82 lines
4.2 KiB
Markdown
82 lines
4.2 KiB
Markdown
Mbassador
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=========
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Mbassador is a very light-weight message bus implementation following the publish subscribe pattern. It is designed
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for ease of use and aims to be resource efficient and very fast. It was inspired by google guava's event bus which lacked some
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features like weak references at that time. At its core it offers the following:
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+ Delivers everything: Messages must not implement any interface and can be of any type (-> message bus is typed using generics with upper
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bound being Object.class)
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+ Follows type hierarchy: Class hierarchy of messages are considered during message delivery. This means that listeners will also receive
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subtypes of the message type they are listening for, e.g. a listener for Object.class receives everything.
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+ Annotation driven: To define and customize a message handler simply mark it with @Listener annotation
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+ Synchronous and asynchronous message delivery: A handler can be invoked to handle a message either synchronously or
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asynchronously. This is configurable for each handler via annotations. Message publication itself supports synchronous (method
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blocks until messages are delivered to all handlers) or asynchronous (fire and forget) dispatch
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+ Weak references: Mbassador uses weak references to all listening objects to relieve the programmer of the burden to explicitly unregister
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listeners that are not used anymore (of course it is also possible to explicitly unregister a listener if needed). This is very comfortable
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in certain environments where objects are created by frameworks, i.e. spring, guice etc. Just stuff everything into the message bus, it will
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ignore objects without message handlers and automatically clean-up orphaned weak references after the garbage collector has done its job.
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+ Filtering: Mbassador offers static message filtering. Filters are configured using annotations and multiple filters can be attached to
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a single message handler
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+ Error handling: Errors during message delivery are sent to an error handler of which a custom implementation can easily be plugged-in.
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+ Ease of Use: Using Mbassador in your project is very easy. Create as many instances of Mbassador as you like (usually a singleton will do),
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mark and configure your message handlers with @Listener annotations and finally register the listeners at any Mbassador instance. Start
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sending messages to your listeners using one of Mbassador's publication methods (sync or async). Done!
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---------------------------------------
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<h2>Usage</h2>
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Listener definition (in any bean):
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// every event of type TestEvent or any subtype will be delivered
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// to this handler
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@Listener
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public void handleTestEvent(TestEvent event) {
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// do something
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}
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// this handler will be invoked asynchronously
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@Listener(mode = Listener.Dispatch.Asynchronous)
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public void handleSubTestEvent(SubTestEvent event) {
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// do something more expensive here
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}
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// this handler will receive events of type SubTestEvent
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// or any subtabe and that passes the given filter(s)
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@Listener({@Filter(value = SpecialEventsOnly.class),@Filter(value = SpecialEventsOnly.class)})
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public void handleFiltered(SubTestEvent event) {
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//do something special here
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}
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Creation of message bus and registration of listeners:
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// create as many instances as necessary
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// bind it to any upper bound
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MBassador<TestEvent> bus = new MBassador<TestEvent();
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ListeningBean listener = new ListeningBean();
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// the listener will be registered using a weak-reference
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bus.subscribe(listener);
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// objects without handlers will be ignored
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bus.subscribe(new ClassWithoutAnyDefinedHandlers());
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Message puclication:
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TestEvent event = new TestEvent();
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TestEvent subEvent = new SubTestEvent();
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bus.publishAsync(event); //returns immediately, publication will continue asynchronously
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bus.publish(subEvent); // will return after each handler has been invoked
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---------------------------------------
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<h2>Planned features</h2>
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+ Maven dependency: Add Mbassador to your project using maven. Coming soon!
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+ Message handler priority: Message handlers can specify priority to influence order of message delivery
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