ConfigurableApplicationContext context = SpringApplication.run(NacosSpringBootYamlApplication.class, args);

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

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

public SpringApplication(Class<?>... primarySources) {
  this(null, primarySources);
 }

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
  this.resourceLoader = resourceLoader;
  Assert.notNull(primarySources, "PrimarySources must not be null");
  this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources)); // 這里就是SpringMvcApplication的實例
  this.webApplicationType = WebApplicationType.deduceFromClasspath();// deduce(推斷)web類型(servlet、reactive、NoWeb)
  setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));// 這里會處理加載所有的spring.factories文件的內容到緩存 找到*META-INF/spring.factories*中聲明的所有ApplicationContextInitializer的實現(xiàn)類并將其實例化
  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); //找到*META-INF/spring.factories*中聲明的所有ApplicationListener的實現(xiàn)類并將其實例化
  this.mainApplicationClass = deduceMainApplicationClass(); //獲得當前執(zhí)行main方法的類對象，這里就是SpringMvcApplication的實例
 }

/** key:是spring.factories的key    value:是根據key分組，把同個key的不同value放到list里面 */
 private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
  MultiValueMap<String, String> result = cache.get(classLoader);
  if (result != null) { //已經處理過了  直接返回
   return result;
  }
//url: // file:/C:/Users/kongqi/.m2/repository/org/springframework/spring-beans/5.1.9.RELEASE/spring-beans-5.1.9.RELEASE.jar!/META-INF/spring.factories
  try { //得到classloader下的所有jar包中的spring.factories的文件
   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); // 得到spring.factories的內容
    for (Map.Entry<?, ?> entry : properties.entrySet()) { // key: spring.factories的key  value: spring.factories的value
     String factoryClassName = ((String) entry.getKey()).trim(); // spring.factories的key
     for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {//value根據逗號，分隔
      result.add(factoryClassName, factoryName.trim()); //factoryClassName其實就是spring.factories的key   由于value是List類型 MultiValueMap value有多個
     }
    }
   }
   cache.put(classLoader, result);
   return result;
  }
  catch (IOException ex) {
   throw new IllegalArgumentException("Unable to load factories from location [" +
     FACTORIES_RESOURCE_LOCATION + "]", ex);
  }
 }

@Configuration
@EnableConfigurationProperties(HelloProperties.class)
public class HelloServiceAutoConfiguration {
    @Autowired
    HelloProperties helloProperties;
    @Bean
    public HelloService helloService() {
        HelloService service = new HelloService();
        service.setHelloProperties( helloProperties  );
        return service;
    }
}

@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 {

    /**
     * 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;
    }