SpringBoot源码分析之监听器

1. spring监听器的设计

主要是基于发布订阅者设计模式，由广播器(MultiCaster)、事件（Event）、监听器(Listener)三个元素组成。
监听器：ApplicationListener
广播器：SpringApplicationRunListeners
事件： SpringApplicationEvent

2. spring监听器源码

SpringBoot中事件类型以及发布顺序：

1. ApplicationStartingEvent : is sent at the start of a run but before any processing, except for the registration of listeners and initializers.
2. ApplicationEnvironmentPreparedEvent : is sent when the Environment to be used in the context is known but before the context is created.
3. ApplicationContextInitializedEvent : is sent when the ApplicationContext is prepared and ApplicationContextInitializers have been called but before any bean definitions are loaded.
4. ApplicationPreparedEvent : is sent just before the refresh is started but after bean definitions have been loaded.
5. ApplicationStartedEvent : is sent after the context has been refreshed but before any application and command-line runners have been called.
6. ApplicationReadyEvent : is sent after any application and command-line runners have been called.
7. ApplicationFailedEvent : is sent if there is an exception on startup.

SpringBoot中的监听器注册：
在在/resources/META-INF新建spring.factories文件中配置监听器，通过SpringFactoriesLoader进行加载。
SpringBoot中的监听器调用：
通过SpringApplicationRunListeners在对应事件发生的时候进行广播。

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();
            exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            prepareContext(context, environment, listeners, applicationArguments, printedBanner);
            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;
    }