ApplicationListener是Spring事件機(jī)制的一部分，與抽象類ApplicationEvent類配合來完成ApplicationContext的事件機(jī)制。
如果容器中存在ApplicationListener的Bean，當(dāng)ApplicationContext調(diào)用publishEvent方法時(shí)，對(duì)應(yīng)的Bean會(huì)被觸發(fā)。這一過程是典型的觀察者模式的實(shí)現(xiàn)。

// 使用applicationListener監(jiān)聽ContextRefreshedEvent事件
// 需對(duì)該監(jiān)聽類進(jìn)行Bean的實(shí)例化
@Component
public class ContentListener implements ApplicationListener<ContextRefreshedEvent> {
    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        // 打印容器中出事Bean的數(shù)量
        System.out.println("監(jiān)聽器獲得容器中初始化Bean數(shù)量：" + event.getApplicationContext().getBeanDefinitionCount());
    }
}
控制臺(tái)輸出：
  監(jiān)聽器獲得容器中初始化Bean數(shù)量：127

如果在實(shí)現(xiàn)ApplicationListener接口時(shí)不寫入event 則會(huì)堅(jiān)挺到所有事件

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

如果要自定義event 需要實(shí)現(xiàn)抽象類ApplicationEvent

// 自定義事件
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 + '\'' +
                '}';
    }
}

// 監(jiān)聽
@Component
public class UserLoginListener implements ApplicationListener<UserLoginEvent> {
    @Override
    public void onApplicationEvent(UserLoginEvent event) {
        System.out.println("用戶登錄事件觸發(fā)");
        System.out.println(event);
        System.out.println("執(zhí)行通知");
    }
}

// 觸發(fā)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 "登錄成功";
    }
}

調(diào)用http://localhost:8080/user/login 控制臺(tái)輸出
用戶登錄事件觸發(fā)
UserLoginEvent{id='kim', userName='000010', phone='188****8778'}
執(zhí)行通知

以上方式是通過實(shí)現(xiàn)ApplicationListener接口實(shí)現(xiàn)監(jiān)聽，同時(shí)spring還提供了@EventListener注解用于對(duì)事件進(jìn)行監(jiān)聽 如下代碼效果同實(shí)現(xiàn)ApplicationListener相同

@Component
public class ListenerConfiguration {
    @EventListener
    public void UserLoginListener(UserLoginEvent event){
        System.out.println("用戶登錄事件觸發(fā),基于注解");
        System.out.println(event);
        System.out.println("執(zhí)行通知,基于注解");
    }
}