CODESAMPLE

Message Broker - Java

Message Broker Java

The Message Broker pattern decouples application components by enabling asynchronous communication through a central message store. Components don’t directly interact; instead, they publish messages to the broker, and other components subscribe to receive messages of interest. This promotes scalability, resilience, and flexibility.

This Java implementation uses a simple in-memory MessageBroker class to store and deliver messages. Publisher components publish messages with a topic, and Subscriber components register for specific topics. When a message is published, the broker iterates through its subscribers and delivers the message to those interested in the topic. Using interfaces (Publisher, Subscriber) allows for loose coupling and easy extension. The use of java.util.HashMap and java.util.ArrayList are standard Java collections for this purpose, making the code idiomatic.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

interface Publisher {
    void publish(String topic, String message);
}

interface Subscriber {
    void receive(String topic, String message);
}

class MessageBroker {
    private final Map<String, List<Subscriber>> subscribers = new HashMap<>();

    public void subscribe(String topic, Subscriber subscriber) {
        subscribers.computeIfAbsent(topic, k -> new ArrayList<>()).add(subscriber);
    }

    public void unsubscribe(String topic, Subscriber subscriber) {
        subscribers.getOrDefault(topic, new ArrayList<>()).remove(subscriber);
    }

    public void publish(String topic, String message) {
        subscribers.getOrDefault(topic, new ArrayList<>())
                .forEach(subscriber -> subscriber.receive(topic, message));
    }
}

class ConcreteSubscriber implements Subscriber {
    private final String name;

    public ConcreteSubscriber(String name) {
        this.name = name;
    }

    @Override
    public void receive(String topic, String message) {
        System.out.println(name + " received on topic " + topic + ": " + message);
    }
}

public class MessageBrokerExample {
    public static void main(String[] args) {
        MessageBroker broker = new MessageBroker();

        ConcreteSubscriber sub1 = new ConcreteSubscriber("Subscriber 1");
        ConcreteSubscriber sub2 = new ConcreteSubscriber("Subscriber 2");

        broker.subscribe("news", sub1);
        broker.subscribe("news", sub2);
        broker.subscribe("sports", sub1);

        Publisher publisher = broker::publish;
        publisher.publish("news", "Breaking news: Java is awesome!");
        publisher.publish("sports", "Local team wins championship!");
    }
}