启动项目

SpringBoot项目有一个主程序启动类，在主程序启动勒种有个启动项目main()方法，通过执行SpringApplication.run()启动整个SpringBoot程序

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

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

从源码中可以看出来，SpringApplication.run()方法内部执行了两个操作，分别是SpringApplication实例的初始化和调用run()启动项目

SpringApplication实例的初始化

    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
    //把项目启动类.class设置为属性存储起来
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    //判断当前webApplicationType应用的类型
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
    //初始化器，最后会调用这些初始化器
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    //设置监听器
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    //用于推断并设置项目main()方法启动主程序启动类
        this.mainApplicationClass = deduceMainApplicationClass();
    }

从上诉源码中可以看出来，SpringApplication的初始化过程包括4部分

1. this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));

用于判断webApplicationType应用的类型。deduceFromClasspath()方法用于插手Classpath类路径下是否存在某个特征类，从而判断当前webApplicationType类似是SERVLETyingy (Spring5之前的传统MVC应用）还是REACTIVE应用（Spring5开始出现的WebFlux交互式应用）

1. setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));

用于SpringApplication应用的初始化器设置。在初始化器设置的过程中，会使用Spring类加载器SpringFactoriesLoader从META-INF/spring.factories类路径下的ጱMETA-INF/spring.factores文件中获取所有可用的应用初始化器类ApplicationContextInitializer

1. setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

用于SpringApplication应用的监听器设置。监听器设置的过程与上一步初始化器设置基本一样，也是使用SpringFactoriesLoader从META-INF/spring.factories类路径下的ጱMETA-INF/spring.factores文件中获取所有可用的应用初始化器类ApplicationListener。

1. this.mainApplicationClass = deduceMainApplicationClass();

用于推断并设置项目main(0方法启动的主程序启动类

项目初始化

    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
    //第一步 获取并启动监听器
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            //第二步 根据SpringApplicationRunListeners以及参数来准备环境
      ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
            //第三步 创建Spring容器
      context = createApplicationContext();
            exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            //第四步 容器前置处理
      prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            //第五步 刷新容器
      refreshContext(context);
            //第六步 容器后置处理
      afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
            }
      //第七步 发出结束执行的事件
            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;
    }

SpringBoot执行流程图