Spring Boot自帶了訊息機制，可以讓我們在一個地方釋出訊息，多個地方同時接收訊息並處理訊息，當然這是在同一個JVM記憶體下進行的，不同的程序還需要使用MQ來實現。我覺得該訊息模式跟觀察者模式有一定的區別，觀察者模式一般觀察的是一個物件內部屬性發生變化的時候使用。而該訊息機制可以在任意地方使用。

訊息事件本身是一個物件，繼承於ApplicationEvent

@Data
public class DemoEvent extends ApplicationEvent {
    private String type;
    private List<Map> msg
 
;

    public DemoEvent(Object source,String type,List<Map> msg) {
        super(source);
        this.type = type;
        this.msg = msg;
    }
}

還需要有一個訊息事件釋出者，將這個訊息事件給釋出出去

@Component
public class DemoPublisher {
    @Autowired
    ApplicationContext applicationContext;

    public void 
 
publish(DemoEvent demoEvent) {
        applicationContext.publishEvent(demoEvent);
    }
}

然後就是我們的偵聽者，偵聽者可以有任意個根據業務不同做不同的處理，他的寫法分兩種，一個是實現了ApplicationListener介面，一個是在方法上打上@EventListener標籤

@Component
@Slf4j
public class DemoListener implements ApplicationListener<DemoEvent> {
    @Override
 
    @Async
    public void onApplicationEvent(DemoEvent demoEvent) {
        log.info("接收到publisher傳送到訊息，時間" + Time.getTime());
        List<Map> msg = demoEvent.getMsg();
        String type = demoEvent.getType();
        try {
            Thread.sleep(3000);
        }catch (Exception e) {
            e.printStackTrace();
        }
        log.info("型別" + type + ",訊息內容:" + msg);
    }
}

@Component
@Slf4j
public class DemoListener1 {
    @EventListener
    public void onDemoEvent(DemoEvent demoEvent) {
        log.info("listener1通過註解接收到了publisher傳送的訊息，時間" + Time.getTime());
        String type = demoEvent.getType();
        List<Map> msg = demoEvent.getMsg();
        try {
            Thread.sleep(2000);
        }catch (Exception e) {
            e.printStackTrace();
        }
        log.info("listener1:型別" + type + ",訊息內容：" + msg);
    }
}

但是我們需要知道的是，多個訊息監聽是同步執行的，他們會發生阻塞，所以我們需要進行非同步監聽，實現非同步監聽只需要在方法上打上@Async標籤，同時在Springboot主程式中開啟允許非同步

@EnableAsync
@SpringBootApplication
public class LanmdaApplication {

   public static void main(String[] args) {
      SpringApplication.run(LanmdaApplication.class, args);
   }

}

最後寫一個測試的Controller

@Slf4j
@RestController
public class TestController {
    @Autowired
    private DemoPublisher publisher;

    @GetMapping("/test")
    public String testListener() {
        List<Map> list = new ArrayList<>();
        Map<String,String> m1 = new HashMap<>();
        m1.put("1","2");
        Map<String,String> m2 = new HashMap<>();
        m2.put("3","4");
        Map<String,String> m3 = new HashMap<>();
        m3.put("5","6");
        list.add(m1);
        list.add(m2);
        list.add(m3);
        log.info("開始釋出訊息:" + Time.getTime());
        publisher.publish(new DemoEvent(this,"測試訊息",list));
        log.info("訊息釋出結束:" + Time.getTime());
        return "訊息釋出成功";
    }
}

執行後，日誌如下

2019-07-21 10:42:39.686  INFO 1756 --- [nio-8080-exec-1] c.g.lanmda.controller.TestController     : 開始釋出訊息:10:42:39
2019-07-21 10:42:39.687  INFO 1756 --- [nio-8080-exec-1] com.guanjian.lanmda.event.DemoListener1  : listener1通過註解接收到了publisher傳送的訊息，時間10:42:39
2019-07-21 10:42:41.690  INFO 1756 --- [nio-8080-exec-1] com.guanjian.lanmda.event.DemoListener1  : listener1:型別測試訊息,訊息內容：[{1=2}, {3=4}, {5=6}]
2019-07-21 10:42:41.695  INFO 1756 --- [nio-8080-exec-1] .s.a.AnnotationAsyncExecutionInterceptor : No task executor bean found for async processing: no bean of type TaskExecutor and no bean named 'taskExecutor' either
2019-07-21 10:42:41.697  INFO 1756 --- [nio-8080-exec-1] c.g.lanmda.controller.TestController     : 訊息釋出結束:10:42:41
2019-07-21 10:42:41.697  INFO 1756 --- [cTaskExecutor-1] com.guanjian.lanmda.event.DemoListener   : 接收到publisher傳送到訊息，時間10:42:41
2019-07-21 10:42:44.701  INFO 1756 --- [cTaskExecutor-1] com.guanjian.lanmda.event.DemoListener   : 型別測試訊息,訊息內容:[{1=2}, {3=4},