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POWhy people use message/event buses in their code?
 

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    <p>I think that you have heard of message/event buses, it's the single place when all events in the system flow. Similar architectures are found in computer's motherboards and LAN networks. It's a good approach for motherboards and networks as it reduces the number of wires, but is it good for software development? We don't have such restrictions as electronics does.</p> <p>The simplest implementation of message bus/event bus can be like:</p> <pre><code>class EventBus { void addListener(EventBusListener l}{...} void fireEvent(Event e) {...} } </code></pre> <p>Posting events is done with bus.fireEvent(event), receiving messages is enabled by bus.addListener(listener). Such architectures are sometimes used for software development, for example MVP4G implements similar message bus for GWT.</p> <p>Active projects:</p> <ul> <li>Google <a href="https://code.google.com/p/guava-libraries/wiki/EventBusExplained" rel="noreferrer">Guava EventBus</a></li> <li><a href="https://github.com/bennidi/mbassador" rel="noreferrer">MBassador</a> by <a href="https://github.com/bennidi" rel="noreferrer">Benjamin Diedrichsen</a> </li> <li><a href="http://code.mycila.com/" rel="noreferrer">Mycila PubSub</a> by <a href="http://about.me/mathieucarbou" rel="noreferrer">Mathieu Carbou</a></li> <li><a href="http://code.google.com/p/mvp4g/wiki/EventBus" rel="noreferrer">mvp4g Event Bus</a></li> <li><a href="http://code.google.com/p/simpleeventbus/" rel="noreferrer">Simple Java Event Bus</a></li> </ul> <p>Dormant/Dead projects:</p> <ul> <li>Sun/Oracle <a href="http://www.oracle.com/technetwork/java/javase/faq-139166.html" rel="noreferrer">JavaBeans InfoBus</a></li> <li><a href="https://eventbus.dev.java.net/" rel="noreferrer">https://eventbus.dev.java.net/</a> [Broken link]</li> </ul> <p>It's just the popular Observer (Listener) pattern made 'globally' - each object in the system can listen to each message, and I think it's bad, it breaks the Encapsulation principle (each object knows about everything) and Single Responsibility principle (eg when some object needs to a new type of message, event bus often needs to be changed for example to add a new Listener class or a new method in the Listener class).</p> <p>For these reasons I think, that for most software, Observer pattern is better than event bus. What do you think about event bus, does it make any good sense for typical applications?</p> <p>EDIT: I'm not talking about 'big' enterprise solutions like ESB - they can be useful (what's more ESB offers much, much more than just an event bus). I'm asking about usefulness of using message bus in 'regular' Java code for object-to-object connection - some people do it, check the links above. Event bus is probably best solution for telephone-to-telephone communication or computer-to-computer communication because each telefone (or computer) in a network can typically talk to each other, and bus reduces the number of wires. But objects rarely talk to each other - how many collaborators one object can have - 3, 5?</p>
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