1、springBoot项目启动过程中的监听机制

springBoot项目启动过程有两层监听，第一层由组件【SpringApplicationRunListener】实现，第二层由组件【ApplicationListener】实现；

public interface SpringApplicationRunListener {
    default void starting() {}
    default void environmentPrepared(ConfigurableEnvironment environment) {}
    default void contextPrepared(ConfigurableApplicationContext context) {}
    default void contextLoaded(ConfigurableApplicationContext context) {}
    default void started(ConfigurableApplicationContext context) {}
    default void running(ConfigurableApplicationContext context) {}
    default void failed(ConfigurableApplicationContext context, Throwable exception) {}
}

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    void onApplicationEvent(E event);
}

接下来我们从springBoot项目启动的核心方法【org.springframework.boot.SpringApplication#run】看监听机制的具体实现：

public ConfigurableApplicationContext run(String... args) {
    ......
        /** 加载第一层监听组件，
             第二层监听组件在【SpringApplication】实例化的时候加载 **/
        SpringApplicationRunListeners listeners = getRunListeners(args);
               // mark1
        listeners.starting();
        try {
            .......
            ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
            configureIgnoreBeanInfo(environment);
              ......
            prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            refreshContext(context);
            afterRefresh(context, applicationArguments);
                //  mark2
            listeners.started(context);
        }
        try {
               //  mark3
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }
private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
         ......
         // mark4
        listeners.environmentPrepared(environment);
         ......
    }
private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
            SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
        ......
      // mark5
        listeners.contextPrepared(context);
        ......
    // mark6
        listeners.contextLoaded(context);
    }
private void handleRunFailure(ConfigurableApplicationContext context, Throwable exception,
            Collection<SpringBootExceptionReporter> exceptionReporters, SpringApplicationRunListeners listeners) {
     ......
    // mark7    
    listeners.failed(context, exception);
    ....... 
    }

接下来，我们主要看下mark4处的处理：

void environmentPrepared(ConfigurableEnvironment environment) {
          //循环所有的【SpringApplicationRunListener】，目前只有一个实现【EventPublishingRunListener】
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.environmentPrepared(environment);
        }
    }
public void environmentPrepared(ConfigurableEnvironment environment) {
        // 委托自己的事件发布器发布事件
        this.initialMulticaster
                .multicastEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args, environment));
    }
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        Executor executor = getTaskExecutor();
        /**循环所有的【ApplicationListener】；
           SpringApplication实例化的时候加载所有【ApplicationListener】的实现;
           实例化【EventPublishingRunListener】的时候，把所有【ApplicationListener】的实现传递给了前者的事件发布器【SimpleApplicationEventMulticaster】**/
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);
            }
        }
    }

2、【bootstrap】的栈帧分析

/**
6、获取配置文件名称
**/
getSearchNames:716, ConfigFileApplicationListener$Loader (org.springframework.boot.context.config)
lambda$load$8:446, ConfigFileApplicationListener$Loader (org.springframework.boot.context.config)
accept:-1, 1398260359 (org.springframework.boot.context.config.ConfigFileApplicationListener$Loader$$Lambda$115)
forEach:75, Iterable (java.lang)
load:444, ConfigFileApplicationListener$Loader (org.springframework.boot.context.config)
lambda$load$0:347, ConfigFileApplicationListener$Loader (org.springframework.boot.context.config)
accept:-1, 1037955032 (org.springframework.boot.context.config.ConfigFileApplicationListener$Loader$$Lambda$99)
apply:54, FilteredPropertySource (org.springframework.boot.context.config)
load:335, ConfigFileApplicationListener$Loader (org.springframework.boot.context.config)
addPropertySources:226, ConfigFileApplicationListener (org.springframework.boot.context.config)
postProcessEnvironment:210, ConfigFileApplicationListener (org.springframework.boot.context.config)
onApplicationEnvironmentPreparedEvent:200, ConfigFileApplicationListener (org.springframework.boot.context.config)
/**
5、第二个SpringApplication启动的时候，又会执行到BootstrapApplicationListener，但因为此时我们的Environment中有属性【spring.config.name】，
所以会跳过，接着执行下一个监听器【ConfigFileApplicationListener】
**/
onApplicationEvent:188, ConfigFileApplicationListener (org.springframework.boot.context.config)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:127, SimpleApplicationEventMulticaster (org.springframework.context.event)
environmentPrepared:80, EventPublishingRunListener (org.springframework.boot.context.event)
environmentPrepared:53, SpringApplicationRunListeners (org.springframework.boo
prepareEnvironment:345, SpringApplication (org.springframework.boot)
run:308, SpringApplication (org.springframework.boot)
/**
 4、在执行第二层监听器【BootstrapApplicationListener】的时候，会创建一个新的SpringApplication然后启动，
 这次传入的主类会是null，会创建新的StandardEnvironment且设置属性【spring.config.name】为【bootstrap】
 **/
run:140, SpringApplicationBuilder (org.springframework.boot.builder)
bootstrapServiceContext:212, BootstrapApplicationListener (org.springframework.cloud.bootstrap)
onApplicationEvent:117, BootstrapApplicationListener (org.springframework.cloud.bootstrap)
// 3、执行第二层监听器    
onApplicationEvent:74, BootstrapApplicationListener (org.springframework.cloud.bootstrap)
doInvokeListener:172, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:165, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:139, SimpleApplicationEventMulticaster (org.springframework.context.event)
// 2、监听器委托事件广播器【SimpleApplicationEventMulticaster】广播事件   
multicastEvent:127, SimpleApplicationEventMulticaster (org.springframework.context.event)
// 1、 执行第一层监听器列表，目前只有一个【EventPublishingRunListener】    
environmentPrepared:80, EventPublishingRunListener (org.springframework.boot.context.event)
environmentPrepared:53, SpringApplicationRunListeners (org.springframework.boot)
prepareEnvironment:345, SpringApplication (org.springframework.boot)
run:308, SpringApplication (org.springframework.boot)
run:1237, SpringApplication (org.springframework.boot)
run:1226, SpringApplication (org.springframework.boot)
main:42, GfundActbApiApplication (com.gome.gfund.actb)