概述

本文所涉及程式碼基於 spring-boot 2.1.0

ApplicationListener是Spring框架提供的一個用於監聽應用事件(application event)的事件監聽器。它繼承自Java標準觀察者模式的EventListener介面。從Spring 3.0之後，一個ApplicationListener實現可以宣告自己所關心的事件型別。當一個ApplicationListener被註冊到Spring ApplicationContext之後，應用執行時應用事件會陸續發生，對應響應型別的事件監聽器會被呼叫。

介面ApplicationListener

// 泛型介面，E定義了監聽器實現類所真正關心的應用事件型別
@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

	/**
	 * Handle an application event.
	 * @param event the event to respond to
	 */
	void onApplicationEvent(E event);

}

分析

當Springboot應用程式啟動時，它會載入一組內建的ApplicationListener

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
	this.resourceLoader = resourceLoader;
	Assert.notNull(primarySources, "PrimarySources must not be null");
	this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
	this.webApplicationType =
 
 WebApplicationType.deduceFromClasspath();
	setInitializers((Collection) getSpringFactoriesInstances(
			ApplicationContextInitializer.class));
	setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));//<====
	this.mainApplicationClass = deduceMainApplicationClass();
}

最終這些ApplicationListener會在下面位置被使用 :

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();
	// ... 省略其他程式碼
}

這些內建的ApplicationListener是:

上表中包名springframework縮寫為sf