ApplicationListener是Spring事件机制的一部分，与抽象类ApplicationEvent类配合来完成ApplicationContext的事件机制。
如果容器中存在ApplicationListener的Bean，当ApplicationContext调用publishEvent方法时，对应的Bean会被触发。这一过程是典型的观察者模式的实现。

// 使用applicationListener监听ContextRefreshedEvent事件
// 需对该监听类进行Bean的实例化
@Component
public class ContentListener implements ApplicationListener<ContextRefreshedEvent> {
    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        // 打印容器中出事Bean的数量
        System.out.println("监听器获得容器中初始化Bean数量：" + event.getApplicationContext().getBeanDefinitionCount());
    }
}
控制台输出：
  监听器获得容器中初始化Bean数量：127

如果在实现ApplicationListener接口时不写入event 则会坚挺到所有事件

@Component
public class AllListener implements ApplicationListener {
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println("监听到事件，"+event);
    }
}
控制台输出：
  监听到事件，org.springframework.boot.web.servlet.context.ServletWebServerInitializedEvent[source=org.springframework.boot.web.embedded.tomcat.TomcatWebServer@71a06021]
  监听到事件，org.springframework.context.event.ContextRefreshedEvent[source=org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@3b96c42e, started on Sat Dec 11 17:48:29 CST 2021]

如果要自定义event 需要实现抽象类ApplicationEvent

// 自定义事件
public class UserLoginEvent extends ApplicationEvent {
    private String id;
    private String userName;
    private String phone;
    /**
     * Create a new {@code ApplicationEvent}.
     *
     * @param source the object on which the event initially occurred or with
     *               which the event is associated (never {@code null})
     */
    public UserLoginEvent(Object source) {
        super(source);
    }

    public UserLoginEvent(Object source, String id, String userName, String phone) {
        super(source);
        this.id = id;
        this.userName = userName;
        this.phone = phone;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    @Override
    public String toString() {
        return "UserLoginEvent{" +
                "id='" + id + '\'' +
                ", userName='" + userName + '\'' +
                ", phone='" + phone + '\'' +
                '}';
    }
}

// 监听
@Component
public class UserLoginListener implements ApplicationListener<UserLoginEvent> {
    @Override
    public void onApplicationEvent(UserLoginEvent event) {
        System.out.println("用户登录事件触发");
        System.out.println(event);
        System.out.println("执行通知");
    }
}

// 触发事件
@RestController
@RequestMapping("/user")
public class UserController {
    @Autowired
    private ApplicationContext applicationContext;

    @RequestMapping("/login")
    public String login(){
        String username = "kim";
        String id = "000010";
        String phone = "188****8778";
        UserLoginEvent userLoginEvent = new UserLoginEvent("loginEvent",username,id,phone);
        applicationContext.publishEvent(userLoginEvent);
        return "登录成功";
    }
}

调用http://localhost:8080/user/login 控制台输出
用户登录事件触发
UserLoginEvent{id='kim', userName='000010', phone='188****8778'}
执行通知

以上方式是通过实现ApplicationListener接口实现监听，同时spring还提供了@EventListener注解用于对事件进行监听 如下代码效果同实现ApplicationListener相同

@Component
public class ListenerConfiguration {
    @EventListener
    public void UserLoginListener(UserLoginEvent event){
        System.out.println("用户登录事件触发,基于注解");
        System.out.println(event);
        System.out.println("执行通知,基于注解");
    }
}