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+## SpringBoot监听器实现
+
+
+
+### SpringBoot 系统监听器介绍
+
+```java
+/**
+ * Interface to be implemented by application event listeners.
+ * Based on the standard {@code java.util.EventListener} interface
+ * for the Observer design pattern.
+ *
+ * <p>As of Spring 3.0, an ApplicationListener can generically declare the event type
+ * that it is interested in. When registered with a Spring ApplicationContext, events
+ * will be filtered accordingly, with the listener getting invoked for matching event
+ * objects only.
+ *
+ * @author Rod Johnson
+ * @author Juergen Hoeller
+ * @param <E> the specific ApplicationEvent subclass to listen to
+ * @see org.springframework.context.event.ApplicationEventMulticaster
+ */
+@FunctionalInterface
+public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
+
+	/**
+	 * Handle an application event.
+	 * @param event the event to respond to
+	 */
+	void onApplicationEvent(E event);
+}
+```
+
+- SpringBoot中可以通过实现ApplicationListener接口类，来完成系统事件的监听
+- 这个监听器是机遇java.util.EventListener标准来实现的，是一个ObServer观察者模式
+- 从Spring3.0开始，ApplicationListener可以通过声明感兴趣的事件进行触发
+
+
+
+###  SpringBoot 系统广播器介绍
+
+ ```java
+ /**
+  * Interface to be implemented by objects that can manage a number of
+  * {@link ApplicationListener} objects, and publish events to them.
+  *
+  * <p>An {@link org.springframework.context.ApplicationEventPublisher}, typically
+  * a Spring {@link org.springframework.context.ApplicationContext}, can use an
+  * ApplicationEventMulticaster as a delegate for actually publishing events.
+  *
+  * @author Rod Johnson
+  * @author Juergen Hoeller
+  * @author Stephane Nicoll
+  */
+ public interface ApplicationEventMulticaster {
+ ```
+
+- 用来管理ApplicationListener事件监听器，比如事件监听器的添加、删除等
+- 用来遍历所有的ApplicationListener事件监听器，触发ApplicationEvent事件
+
+
+
+####  （1）Spring 系统广播器的具体方法介绍
+
+```java
+public interface ApplicationEventMulticaster {
+
+  // 添加一个 ApplicationListener 事件监听器
+	void addApplicationListener(ApplicationListener<?> listener);
+
+  // 删除一个 ApplicationListener 事件监听器
+	void removeApplicationListener(ApplicationListener<?> listener);
+
+	// 删除所有事件监听器
+	void removeAllListeners();
+
+  // 广播 ApplicationEvent 事件
+	void multicastEvent(ApplicationEvent event);
+}
+```
+
+
+
+### Spring 系统事件介绍
+
+<img src="https://tva1.sinaimg.cn/large/008i3skNly1gt3puhrolmj323g0u041t.jpg" alt="image-20210803171306277" align="left" width="1000" />
+
+- EventObject：具体的事件描述
+- ApplicationEvent：应用事件描述
+- SpringApplicationEvent：框架应用事件描述
+- ApplicationContextInitializedEvent：框架应用上下文初始化阶段的事件
+- ApplicationEnvironmentPreparedEvent：框架应用环境属性准备好阶段的事件
+- ApplicationFeiledEvent：框架启动失败时的事件
+- ApplicationPreparedEvent：框架准备阶段的事件
+- ApplicationReadyEvent：框架准备好时的事件
+- ApplicationStartedEvent：框架启动完成时的事件
+- ApplicationStartingEvent：框架启动中的事件
+
+
+
+### Spring 事件发送的顺序
+
+<img src="https://tva1.sinaimg.cn/large/008i3skNly1gt3q2eu1b0j322x0u0q5p.jpg" alt="image-20210803172047686" align="left" width="1000" />
+
+- 框架准备启动
+- 触发ApplicationStaringEvent事件，告知框架已开始启动
+- 触发ApplicationEnvironmentPreparedEvent事件，告知框架环境属性、我们指定的一些属性已经准备完成
+- 触发ApplicationContextInitializedEvent事件，告知框架ApplicationContextInitializer的系统上下文已经初始化好
+- Bean的准备、启动、准备好阶段，会分别触发ApplicationPreparedEvent、ApplicationStartedEvent、ApplicationReadyEvent事件；
+- 整个SpringBoot启动失败时，会触发ApplicationFailedEvent事件；
+- 框架启动完毕
+
+
+