1. 了解事件监听器

事件监听器包括3个部分，事件、事件源、事件监听器。

事件，主要用于传递参数，例如用户登录，可以传递用户的名字等参数。

事件源，就是触发监听的方法或事件，用于抛出一个事件，通知事件监听器。

事件监听器，监听某个事件，接收到这个事件后，处理相应的逻辑。

2. 监听器的处理流程

• 给事件注册监听
• 触发某个事件
• 触发的事件通知对应的监听器，传递事件参数
• 事件监听器处理对应的逻辑

3. 基于SpringBoot和注解实现优雅的事件监听器

• 定义事件IEvent 事件的抽象接口，用于传递事件参数

  public interface IEvent {
  }
  

• 定义事件监听处理器IEventListener 事件触发监听后，处理对应的逻辑

  public interface IEventListener {
      void handle(IEvent event);
  }
  

• 定义@Listener和@EventAnno 用于自动注册事件

  @Retention(RetentionPolicy.RUNTIME)
  @Target({ElementType.TYPE})
  public @interface Listener {
  }
  

  @Retention(RetentionPolicy.RUNTIME)
  @Target({ElementType.METHOD})
  public @interface EventAnno {
  }
  

• 定义监听方法处理器 EventMethodListener 用反射调用触发的监听方法

  public class EventMethodListener implements IEventListener {
  
      private Class<?> eventClass;
  
      private Object invoker;
  
      private Method method;
  
      public EventMethodListener(Class<?> eventClass, Object invoker, Method method) {
          this.eventClass = eventClass;
          this.invoker = invoker;
          this.method = method;
      }
  
      @Override
      public void handle(IEvent event) {
          try {
              method.invoke(invoker, event);
          } catch (IllegalAccessException e) {
              e.printStackTrace();
          } catch (InvocationTargetException e) {
              e.printStackTrace();
          }
      }
  
      public Class<?> getEventClass() {
          return eventClass;
      }
  
      public void setEventClass(Class<?> eventClass) {
          this.eventClass = eventClass;
      }
  
      public Object getInvoker() {
          return invoker;
      }
  
      public void setInvoker(Object invoker) {
          this.invoker = invoker;
      }
  
      public Method getMethod() {
          return method;
      }
  
      public void setMethod(Method method) {
          this.method = method;
      }
  }
  
  

• 定义事件处理中心，统一处理事件的注册，事件触发后通知事件监听器。

  Component
  public class EventBus implements ApplicationContextAware {
  
      private Logger logger = LoggerFactory.getLogger(EventBus.class);
  
      private ApplicationContext applicationContext;
      private static EventBus instance;
  
      private Map<Class<?>, List<IEventListener>> listenerMap = new HashMap<>();
  
      public static EventBus getInstance() {
          return instance;
      }
  
      @PostConstruct
      public void init() {
          instance = this;
          initListeners();
      }
  
      /**
       * 自动注册
       */
      private void initListeners() {
          Map<String, Object> beans = applicationContext.getBeansWithAnnotation(Listener.class);
          beans.values().forEach(this::register);
      }
  
      /**
       * 注册用@Listener注解的bean，扫描所有用@EventAnno注解的方法
       * @param object
       */
      public void register(Object object) {
          Method[] declaredMethods = object.getClass().getDeclaredMethods();
          for (Method method : declaredMethods) {
              EventAnno annotation = method.getAnnotation(EventAnno.class);
              if (annotation == null) {
                  continue;
              }
              if (!Modifier.isPublic(method.getModifiers())) {
                  logger.error("The Listener Method Modifier Must be public" + method.getName());
                  continue;
              }
              checkListenerMethod(method);
              Class<?> eventClass = method.getParameterTypes()[0];
              EventMethodListener eventMethodListener = new EventMethodListener(eventClass, object, method);
              addListener(eventMethodListener);
          }
      }
  
      private void checkListenerMethod(Method method) {
          if (method.getParameterTypes().length != 1) {
              throw new IllegalArgumentException("The method params wrong on" + method.getName()
                      + ", The method params length need equals 1");
          }
          if (!IEvent.class.isAssignableFrom(method.getParameterTypes()[0])) {
              throw new IllegalArgumentException("The method params wrong on" + method.getName()
                      + ", The method params need to subClass of IEvent");
          }
      }
  
      /**
       * 事件触发后通知事件监听器
       * @param event
       */
      public void fire(IEvent event) {
          Class<?> clz = event.getClass();
          while (clz != Object.class && clz != null) {
              List<IEventListener> listeners = listenerMap.get(clz);
              if (listeners != null) {
                  listeners.forEach(listener -> handleListener0(listener, event));
              }
              clz = clz.getSuperclass();
          }
      }
  
      /**
       * 处理对应的逻辑
       * @param listener
       * @param event
       */
      private void handleListener0(IEventListener listener, IEvent event) {
          listener.handle(event);
      }
  
      private void addListener(EventMethodListener listener) {
          List<IEventListener> listeners = listenerMap.computeIfAbsent(listener.getEventClass(), a -> new ArrayList<>());
          listeners.add(listener);
      }
  
      @Override
      public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
          this.applicationContext = applicationContext;
      }
  }
  

• 定义一个测试事件

  public class TestEvent implements IEvent {
  
      private int id;
  
      public TestEvent(int id) {
          this.id = id;
      }
  
      public int getId() {
          return id;
      }
  
      public void setId(int id) {
          this.id = id;
      }
  }
  

• 定义用于处理监听事件的Listener

  @Listener
  @Component
  public class TestListener {
  
      @EventAnno
      public void testEvent(TestEvent event) {
          int id = event.getId();
          System.out.println("+++++++++" + id);
      }
  
      @EventAnno
      private void testEvent1(TestEvent event) {
          int id = event.getId();
          System.out.println("testEvent1" + id);
      }
  }
  

• 最后启动类，抛出对应的事件

  @SpringBootApplication
  public class EventApplication {
  
      public static void main(String[] args) {
          SpringApplication.run(EventApplication.class, args);
          EventBus.getInstance().fire(new TestEvent(22222));
      }
  
  }
  

谢谢大家，有错误或者不理解的地方，方法大家纠正讨论。

最后附上传送门，https://github.com/Zengzehao/easy-event