目标：

1. @SpringBootApplication注解认识
2. 了解SpringBoot的启动流程
   • 了解SpringFactoriesLoader对META-INF/spring.factories的反射加载
   • 认识AutoConfigurationImportSelector这个ImportSelector
3. starter的认识和使用

SpringBoot 启动原理

springboot项目启动主类如下

@SpringBootApplication
@ComponentScan("com.example")
@EnableScheduling
public class DemoApiApplication {public static void main(String[] args) {SpringApplication.run(DemoApiApplication.class, args);}
}

主要就是@SpringBootApplication这个注解

@SpringBootApplication 注解分析

• @SpringBootApplication

@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 {

• @SpringBootConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {}

可以看到全注解类的@Configuration注解

• @EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

@EnableAutoConfiguration注解

EnableAutoConfiguration是一个组合注解，用@Import把AutoConfigurationImportSelector导入容器中，SpringBoot启动的时候会加载所有的selector并执行selectImports方法，这个方法会加载META-INF/spring.factories中配置的EnableAutoConfiguration，从而实现加载自动配置

而mybatis-spring-boot-starter、spring-boot-starter-web等组件的META-INF文件下均含有spring.factories文件，自动配置模块中，SpringFactoriesLoader收集到文件中的类全名并返回一个类全名的数组，返回的类全名通过反射被实例化，就形成了具体的工厂实例，工厂实例来生成组件具体需要的bean。

AutoConfigurationImportSelector类

public class AutoConfigurationImportSelectorimplements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,BeanFactoryAware, EnvironmentAware, Ordered {

AutoConfigurationImportSelector实现了ImportSelector(选择器)和BeanClassLoaderAware(bean类加载器中间件)

关于ImportSelector在https://blog.csdn.net/qq_26437925/article/details/144865082 这篇文章中有分析到，即在使用@Import注解来注册bean的时候（Import注解的值可以是ImportSelector或者DeferredImportSelector的实现类）spring容器会实例化这个实现类，并执行其selectImports方法；

具体是在 spring refresh 12 个方法中的invokeBeanFactoryPostProcessors(beanFactory);会执行ConfigurationClassPostProcessor的processConfigBeanDefinitions方法中

eg:AutoConfigurationImportSelector扫描出mybatis-spring-starter的spring.factories配置

debug Springboot启动过程

new SpringApplication类

/*** Create a new {@link SpringApplication} instance. The application context will load* beans from the specified primary sources (see {@link SpringApplication class-level}* documentation for details. The instance can be customized before calling* {@link #run(String...)}.* @param resourceLoader the resource loader to use* @param primarySources the primary bean sources* @see #run(Class, String[])* @see #setSources(Set)*/@SuppressWarnings({ "unchecked", "rawtypes" })public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {this.resourceLoader = resourceLoader;Assert.notNull(primarySources, "PrimarySources must not be null");this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));this.webApplicationType = WebApplicationType.deduceFromClasspath();// 通过SpringFactoriesLoader，获取到所有META-INF/spring.factories中的ApplicationContextInitializer，并实例化setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));// 通过SpringFactoriesLoader，获取到所有META-INF/spring.factories中的ApplicationListener，并实例化setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));// 获取执行当前main方法的类，也就是启动类this.mainApplicationClass = deduceMainApplicationClass();}

• 判断应用类型为 WebApplicationType.SERVLET

---

• 利用SpringFactoriesLoader加载所有META-INF/spring.factories中的ApplicationContextInitializer
• SpringFactoriesLoader加载所有META-INF/spring.factories中的ApplicationListener

附：SpringFactoriesLoader

其原理AI回答如下：

附：spring-boot的spring.factories内容

# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

SpringApplication类的run方法流程

使用的spring-boot-starter版本如下

<parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.1.4.RELEASE</version></parent>

• org.springframework.boot.SpringApplication#run(java.lang.String…)

/*** Run the Spring application, creating and refreshing a new* {@link ApplicationContext}.* @param args the application arguments (usually passed from a Java main method)* @return a running {@link ApplicationContext}*/public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();configureHeadlessProperty();SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting();try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners,applicationArguments);configureIgnoreBeanInfo(environment);Banner printedBanner = printBanner(environment);context = createApplicationContext();exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,new Class[] { ConfigurableApplicationContext.class }, context);prepareContext(context, environment, listeners, applicationArguments,printedBanner);refreshContext(context);afterRefresh(context, applicationArguments);stopWatch.stop();if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);}listeners.started(context);callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, listeners);throw new IllegalStateException(ex);}try {listeners.running(context);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, null);throw new IllegalStateException(ex);}return context;}

1. 首先遍历执行所有通过SpringFactoriesLoader，在当前classpath下的META-INF/spring.factories中查找所有可用的SpringApplicationRunListeners并实例化。调用它们的starting()方法，通知这些监听器SpringBoot应用启动。

2. 创建并配置当前SpringBoot应用将要使用的Environment，包括当前有效的PropertySource以及Profile。

3. 遍历调用所有的SpringApplicationRunListeners的environmentPrepared()的方法，通知这些监听器SpringBoot应用的Environment已经完成初始化。

4. 打印SpringBoot应用的banner，SpringApplication的showBanner属性为true时，如果classpath下存在banner.txt文件，则打印其内容，否则打印默认banner。

5. 根据启动时设置的applicationContextClass和在initialize方法设置的webEnvironment，创建对应的applicationContext。

6. 创建异常解析器，用在启动中发生异常的时候进行异常处理(包括记录日志、释放资源等)。

7. 设置SpringBoot的Environment，注册Spring Bean名称的序列化器BeanNameGenerator，并设置资源加载器ResourceLoader，通过SpringFactoriesLoader加载ApplicationContextInitializer初始化器，调用initialize方法，对创建的ApplicationContext进一步初始化。

8. 调用所有的SpringApplicationRunListeners的contextPrepared方法，通知这些Listener当前ApplicationContext已经创建完毕。

9. 最核心的一步，将之前通过@EnableAutoConfiguration获取的所有配置以及其他形式的IoC容器配置加载到已经准备完毕的ApplicationContext。

10. 调用所有的SpringApplicationRunListener的contextLoaded方法，加载准备完毕的ApplicationContext。

11. 调用refreshContext（即spring容器refresh方法），并注册一个关闭Spring容器的钩子ShutdownHook，当程序在停止的时候释放资源（包括：销毁Bean，关闭SpringBean的创建工厂等）
    注： 钩子可以在以下几种场景中被调用：
    1）程序正常退出
    2）使用System.exit()
    3）终端使用Ctrl+C触发的中断
    4）系统关闭
    5）使用Kill pid命令杀死进程

获取当前所有ApplicationRunner和CommandLineRunner接口的实现类，执行其run方法
遍历所有的SpringApplicationRunListener的finished()方法，完成SpringBoot的启动。

prepareEnvironment

环境准备的时候会new ApplicationEnvironmentPreparedEvent()并广播这个事件

而ConfigFileApplicationListener‌是Spring Boot中的一个监听器，主要负责配置文件的加载和处理。它通过监听特定的应用事件，来加载和解析配置文件，并将解析后的配置信息添加到Spring的环境中。会监听到

如下：

加载application.properties文件，具体使用的是spring-boot的spring.factories中配置的org.springframework.boot.env.PropertiesPropertySourceLoader

org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

PropertiesPropertySourceLoader怎么实例化出来的？

1. 事件监听触发后，使用EnvironmentPostProcessor后处理加载配置，后处理通过SpringFactoriesLoader加载获得到
   
2. 然后添加配置new Loader()

/*** Add config file property sources to the specified environment.* @param environment the environment to add source to* @param resourceLoader the resource loader* @see #addPostProcessors(ConfigurableApplicationContext)*/
protected void addPropertySources(ConfigurableEnvironment environment,ResourceLoader resourceLoader) {RandomValuePropertySource.addToEnvironment(environment);new Loader(environment, resourceLoader).load();
}

Loader构造函数：

Loader(ConfigurableEnvironment environment, ResourceLoader resourceLoader) {this.environment = environment;this.placeholdersResolver = new PropertySourcesPlaceholdersResolver(this.environment);this.resourceLoader = (resourceLoader != null) ? resourceLoader: new DefaultResourceLoader();this.propertySourceLoaders = SpringFactoriesLoader.loadFactories(PropertySourceLoader.class, getClass().getClassLoader());
}

3. PropertySourcesPlaceholdersResolver通过SpringFactoriesLoader load出来

this.propertySourceLoaders = SpringFactoriesLoader.loadFactories(PropertySourceLoader.class, getClass().getClassLoader());

printBanner

createApplicationContext

根据webApplicationType创建ConfigurableApplicationContext，具体为：
org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext

prepareContext

• profile信息设置并打印，否则使用默认的default
  
• 加载启动类

refreshContext

private void refreshContext(ConfigurableApplicationContext context) {refresh(context);if (this.registerShutdownHook) {try {context.registerShutdownHook();}catch (AccessControlException ex) {// Not allowed in some environments.}}
}

如下图走到spring容器的refresh方法：

ImportSelector 的 selectImports 方法

其中List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);一句如下

/*** Return the auto-configuration class names that should be considered. By default* this method will load candidates using {@link SpringFactoriesLoader} with* {@link #getSpringFactoriesLoaderFactoryClass()}.* @param metadata the source metadata* @param attributes the {@link #getAttributes(AnnotationMetadata) annotation* attributes}* @return a list of candidate configurations*/
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,AnnotationAttributes attributes) {List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), 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;
}

使用SpringFactoryLoader，读取META-INF/spring.factories文件里所配置的EnableAutoConfiguration。经过exclude和filter等操作，最终确定要装配的类

Spring的SpringFactoriesLoader工厂的加载机制类似java提供的SPI机制一样，是Spring提供的一种加载方式。只需要在classpath路径下新建一个文件META-INF/spring.factories，并在里面按照properties格式填写好接口和实现类即可通过SpringFactoriesLoader来实例化相应的Bean。其中key可以是接口、注解、或者抽象类的全名。value为相应的实现类，当存在多个实现类时，用","进行分割。

loadFactoryNames方法如下：

/*** Load the fully qualified class names of factory implementations of the* given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given* class loader.* @param factoryClass the interface or abstract class representing the factory* @param classLoader the ClassLoader to use for loading resources; can be* {@code null} to use the default* @see #loadFactories* @throws IllegalArgumentException if an error occurs while loading factory names*/
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {String factoryClassName = factoryClass.getName();return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {MultiValueMap<String, String> result = cache.get(classLoader);if (result != null) {return result;}try {Enumeration<URL> urls = (classLoader != null ?classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));result = new LinkedMultiValueMap<>();while (urls.hasMoreElements()) {URL url = urls.nextElement();UrlResource resource = new UrlResource(url);Properties properties = PropertiesLoaderUtils.loadProperties(resource);for (Map.Entry<?, ?> entry : properties.entrySet()) {List<String> factoryClassNames = Arrays.asList(StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));result.addAll((String) entry.getKey(), factoryClassNames);}}cache.put(classLoader, result);return result;}catch (IOException ex) {throw new IllegalArgumentException("Unable to load factories from location [" +FACTORIES_RESOURCE_LOCATION + "]", ex);}
}

后续就能完成bean的自动注入了

再次说明starter

SpringBoot将所有的常见开发功能，分成了一个个场景启动器（starter），这样我们需要开发什么功能，就导入什么场景启动器依赖即可

SpringBoot starter机制

SpringBoot中的starter能够抛弃以前繁杂的配置，将其统一集成进starter，应用者只需要在maven中引入starter依赖，SpringBoot就能自动扫描到要加载的信息并启动相应的默认配置。starter让我们摆脱了各种依赖库的处理，需要配置各种信息的困扰。SpringBoot会自动通过classpath路径下的类发现需要的Bean，并注册进IOC容器。SpringBoot提供了针对日常企业应用研发各种场景的spring-boot-starter依赖模块。所有这些依赖模块都遵循着约定成俗的默认配置，并允许我们调整这些配置，即遵循"约定大于配置"的理念。

自定义一个starter并引用

• 其/META-INF/spring.factories的内容

#-------starter自动装配---------
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.demo.starter.config.DemoConfig

• DemoConfig类如下

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//package com.demo.starter.config;import com.demo.starter.properties.DemoProperties;
import com.demo.starter.service.DemoService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;@Configuration
@EnableConfigurationProperties({DemoProperties.class})
@ConditionalOnProperty(prefix = "demo",name = {"isopen"},havingValue = "true"
)
public class DemoConfig {@Autowiredprivate DemoProperties demoProperties;public DemoConfig() {}@Bean(name = {"demo"})public DemoService demoService() {return new DemoService(this.demoProperties.getSayWhat(), this.demoProperties.getToWho());}
}

• 引用并测试

	// 引入自定义的startercompile files('src/libs/demo-spring-boot-starter-0.0.1-RELEASE.jar')

• 使用&测试

@RestController
public class DemoControl {@Resource(name = "demo")private DemoService demoService;@GetMapping("/say")public String sayWhat() {return demoService.say();}}