Springboot之事件管家SpringApplicationRunListeners事件管理机制源码分析

Springboot的SpringApplication事件监听是独立于spring容器事件管理的另一套机制，分为：
starting、environmentPrepared、contextPrepared、contextLoaded、started、running、failed等应用阶段的监听

springboot启动的核心方法：

public ConfigurableApplicationContext run(String... args) {
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
		// @A
		SpringApplicationRunListeners listeners = getRunListeners(args);
		listeners.starting();
		try {
			...省略...
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}
		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}
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@A：初步看来，spring-boot使用了 SpringApplicationRunListeners 这个类来抽象了spring应用运行时的一些event；SpringApplicationRunListeners 实则是对SpringApplicationRunListener子类集合的封装

SpringApplication构造方法

	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();
		// @A
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		// @B
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}
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@A、@B：在这里我先讲SpringApplication的构造方法，是因为在构造方法里做了很多事情，跟本文有关的就是框架会在构造方法里从配置文件spring.factories里拿到key为org.springframework.context.ApplicationListener 和 key为org.springframework.context.ApplicationContextInitializer的类，实例化好后缓存起来

这一点很重要，否则后面看代码会很绕；getSpringFactoriesInstances方法源码如下

	private  Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) {
		ClassLoader classLoader = getClassLoader();
		// Use names and ensure unique to protect against duplicates
		
		// @A
		Set names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
       	List instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
		AnnotationAwareOrderComparator.sort(instances);
		return instances;
	}
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@A：解析配置文件并实例化，返回对象

SpringApplicationRunListeners 初始化

在调用spring-boot SpringApplication的run方法流程中，有一行代码：SpringApplicationRunListeners listeners = getRunListeners(args);

getRunListeners方法代码如下：

	private SpringApplicationRunListeners getRunListeners(String[] args) {
		Class[] types = new Class[] { SpringApplication.class, String[].class };
		return new SpringApplicationRunListeners(logger,
				getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
	}

	private  Collection getSpringFactoriesInstances(Class type, Class[] parameterTypes, Object... args) {
		ClassLoader classLoader = getClassLoader();
		// Use names and ensure unique to protect against duplicates
		
		// @A
		Set names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        // @B
		List instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
		AnnotationAwareOrderComparator.sort(instances);
		return instances;
	}

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@A：这个方法会到 spring.factories 文件里去拿 org.springframework.boot.SpringApplicationRunListener 为key 的value值
@B：创建对应的实例集合，框架默认为SpringApplicationRunListener提供了一个子类 EventPublishingRunListener，所以instances的size是1

EventPublishingRunListener

我们来看看其构造方法

	public EventPublishingRunListener(SpringApplication application, String[] args) {
		this.application = application;
		this.args = args;
		this.initialMulticaster = new SimpleApplicationEventMulticaster();
		// @A
		for (ApplicationListener listener : application.getListeners()) {
			this.initialMulticaster.addApplicationListener(listener);
		}
	}
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@A：将application里的Listeners加载到自己类中，那么这些Listeners从哪里来的呢？
这就跟上面的 SpringApplication 构造方法有关了，在构造方法里已经将这些listeners提前加载了！（我反正是debug了一大圈才发现！）
这就完成了 EventPublishingRunListener 对象的初始化，后面springapplication的事件体系都由这个类来管控了！

扩展点：spring cloud 就是扩展了 org.springframework.context.ApplicationListener 实现了双容器！下一篇再详细分析一下双容器。

over~~