pom.xml：

spring-boot-dependencies -> spring-boot-starter-parent

spring-boot-dependencies：核心依赖在父工程，引入依赖时不需要指定版本

启动器：SpringBoot 的启动场景

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

比如 spring-boot-starter-web，自动导入 web 环境所有依赖

SpringBoot 会将所有的功能场景都变成一个个的启动器

主程序：

@SpringBootApplication，标注这个类是一个 SpringBoot 的应用，启动类下的所有资源被导入

Application.class，反射加载这个类的对象

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

//@SpringBootApplication：标注这个类是一个SpringBoot的应用，启动类下的所有资源被导入
@SpringBootApplication
public class HelloWorldApplication {

    public static void main(String[] args) {
        //将SpringBoot应用启动，反射加载这个类的对象
        SpringApplication.run(HelloWorldApplication.class, args);
    }

}

 注解：（源码）

1. @SpringBootApplication -> @SpringBootConfiguration（SpringBoot 的配置）

-> @Configuration（Spring 的配置类）-> @Component（Spring 的一个组件）

2. @SpringBootApplication -> @EnableAutoConfiguration（自动装配）

-> @AutoConfigurationPackage（自动装配包)

-> @Import({AutoConfigurationPackages.Registrar.class})（自动配置包注册，自动注册包）

3. @SpringBootApplication -> @EnableAutoConfiguration（自动装配）

-> @Import({AutoConfigurationImportSelector.class})（自动配置导入选择器，自动导入包的核心）

获取所有的配置：

List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);

获取候选的配置：

    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = ImportCandidates.load(AutoConfiguration.class, this.getBeanClassLoader()).getCandidates();
        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports. If you are using a custom packaging, make sure that file is correct.");
        return configurations;
    }

所有的自动配置类都在这

# ApplicationContext 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

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.autoconfigure.integration.IntegrationPropertiesEnvironmentPostProcessor

# 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

# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.data.redis.RedisUrlSyntaxFailureAnalyzer,\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jooq.NoDslContextBeanFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.MissingR2dbcPoolDependencyFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.MultipleConnectionPoolConfigurationsFailureAnalyzer,\
org.springframework.boot.autoconfigure.r2dbc.NoConnectionFactoryBeanFailureAnalyzer,\
org.springframework.boot.autoconfigure.ssl.BundleContentNotWatchableFailureAnalyzer

# 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

# DataSource Initializer Detectors
org.springframework.boot.sql.init.dependency.DatabaseInitializerDetector=\
org.springframework.boot.autoconfigure.flyway.FlywayMigrationInitializerDatabaseInitializerDetector

# Depends on Database Initialization Detectors
org.springframework.boot.sql.init.dependency.DependsOnDatabaseInitializationDetector=\
org.springframework.boot.autoconfigure.batch.JobRepositoryDependsOnDatabaseInitializationDetector,\
org.springframework.boot.autoconfigure.quartz.SchedulerDependsOnDatabaseInitializationDetector,\
org.springframework.boot.autoconfigure.session.JdbcIndexedSessionRepositoryDependsOnDatabaseInitializationDetector

SpringBoot 所有自动装配都是在启动的时候扫描并加载

spring.factories 所有的自动装配类都在这，但不一定生效，要判断条件是否成立

只有导入了对应的 start，就有对应的启动器，自动装配就会生效，最后配置成功

SpringBoot 在启动时，从类路径 /META-INF/spring.factories 获取指定的值

spring-boot-autoconfigure.jar 会把所有需要导入的组件，以类名的方式返回，这些组件就会被添加到容器

将这些自动配置的类导入容器，自动配置就会生效

（一个小插件 Smart Input，注释自动转换中文）