MessageBus

The MessageBus is an extremely light-weight message/event bus implementation that follows the publish/subscribe pattern and is based on the MBassador project. It is designed for ease of use and simplicity, and aims for maximum performance and zero garbage during message publication. At the core of this project is the use of the single writer principle as described by Nitsan Wakart on his blog and the fantastic LMAX Disruptor.

Using the MessageBus in your project is very easy.
1 Create an instance of the MessageBus (usually a singleton will do) MessageBus bus = new MessageBus()
2 Mark and configure your message subscription handlers (the methods that will receive the messages) with @Subscribe notations
3 Register these via bus.subscribe(listener)
4 Send messages to these listeners via bus.publish(message) for synchronous publication, or bus.publishAsync(message) for asynchronous publication
5 (Optional) Free resources and threads via bus.shutdown() when you are finished (usually on application exit)

You're done!

Notes

The difference between the sync and async is that with synchronous publication, all of the logic and method calls occur on the same thread that calls it; while with an asynchronous publication, all of these actions occur on a separate thread. Please note that asynchronous publication is not in a guaranteed order.

bus.shutdown(). It is not necessary if exiting the JVM (which is most use-cases), but it is extremely useful in situations where you are reloading classes (ie: a webserver), as it will guarantee freeing all used resources and threads.

Table of contents:

• Features
• Usage
• Installation
• License

Features

Annotations

Annotation	Function
@Subscribe	Defines and customizes a message subscription handler. Any well-formed method annotated with @Subscribe will cause instances of the defining class to be treated as event receivers
@Listener	Can be used to customize listener wide configuration like the used reference type
@Synchronized	Specifies that the handler/method will be accessed in a synchronized block

Delivers everything

Messages do not need to implement any interface and can be of any type. It is possible though to define an upper bound of the message type using generics. The class hierarchy of a message is considered during message delivery, such that handlers will also receive subtypes of the message type they consume for - e.g. a handler of Object.class receives everything. Messages that do not match any handler result in the publication of a DeadMessage object which wraps the original message. DeadMessage events can be handled by registering listeners that handle DeadMessage.

Configurable reference types

By default, the MessageBus uses strong references for listeners. If the programmer wants to relieve the need to explicitly unsubscribe listeners that are not used anymore and avoid memory-leaks, it is trivial to configure via setting the SubscriptionMode. Using strong references is the fastest, most robust method for dispatching messages, however weak references are very comfortable in container managed environments where listeners are created and destroyed by frameworks, i.e. Spring, Guice etc. Just stuff everything into the message bus, it will ignore objects without message handlers and automatically clean-up orphaned weak references after the garbage collector has done its job. Strongly referenced listeners will stick around until explicitly unsubscribed.

Custom error handling

Errors during message delivery are sent to all registered error handlers which can be added to the bus as necessary.

Usage

Handler definition (in any bean):

    // every message of type TestMessage or any subtype will be delivered to this subscription handler
    @Subscribe
	public void handleTestMessage(TestMessage message) {
		// do something
	}

	// every message of type TestMessage or any subtype will be delivered to this subscription handler
    @Subscribe
    public void handleTestMessage(TestMessage message) {
        // do something
    }

    // this subscription handler will not accept subtypes of the TestMessage.
    @Subscribe(acceptSubtypes = false})
    public void handleNoSubTypes(TestMessage message) {
       //do something
    }

    // this handler will be accessed in a "syncrhonized" manner (only one thread at a time may access it)
    @Subscribe
    @Synchronized
    public void handleSynchronzied(TeastMessage message) {
        //do something
    }

    // configure a listener to be stored using strong/weak references
    @Listener(references = References.Strong)
    public class MessageListener{
        @Subscribe
        public void handleTestMessage(TestMessage message) {
            // do something
        }
    }

Creation of message bus and registration of listeners:

    // create as many instances as necessary (usually a singleton is best)
    MessageBus bus = new MessageBus();
    
    ListeningBean listener = new ListeningBean();
    
    // the listener will be registered using a weak-reference if not configured otherwise via @Listener
    bus.subscribe(listener);
    
    // this listener without handlers will be ignored
    bus.subscribe(new ClassWithoutAnyDefinedHandlers());
    
    // do stuff....
    
    
    // and when FINSIHED with the messagebus, to shutdown all of the in-use threads and clean the data-structures
    bus.shutdown();

Message publication:

    TestMessage message = new TestMessage();
    TestMessage message2 = new TestMessage();
    TestMessage message3 = new TestMessage();
    TestMessage subMessage = new SubTestMessage();

    bus.publishAsync(message); // returns immediately, publication will continue asynchronously
    bus.publish(subMessage);   // will return after all the handlers have been invoked
    
    bus.publish(message, message2);   // will return after all the handlers have been invoked, but for two messages at the same time
    bus.publish(message, message2, message3);   // will return after all the handlers have been invoked, but for three messages 

   

Release Notes

Maven Info

<dependencies>
    ...
    <dependency>
        <groupId>com.dorkbox</groupId>
        <artifactId>MessageBus</artifactId>
        <version>2.7</version>
    </dependency>
</dependencies>

Gradle Info

dependencies {
    ...
    implementation("com.dorkbox:MessageBus:2.7")
}

License

This project is © 2012 Benjamin Diedrichsen and © 2021 dorkbox llc, and is distributed under the terms of the Apache v2.0 License. See file "LICENSE" for further references.