spring 事件监听同时支持同步事件及异步事件

spring的事件监听机制这里就不再赘述了，不清楚的可以自行google，我们知道，事件发布是依靠调用实现了ApplicationEventPublisher接口类的publishEvent方法进行发布事件，而publishEvent 方法又是通过调用实现了ApplicationEventMulticaster接口的类的multicastEvent方法进行事件的广播的，ApplicationEventMulticaster中保存了所有的实现了ApplicationListener接口的监听器，我们看一下spring内部实现ApplicationEventMulticaster接口的一个广播器SimpleApplicationEventMulticaster的源码

public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {
    private Executor taskExecutor;

    public SimpleApplicationEventMulticaster() {
    }

    public SimpleApplicationEventMulticaster(BeanFactory beanFactory) {
        this.setBeanFactory(beanFactory);
    }

    public void setTaskExecutor(Executor taskExecutor) {
        this.taskExecutor = taskExecutor;
    }

    protected Executor getTaskExecutor() {
        return this.taskExecutor;
    }

    public void multicastEvent(final ApplicationEvent event) {
        Iterator i$ = this.getApplicationListeners(event).iterator();

        while(i$.hasNext()) {
            final ApplicationListener listener = (ApplicationListener)i$.next();
            Executor executor = this.getTaskExecutor();
            if (executor != null) {
                executor.execute(new Runnable() {
                    public void run() {
                        listener.onApplicationEvent(event);
                    }
                });
            } else {
                listener.onApplicationEvent(event);
            }
        }

    }
}

可以看到，异步事件通知主要依靠SimpleApplicationEventMulticaster 类中的Executor去实现的，如果这个变量不配置的话默认事件通知是同步的， 否则就是异步通知了，要实现同时支持同步通知和异步通知就得从这里下手；

我的实现方式是为每个监听方法加个自定义注解，然后在multicastEvent方法中获取对应监听器上的注解，根据注解去决定是同步通知还是异步通知，这样就可以同时支持了，废话不多说，直接看代码

/**
 * event listener 专用注解，只能加在实现了ApplicationListener 的onApplicationEvent方法上
 * 用来标识是同步监听还是异步监听
 */
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface EventType {
    EventTypeEnum value() default EventTypeEnum.ASYNC;
}

/**
 * @Description: 事件监听枚举类
 * @Author Mr.huang
 * @Date 2020/3/20 0020
 * @Version V1.0
 **/
public enum  EventTypeEnum {
    ASYNC,  //异步
    SYNC;   //同步
}

/**
 * @Description: 事件广播基础类，支持自定义注解实现同步或异步的事件监听
 * @Author Mr.huang
 * @Date 2020/3/20 0020
 * @Version V1.0
 **/
public class BaseApplicationEventMulticaster extends SimpleApplicationEventMulticaster {
    private static Logger log = LoggerFactory.getLogger(BaseApplicationEventMulticaster.class);
    @SuppressWarnings("unchecked")
    public void multicastEvent(final ApplicationEvent event) {
        //默认异步
        EventTypeEnum defaultEventType = EventTypeEnum.ASYNC;
        for (final ApplicationListener listener : getApplicationListeners(event)) {
            try {
                Class listenerClass = Class.forName(listener.getClass().getName());
                if(listenerClass!=null){
                    Method onApplicationEventMethod = listenerClass.getMethod("onApplicationEvent",ApplicationEvent.class);
                    if(onApplicationEventMethod.isAnnotationPresent(EventType.class)){
                        //获取该元素上指定类型的注解
                        EventType eventMethodAnnotation = onApplicationEventMethod.getAnnotation(EventType.class);
                        defaultEventType = eventMethodAnnotation.value();
                    }
                }
            } catch (Exception e) {
                log.error("获取监听类实例出错~");
            }

            Executor executor = getTaskExecutor();
            if (executor != null&&defaultEventType==EventTypeEnum.ASYNC) {
                executor.execute(new Runnable() {
                    public void run() {
                        listener.onApplicationEvent(event);
                    }
                });
            }
            else {
                listener.onApplicationEvent(event);
            }
        }
    }
}

最后在实现了ApplicationListener接口类中的onApplicationEvent方法上加上注解即可

/**
 * @Description: TODO
 * @Author Mr.huang
 * @Date 2020/3/21 0021
 * @Version V1.0
 **/
public class UserChargeService implements ApplicationListener<UserChargeEvent> {
    @EventType(value = EventTypeEnum.ASYNC)
    @Override
    public void onApplicationEvent(UserChargeEvent userChargeEvent) {
        //业务逻辑
    }
}