1，来到代码入口：

package com.example.demo;

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

@SpringBootApplication
public class DemoApplication {

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

}

2，跟踪run方法：

2.1 实例化SpringApplication

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
   this.resourceLoader = resourceLoader; //这里是null
   Assert.notNull(primarySources, "PrimarySources must not be null");
   this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));//这里是DemoApplication.class,
   this.webApplicationType = WebApplicationType.deduceFromClasspath();//通过是否能加载相应的Class来判断应用的类型
   this.bootstrappers = new ArrayList<>(getSpringFactoriesInstances(Bootstrapper.class)); //加载SpringFactories文件中的配置类
   /*
         

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
   ClassLoader classLoader = getClassLoader();
   // Use names and ensure unique to protect against duplicates
   Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader)); //加载SpringFactories文件中的类名
   List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names); //创建实例
   AnnotationAwareOrderComparator.sort(instances);//按注解排序
   return instances;
}

    */
   setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class)); //加载初始化类
   setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//加载监听器
   this.mainApplicationClass = deduceMainApplicationClass();//更新main类
}

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
    Map<String, List<String>> result = (Map)cache.get(classLoader);
    if (result != null) {
        return result;
    } else {
        HashMap result = new HashMap();

        try {
            Enumeration urls = classLoader.getResources("META-INF/spring.factories");

            while(urls.hasMoreElements()) {
//jar:file:/C:/Users/Administrator/.m2/repository/org/springframework/boot/spring-boot/2.4.3/spring-boot-2.4.3.jar!/META-INF/spring.factories
                URL url = (URL)urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                /*
                     "org.springframework.boot.diagnostics.FailureAnalysisReporter" -> "org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter"
"org.springframework.boot.diagnostics.FailureAnalyzer" -> "org.springframework.boot.context.config.ConfigDataNotFoundFailureAnalyzer,org.springframework.boot.context.properties.IncompatibleConfigurationFailureAnalyzer,org.springframework.boot.context.properties.NotConstructorBoundInjectionFailureAnalyzer,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.diagnostic"
"org.springframework.boot.SpringBootExceptionReporter" -> "org.springframework.boot.diagnostics.FailureAnalyzers"
"org.springframework.boot.SpringApplicationRunListener" -> "org.springframework.boot.context.event.EventPublishingRunListener"
"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.DelegatingApplicationListener,org.springframework.boot.context.logging.LoggingApplicationListener,org.springframework.boot.env.EnvironmentPostProcessorApplicationListener,org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener"
"org.springframework.boot.logging.LoggingSystemFactory" -> "org.springframework.boot.logging.logback.LogbackLoggingSystem.Factory,org.springframework.boot.logging.log4j2.Log4J2LoggingSystem.Factory,org.springframework.boot.logging.java.JavaLoggingSystem.Factory"
"org.springframework.boot.context.config.ConfigDataLocationResolver" -> "org.springframework.boot.context.config.ConfigTreeConfigDataLocationResolver,org.springframework.boot.context.config.StandardConfigDataLocationResolver"
"org.springframework.boot.env.PropertySourceLoader" -> "org.springframework.boot.env.PropertiesPropertySourceLoader,org.springframework.boot.env.YamlPropertySourceLoader"
"org.springframework.boot.context.config.ConfigDataLoader" -> "org.springframework.boot.context.config.ConfigTreeConfigDataLoader,org.springframework.boot.context.config.StandardConfigDataLoader"
"org.springframework.context.ApplicationContextInitializer" -> "org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,org.springframework.boot.context.ContextIdApplicationContextInitializer,org.springframework.boot.context.config.DelegatingApplicationContextInitializer,org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer"
"org.springframework.boot.env.EnvironmentPostProcessor" -> "org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,org.springframework.boot.context.config.ConfigDataEnvironmentPostProcessor,org.springframework.boot.env.RandomValuePropertySourceEnvironmentPostProcessor,org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor,org.springframework.boot.reactor.DebugAgentEnvironmentPostProcessor" 
                  */
                Iterator var6 = properties.entrySet().iterator();

                while(var6.hasNext()) {
                    Entry<?, ?> entry = (Entry)var6.next();
                    String factoryTypeName = ((String)entry.getKey()).trim();
                    String[] factoryImplementationNames = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                    String[] var10 = factoryImplementationNames;
                    int var11 = factoryImplementationNames.length;

                    for(int var12 = 0; var12 < var11; ++var12) {
                        String factoryImplementationName = var10[var12];
                        ((List)result.computeIfAbsent(factoryTypeName, (key) -> {
                            return new ArrayList();
                        })).add(factoryImplementationName.trim());
                    }
                }
            } 
            result.replaceAll((factoryType, implementations) -> {
                return (List)implementations.stream().distinct().collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList));
            });
            cache.put(classLoader, result);
            return result;
        } catch (IOException var14) {
            throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var14);
        }
    }
}

2.2 进入run方法：

public ConfigurableApplicationContext run(String... args) {
   StopWatch stopWatch = new StopWatch();
   stopWatch.start(); //记录程序开始时间
   DefaultBootstrapContext bootstrapContext = createBootstrapContext();//执行Bootstrap初始化方法
   ConfigurableApplicationContext context = null;
   configureHeadlessProperty();
   SpringApplicationRunListeners listeners = getRunListeners(args);
   listeners.starting(bootstrapContext, this.mainApplicationClass); //启动监听类
   try {
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
      ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments); //配置环境变量
      configureIgnoreBeanInfo(environment);
      Banner printedBanner = printBanner(environment);
      context = createApplicationContext();//创建上下文
      context.setApplicationStartup(this.applicationStartup);
      prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);//加载项目代码
      refreshContext(context);//更新上下文
      afterRefresh(context, applicationArguments);
      stopWatch.stop();
      if (this.logStartupInfo) {
         new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
      }
      listeners.started(context);//运行监听器
      callRunners(context, applicationArguments);
   }
   catch (Throwable ex) {
      handleRunFailure(context, ex, listeners);
      throw new IllegalStateException(ex);
   }

   try {
      listeners.running(context);//运行监听器
   }
   catch (Throwable ex) {
      handleRunFailure(context, ex, null);
      throw new IllegalStateException(ex);
   }
   return context;
}