SpringBoot（Ⅱ）——@SpringBootApplication注解+自动装配原理+约定大于配置

1. @SpringBootApplication注解

@SpringBootApplication标注在某个类上说明这个类是SpringBoot的主配置类，SpringBoot就通过运行这个类的main方法来启动SpringBoot应用；

并且@Configuration注解中也有@Component注解，所以这个主启动类/主配置类本质上也是一个bean类

1.1 注解源码分析

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//package org.springframework.boot.autoconfigure;import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.beans.factory.support.BeanNameGenerator;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.FilterType;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.core.annotation.AliasFor;@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {@Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}
), @Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {@AliasFor(annotation = EnableAutoConfiguration.class)Class<?>[] exclude() default {};@AliasFor(annotation = EnableAutoConfiguration.class)String[] excludeName() default {};@AliasFor(annotation = ComponentScan.class,attribute = "basePackages")String[] scanBasePackages() default {};@AliasFor(annotation = ComponentScan.class,attribute = "basePackageClasses")Class<?>[] scanBasePackageClasses() default {};@AliasFor(annotation = ComponentScan.class,attribute = "nameGenerator")Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;@AliasFor(annotation = Configuration.class)boolean proxyBeanMethods() default true;
}

1.1.1 @Target

@Target({ElementType.TYPE})：表示只能标注在类上

1.1.2 @Retention

@Retention(RetentionPolicy.RUNTIME)：表示此注解可以一直保留到JVM内存中
@Documented：运行javadoc命令时会带上@SpringBootApplication注解，不标注则不会带上

1.1.3 @Inherited

@Inherited：这个注解只能标注在注解上，表示如果一个类用上了@Inherited修饰的注解，那么其子类也会继承这个注解（接口和实现类的父子关系不生效不行）

1.1.4 @SpringBootConfiguration

@SpringBootConfiguration：加载SpringBoot的配置。继承自@Configuration，二者功能也一致，标注当前类是配置类，并会将当前类内声明的一个或多个以@Bean注解标记的方法的实例对象纳入到spring容器中，并且实例对象名就是方法名。

@SpringBootConfiguration是springboot的注解，而@Configuration是spring的注解

纳入spring容器的方式是通过动态代理，proxyBeanMethods即是此含义（CGLIB）

更详细的解释：Spring框架中的@Configuration参数proxyBeanMethods

1.1.5 @EnableAutoConfiguration(自动装配的核心注解，非常非常重要)

SpringBoot有两大核心：依赖大于配置&&自动装配，

@EnableAutoConfiguration：这个注解表示开启自动装配功能

在这里插入图片描述

当spring boot扫描到@EnableAutoConfiguration注解时则会将spring-boot-autoconfigure.jar/META-INF/spring.factories文件中org.springframework.boot.autoconfigure.EnableAutoConfiguration对应的value里的所有xxxConfiguration类加载到IOC容器中。而xxxAutoConfiguration类一般都会有@ConditionalOnxxx注解，通过这些条件注解来判断是否真正的创建xxxConfiguration对象。spring boot的各种spring-boot-starter-xxx.jar也正是居于此注解来达到自动装配的目的。

在这里插入图片描述

# 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.Neo4jReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
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.netty.NettyAutoConfiguration,\
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.sql.init.SqlInitializationAutoConfiguration,\
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.ReactiveMultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebSessionIdResolverAutoConfiguration,\
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

这些标签都是可以ctrl点进去的，每一个都对应一个xxxAutoConfiguration配置类，这些类在这些包中
在这里插入图片描述

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";Class<?>[] exclude() default {};String[] excludeName() default {};
}

1.1.5.1 @AutoConfigurationPackage

@AutoConfigurationPackage：自动配置包，导入的组件由AutoConfigurationPackages.Registrar.class决定

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import({AutoConfigurationPackages.Registrar.class})
public @interface AutoConfigurationPackage {String[] basePackages() default {};Class<?>[] basePackageClasses() default {};
}

AutoConfigurationPackages是一个抽象类，Registrar是其静态内部类。

AutoConfigurationPackage注解的作用是将添加该注解的类所在的package 作为自动配置package 进行管理。

将启动类所在的包+子包都注册到自动装配包中
在这里插入图片描述

        public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {AutoConfigurationPackages.register(registry, (String[])(new PackageImports(metadata)).getPackageNames().toArray(new String[0]));}

通过元数据AnnotationMetadata获取被 @Import 注解的类的注解信息，即注解元数据，进而获取包名，将包名的string数组注册到自动导入包中

这个注解不是很重要，只是定义了一些自动装配的包规则，AutoConfigurationImportSelector才是完成自动装配的重点

1.1.5.2 AutoConfigurationImportSelector

@Import，给容器中导入一个组件；
将所有需要导入的组件以全类名的方式返回；这些组件就会被添加到容器中；会给容器中导入非常多的自动配置类（xxxAutoConfiguration）；就是给容器中导入这个场景需要的所有组件，并配置好这些组件；

将spring-boot-autoconfigure.jar/META-INF/spring.factories文件中org.springframework.boot.autoconfigure.EnableAutoConfiguration对应的value里的所有xxxConfiguration类加载到IOC容器中。而xxxAutoConfiguration类一般都会有@ConditionalOnxxx注解，通过这些条件注解来判断是否真正的创建xxxConfiguration对象。

2. 自动装配原理

自动装配主要是装配两部分的bean组件：本项目内的+依赖jar包内的

依赖jar包又可以分为spring-boot-autoconfigure包+其他

@AutoConfigurationPackage注解将添加该注解的类所在的package 作为自动配置package 进行管理，保存自动配置类的全限定包名以供之后的使用，比如给JPA entity扫描器用来扫描开发人员通过注解@Entity定义的entity类

SpringBoot启动类自动标注了该注解，所以当SpringBoot应用启动时默认会将启动类所在的package作为自动配置的package，提供给后续扫描bean组件的注解：@Controller,@Mapper,@Service等使用

所以@AutoConfigurationPackage负责自动装配本项目内的bean组件，@Import(AutoConfigurationImportSelector.class)负责自动装配pom依赖项目的bean组件

SpringBoot启动的时候加载主配置类，开启了自动配置功能 @EnableAutoConfiguration
@EnableAutoConfiguration注解利用EnableAutoConfigurationImportSelector给容器中导入一些组件。查看selectImports()方法的内容；

// 获取候选的配置
List configurations = getCandidateConfigurations(annotationMetadata, attributes);

SpringFactoriesLoader.loadFactoryNames()
扫描所有jar包类路径下 META‐INF/spring.factories
把扫描到的这些文件的内容包装成properties对象

从properties中获取到EnableAutoConfiguration.class类（类名）对应的值，然后把他们添加在容器中
对每一个自动配置类进行自动配置功能

根据当前不同的条件判断，决定这个配置类是否生效；

一旦这个配置类生效，这个配置类就会给容器中添加各种组件

这些组件的属性是从对应的properties类中获取的，这些类里面的每一个属性又是和配置文件绑定的；

3. 约定大于配置

如果有些值你没有配置的话，那程序会取一个默认值，换句话说，能取默认值的配置，就不需要配置了，这个默认值就是约定。

约定可以减少很多配置

约定优于配置（convention over configuration），也称作按约定编程，是一种软件设计范式，旨在减少软件开发人员需做决定的数量，获得简单的好处，而又不失灵活性。

在SpringBoot中，约定大于配置可以从以下几方面来理解：

SpringBoot会加载大量的自动配置类，这些配置类会根据@ConditionalXXX注解默认加载到bean容器中（自动装配+默认配置）

想要修改，可以从resources下的application.yaml中修改配置

在spring boot中提供了一套默认配置，不需要手动去写xml配置文件，不写xml项目也能跑起来。

只有默认配置不能满足我们的需求时，才会去修改配置（自定义yaml文件）

maven的默认目录结构也体现了约定大于配置的思想

 /src/main/java目录用来存放java源文件src/main/resources目录用来存放资源文件，如application.yml文件，mybatis的*mapper.xml文件/src/test/java目录用来存放java测试文件/src/test/resources目录用来存放测试资源文件/target目录为项目的输出位置

约定：springboot配置默认值使用，除非你自己写了，这样可以减少很多配置，而且更加灵活
配置：没有默认配置，完全用自己的