MBassador is a very light-weight message (event) bus implementation following the publish subscribe pattern. It is designed for ease of use and aims to be feature rich and extensible while preserving resource efficiency and performance. The core of MBassador's high performance is a specialized data structure that minimizes lock contention such that performance degradation of concurrent access is minimal. The performance win of this design is illustrated in <ahref="http://codeblock.engio.net/?p=37"target="_blank">performance comparison</a> and more recently in the [eventbus-performance](https://github.com/bennidi/eventbus-performance) github repository.
Using MBassador in your project is very easy. Create as many instances of MBassador as you like (usually a singleton will do) ` bus = new MBassador(BusConfiguration.SyncAsync())`, mark and configure your message handlers with `@Handler` annotations and finally register the listeners at any MBassador instance `bus.subscribe(aListener)`. Start sending messages to your listeners using one of MBassador's publication methods `bus.post(message).now()` or `bus.post(message).asynchronously()`. Done!
There is a [spring-extension](https://github.com/bennidi/mbassador-spring) available to support CDI-like transactional message sending in a Spring environment. It's beta but stable enough to give it a try.
|`@Handler`|Defines and customizes a message handler. Any well-formed method annotated with `@Handler` will cause instances of the defining class to be treated as message listeners|
|`@Listener`|Can be used to customize listener wide configuration like the used reference type|
|`@Enveloped`|A message envelope can be used to pass messages of different types into a single handler|
|`@Filter`|Add filtering to prevent certain messages from being published|
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.
A handler can be invoked to handle a message either synchronously or asynchronously. This is configurable for each handler via annotations. Message publication itself supports synchronous (method blocks until messages are delivered to all handlers) or asynchronous (fire and forget) dispatch
By default, MBassador uses weak references for listeners to relieve the programmer of the need to explicitly unsubscribe listeners that are not used anymore and avoid memory-leaks. This is 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. Instead of using weak references, a listener can be configured to be referenced using strong references using `@Listener(references=References.Strong)`. Strongly referenced listeners will stick around until explicitly unsubscribed.
> Filtering
MBassador offers static message filtering. Filters are configured using annotations and multiple filters can be attached to a single message handler. Since version 1.2.0 Java EL expressions in `@Handler` are another way to define conditional message dispatch. Messages that have matching handlers but do not pass the configured filters result in the publication of a FilteredMessage object which wraps the original message. FilteredMessage events can be handled by registering listeners that handle FilteredMessage.
> Enveloped messages
Message handlers can declare to receive an enveloped message using `Enveloped`. The envelope can wrap different types of messages to allow a single handler to handle multiple, unrelated message types.
> Handler priorities
A handler can be associated with a priority to influence the order in which messages are delivered when multiple matching handlers exist
> Custom error handling
Errors during message delivery are sent to all registered error handlers which can be added to the bus as necessary.
> Extensibility
MBassador is designed to be extensible with custom implementations of various components like message dispatchers and handler invocations (using the decorator pattern), metadata reader (you can add your own annotations) and factories for different kinds of objects. A configuration object is used to customize the different configurable parts, see [Features](https://github.com/bennidi/mbassador/wiki/Components#Feature)
You can also download the latest binary release from the official [maven repository](http://mvnrepository.com/artifact/net.engio/mbassador). Of course you can always clone the repository and build from source.
There is ongoing effort to extend documentation and provide code samples and detailed explanations of how the message bus works. Code samples can also be found in the various test cases. Please read about the terminology used in this project to avoid confusion and misunderstanding.
++ Complete redesign of configuration setup using Features instead of simple get/set parameters. This will allow
to flexibly combine features and still be able to exclude those not available in certain environments,for example, threading and reflection in GWT (this will be part of future releases)
++ Properties formerly located in BusConfiguration now moved to their respective Feature class
++ Removed all SyncXX related interfaces and config implementations. There is now only one `BusConfiguration`
with its corresponding interface which will be used for all types of message bus implementations
Many thanks also to ej-technologies for providing me with an open source license of [![JProfiler](http://www.ej-technologies.com/images/banners/jprofiler_small.png)](http://www.ej-technologies.com/products/jprofiler/overview.html)