初始化基本流程

SpringApplication的prepareEnvironment准备环境

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments) {
   
     
		// Create and configure the environment
		ConfigurableEnvironment environment = getOrCreateEnvironment();//获取环境
		configureEnvironment(environment, applicationArguments.getSourceArgs());
		ConfigurationPropertySources.attach(environment);//ConfigurationPropertySources附加到环境一次
		listeners.environmentPrepared(environment);//广播环境准备好的事件
		bindToSpringApplication(environment);//绑定到SpringApplication
		if (!this.isCustomEnvironment) {
   
     
			environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
					deduceEnvironmentClass());//推断环境，如果不是这个类型，要进行切换
		}
		ConfigurationPropertySources.attach(environment);//又附加一次，主要是为了放最前面
		return environment;
	}

SpringApplication的getOrCreateEnvironment创建环境

根据前面的webApplicationType判断要创建哪种环境。

	private ConfigurableEnvironment getOrCreateEnvironment() {
   
     
		if (this.environment != null) {
   
     
			return this.environment;
		}
		switch (this.webApplicationType) {
   
     
		case SERVLET:
			return new StandardServletEnvironment();
		case REACTIVE:
			return new StandardReactiveWebEnvironment();
		default:
			return new StandardEnvironment();
		}
	}

configureEnvironment配置环境

这里主要是配置了很多的类型转换器和格式转换器，另外两个跟换进属性相关

	protected void configureEnvironment(ConfigurableEnvironment environment, String[] args) {
   
     
		if (this.addConversionService) {
   
     
			ConversionService conversionService = ApplicationConversionService.getSharedInstance();//配置转换器
			environment.setConversionService((ConfigurableConversionService) conversionService);//设置到环境中去
		}
		configurePropertySources(environment, args);//配置默认属性
		configureProfiles(environment, args);//配置代码添加的环境
	}

ApplicationConversionService的getSharedInstance配置转换器

这个里面有配置很多转换器，我就不多讲了，自己看就行了。

	/**
	 * Add converters appropriate for most environments.
	 * @param converterRegistry the registry of converters to add to
	 * (must also be castable to ConversionService, e.g. being a {@link ConfigurableConversionService})
	 * @throws ClassCastException if the given ConverterRegistry could not be cast to a ConversionService
	 */
	public static void addDefaultConverters(ConverterRegistry converterRegistry) {
		addScalarConverters(converterRegistry);
		addCollectionConverters(converterRegistry);

		converterRegistry.addConverter(new ByteBufferConverter((ConversionService) converterRegistry));
		converterRegistry.addConverter(new StringToTimeZoneConverter());
		converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
		converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());

		converterRegistry.addConverter(new ObjectToObjectConverter());
		converterRegistry.addConverter(new IdToEntityConverter((ConversionService) converterRegistry));
		converterRegistry.addConverter(new FallbackObjectToStringConverter());
		converterRegistry.addConverter(new ObjectToOptionalConverter((ConversionService) converterRegistry));
	}

	/**
	 * Add converters useful for most Spring Boot applications.
	 * @param registry the registry of converters to add to (must also be castable to
	 * ConversionService, e.g. being a {@link ConfigurableConversionService})
	 * @throws ClassCastException if the given ConverterRegistry could not be cast to a
	 * ConversionService
	 */
	public static void addApplicationConverters(ConverterRegistry registry) {
		addDelimitedStringConverters(registry);
		registry.addConverter(new StringToDurationConverter());
		registry.addConverter(new DurationToStringConverter());
		registry.addConverter(new NumberToDurationConverter());
		registry.addConverter(new DurationToNumberConverter());
		registry.addConverter(new StringToPeriodConverter());
		registry.addConverter(new PeriodToStringConverter());
		registry.addConverter(new NumberToPeriodConverter());
		registry.addConverter(new StringToDataSizeConverter());
		registry.addConverter(new NumberToDataSizeConverter());
		registry.addConverter(new StringToFileConverter());
		registry.addConverter(new InputStreamSourceToByteArrayConverter());
		registry.addConverterFactory(new LenientStringToEnumConverterFactory());
		registry.addConverterFactory(new LenientBooleanToEnumConverterFactory());
		if (registry instanceof ConversionService) {
			addApplicationConverters(registry, (ConversionService) registry);
		}
	}

其他剩下的暂时不讲，因为比较复杂，很多都是一些属性的配置，比如会获取你的spring.profiles.active信息，而且是ConfigFileApplicationListener收到环境准备好的事件后做的事，当然还有其他的一些配置信息，我们还是先把主线弄明白再搞细节，不过这里配置完成会进行广播，这次的事件是ApplicationEnvironmentPreparedEvent：

SpringApplication的printBanner打印banner

内部支持 "gif", "jpg", "png"，"txt"：

默认把资源放到resources下，名字为banner.xxx就可以了，自己可以去试试：

我们经常看到的默认的在这里面：

  public static void main(String[] args){
        SpringApplication.run(SteamGridApplication.class, args);
        System.out.println("(♥◠‿◠)ﾉﾞ  启动成功   ლ(´ڡ`ლ)ﾞ  \n" +
                "  _, ___ __,  _, _, _     _, __, _ __,\n" +
                " (_   |  |_  /_\\ |\\/|    / _ |_) | | \\\n" +
                " , )  |  |   | | |  | ~~ \\ / | \\ | |_/\n" +
                "  ~   ~  ~~~ ~ ~ ~  ~     ~  ~ ~ ~ ~  ");
    }

createApplicationContext创建上下文

根据类型创建对应的上下文对象，默认全是注解

	protected ConfigurableApplicationContext createApplicationContext() {

		Class<?> contextClass = this.applicationContextClass;
		if (contextClass == null) {

			try {
					switch (this.webApplicationType) {
   
     
				case SERVLET://AnnotationConfigServletWebServerApplicationContext
					contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
					break;
				case REACTIVE://AnnotationConfigReactiveWebServerApplicationContext
					contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
					break;
				default://AnnotationConfigApplicationContext
					contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
				}
			}
			catch (ClassNotFoundException ex) {
   
     
				throw new IllegalStateException(
						"Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);
			}
		}
		return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
	}

准备环境这里比较复杂，可以写很多，这个还是细节的时候去看，先把脉络理完吧，后面讲上下文的准备，这个也很重要，准备完了就是spring的核心refresh了。