首先我们新建一个springboot，先从springboot的启动入口开始看

@SpringBootApplication
public class Demo2020Application {public static void main(String[] args) {SpringApplication.run(Demo2020Application.class, args);}}

我们直接运行这个main方法就能启动项目。

首先我们在run()这一行打断点，启动，进去(SpringApplication.java)，

传入的是class和args，继续

这是创建一个SpringApplication，继续看看这个方法的第一部分：

public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
// 设置java.awt.headless环境变量，是否图形化显示？configureHeadlessProperty();
//获取加载的SpringApplicationRunListener，然后调用他们的starting方法SpringApplicationRunListeners listeners = getRunListeners(args);
//EventPublishingRunListener（只加载了这一个Listener）listeners.starting();

进入到starting方法（EventPublishingRunListener.java）

SimpleApplicationEventMulticaster.java
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
//class org.springframework.boot.context.event.ApplicationStartingEvent
//看名字就知道是springboot应用启动事件ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));Executor executor = getTaskExecutor();
//从最启动获取所有的监听器for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {if (executor != null) {executor.execute(() -> invokeListener(listener, event));}else {invokeListener(listener, event);}}
}

然后看看第二部分（SpringApplication.java）

try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);

进入到

//将所有的命令行启动参数封装成ConfigurableEnvironment 对象
private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments) {// Create and configure the environment
// 获得或者是创建环境（这是一个web项目所以会创建一个servlet环境，往下看这个方法，下文给出）ConfigurableEnvironment environment = getOrCreateEnvironment();
// 配置环境看下文configureEnvironment(environment, applicationArguments.getSourceArgs());
// 转换环境ConfigurationPropertySources.attach(environment);
// 准备环境listeners.environmentPrepared(environment);
// 绑定环境bindToSpringApplication(environment);
// 转换成StandardEnvironmentif (!this.isCustomEnvironment) {environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,deduceEnvironmentClass());}ConfigurationPropertySources.attach(environment);return environment;
}

private ConfigurableEnvironment getOrCreateEnvironment() {if (this.environment != null) {return this.environment;}
// 根据断点显示webApplicationType是"SERVLET"switch (this.webApplicationType) {case SERVLET:return new StandardServletEnvironment();case REACTIVE:return new StandardReactiveWebEnvironment();default:return new StandardEnvironment();}
}

进入到StandardServletEnvironment.java

@Override
protected void customizePropertySources(MutablePropertySources propertySources) {
// servletContextInitParamspropertySources.addLast(new StubPropertySource(SERVLET_CONFIG_PROPERTY_SOURCE_NAME));
// servletConfigInitParamspropertySources.addLast(new StubPropertySource(SERVLET_CONTEXT_PROPERTY_SOURCE_NAME));if (JndiLocatorDelegate.isDefaultJndiEnvironmentAvailable()) {propertySources.addLast(new JndiPropertySource(JNDI_PROPERTY_SOURCE_NAME));}super.customizePropertySources(propertySources);
}

下面是他的父类的customizePropertySources方法

StandardEnvironment.javaprotected void customizePropertySources(MutablePropertySources propertySources) {propertySources.addLast(new PropertiesPropertySource("systemProperties", this.getSystemProperties()));propertySources.addLast(new SystemEnvironmentPropertySource("systemEnvironment", this.getSystemEnvironment()));
}

可以看出StandardServletEnvironment往propertySources添加了两个StubPropertySource对象，然后调用父类添加了系统环境变量

配置环境

protected void configureEnvironment(ConfigurableEnvironment environment, String[] args) {if (this.addConversionService) {ConversionService conversionService = ApplicationConversionService.getSharedInstance();environment.setConversionService((ConfigurableConversionService) conversionService);}
// 配置PropertySourcesconfigurePropertySources(environment, args);
//配置ProfilesconfigureProfiles(environment, args);
}
下面看看这两个方法

protected void configurePropertySources(ConfigurableEnvironment environment, String[] args) {MutablePropertySources sources = environment.getPropertySources();
// 如果存在默认配置就放在最后位置if (this.defaultProperties != null && !this.defaultProperties.isEmpty()) {sources.addLast(new MapPropertySource("defaultProperties", this.defaultProperties));}
// 如果有命令行参数就将原有的替换掉（这里就说明命令行参数比配置文件的优先级高）if (this.addCommandLineProperties && args.length > 0) {String name = CommandLinePropertySource.COMMAND_LINE_PROPERTY_SOURCE_NAME;if (sources.contains(name)) {PropertySource<?> source = sources.get(name);CompositePropertySource composite = new CompositePropertySource(name);composite.addPropertySource(new SimpleCommandLinePropertySource("springApplicationCommandLineArgs", args));composite.addPropertySource(source);sources.replace(name, composite);}
// 如果没有命令行参数就放在第一个else {sources.addFirst(new SimpleCommandLinePropertySource(args));}}
}

//从PropertySources中查找spring.profiles.active属性，存在则将其值添加activeProfiles集合中
protected void configureProfiles(ConfigurableEnvironment environment, String[] args) {Set<String> profiles = new LinkedHashSet<>(this.additionalProfiles);profiles.addAll(Arrays.asList(environment.getActiveProfiles()));environment.setActiveProfiles(StringUtils.toStringArray(profiles));
}

转换环境（就是重新配置属性）

public static void attach(Environment environment) {Assert.isInstanceOf(ConfigurableEnvironment.class, environment);MutablePropertySources sources = ((ConfigurableEnvironment) environment).getPropertySources();PropertySource<?> attached = sources.get(ATTACHED_PROPERTY_SOURCE_NAME);
//有了就移除，并设置为空if (attached != null && attached.getSource() != sources) {sources.remove(ATTACHED_PROPERTY_SOURCE_NAME);attached = null;}
// 没有就创建一个并在第一位置if (attached == null) {sources.addFirst(new ConfigurationPropertySourcesPropertySource(ATTACHED_PROPERTY_SOURCE_NAME,new SpringConfigurationPropertySources(sources)));}
}

第三部分

//spring.beaninfo.ignoreconfigureIgnoreBeanInfo(environment);
//打印spring 就是控制台的图标Banner printedBanner = printBanner(environment);context = createApplicationContext();
//这个类里包含了SpringBoot启动失败后异常处理相关的组件exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,new Class[] { ConfigurableApplicationContext.class }, context);prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//初始化容器refreshContext(context);afterRefresh(context, applicationArguments);stopWatch.stop();if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);}listeners.started(context);callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, listeners);throw new IllegalStateException(ex);}try {listeners.running(context);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, null);throw new IllegalStateException(ex);}return context;
}

//指示Spring Introspector.IGNORE_ALL_BEANINFO 在调用JavaBeans时使用该模式的系统属性
//Introspector：“ spring.beaninfo.ignore”，其值为“ true”会跳过对BeanInfo类的搜索（通常用于在应
//用程序中未为bean定义此类的情况下）首先）。
private void configureIgnoreBeanInfo(ConfigurableEnvironment environment) {if (System.getProperty(CachedIntrospectionResults.IGNORE_BEANINFO_PROPERTY_NAME) == null) {Boolean ignore = environment.getProperty("spring.beaninfo.ignore", Boolean.class, Boolean.TRUE);System.setProperty(CachedIntrospectionResults.IGNORE_BEANINFO_PROPERTY_NAME, ignore.toString());}
}

根据当前环境创建ApplicationContext

protected ConfigurableApplicationContext createApplicationContext() {Class<?> contextClass = this.applicationContextClass;if (contextClass == null) {try {//SERVLETswitch (this.webApplicationType) {case SERVLET://AnnotationConfigServletWebServerApplicationContextcontextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);break;case REACTIVE:contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);break;default:contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);}}catch (ClassNotFoundException ex) {throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", ex);}}return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

继续下一行

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
//重新设置环境context.setEnvironment(environment);
//上下文处理
//beanNameGenerator和resourceLoader默认都是空的，只有自定义这两个对象时才会把容器内的bean替换
//postProcessApplicationContext(context);applyInitializers(context);
//待扩展listeners.contextPrepared(context);if (this.logStartupInfo) {logStartupInfo(context.getParent() == null);logStartupProfileInfo(context);}// Add boot specific singleton beansConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
//注册Bean-->springApplicationArgumentsbeanFactory.registerSingleton("springApplicationArguments", applicationArguments);if (printedBanner != null) {beanFactory.registerSingleton("springBootBanner", printedBanner);}if (beanFactory instanceof DefaultListableBeanFactory) {((DefaultListableBeanFactory) beanFactory).setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);}if (this.lazyInitialization) {context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());}// Load the sourcesSet<Object> sources = getAllSources();Assert.notEmpty(sources, "Sources must not be empty");load(context, sources.toArray(new Object[0]));listeners.contextLoaded(context);
}

这里的第一行

@Override
public void setEnvironment(ConfigurableEnvironment environment) {super.setEnvironment(environment);this.reader.setEnvironment(environment);this.scanner.setEnvironment(environment);
}

继续下一行

protected void postProcessApplicationContext(ConfigurableApplicationContext context) {if (this.beanNameGenerator != null) {context.getBeanFactory().registerSingleton(AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR,this.beanNameGenerator);}if (this.resourceLoader != null) {if (context instanceof GenericApplicationContext) {((GenericApplicationContext) context).setResourceLoader(this.resourceLoader);}if (context instanceof DefaultResourceLoader) {((DefaultResourceLoader) context).setClassLoader(this.resourceLoader.getClassLoader());}}if (this.addConversionService) {context.getBeanFactory().setConversionService(ApplicationConversionService.getSharedInstance());}
}

继续--执行所有的ApplicationContextInitializer的initialize()

protected void applyInitializers(ConfigurableApplicationContext context) {for (ApplicationContextInitializer initializer : getInitializers()) {Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(initializer.getClass(),ApplicationContextInitializer.class);Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");initializer.initialize(context);}
}

继续-->循环往ApplicationContextAware接口的监听器中注入了context容器，并且发布事件

@Override
public void contextLoaded(ConfigurableApplicationContext context) {for (ApplicationListener<?> listener : this.application.getListeners()) {if (listener instanceof ApplicationContextAware) {((ApplicationContextAware) listener).setApplicationContext(context);}context.addApplicationListener(listener);}this.initialMulticaster.multicastEvent(new ApplicationPreparedEvent(this.application, this.args, context));
}