java 实现消息通知后端代码

Java实现消息通知后端代码可以基于消息队列和推送服务等方式来实现。下面是一个基于RabbitMQ消息队列的Java后端代码示例，实现了向指定用户发送站内信通知的功能：

1. 添加依赖

<dependency>
    <groupId>com.rabbitmq</groupId>
    <artifactId>amqp-client</artifactId>
    <version>5.7.2</version>
</dependency>

2. 配置RabbitMQ连接信息

@Configuration
public class RabbitMQConfig {
    @Bean
    public ConnectionFactory connectionFactory() {
        CachingConnectionFactory connectionFactory = new CachingConnectionFactory("localhost");
        connectionFactory.setUsername("guest");
        connectionFactory.setPassword("guest");
        connectionFactory.setVirtualHost("/");
        connectionFactory.setPublisherConfirms(true);
        return connectionFactory;
    }

    @Bean
    public RabbitTemplate rabbitTemplate() {
        return new RabbitTemplate(connectionFactory());
    }

    @Bean
    public DirectExchange directExchange() {
        return new DirectExchange("directExchange");
    }

    @Bean
    public Queue queue() {
        return new Queue("messageQueue");
    }

    @Bean
    public Binding binding() {
        return BindingBuilder.bind(queue()).to(directExchange()).with("messageRoutingKey");
    }
}

3. 发送消息

@RestController
@RequestMapping("/message")
public class MessageController {
    @Autowired
    private RabbitTemplate rabbitTemplate;

    @PostMapping("/send")
    public String sendMessage(@RequestBody MessageDto messageDto) {
        String userId = messageDto.getUserId();
        String content = messageDto.getContent();
        rabbitTemplate.convertAndSend("directExchange", "messageRoutingKey", new Message(userId, content));
        return "success";
    }
}

public class Message {
    private String userId;
    private String content;

    public Message(String userId, String content) {
        this.userId = userId;
        this.content = content;
    }

    // getters and setters
}

public class MessageDto {
    private String userId;
    private String content;

    // getters and setters
}

4. 接收消息并处理

@Component
public class MessageReceiver {
    @RabbitListener(queues = "messageQueue")
    public void handleMessage(Message message) {
        String userId = message.getUserId();
        String content = message.getContent();
        // 根据userId获取用户信息，向用户发送站内信通知
    }
}

以上仅是一个简单的示例，具体的实现方式需要根据具体的业务需求来确定。