[Java知识库]设计模式-spring自带观察者模式

参考资料
1.https://www.cnblogs.com/cq-yangzhou/p/14648810.html
2.如果你还不知道如何控制springboot中bean的加载顺序，那你一定要看此篇

一. 使用场景

观察者模式主要作用在于实现业务的解耦。以用户注册的场景来举例子:
假设在用户注册完成时，需要给该用户发送邮件、发送优惠劵等等操作.

二.示例

2.1 创建自定义用户注册事件

import org.springframework.context.ApplicationEvent;

// 继承ApplicationEvent类,创建自定义用户注册事件
public class UserRegisterEvent extends ApplicationEvent {
	
	// 注册成功的用户id
    private String userId;

    public UserRegisterEvent(Object source) {
        super(source);
    }

    public UserRegisterEvent(Object source, String userId) {
        super(source);
        this.userId = userId;
    }
	
	// 获取用户id
    public String getUserId() {
        return userId;
    }
}

2.2 监听用户自定义事件

?当事件发布者会触发多个事件的时候,可通过@Order注解指定事件触发的顺序
?order越小,执行优先度越高
?不实现ApplicationListener接口,给方法添加@EventListener注解,也可以触发事件
?通过@Order注解指定执行顺序的类,必须要实现ApplicationListener接口,ApplicationListener接口和@EventListener注解搭配的使用的话,@Order注解会失效

import org.springframework.context.ApplicationListener;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;

@Component
@Order(1)
public class EmailService implements ApplicationListener<UserRegisterEvent>  {

    // 监听UserRegisterEvent用户注册事件,当用户注册后,给用户发送邮件
    @Override
    public void onApplicationEvent(UserRegisterEvent event) {

        String userId = event.getUserId();
		
		// 模拟耗发送邮件耗时操作
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("给ID为" + userId + "的用户发送邮件");
    }
}

import org.springframework.context.ApplicationListener;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;

@Component
@Order(2)
public class CouponService implements ApplicationListener<UserRegisterEvent> {

    @Override
    public void onApplicationEvent(UserRegisterEvent userRegisterEvent) {

        String userId = userRegisterEvent.getUserId();
        System.out.println("给ID为" + userId + "的用户发送优惠券");
    }
}

2.3 创建事件发布对象

import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.ApplicationEventPublisherAware;
import org.springframework.stereotype.Component;

@Component
public class UserService implements ApplicationEventPublisherAware {
	
	// 事件发布对象
    private ApplicationEventPublisher applicationEventPublisher;

    @Override
    public void setApplicationEventPublisher(ApplicationEventPublisher applicationEventPublisher) {
        this.applicationEventPublisher = applicationEventPublisher;
    }

    public void register(String userId) {

        // 当用户注册成功之后,发布自定义事件UserRegisterEvent,
        applicationEventPublisher.publishEvent(new UserRegisterEvent(this, userId));
    }
}

2.4 调用

@Component
public class ParamExecute implements CommandLineRunner {

    @Autowired
    private UserService service;

    @Override
    public void run(String... args) throws Exception {
        
        // 模拟用户注册成功
        String userId = "F202001";
        service.register(userId);
    }
}