springboot启动流程源码解析（带流程图）

本文自己写的（头条也有这篇文章），若有问题，请指正。

大致流程如下：

1. 初始化SpringApplication,从META-INF下的spring.factories读取
ApplicationListener/ApplicationContextInitializer

2.运行SpringApplication的run方法

3.读取项目中环境变量、jvm配置信息、配置文件信息等

4.创建Spring容器对象（ApplicationContext）

5. 利用
ApplicationContextInitializer初始化Spring容器对象，读取启动类

6.调用spring的refresh加载IOC容器、自动配置类，并创建bean、servlet容器等信息

7.springboot会调用很多监听器

8.如果启动时发生异常，则发送ApplicationFailedEvent事件

下面对上述流程进行源码细化分析，首先调用SpringApplication.run启动springboot应用：

@SpringBootApplication(exclude = DruidDataSourceAutoConfigure.class)@EnableTransactionManagement(proxyTargetClass = true)@Import(MyDynamicDataSourceConfig.class)public class MySpringBootApplication {public static void main(String[] args) {SpringApplication.run(MySpringBootApplication.class, args);}}

进入run方法后，会进行SpringApplication进行启动，分两大步，第一步初始化SpringApplication，第二步调用run方法：

public static ConfigurableApplicationContext run(Class[] primarySources, String[] args) { return new SpringApplication(primarySources).run(args);}

第一步初始化SpringApplication，new SpringApplication(primarySources)的流程如下(具体方法含义参考注释)：

public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) { this.resourceLoader = resourceLoader; Assert.notNull(primarySources, "PrimarySources must not be null"); this.primarySources = new LinkedHashSet(Arrays.asList(primarySources));// 根据classpath下的类，推算当前web应用类型(REACTIVE/SERVLET/NONE) this.webApplicationType = WebApplicationType.deduceFromClasspath();// 获取BootstrapRegistryInitializer对象,从META-INF/spring.factories中读取key为BootstrapRegistryInitializer，并实例化出对象 // BootstrapRegistryInitializer的作用是可以初始化BootstrapRegistrythis.bootstrapRegistryInitializers = new ArrayList( getSpringFactoriesInstances(BootstrapRegistryInitializer.class));//去spring.factories中去获取所有key:org.springframework.context.ApplicationContextInitializer为了初始化Spring容器ApplicationContext对象（可以利用 //ApplicationContextInitializer向Spring容器中添加ApplicationListener）setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));//去spring.factories中去获取所有key: org.springframework.context.ApplicationListener,ApplicationListener是Spring中的监听器 setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//推测main()方法所在的类 this.mainApplicationClass = deduceMainApplicationClass();}

第二步调用run方法，初始化完SpringApplication开始运行run方法，源码如下：

public ConfigurableApplicationContext run(String... args) { long startTime = System.nanoTime();//记录时间 //创建DefaultBootstrapContext对象,利用BootstrapRegistryInitializer初始化DefaultBootstrapContext对象 DefaultBootstrapContext bootstrapContext = createBootstrapContext(); ConfigurableApplicationContext context = null;// 开启了Headless模式： configureHeadlessProperty();//获取SpringApplicationRunListeners, //SpringBoot提供了一个EventPublishingRunListener，它实现了SpringApplicationRunListener接口//spring会利用这个类，发布一个ApplicationContextInitializedEvent事件，可以通过定义ApplicationListener来消费这个事件 SpringApplicationRunListeners listeners = getRunListeners(args);// 发布ApplicationStartingEvent事件，在运行开始时发送 listeners.starting(bootstrapContext, this.mainApplicationClass); try { // 根据命令行参数 实例化一个ApplicationArgumentsApplicationArguments applicationArguments = new DefaultApplicationArguments(args); // 预初始化环境（见下面的源码分析）： 读取环境变量（操作系统的环境变量/JVM的环境变量），读取配置文件信息（基于监听器，会利用EventPublishingRunListener发布一个ApplicationEnvironmentPreparedEvent事件，//默认会有一个EnvironmentPostProcessorApplicationListener来处理这个事件，当然也可以通过自定义ApplicationListener来处理这个事件，当ApplicationListener接收到这个事件之后，就会解析application.properties、application.yml文件， //并添加到Environment中）ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);configureIgnoreBeanInfo(environment);Banner printedBanner = printBanner(environment);// 打印Banner //据webApplicationType创建不同的Spring上下文容器（有三种）context = createApplicationContext();context.setApplicationStartup(this.applicationStartup); //预初始化spring上下文，见下面的源码分析prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner); //刷新Spring容器,这一步中创建并初始化bean，创建并启动tomacat等（以tomcat为例，调到ServletWebServerApplicationContext的createWebServer()方法 //最后执行TomcatServletWebServerFactory的getWebServer方法） refreshContext(context);afterRefresh(context, applicationArguments);Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);if (this.logStartupInfo) { new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);} //发布ApplicationStartedEvent事件和AvailabilityChangeEvent事件listeners.started(context, timeTakenToStartup); // 获取Spring容器中的ApplicationRunner/CommandLineRunner类型的Bean,并执行run方法callRunners(context, applicationArguments); } catch (Throwable ex) {handleRunFailure(context, ex, listeners);//发布ApplicationFailedEvent事件throw new IllegalStateException(ex); } try {Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime); //发布ApplicationReadyEvent事件和AvailabilityChangeEvent事件listeners.ready(context, timeTakenToReady); } catch (Throwable ex) {handleRunFailure(context, ex, null);//发布ApplicationFailedEvent事件throw new IllegalStateException(ex); } return context;}

预初始化环境，创建Environment对象源码解析：

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,DefaultBootstrapContext bootstrapContext, ApplicationArguments applicationArguments) { // 根据webApplicationType 创建Environment 创建就会读取： java环境变量和系统环境变量 ConfigurableEnvironment environment = getOrCreateEnvironment();// 将命令行参数读取环境变量中 configureEnvironment(environment, applicationArguments.getSourceArgs());// 将@PropertieSource的配置信息 放在第一位，它的优先级是最低的 ConfigurationPropertySources.attach(environment);// 发布了ApplicationEnvironmentPreparedEvent 的监听器 读取了全局配置文件 listeners.environmentPrepared(bootstrapContext, environment);// 将所有spring.main 开头的配置信息绑定到SpringApplication中 DefaultPropertiesPropertySource.moveToEnd(environment); Assert.state(!environment.containsProperty("spring.main.environment-prefix"), "Environment prefix cannot be set via properties."); bindToSpringApplication(environment); if (!this.isCustomEnvironment) {EnvironmentConverter environmentConverter = new EnvironmentConverter(getClassLoader());environment = environmentConverter.convertEnvironmentIfNecessary(environment, deduceEnvironmentClass()); } ConfigurationPropertySources.attach(environment);//更新PropertySources return environment;}

预初始化spring上下文源码解析：

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments, Banner printedBanner) { context.setEnvironment(environment); postProcessApplicationContext(context); applyInitializers(context);// 拿到之前读取到所有ApplicationContextInitializer的组件， 循环调用initialize方法 listeners.contextPrepared(context);// 发布了ApplicationContextInitializedEvent bootstrapContext.close(context); if (this.logStartupInfo) {logStartupInfo(context.getParent() == null);logStartupProfileInfo(context); }// 获取当前spring上下文beanFactory (负责创建bean) 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) {//在SpringBoot 在这里设置了不允许覆盖， 当出现2个重名的bean 会抛出异常 ((DefaultListableBeanFactory) beanFactory) .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);} }// 设置当前spring容器是不是要将所有的bean设置为懒加载 if (this.lazyInitialization) {context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor()); } context.addBeanFactoryPostProcessor(new PropertySourceOrderingBeanFactoryPostProcessor(context)); // Load the sources Set