public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.sources = new LinkedHashSet();
        this.bannerMode = Mode.CONSOLE;
        this.logStartupInfo = true;
        this.addCommandLineProperties = true;
        this.addConversionService = true;
        this.headless = true;

        this.registerShutdownHook = true;
        this.additionalProfiles = Collections.emptySet();
        this.isCustomEnvironment = false;
        this.lazyInitialization = false;
        this.applicationContextFactory = ApplicationContextFactory.DEFAULT;
        this.applicationStartup = ApplicationStartup.DEFAULT;

        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet(Arrays.asList(primarySources));
        //1.推測web應用類型（NONE   REACTIVE   SERVLET）
        this.webApplicationType = WebApplicationType.deduceFromClasspath();

         //2.從spring.factories中獲取BootstrapRegistryInitializer對象
        this.bootstrapRegistryInitializers =             
       this.getBootstrapRegistryInitializersFromSpringFactories();
        
 //3.從spring.factories中獲取ApplicationContextInitializer對象
 this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
         //4.從spring.factories中獲取ApplicationListener對象
        this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
        //5.推測出Main類 (main()方法所在的類)
        this.mainApplicationClass = this.deduceMainApplicationClass();
    }

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        DefaultBootstrapContext bootstrapContext = this.createBootstrapContext();
        ConfigurableApplicationContext context = null;
        this.configureHeadlessProperty();
           /**從spring.factories中獲取SpringApplicationRunListeners 對象
            * 默認會拿到一個EventPublishingRunListener ，他會啟動過程的各個階段發(fā)布對應的事件
            **/
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        listeners.starting(bootstrapContext, this.mainApplicationClass);

        try {
            //將run()的參數(shù)封裝為DefaultApplicationArguments對象
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
                //配置文件的入口
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
            this.configureIgnoreBeanInfo(environment);
            Banner printedBanner = this.printBanner(environment);
            //根據(jù)應用類型創(chuàng)建Spring容器
            context = this.createApplicationContext();
            context.setApplicationStartup(this.applicationStartup);
            this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
               //刷新Spring容器， 會解析配置類  掃描  啟動Webserver
            this.refreshContext(context);
            this.afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }

            listeners.started(context);

            //調用applicationArguments 和CommandLineRunner
            this.callRunners(context, applicationArguments);
        } catch (Throwable var10) {
            this.handleRunFailure(context, var10, listeners);
            throw new IllegalStateException(var10);
        }

        try {
            listeners.running(context);
            return context;
        } catch (Throwable var9) {
            this.handleRunFailure(context, var9, (SpringApplicationRunListeners)null);
            throw new IllegalStateException(var9);
        }
    }

public ConfigurableApplicationContext run(String... args) {
    // ...
    ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
    // ...
}

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments) {
    // 創(chuàng)建 Environment 對象
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    // 配置 Environment，加載配置文件
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    // 觸發(fā)環(huán)境準備事件
    listeners.environmentPrepared(environment);
    // 將 Environment 綁定到 SpringApplication
    bindToSpringApplication(environment);
    return environment;
}

protected void configureEnvironment(ConfigurableEnvironment environment, String[] args) {
    configurePropertySources(environment, args);
    configureProfiles(environment, args);
}

public void onApplicationEvent(ApplicationEvent event) {
    if (event instanceof ApplicationEnvironmentPreparedEvent) {
        onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
    }
}

void load() {
    for (String location : getSearchLocations()) {
        for (String name : getSearchNames()) {
            load(location, name);
        }
    }
}

public PropertySource<?> load(Resource resource) throws IOException {
    if (resource.getFilename().endsWith(".yml")) {
        return loadYaml(resource);
    } else {
        return loadProperties(resource);
    }
}

public static Profiles of(String... profiles) {
    return new Profiles() {
        @Override
        public boolean matches(Predicate<String> predicate) {
            // ...
        }
    };
}

server.port=8080
spring.profiles.active=dev

server.port=8081

@RestController
public class MyController {
?
    @Value("${server.port}")
    private String port;
?
    @GetMapping("/port")
    public String getPort() {
        return "Server port: " + port;
    }
}