutoConfiguration：自動配置類；給容器中添加組件
xxxProperties：封裝配置文件中的相關(guān)屬性

參考-雷豐陽老師

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