Spring源码之Springboot中监听器介绍

https://www.bilibili.com/video/BV12C4y1s7dR?p=11

监听器模式要素

• 事件
• 监听器
• 广播器
• 触发机制

Springboot中监听模式总结

1. 在SpringApplication初始化中从META_INF/spring.factories获取Listeners
2. 创建监听器
3. 创建广播器
4. 将监听器在广播器中进行注册
5. 事件触发时，监听器监听到，同时将创建的事件传给广播器，执行广播器，通过回调调用实际事件中方法

Springboot中监听器各要素

事件

public abstract class ApplicationEvent extends EventObject {

	/** use serialVersionUID from Spring 1.2 for interoperability. */
	private static final long serialVersionUID = 7099057708183571937L;

	/** System time when the event happened. */
	private final long timestamp;


	/**
	 * Create a new {@code ApplicationEvent}.
	 * @param source the object on which the event initially occurred or with
	 * which the event is associated (never {@code null})
	 */
	public ApplicationEvent(Object source) {
		super(source);
		this.timestamp = System.currentTimeMillis();
	}


	/**
	 * Return the system time in milliseconds when the event occurred.
	 */
	public final long getTimestamp() {
		return this.timestamp;
	}

}

监听器

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {

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

}

广播器

public interface ApplicationEventMulticaster {
	void addApplicationListener(ApplicationListener<?> listener);
	void addApplicationListenerBean(String listenerBeanName);
	void removeApplicationListener(ApplicationListener<?> listener);
	void removeApplicationListenerBean(String listenerBeanName);
	void removeAllListeners();
	void multicastEvent(ApplicationEvent event);
	void multicastEvent(ApplicationEvent event, @Nullable ResolvableType eventType);
}

事件触发，如：

SpringAppliation中listeners.starting();

源码梳理

在SpringApplication初始化中setListeners

在SpringApplication构造函数中，执行setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

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();
}

执行SpringFactoriesLoader类中loadSpringFactories方法，将jar包中所有META_INF/spring.factories文件中的属性进行加载(在setInitializers中已经加载，所以在setListeners直接从MultiValueMap缓存中取)

Enumeration<URL> urls = (classLoader != null ?
		classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
		ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

调用链：

SpringApplication#init() --> SpringApplication#getSpringFactoriesInstances() --> SpringFactoriesLoader#loadFactoryNames() --> SpringFactoriesLoader#loadSpringFactories()

loadSpringFactories代码：

	private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
		MultiValueMap<String, String> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		try {
			Enumeration<URL> urls = (classLoader != null ?
					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
			result = new LinkedMultiValueMap<>();
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					String factoryTypeName = ((String) entry.getKey()).trim();
					for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryTypeName, factoryImplementationName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}

调用createSpringFactoriesInstances方法进行实例化

private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,
		ClassLoader classLoader, Object[] args, Set<String> names) {
	List<T> instances = new ArrayList<>(names.size());
	for (String name : names) {
		try {
			Class<?> instanceClass = ClassUtils.forName(name, classLoader);
			Assert.isAssignable(type, instanceClass);
			Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
			T instance = (T) BeanUtils.instantiateClass(constructor, args);
			instances.add(instance);
		}
		catch (Throwable ex) {
			throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
		}
	}
	return instances;
}

将获取到的listeners注册到List<ApplicationListener<?>> listeners中

public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
	this.listeners = new ArrayList<>(listeners);
}

获取SpringApplicationRunListener

SpringApplicationRunListeners listeners = getRunListeners(args);

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

key为org.springframework.boot.SpringApplicationRunListener，从之前缓存的map中取出SpringApplicationRunListener,value为EventPublishingRunListener

对EventPublishingRunListener进行初始化，创建SimpleApplicationEventMulticaster广播器

public EventPublishingRunListener(SpringApplication application, String[] args) {
	this.application = application;
	this.args = args;
	this.initialMulticaster = new SimpleApplicationEventMulticaster();
	for (ApplicationListener<?> listener : application.getListeners()) {
		this.initialMulticaster.addApplicationListener(listener);
	}
}

将监听器注册到广播器中：

for (ApplicationListener<?> listener : application.getListeners()) {
	this.initialMulticaster.addApplicationListener(listener);
}

事件触发

SpringApplication中listeners.starting();
执行到EventPublishingRunListener中

@Override
public void starting() {
	this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
}

进行广播事件：

@Override
public void multicastEvent(ApplicationEvent event) {
	multicastEvent(event, resolveDefaultEventType(event));
}

@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
	ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
	Executor executor = getTaskExecutor();
	for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
		if (executor != null) {
			executor.execute(() -> invokeListener(listener, event));
		}
		else {
			invokeListener(listener, event);
		}
	}
}

然后去调用具体事件的onApplicationEvent方法