SpringApplication作為Spring Boot的啟動類。

一、入口方法run

public static ConfigurableApplicationContext run(Object source, String... args) {
            return run(new Object[] { source }, args);
}
public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
    return new SpringApplication(sources).run(args);
}
 

例項SpringApplication執行run。

二、初始化函式initialize

SpringApplication構造方法，會呼叫initialize方法進行初始化。

@SuppressWarnings({ "unchecked", "rawtypes" })
private void initialize(Object[] sources) {
    if (sources != null && sources.length > 0) {
        this.sources.addAll(Arrays.asList(sources));
    }
    this
 
.webEnvironment = deduceWebEnvironment();
    setInitializers((Collection) getSpringFactoriesInstances(
            ApplicationContextInitializer.class));
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}
 

流程分析：
1.設定配置類
2.推斷是否為WEB環境
3.例項化META-INF/spring.factories檔案中的ApplicationContextInitializer類
4.例項化META-INF/spring.factories檔案中的ApplicationListener類
5.推斷主類

SpringBoot例項化的5個ApplicationContextInitializer（另開一篇寫這5個類）

SpringBoot例項化的9個ApplicationListener（另開一篇寫這9個類）

三、執行方法run

原始碼如下：

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;

    //設定java.awt.headless未配置時預設為true
    //是J2SE的一種Headless模式
    configureHeadlessProperty();
    //例項化SpringApplicationRunListeners
    //<1>例項化META-INF/spring.factories檔案中的SpringApplicationRunListener類
    SpringApplicationRunListeners listeners = getRunListeners(args);
    //這裡看起來是一個啟動，其實根據不同的listener不一樣。
    listeners.started();
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                args);
        //<2>建立並重新整理spring上下文     
        context = createAndRefreshContext(listeners, applicationArguments);
        afterRefresh(context, applicationArguments);
        // EventPublishingRunListener廣播ApplicationReadyEvent或ApplicationFailedEvent事件
        listeners.finished(context, null);
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass)
                    .logStarted(getApplicationLog(), stopWatch);
        }
        return context;
    }
    catch (Throwable ex) {
        handleRunFailure(context, listeners, ex);
        throw new IllegalStateException(ex);
    }
}

1.啟動所有SpringApplicationRunListener
SpringBoot例項化的1個SpringApplicationRunListener

EventPublishingRunListener會向SpringApplication的所有ApplicationListener廣播事件。
事件有：ApplicationStartedEvent；ApplicationEnvironmentPreparedEvent；ApplicationPreparedEvent；ApplicationFailedEvent；ApplicationReadyEvent
原始碼中listeners.started(); 廣播ApplicationStartedEvent事件。

2.建立並重新整理spring上下文
原始碼如下：

private ConfigurableApplicationContext createAndRefreshContext(
        SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments) {
    ConfigurableApplicationContext context;
    // Create and configure the environment
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    //EventPublishingRunListener廣播ApplicationEnvironmentPreparedEvent
    listeners.environmentPrepared(environment);
    if (isWebEnvironment(environment) && !this.webEnvironment) {
        environment = convertToStandardEnvironment(environment);
    }

    if (this.bannerMode != Banner.Mode.OFF) {
        printBanner(environment);
    }

    // <2.1>Create, load, refresh and run the ApplicationContext
    context = createApplicationContext();
    context.setEnvironment(environment);
    postProcessApplicationContext(context);
    // 初始化所有ApplicationContextInitializer
    // 載入BeanFactoryPostProcessor
    applyInitializers(context);
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }

    // Add boot specific singleton beans
    context.getBeanFactory().registerSingleton("springApplicationArguments",
            applicationArguments);

    // Load the sources
    Set<Object> sources = getSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    load(context, sources.toArray(new Object[sources.size()]));
    // 對ApplicationContextAware的ApplicationListener設定context
    // context新增所有ApplicationListener
    // EventPublishingRunListener廣播ApplicationPreparedEvent事件
    listeners.contextLoaded(context);

    //<2.2> Refresh the context
    refresh(context);
    if (this.registerShutdownHook) {
        try {
            context.registerShutdownHook();
        }
        catch (AccessControlException ex) {
            // Not allowed in some environments.
        }
    }
    return context;
}

2.1建立context
根據不同的environment會建立不同的ApplicationContext。
Web：AnnotationConfigEmbeddedWebApplicationContext
Standard：AnnotationConfigApplicationContext

2.2重新整理上下文
重點後面另開一篇