/**

     * Static helper that can be used to run a {@link SpringApplication} from the

     * specified sources using default settings and user supplied arguments.

     *

     * @param primarySources the primary sources to load

     * @param args           the application arguments (usually passed from a Java main method)

     * @return the running {@link ApplicationContext}

     */

    public static ConfigurableApplicationContext run(Class<?>[] primarySources,

                                                     String[] args) {

        return new SpringApplication(primarySources).run(args);

    }

 public SpringApplication(Class<?>... primarySources) {

     this(null, primarySources);

 }

 @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));

     //推断应用类型，后面会根据类型初始化对应的环境。常用的一般都是servlet环境

     this.webApplicationType = deduceWebApplicationType();//2.2.1

     //初始化classpath下 META-INF/spring.factories中已配置的ApplicationContextInitializer

     setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));//2.2.2

     //初始化classpath下所有已配置的 ApplicationListener

     setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//2.2.3

     //根据调用栈，推断出 main 方法的类名

     this.mainApplicationClass = deduceMainApplicationClass();

 }

 //常量值

 private static final String[] WEB_ENVIRONMENT_CLASSES = {"javax.servlet.Servlet",

             "org.springframework.web.context.ConfigurableWebApplicationContext"};

 private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework."

         + "web.reactive.DispatcherHandler";

 private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework."

         + "web.servlet.DispatcherServlet";

 private static final String JERSEY_WEB_ENVIRONMENT_CLASS = "org.glassfish.jersey.server.ResourceConfig";

 /**

  * 判断 应用的类型

  * NONE: 应用程序不是web应用，也不应该用web服务器去启动

  * SERVLET: 应用程序应作为基于servlet的web应用程序运行，并应启动嵌入式servlet web（tomcat）服务器。

  * REACTIVE: 应用程序应作为 reactive web应用程序运行，并应启动嵌入式 reactive web服务器。

  * @return

  */

 private WebApplicationType deduceWebApplicationType() {

     //classpath下必须存在org.springframework.web.reactive.DispatcherHandler

     if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)

             && !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)

             && !ClassUtils.isPresent(JERSEY_WEB_ENVIRONMENT_CLASS, null)) {

         return WebApplicationType.REACTIVE;

     }

     for (String className : WEB_ENVIRONMENT_CLASSES) {

         if (!ClassUtils.isPresent(className, null)) {

             return WebApplicationType.NONE;

         }

     }

     //classpath环境下存在javax.servlet.Servlet或者org.springframework.web.context.ConfigurableWebApplicationContext

     return WebApplicationType.SERVLET;

 }

 private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {

     return getSpringFactoriesInstances(type, new Class<?>[]{});

 }

 /**

  * 通过指定的classloader 从META-INF/spring.factories获取指定的Spring的工厂实例

  * @param type

  * @param parameterTypes

  * @param args

  * @param <T>

  * @return

  */

 private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,

                                                       Class<?>[] parameterTypes, Object... args) {

     ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

     // Use names and ensure unique to protect against duplicates

     //通过指定的classLoader从 META-INF/spring.factories 的资源文件中，

     //读取 key 为 type.getName() 的 value

     Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));

     //创建Spring工厂实例

     List<T> instances = createSpringFactoriesInstances(type, parameterTypes,

             classLoader, args, names);

     //对Spring工厂实例排序（org.springframework.core.annotation.Order注解指定的顺序）

     AnnotationAwareOrderComparator.sort(instances);

     return instances;

 }