SpringBoot监听器

介绍

类名

org.springframework.context.ApplicationListener

监听器模式

参照 SpringBoot 监听器源码写的 监听天气功能，可跳过忽略

事件对象

// 事件 接口
public abstract class WeatherEvent {
   // 事件本身不同的方法
    abstract String getWeather();
}

// 两个事件实现
// 下雪事件
public class SnowEvent extends WeatherEvent{
    @Override
    String getWeather() {
        return "下雪";
    }
}
// 下雨事件
public class RainEvent extends WeatherEvent{
    @Override
    String getWeather() {
        return "下雨";
    }
}

事件监听器

// 监听器接口
public interface WeatherListener {
    /**
     *  监听到天气时，做出的行为
     * @param event
     */
    void onWeatherEvent(WeatherEvent event);
}

// 监听器两个实现
// 下雪事件监听器
public class SnowListener implements WeatherListener{
    @Override
    public void onWeatherEvent(WeatherEvent event) {
        if (event instanceof SnowEvent){
            System.out.println("执行事件的方法：" + event.getWeather());
        }
    }
}
// 下雨事件监听器
public class RainListener implements WeatherListener{
    @Override
    public void onWeatherEvent(WeatherEvent event) {
        if (event instanceof  RainEvent){
            System.out.println("执行事件的方法" + event.getWeather());
        }
    }
}

事件源

// 事件源接口
public interface EventMulticaster {
    void multicastEvent(WeatherEvent event);

    void addListener(WeatherListener listener);

    void removeListener(WeatherListener listener);
}

//事件源接口实现
public class WeatherEventMulticaster implements EventMulticaster{
    @Autowired
    private List<WeatherListener> listenerList;

    @Override
    public void multicastEvent(WeatherEvent event) {
        System.out.println("事件开始");
        listenerList.forEach(i -> i.onWeatherEvent(event));
        System.out.println("事件结束");
    }

    @Override
    public void addListener(WeatherListener listener) {
        listenerList.add(listener);
    }

    @Override
    public void removeListener(WeatherListener listener) {
        listenerList.remove(listener);
    }
}

WeatherRunListener 类比于 SpringApplicationRunListener 之后介绍

public class WeatherRunListener {
    @Autowired
    private WeatherEventMulticaster weatherEventMulticaster;

    public void snow(){
        weatherEventMulticaster.multicastEvent(new SnowEvent());
    }

    public void rain(){
        weatherEventMulticaster.multicastEvent(new RainEvent());
    }
}

测试类

    @Autowired
    WeatherRunListener weatherRunListener;
    @Test
    public void weatherEventMultica(){
        weatherRunListener.snow();
        weatherRunListener.rain();
    }

运行结果

事件开始
调用事件：下雪
事件结束
事件开始
调用事件：下雨
事件结束

SpringBoot监听器

事件

常用事件类

事件event关系

事件加载顺序 项目启动事件加载顺序

### 事件监听器
#### 类名及继承关系
![事件监听器Listener类](https://img.haomeiwen.com/i14214388/4f5487c64789d03b.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

#### 事件源
```java
// 传入事件对象，触发不同的  Listener
public class SimpleApplicationEventMulticaster extends AbstractApplicationEventMulticaster {

    public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
        Executor executor = getTaskExecutor();
        for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
            if (executor != null) {
                executor.execute(() -> invokeListener(listener, event));
            }
            else {
                invokeListener(listener, event);
            }
        }
    }
}

原理

加载机制

SpringFactoriesLoader

SpringFactoriesLoader具体实现过程可参考前一篇系统初始化器，逻辑一致，只是 SpringFactoriesLoader.loadFactoryNames(type, classLoader)中的 type由 ApplicationContextInitializer.class变为ApplicationListener.class

自定义实现

有四种方式来实现自定义的监听器

方式一

1、// 实现 ApplicationListener 触发事件是  ApplicationStartedEvent
public class FirstListener implements ApplicationListener<ApplicationStartedEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        System.out.println("hello first listener");
    }
}
2、// spring.factories文件中指定
org.springframework.context.ApplicationListener=com.lanmi17.sb.code.listener.FirstListener

方式二

1、public class SecondListener implements ApplicationListener<ApplicationStartedEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        System.out.println("hello second listener");
    }
}
2、// SpringBoot主类中指定
    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(SbCodeApplication.class);
        application.addListeners(new SecondListener());
        application.run(args);
    }

方式三

1、public class ThirdListener implements ApplicationListener<ApplicationStartedEvent> {
    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        System.out.println("hello third listener");
    }
}
2、application.yml 中指定
context:
  listener:
    classes: com.lanmi17.sb.code.listener.ThirdListener,com.lanmi17.sb.code.listener.FourthListener

方式四

// 实现 SmartApplicationListener 类，可以自定义对哪些 event 感兴趣，触发事件
public class FourthListener implements SmartApplicationListener {
    //对哪类事件感兴趣
    @Override
    public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
        return ApplicationStartedEvent.class.isAssignableFrom(eventType)
                || ApplicationPreparedEvent.class.isAssignableFrom(eventType);
    }

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println("hello fourth lintener");
    }
}