Implementing a Publish-Subscribe (Pub/Sub) pattern in Java with Spring Boot can be done in multiple ways, depending on your needs:

The Pub/Sub (Publisher/Subscriber) model is a messaging pattern used in software architecture to facilitate asynchronous communication between different components or systems. In this model, publishers produce messages that are then consumed by subscribers.

Common Approaches in Spring Boot:

1. In-memory Pub/Sub using Spring Events
2. Distributed Pub/Sub using Apache Kafka (recommended for production)
3. Other options: RabbitMQ, Redis Pub/Sub, Google Cloud Pub/Sub, etc.

Spring Boot + Kafka (Production-grade)

1. Create a Publisher Service

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.stereotype.Service;

@Service
public class EventPublisher {

    private final KafkaTemplate<String, String> kafkaTemplate;

    @Autowired
    public EventPublisher(KafkaTemplate<String, String> kafkaTemplate) {
        this.kafkaTemplate = kafkaTemplate;
    }

    public void publish(String topic, String message) {
        kafkaTemplate.send(topic, message);
    }
}

2. Create a subscriber/consumer service

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import org.springframework.kafka.annotation.KafkaListener;
import org.springframework.stereotype.Service;

@Service
public class EventSubscriber {

    @KafkaListener(topics = "demo-topic", groupId = "test-group")
    public void handleMessage(String message) {
        System.out.println("Received Message: " + message);
    }
}

Redis Pub/Sub Spring Boot

1. Create Publisher

@Service
public class RedisPublisher {

    @Autowired
    private RedisTemplate<String, Object> redisTemplate;

    @Autowired
    private ChannelTopic topic;

    public void publish(String message) {
        redisTemplate.convertAndSend(topic.getTopic(), message);
        System.out.println("Published: " + message);
    }
}

2. Create Subscriber

@Service
public class RedisSubscriber implements MessageListener {

    @Override
    public void onMessage(Message message, byte[] pattern) {
        String msg = new String(message.getBody());
        System.out.println("Received Message: " + msg);
    }
}

In-Memory Spring Events (for internal async)

1.Add Listener

@Component
public class NotificationListener {

    @EventListener
    public void onEvent(String message) {
        System.out.println("Handled in-memory event: " + message);
    }
}

2. Publish Event

@Autowired
private ApplicationEventPublisher eventPublisher;

public void publishMessage(String msg) {
    eventPublisher.publishEvent(msg);
}