public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
    return new SpringApplication(primarySources).run(args);
  }

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
　　　　 //保存主配置類
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
　　　　 //判斷當(dāng)前是否一個(gè)web應(yīng)用
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
　　　　 //從類路徑下找到META-INF/spring.factories配置的所有ApplicationContextInitializer；然后保存起來(lái)
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
　　　　　//從類路徑下找到ETA-INF/spring.factories配置的所有ApplicationListener　
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
　　　　　//從多個(gè)配置類中找到有main方法的主配置類　
    this.mainApplicationClass = deduceMainApplicationClass();
  }

public final class SpringFactoriesLoader {
  /**spring.factories的位置*/
  public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

  private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);
  /**
  * 緩存掃描后的結(jié)果， 注意這個(gè)cache是static修飾的，說(shuō)明是多個(gè)實(shí)例共享的
  * 其中MultiValueMap的key就是spring.factories中的key（比如org.springframework.boot.autoconfigure.EnableAutoConfiguration）, 
  * 其值就是key對(duì)應(yīng)的value以逗號(hào)分隔后得到的List集合（這里用到了MultiValueMap，他是guava的一鍵多值map， 類似Map<String, List<String>>）
  */
  private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();

  private SpringFactoriesLoader() {
  }
  
  /**
  * AutoConfigurationImportSelector及應(yīng)用的初始化器和監(jiān)聽(tīng)器里最終調(diào)用的就是這個(gè)方法，
  * 這里的factoryType是EnableAutoConfiguration.class、ApplicationContextInitializer.class、或ApplicationListener.class
  * classLoader是AutoConfigurationImportSelector、ApplicationContextInitializer、或ApplicationListener里的beanClassLoader
  */
  public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    String factoryTypeName = factoryType.getName();
    return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
  }
  
  /**
  * 加載 spring.factories文件的核心實(shí)現(xiàn)
  */
  private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    // 先從緩存獲取，如果獲取到了說(shuō)明之前已經(jīng)被加載過(guò)
    MultiValueMap<String, String> result = cache.get(classLoader);
    if (result != null) {
      return result;
    }

    try {
      // 找到所有jar中的spring.factories文件的地址
      Enumeration<URL> urls = (classLoader != null ?
          classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
          ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
      result = new LinkedMultiValueMap<>();
      // 循環(huán)處理每一個(gè)spring.factories文件
      while (urls.hasMoreElements()) {
        URL url = urls.nextElement();
        UrlResource resource = new UrlResource(url);
        // 加載spring.factories文件中的內(nèi)容到Properties對(duì)象中
        Properties properties = PropertiesLoaderUtils.loadProperties(resource);
        // 遍歷spring.factories內(nèi)容中的所有的鍵值對(duì)
        for (Map.Entry<?, ?> entry : properties.entrySet()) {
          // 獲得spring.factories內(nèi)容中的key（比如org.springframework.boot.autoconfigure.EnableAutoConfiguratio）
          String factoryTypeName = ((String) entry.getKey()).trim();
          // 獲取value， 然后按英文逗號(hào)(,)分割得到value數(shù)組并遍歷
          for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
            // 存儲(chǔ)結(jié)果到上面的多值Map中(MultiValueMap<String, String>)
            result.add(factoryTypeName, factoryImplementationName.trim());
          }
        }
      }
      cache.put(classLoader, result);
      return result;
    }
    catch (IOException ex) {
      throw new IllegalArgumentException("Unable to load factories from location [" +
          FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
  }
}

public ConfigurableApplicationContext run(String... args) {
　　//開(kāi)始停止的監(jiān)聽(tīng)
  StopWatch stopWatch = new StopWatch();
  stopWatch.start();
　　//聲明一個(gè)可配置的ioc容器
  ConfigurableApplicationContext context = null;
  FailureAnalyzers analyzers = null;
  //配置awt相關(guān)的東西
  configureHeadlessProperty();
  
  //獲取SpringApplicationRunListeners；從類路徑下META-INF/spring.factories
  SpringApplicationRunListeners listeners = getRunListeners(args);
  //回調(diào)所有的獲取SpringApplicationRunListener.starting()方法
  listeners.starting();
  try {
    //封裝命令行參數(shù)
   ApplicationArguments applicationArguments = new DefaultApplicationArguments(
      args);
   //準(zhǔn)備環(huán)境
   ConfigurableEnvironment environment = prepareEnvironment(listeners,
      applicationArguments);
   //創(chuàng)建環(huán)境完成后回調(diào)SpringApplicationRunListener.environmentPrepared()；表示環(huán)境準(zhǔn)備完成
   Banner printedBanner = printBanner(environment);
    
    //創(chuàng)建ApplicationContext；決定創(chuàng)建web的ioc還是普通的ioc，
　　　　//通過(guò)反射創(chuàng)建ioc容器（(ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);）
   context = createApplicationContext();
    //出現(xiàn)異常之后做異常分析報(bào)告
   analyzers = new FailureAnalyzers(context);
    //準(zhǔn)備上下文環(huán)境;將environment保存到ioc中；而且applyInitializers()；
    //applyInitializers()：回調(diào)之前保存的所有的ApplicationContextInitializer的initialize方法
    //回調(diào)所有的SpringApplicationRunListener的contextPrepared()；
    //
   prepareContext(context, environment, listeners, applicationArguments,
      printedBanner);
    //prepareContext運(yùn)行完成以后回調(diào)所有的SpringApplicationRunListener的contextLoaded（）；
    
    //刷新容器；ioc容器初始化（如果是web應(yīng)用還會(huì)創(chuàng)建嵌入式的Tomcat）；Spring注解版
    //掃描，創(chuàng)建，加載所有組件的地方；（配置類，組件，自動(dòng)配置）
   refreshContext(context);
    //從ioc容器中獲取所有的ApplicationRunner和CommandLineRunner進(jìn)行回調(diào)
    //ApplicationRunner先回調(diào)，CommandLineRunner再回調(diào)
   afterRefresh(context, applicationArguments);
    //所有的SpringApplicationRunListener回調(diào)finished方法
   listeners.finished(context, null);
   stopWatch.stop();
   if (this.logStartupInfo) {
     new StartupInfoLogger(this.mainApplicationClass)
        .logStarted(getApplicationLog(), stopWatch);
   }
    //整個(gè)SpringBoot應(yīng)用啟動(dòng)完成以后返回啟動(dòng)的ioc容器；
   return context;
  }
  catch (Throwable ex) {
   handleRunFailure(context, listeners, analyzers, ex);
   throw new IllegalStateException(ex);
  }
}