SpringBoot原理:run方法解读

接着我们来看run方法里面做了什么事情。
 

public ConfigurableApplicationContext run(String... args) {
    long startTime = System.nanoTime();
    DefaultBootstrapContext bootstrapContext = this.createBootstrapContext();
    ConfigurableApplicationContext context = null;
    this.configureHeadlessProperty();
  	//获取所有的SpringApplicationRunListener，并通知启动事件，默认只有一个实现类EventPublishingRunListener
  	//EventPublishingRunListener会将初始化各个阶段的事件转发给所有监听器
    SpringApplicationRunListeners listeners = this.getRunListeners(args);
    listeners.starting(bootstrapContext, this.mainApplicationClass);

    try {
      	//环境配置
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
        this.configureIgnoreBeanInfo(environment);
      	//打印Banner
        Banner printedBanner = this.printBanner(environment);
      	//创建ApplicationContext，注意这里会根据是否为Web容器使用不同的ApplicationContext实现类
        context = this.createApplicationContext();
        context.setApplicationStartup(this.applicationStartup);
      	//初始化ApplicationContext
        this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
      	//执行ApplicationContext的refresh方法
        this.refreshContext(context);
        this.afterRefresh(context, applicationArguments);
        Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
        if (this.logStartupInfo) {
            (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), timeTakenToStartup);
        }
        ....
}

我们发现，实际上SpringBoot就是Spring的一层壳罢了，离不开最关键的ApplicationContext，也就是说，在启动后会自动配置一个ApplicationContext，只不过是进行了大量的扩展。

我们来看ApplicationContext是怎么来的，打开`createApplicationContext`方法：

protected ConfigurableApplicationContext createApplicationContext() {
    return this.applicationContextFactory.create(this.webApplicationType);
}

 我们发现在构造方法中`applicationContextFactory`直接使用的是DEFAULT：

this.applicationContextFactory = ApplicationContextFactory.DEFAULT;

ApplicationContextFactory DEFAULT = (webApplicationType) -> {
    try {
        switch(webApplicationType) {
        case SERVLET:
            return new AnnotationConfigServletWebServerApplicationContext();
        case REACTIVE:
            return new AnnotationConfigReactiveWebServerApplicationContext();
        default:
            return new AnnotationConfigApplicationContext();
        }
    } catch (Exception var2) {
        throw new IllegalStateException("Unable create a default ApplicationContext instance, you may need a custom ApplicationContextFactory", var2);
    }
};

ConfigurableApplicationContext create(WebApplicationType webApplicationType);

DEFAULT是直接编写的一个匿名内部类，其实已经很明确了，正是根据`webApplicationType`类型进行判断，如果是SERVLET，那么久返回专用于Web环境的AnnotationConfigServletWebServerApplicationContext对象（SpringBoot中新增的），否则返回普通的AnnotationConfigApplicationContext对象，也就是到这里为止，Spring的容器就基本已经确定了。

注意AnnotationConfigApplicationContext是Spring框架提供的类，从这里开始相当于我们在讲Spring的底层源码了，我们继续深入，AnnotationConfigApplicationContext对象在创建过程中会创建`AnnotatedBeanDefinitionReader`，它是用于通过注解解析Bean定义的工具类：

public AnnotationConfigApplicationContext() {
    StartupStep createAnnotatedBeanDefReader = this.getApplicationStartup().start("spring.context.annotated-bean-reader.create");
    this.reader = new AnnotatedBeanDefinitionReader(this);
    createAnnotatedBeanDefReader.end();
    this.scanner = new ClassPathBeanDefinitionScanner(this);
}

其构造方法：

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
    ...
    //这里会注册很多的后置处理器
    AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}

public static Set registerAnnotationConfigProcessors(BeanDefinitionRegistry registry, @Nullable Object source) {
    DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
    ....
    Set beanDefs = new LinkedHashSet(8);
    RootBeanDefinition def;
    if (!registry.containsBeanDefinition("org.springframework.context.annotation.internalConfigurationAnnotationProcessor")) {
      	//注册了ConfigurationClassPostProcessor用于处理@Configuration、@Import等注解
      	//注意这里是关键，之后Selector还要讲到它
      	//它是继承自BeanDefinitionRegistryPostProcessor，所以它的执行时间在Bean定义加载完成后，Bean初始化之前
        def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, "org.springframework.context.annotation.internalConfigurationAnnotationProcessor"));
    }

    if (!registry.containsBeanDefinition("org.springframework.context.annotation.internalAutowiredAnnotationProcessor")) {
      	//AutowiredAnnotationBeanPostProcessor用于处理@Value等注解自动注入
        def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, "org.springframework.context.annotation.internalAutowiredAnnotationProcessor"));
    }

 回到SpringBoot，我们最后来看，`prepareContext`方法中又做了什么事情：

private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context, ConfigurableEnvironment environment, SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
  	//环境配置
    context.setEnvironment(environment);
    this.postProcessApplicationContext(context);
    this.applyInitializers(context);
    listeners.contextPrepared(context);
    bootstrapContext.close(context);
    if (this.logStartupInfo) {
        this.logStartupInfo(context.getParent() == null);
        this.logStartupProfileInfo(context);
    }

  	//将Banner注册为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) {
            ((DefaultListableBeanFactory)beanFactory).setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
        }
    }

    if (this.lazyInitialization) {
        context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
    }

  	//这里会获取我们一开始传入的项目主类
    Set