Spring监听器

1. 定义事件

public class DemoEvent extends ApplicationEvent {
    private String name;

    public DemoEvent(Object source, String name) {
        super(source);
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

2. 监听器

• 方式一：接口实现监听器：

@Component
public class DemoListener implements ApplicationListener {
    @Override
    public void onApplicationEvent(DemoEvent event) {
        System.out.println("事件"+event.getName()+"接口触发！");
    }
}

• 方式二：注解监听器：

@Component
public class DemoAnnotationListener {
    @EventListener
    public void onEvent(DemoEvent event) {
        System.out.println("事件"+event.getName()+"注解触发！");
    }
}

3. 发布事件

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
		ConfigurableApplicationContext run = SpringApplication.run(DemoApplication.class, args);
		run.publishEvent(new DemoEvent(run,"发布"));
	}
}

监听器是观察者设计模式的实现。
在refresh()方发中调用了prepareRefresh()方法，预刷新上下文环境。

1. 看下prepareRefresh()方法：

protected void prepareRefresh() {
		
		if (this.earlyApplicationListeners == null) {
			this.earlyApplicationListeners = new linkedHashSet<>(this.applicationListeners);
		}
		else {
			// Reset local application listeners to pre-refresh state.
			this.applicationListeners.clear();
			this.applicationListeners.addAll(this.earlyApplicationListeners);
		}

		
		this.earlyApplicationEvents = new linkedHashSet<>();
	}

2. 创建事件多播器
   initApplicationEventMulticaster()方法：

protected void initApplicationEventMulticaster() {
		//获取我们的bean工厂对象
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		//判断容器中是没有有我们的applicationEventMulticaster 应用多播器组件
		if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
			//直接显示的调用我们的getBean获取出来赋值给我们的applicationContext对象
			this.applicationEventMulticaster =
					beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
			if (logger.isDebugEnabled()) {
				logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
			}
		}
		//容器中没有的话
		else {
			//spring ioc显示的new 一个SimpleApplicationEventMulticaster对象保存在applicatoinContext对象中
			this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
			//并且注入到容器中
			beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
			if (logger.isDebugEnabled()) {
				logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
						APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
						"': using default [" + this.applicationEventMulticaster + "]");
			}
		}
	}

AbstractApplicationEventMulticaster有属性ListenerRetriever，存储了所有的listener。

applicationListenerBeans只存储了监听器的名字，主要是为了捕获懒加载的监听器。
多播器负责存储监听器和后面广播事件。

3. 注册监听器 registerListeners()
   
   以上三步整个监听器就注册完成了。

看下发布事件的流程：

自定义多播器可以选择监听事件是否异步执行。

@Configuration
@ComponentScan(basePackages = {"com.tuling.event"})
@EnableAsync
public class MainConfig {
	
    @Bean(name = "applicationEventMulticaster")
    public ApplicationEventMulticaster simpleApplicationEventMulticaster() {
        SimpleApplicationEventMulticaster eventMulticaster
                = new SimpleApplicationEventMulticaster();
        eventMulticaster.setTaskExecutor(new SimpleAsyncTaskExecutor());
        return eventMulticaster;
    }
}