一、分析spring-boot-starter-parent
按住Ctrl点击pom.xml中的spring-boot-starter-parent，跳转到了spring-boot-starter-parent的pom.xml，xml配
置如下（只摘抄了部分重点配置）：
org.springframework.boot
spring-boot-dependencies
2.0.3.RELEASE
../../spring-boot-dependencies
这里他还有个爸爸也就是SpringBoot的爷爷
按住Ctrl点击pom.xml中的spring-boot-starter-dependencies，跳转到了spring-boot-starter-dependencies的
pom.xml，xml配置如下（只摘抄了部分重点配置）：
org.yaml
snakeyaml
${snakeyaml.version}
redis.clients
jedis
${jedis.version}
org.jetbrains.kotlin
kotlin-maven-plugin
${kotlin.version}
org.springframework.boot
spring-boot-maven-plugin
2.0.3.RELEASE
从上面的spring-boot-starter-dependencies的pom.xml中我们可以发现，一部分坐标的版本、依赖管理、插件管理已经定义好，所以我们的SpringBoot工程继承spring-boot-starter-parent后已经具备版本锁定等配置了。所以起步依赖的作用就是进行依赖的传递
二、分析spring-boot-starter-web
按住Ctrl点击pom.xml中的spring-boot-starter-web，跳转到了spring-boot-starter-web的pom.xml，xml配置如
下（只摘抄了部分重点配置）：
org.springframework.boot
spring-boot-starters
2.0.3.RELEASE
org.springframework.boot
spring-boot-starter-tomcat
2.0.3.RELEASE
compile
org.hibernate.validator
hibernate-validator
6.0.10.Final
compile
org.springframework
spring-web
5.0.7.RELEASE
compile
org.springframework
spring-webmvc
5.0.7.RELEASE
compile
从上面的spring-boot-starter-web的pom.xml中我们可以发现，spring-boot-starter-web就是将web开发要使用的spring-web、spring-webmvc等坐标进行了“打包”，这样我们的工程只要引入spring-boot-starter-web起步依赖的坐标就可以进行web开发了，同样体现了依赖传递的作
三、自动配置原理解析
按住Ctrl点击查看启动类MySpringBootApplication上的注解@SpringBootApplication
@SpringBootApplication
public class DemoApplication {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
}
注解@SpringBootApplication的源码
@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
)
其中，
@SpringBootConfiguration：等同与@Configuration，既标注该类是Spring的一个配置类
@EnableAutoConfiguration：SpringBoot自动配置功能开启
@ComponentScan：组件扫描配置 约定 当前所在的包及子包下的都会被扫描到
详解@EnableAutoConfiguration
按住Ctrl点击查看注解@EnableAutoConfiguration
@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 {};
其中，@Import(AutoConfigurationImportSelector.class) 导入了AutoConfigurationImportSelector类
按住Ctrl点击查看AutoConfigurationImportSelector源码
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
private static final String[] NO_IMPORTS = new String[0];
private static final Log logger = LogFactory.getLog(AutoConfigurationImportSelector.class);
private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
private ConfigurableListableBeanFactory beanFactory;
private Environment environment;
private ClassLoader beanClassLoader;
private ResourceLoader resourceLoader;
public AutoConfigurationImportSelector() {
}
protected List getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
List 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，SpringFactoriesLoader.loadFactoryNames 方法的作用就是从META-INF/spring.factories文件中读取指定类对应的类名称列表
spring.factories 文件中有关自动配置的配置信息如下：
上面配置文件存在大量的以Configuration为结尾的类名称，这些类就是存有自动配置信息的类，而
SpringApplication在获取这些类名后再加载
我们以ServletWebServerFactoryAutoConfiguration为例来分析源码
@Configuration
@AutoConfigureOrder(-2147483648)
@ConditionalOnClass({ServletRequest.class})
@ConditionalOnWebApplication(
type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class})
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, EmbeddedTomcat.class, EmbeddedJetty.class, EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
public ServletWebServerFactoryAutoConfiguration() {
}
@Bean
public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties) {
return new ServletWebServerFactoryCustomizer(serverProperties);
}
其中，
@EnableConfigurationProperties(ServerProperties.class) 代表加载ServerProperties服务器配置属性类
进入ServerProperties.class源码如下：
@ConfigurationProperties(
prefix = "server",
ignoreUnknownFields = true
)
public class ServerProperties {
private Integer port;
private InetAddress address;
@NestedConfigurationProperty
private final ErrorProperties error = new ErrorProperties();
private Boolean useForwardHeaders;
private String serverHeader;
private int maxHttpHeaderSize = 0;
private Duration connectionTimeout;
@NestedConfigurationProperty
private Ssl ssl;
@NestedConfigurationProperty
private final Compression compression = new Compression();
@NestedConfigurationProperty
private final Http2 http2 = new Http2();
private final ServerProperties.Servlet servlet = new ServerProperties.Servlet();
private final ServerProperties.Tomcat tomcat = new ServerProperties.Tomcat();
private final ServerProperties.Jetty jetty = new ServerProperties.Jetty();
private final ServerProperties.Undertow undertow = new ServerProperties.Undertow();
其中，
prefix = "server" 表示SpringBoot配置文件中的前缀，SpringBoot会将配置文件中以server开始的属性映射到该类
的字段中。映射关系如下：