1.启动方式

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

SpringBoot启动方式，这里就不多做说明了

2.@SpringBootApplication

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
	xxxxx
}

1. 可以看到SpringBootApplication注解由三个注解标注，@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan。
2. 简要说明一下这三个注解的作用：
   2.1. SpringBootConfiguration由Configuration标注，而Configuration又是由Component标注，Component表明这个类要被Spring管理。同时Configuration注解还能对当前类进行代理。
   2.2. EnableAutoConfiguration是由@Import注解所标注，而Import注解的作用是告诉Spring容器需要注入哪些bean。
   2.3. ComponentScan是用来配置扫描Component资源的策略，包含策略和剔除策略，剔除策略优先及高。

3.SpringApplication

3.1 构造器SpringApplication

@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));
	this.webApplicationType = WebApplicationType.deduceFromClasspath();
	setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
	setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
	this.mainApplicationClass = deduceMainApplicationClass();
}

1. 参数说明：resourceLoader写死null，primarySources为启动类MyApplication。
2. 通过classLoader判断当前的WebApplicationType，这里返回的是WebApplicationType.SERVLET。
3. 从spring.factories文件中拿到ApplicationContextInitializer和ApplicationListener。spring.factories文件解析可参考 SpringBoot 基础概念：SpringApplication#getSpringFactoriesInstances
4. 推断出主类，通过RuntimeException的栈帧信息找到第一个"main"方法所属的类，也就是MyApplication。

3.2 SpringApplication#run

public ConfigurableApplicationContext run(String... args) {
	StopWatch stopWatch = new StopWatch();
	stopWatch.start();
	ConfigurableApplicationContext context = null;
	Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
	configureHeadlessProperty();
	SpringApplicationRunListeners listeners = getRunListeners(args);
	listeners.starting();
	try {
		ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
		ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
		configureIgnoreBeanInfo(environment);
		Banner printedBanner = printBanner(environment);
		context = createApplicationContext();
		exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
				new Class[] { ConfigurableApplicationContext.class }, context);
		prepareContext(context, environment, listeners, applicationArguments, printedBanner);
		refreshContext(context);
		afterRefresh(context, applicationArguments);
		stopWatch.stop();
		if (this.logStartupInfo) {
			new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
		}
		listeners.started(context);
		callRunners(context, applicationArguments);
	}
	catch (Throwable ex) {
		handleRunFailure(context, ex, exceptionReporters, listeners);
		throw new IllegalStateException(ex);
	}

	try {
		listeners.running(context);
	}
	catch (Throwable ex) {
		handleRunFailure(context, ex, exceptionReporters, null);
		throw new IllegalStateException(ex);
	}
	return context;
}

1. 这是SpringBoot真正启动的总流程。
2. 从spring.factories文件中获取SpringApplicationRunListener对应的类，通过参数类型为SpringApplication.class, String[].class的构造器实例化，其中值为当前的SpringApplication对象和启动参数args，封装在SpringApplicationRunListeners里。其实是实例化了EventPublishingRunListener，从名字上可知，它是用来分发事件的。
   
3. 调用SpringApplicationRunListeners#starting发布"正在启动"事件，事件为ApplicationStartingEvent。
4. prepareEnvironment方法发布"环境准备"事件，由ConfigFileApplicationListener监听，然后解析配置文件（如yml、bootstrap、properties、xml等），将解析的信息存放到Environment里面。
5. printBanner方法打印banner
6. createApplicationContext创建ApplicationContext，判断的依据是webApplicationType。由于是Servlet类型的，创建的是AnnotationConfigServletWebServerApplicationContext。
7. 获取异常报告事件监听器，在catch到异常监听failed。
8. 准备applicationContext
9. refreshContext(context)刷新容器
10. listeners.started(context) 发布"已启动"事件
11. 调用ApplicationRunner和CommandLineRunner
12. listeners.running发送"running事件"

3.3 SpringApplication#run 中关键方法

3.1 SpringApplication#prepareEnvironment

3.2 SpringApplication#prepareContext

3.3 SpringApplication#createApplicationContext

3.4 SpringApplication#prepareContext

3.5 SpringApplication#refreshContext

3.6 SpringApplicationRunListeners#started

3.7 callRunners

3.8 SpringApplicationRunListeners#running