SpringCloud 启动流程一 SpringApplication构造函数

调用SpringApplication.run方法后，第一步会进入到SpringApplication的构造方法中。构造方法源码如下：

    public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

首先进入WebApplicationType.deduceFromClasspath()方法，源码如下：

        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";

	private static final String SERVLET_APPLICATION_CONTEXT_CLASS = "org.springframework.web.context.WebApplicationContext";

	private static final String REACTIVE_APPLICATION_CONTEXT_CLASS = "org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext";

	static WebApplicationType deduceFromClasspath() {
		if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
				&& !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
			return WebApplicationType.REACTIVE;
		}
		for (String className : SERVLET_INDICATOR_CLASSES) {
			if (!ClassUtils.isPresent(className, null)) {
				return WebApplicationType.NONE;
			}
		}
		return WebApplicationType.SERVLET;
	}

ClassUtils.isPresent内部会调用Class.forName方法，如果初始化class成功，就返回true，否则返回false。deduceFromClasspath会根据程序引用的类判断是响应式的web程序还是Servlet式的web程序

接着进入setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class))方法，其中getSpringFactoriesInstances是Spring中经常使用到的一个方法，源码如下：

    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
		ClassLoader classLoader = getClassLoader();
		// Use names and ensure unique to protect against duplicates
		Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
		List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
		AnnotationAwareOrderComparator.sort(instances);
		return instances;
        }

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
		String factoryClassName = factoryClass.getName();
		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
	}

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
		MultiValueMap<String, String> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		try {
			Enumeration<URL> urls = (classLoader != null ?
					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
			result = new LinkedMultiValueMap<>();
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					String factoryClassName = ((String) entry.getKey()).trim();
					for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryClassName, factoryName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}

loadSpringFactories会取出所有依赖的jar包的META-INF/spring.factories中的配置信息，放入map中然后返回。getOrDefault从map中取出key为factoryClassName的value值，这些value值都是全类名。createSpringFactoriesInstances会根据这些全类名，通过反射实例化对象。AnnotationAwareOrderComparator.sort方法会根据指定的规则对这些类进行排序。创建完成后，setInitializers会把这些对象添加到SpringApplication的成员变量List<ApplicationContextInitializer<?>> initializers中。

setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)) 也是一样的逻辑，将创建的对象添加到List<ApplicationListener<?>> listeners中。至此，构造方法结束。

Initializers集合中的6个对象如下：

org.springframework.boot.context.config.DelegatingApplicationContextInitializer
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer
org.springframework.boot.context.ContextIdApplicationContextInitializer
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

listeners集合中的13个对象如下：

org.springframework.cloud.bootstrap.BootstrapApplicationListener
org.springframework.cloud.bootstrap.LoggingSystemShutdownListener
org.springframework.boot.context.config.ConfigFileApplicationListener
org.springframework.boot.context.config.AnsiOutputApplicationListener
org.springframework.boot.context.logging.LoggingApplicationListener
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener
org.springframework.boot.autoconfigure.BackgroundPreinitializer
org.springframework.boot.context.config.DelegatingApplicationListener
org.springframework.cloud.context.restart.RestartListener
org.springframework.boot.builder.ParentContextCloserApplicationListener
org.springframework.boot.ClearCachesApplicationListener
org.springframework.boot.context.FileEncodingApplicationListener
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener