springboot启动流程分析

1.从应用程序启动入口开始追踪代码：

SpringApplication.run(SpringbootDemoApplication.class, args);

2.进入SpringApplication类的构造方法：

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();
		this.bootstrapRegistryInitializers = new ArrayList<>(
				getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

这个步骤做了以下几件事：

a.初始化资源加载器，但是从上下文可以看出是null

b.设置主资源类，即第一步传过来的SpringbootDemoApplication类

c.推断webApplicationType的类型：WebApplicationType.SERVLET

d.设置bootstrapRegistryInitializers数据，通过SpringFactoriesLoader从META-INF/spring.factories目录加载key为BootstrapRegistryInitializer的数据并初始化

e.设置initializers，通过SpringFactoriesLoader从上下文中所以jar根目录META-INF/spring.factories目录加载key为ApplicationContextInitializer的数据并初始化

f.设置listeners，通过SpringFactoriesLoader从上下文中所以jar根目录META-INF/spring.factories目录加载key为ApplicationListener的数据并初始化

j.推断主应用类，即SpringbootDemoApplication类

3.接着继续进入SpringApplication类的run方法中，如下代码：

public ConfigurableApplicationContext run(String... args) {
		long startTime = System.nanoTime();
        // a.创建一个springboot上下文对象
		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
        // b.获取并启动监听的listeners
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting(bootstrapContext, this.mainApplicationClass);
		try {
            // c.初始化应用环境
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
			configureIgnoreBeanInfo(environment);
            // 打印启动banner数据
			Banner printedBanner = printBanner(environment);
            // d.初始化应用上下文
			context = createApplicationContext();
			context.setApplicationStartup(this.applicationStartup);
            // e.刷新应用上下文前的准备工作
			prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
            // f.刷新应用上下文
			refreshContext(context);
            // j.刷新应用上下文之后的扩展功能
			afterRefresh(context, applicationArguments);
			Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);
			}
			listeners.started(context, timeTakenToStartup);
            // h.调用ApplicationRunner和CommandLineRunner的run方法
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}
		try {
			Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);
			listeners.ready(context, timeTakenToReady);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

接下来具体解释一下，每一步的细节：

a.创建springboot上下文对象：创建一个类型为DefaultBootstrapContext的上下文对象，并且将第二步获取过来的bootstrapRegistryInitializers执行初始化操作，执行initialize方法

private DefaultBootstrapContext createBootstrapContext() {
		DefaultBootstrapContext bootstrapContext = new DefaultBootstrapContext();
		this.bootstrapRegistryInitializers.forEach((initializer) -> initializer.initialize(bootstrapContext));
		return bootstrapContext;
	}

BootstrapRegistryInitializer接口类：

@FunctionalInterface
public interface BootstrapRegistryInitializer {
 
	/**
	 * Initialize the given {@link BootstrapRegistry} with any required registrations.
	 * @param registry the registry to initialize
	 */
	void initialize(BootstrapRegistry registry);
 
}

可以自行实现该接口并做某些初始化作用，这个初始化的时机比较早。

b.获取并启动监听的listeners：获取并实例化所有类型为SpringApplicationRunListener的listener

private SpringApplicationRunListeners getRunListeners(String[] args) {
		Class[] types = new Class[] { SpringApplication.class, String[].class };
		return new SpringApplicationRunListeners(logger,
				getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args),
				this.applicationStartup);
	}

启动listener

void starting(ConfigurableBootstrapContext bootstrapContext, Class mainApplicationClass) {
		doWithListeners("spring.boot.application.starting", (listener) -> listener.starting(bootstrapContext),
				(step) -> {
					if (mainApplicationClass != null) {
						step.tag("mainApplicationClass", mainApplicationClass.getName());
					}
				});
	}

进入org.springframework.boot.context.event.EventPublishingRunListener#starting方法中(说明：EventPublishingRunListener这个runlistener是通过反射初始化的，通过getRunListeners反射调用的是EventPublishingRunListener(SpringApplication application, String[] args) 这个构造方法，有兴趣可以看看，里面初始化了多播器：initialMulticaster和当前应用对象：application)：发布了一个ApplicationStartingEvent事件对象

@Override
	public void starting(ConfigurableBootstrapContext bootstrapContext) {
		this.initialMulticaster
				.multicastEvent(new ApplicationStartingEvent(bootstrapContext, this.application, this.args));
	}

继续追踪到org.springframework.context.event.SimpleApplicationEventMulticaster#multicastEvent(org.springframework.context.ApplicationEvent, org.springframework.core.ResolvableType)方法：

public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
		ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
		Executor executor = getTaskExecutor();
		for (ApplicationListener listener : getApplicationListeners(event, type)) {
			if (executor != null) {
				executor.execute(() -> invokeListener(listener, event));
			}
			else {
				invokeListener(listener, event);
			}
		}
	}

继续追到org.springframework.context.event.SimpleApplicationEventMulticaster#doInvokeListener：

private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
		try {
			listener.onApplicationEvent(event);
		}
		catch (ClassCastException ex) {
			// 省略部分代码...
		}
	}

最终执行各个listener的onApplicationEvent方法。

springboot中所有的事件发布都是由org.springframework.boot.context.event.EventPublishingRunListener这个类来发布完成的，它会在容器初始化的各个阶段发布各种不同的事件，关注了这个事件的监听器就能接受到，然后执行对应的方法。

c.初始化应用环境，代码如下：

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,
			ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
			ApplicationArguments applicationArguments, Banner printedBanner) {
		context.setEnvironment(environment);
		postProcessApplicationContext(context);
        // 执行第二步获取的initializers方法
		applyInitializers(context);
        // 发布一个ApplicationContextInitializedEvent类型的事件，告诉listener准备监听
		listeners.contextPrepared(context);
		bootstrapContext.close(context);
		if (this.logStartupInfo) {
			logStartupInfo(context.getParent() == null);
			logStartupProfileInfo(context);
		}
		// Add boot specific singleton beans
		ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
		beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
		if (printedBanner != null) {
			beanFactory.registerSingleton("springBootBanner", printedBanner);
		}
		if (beanFactory instanceof AbstractAutowireCapableBeanFactory) {
			((AbstractAutowireCapableBeanFactory) beanFactory).setAllowCircularReferences(this.allowCircularReferences);
			if (beanFactory instanceof DefaultListableBeanFactory) {
				((DefaultListableBeanFactory) beanFactory)
						.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
			}
		}
		if (this.lazyInitialization) {
			context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
		}
		context.addBeanFactoryPostProcessor(new PropertySourceOrderingBeanFactoryPostProcessor(context));
		// Load the sources
		Set