SpringBoot启动流程（一）：准备SpringApplication

@SpringBootApplication
public class Demo01Application {

    public static void main(String[] args) {
        // һоspringboot
        SpringApplication.run(Demo01Application.class, args);
    }
}

public class SpringApplication {
    ...
    // primarySource Ǵ Demo01Application.class
    // args  main() IJ
    public static ConfigurableApplicationContext run(
            Class<?> primarySource, String... args) {
        //  primarySource װ飬 run(...) 
        return run(new Class<?>[] { primarySource }, args);
    }

    // primarySources Ǵ Demo01Application.class װɵ飬
    // args  main() IJ
    public static ConfigurableApplicationContext run(
            Class<?>[] primarySources, String[] args) {
        // ￪ʼ
        return new SpringApplication(primarySources).run(args);
    }
    ...
}

return new SpringApplication(primarySources).run(args);

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

    /**
     * յõĹ췽
     * resourceLoader Ϊ null
     * primarySources Ϊ Demo01Application.class
     */
    @SuppressWarnings({ "unchecked", "rawtypes" })
    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        // 1\. resourceLoaderõԱֵΪnull
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        // 2\. primarySourcesõԱֵΪ Demo01Application.class
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        // 3\. ǰ web ӦͣREACTIVENONESERVLET
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
        // 4\. óʼgetSpringFactoriesInstances META-INF/spring.factories лȡ
        setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
        // 5\. ügetSpringFactoriesInstances META-INF/spring.factories лȡ
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        // 6\. ذmain()class
        this.mainApplicationClass = deduceMainApplicationClass();
    }
}

public enum WebApplicationType {
    //  web Ӧ
    NONE,

    // servlet ͵ web Ӧ
    SERVLET,

    // reactive ͵ web Ӧ
    REACTIVE;

    ...

    private static final String[] SERVLET_INDICATOR_CLASSES = { 
            "javax.servlet.Servlet",
            "org.springframework.web.context.ConfigurableWebApplicationContext" };

    private static final String WEBMVC_INDICATOR_CLASS 
            = "org.springframework.web.servlet.DispatcherServlet";

    private static final String WEBFLUX_INDICATOR_CLASS 
            = "org.springframework.web.reactive.DispatcherHandler";

    private static final String JERSEY_INDICATOR_CLASS 
            = "org.glassfish.jersey.servlet.ServletContainer";

    static WebApplicationType deduceFromClasspath() {
        // classpath н WEBFLUX 
        if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) 
                && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
                && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
            return WebApplicationType.REACTIVE;
        }
        // classpath  SERVLET 
        for (String className : SERVLET_INDICATOR_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return WebApplicationType.NONE;
            }
        }
        // Ĭ web Ϊ SERVLET
        // Ҳ˵ͬʱ WEBFLUX  SERVLET ࣬շص SERVLET
        return WebApplicationType.SERVLET;
    }

    ...
}

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

public class SpringApplication {
    ...

    // type Ϊ ApplicationContextInitializer.class
    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
        return getSpringFactoriesInstances(type, new Class<?>[] {});
    }

    /**
     * type Ϊ ApplicationContextInitializer.class
     * parameterTypes Ϊ ew Class<?>[] {}
     * args Ϊ null
     */
    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, 
            Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = getClassLoader();
        //  META-INF/spring.factories 
        Set<String> names = new LinkedHashSet<>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        // ʵʹõķ
        List<T> instances = createSpringFactoriesInstances(
                type, parameterTypes, classLoader, args, names);
        // 򣬱Ƚϵ @Order ע⣬ʵֵ Orderd ӿ
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }
    ...
}

public class SpringApplication {
    ...
    public void setInitializers(
            Collection<? extends ApplicationContextInitializer<?>> initializers) {
        this.initializers = new ArrayList<>(initializers);
    }
    ...
}

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

public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
    this.listeners = new ArrayList<>(listeners);
}

private Class<?> deduceMainApplicationClass() {
    try {
        // ȡջ
        StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
        // ջ main
        for (StackTraceElement stackTraceElement : stackTrace) {
            if ("main".equals(stackTraceElement.getMethodName())) {
                return Class.forName(stackTraceElement.getClassName());
            }
        }
    }
    catch (ClassNotFoundException ex) {
        // Swallow and continue
    }
    return null;
}