SpringBoot-关于ContextRefreshedEvent 和 ApplicationReadyEvent 触发时间以及触发机制的研究

在日常开发里我们常常会监听ContextRefreshedEvent 以及ApplicationReadyEvent ，来做一些内容初始化的操作，因为我们通常理解为在这两类Event发生的时候，容器的bean是可用的状态，且在容器启动的过程中，这两个事件只会发生一次。然而事实真的是这样吗？它们的触发顺序又是怎么样的呢？带着疑问我们来看下SpringBoot的源码：

1. ContextRefreshedEvent 

ContextRefreshedEvent 是spring framework所定义的事件，它的触发时间，是在容器刷新操作(refresh方法)的收尾时(finishRefresh)：

	/**
	 * Finish the refresh of this context, invoking the LifecycleProcessor's
	 * onRefresh() method and publishing the
	 * {@link org.springframework.context.event.ContextRefreshedEvent}.
	 */
	protected void finishRefresh() {
		// Clear context-level resource caches (such as ASM metadata from scanning).
		clearResourceCaches();
 
		// Initialize lifecycle processor for this context.
		initLifecycleProcessor();
 
		// Propagate refresh to lifecycle processor first.
		getLifecycleProcessor().onRefresh();
 
		// Publish the final event.
		publishEvent(new ContextRefreshedEvent(this));
 
		// Participate in LiveBeansView MBean, if active.
		LiveBeansView.registerApplicationContext(this);
	}

我们可以清晰地看到ContextRefreshedEvent事件的抛出；

这里就有一个有趣的地方了，没有工程师手动执行的话，refresh方法会仅仅被调用一次，还是可能会被多次触发呢？

这里我们可以来看一看AbstractApplicationContext的实现类

 它的直接子类有两个：AbstractRefreshableApplicationContext 以及 GenericApplicationContext；实际上，这两者都会衍生出很多容器的实现，但二者对refresh触发次数的限制是不同的；

我们先看AbstractRefreshableApplicationContext类的介绍（部分）：

 * Base class for {@link org.springframework.context.ApplicationContext}
 * implementations which are supposed to support multiple calls to {@link #refresh()},
 * creating a new internal bean factory instance every time.
 * Typically (but not necessarily), such a context will be driven by
 * a set of config locations to load bean definitions from.

在第二行，我们可以看到，在这个类以及其子类中，refresh方法是可能被多次触发的！

再来看看GenericApplicationContext：

 * 
In contrast to other ApplicationContext implementations that create a new
 * internal BeanFactory instance for each refresh, the internal BeanFactory of
 * this context is available right from the start, to be able to register bean
 * definitions on it. {@link #refresh()} may only be called once.

在这里的第四行我们可以看到，refresh方法（可能）只会被调用到一次！

2. ApplicationReadyEvent

ApplicationReadyEvent是SpringBoot定义的一个事件，在SpringApplication的run方法中被调用：

    	try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}

在新版本中则running方法被更换为ready方法

	@Override
	public void running(ConfigurableApplicationContext context) {
		context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
	}

3. 总结

ApplicationReadyEvent 是springBoot事件，只会被调用一次，在run方法的执行流程中被调用；

ContextRefreshedEvent 可能被调用多次，如果容器是AbstractRefreshableApplicationContext的子类；（默认SpringBoot启动的容器是GenericWebApplicationContext，不是AbstractRefreshableApplicationContext的子类）

ContextRefreshedEvent的调用在ApplicationReadyEvent之前被调用：

	public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection 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;
	}

ContextRefreshedEvent 的调用点是 refreshContext(context);

ApplicationReadyEvent 的调用点是 listeners.running(context);