打開SampleSimpleApplication.java,我們可以看到

public static void main(String[] args) {
    //SpringBoot啟動函數(shù)
    SpringApplication.run(SampleSimpleApplication.class, args);
}

跟蹤進(jìn)去,可看到

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);
}

其中return new SpringApplication(primarySources).run(args);為核心方法,創(chuàng)建SpringApplication實例并執(zhí)行run()方法,接下來我們對這句話進(jìn)行分析.

1. 創(chuàng)建SpringApplication實例

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

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    //resourceLoader賦值給this.resourceLoader
    this.resourceLoader = resourceLoader;
    //斷言
    Assert.notNull(primarySources, "PrimarySources must not be null");
    //將primarySources存入LinkedHashSet對象并賦值給this.primarySources
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    //推斷當(dāng)前應(yīng)用類型
    this.webApplicationType = deduceWebApplicationType();
    //初始化Initializer
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    //初始化Listener
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    //推斷主類
    this.mainApplicationClass = deduceMainApplicationClass();
    }

2. 執(zhí)行run()完成SpringBoot啟動

public ConfigurableApplicationContext run(String... args) {
    //創(chuàng)建StopWatch對象,用來記錄程序啟動耗時
    StopWatch stopWatch = new StopWatch();
    //啟動
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    //根據(jù)args獲取所有SpringApplicationRunListeners監(jiān)聽器
    SpringApplicationRunListeners listeners = getRunListeners(args);
    //啟動監(jiān)聽器,首次啟動run方法時立即調(diào)用
    listeners.starting();
    try {
        //創(chuàng)建ApplicationArguments對象,并將args封裝至對象實例
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        //準(zhǔn)備環(huán)境
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        //配置需要忽略的bean
        configureIgnoreBeanInfo(environment);
        //打印banner
        Banner printedBanner = printBanner(environment);
        //創(chuàng)建應(yīng)用上下文對象
        context = createApplicationContext();
        //獲取異常報告對象
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
        //準(zhǔn)備上下文環(huán)境
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        //刷新上下文
        refreshContext(context);
        //刷新上下文后續(xù)處理
        afterRefresh(context, applicationArguments);
        //關(guān)閉stopWatch
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        //執(zhí)行監(jiān)聽器started方法
        listeners.started(context);
        callRunners(context, applicationArguments);
        } catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }
        try {
            //在run方法完成之前立即調(diào)用井厌，刷新應(yīng)用程序上下文并且所有的CommandLineRunners和ApplicationRunners已經(jīng)被調(diào)用
            listeners.running(context);
        } catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

至此,我們已經(jīng)可以大概了解到SpringBoot的啟動流程,因為啟動過程中的一些方法比較復(fù)雜,所以我們接下來會分篇幅介紹其中一些核心的方法...