An Open Source Event Bus for Java 1.6+
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README.md

JBus

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An event bus for java 1.6+. It dispatches event to registered listeners.

It is a simple but powerful publish-subscribe event system. It requires object to register themselves with the event bus to receive events. This event bus is safe for concurrent use.

It has the following features:

  • Light-weight but powerful
  • Annotation based API
  • Supports synchronous/asynchronous invocation of subscriber method
  • Handler chain interruption
  • Supports custom error handling
  • Supports strong/weak references of subscriber
  • Supports subscriber inheritance
  • Zero configuration
  • Optional JVM shutdown hook for graceful shutdown

Example

Define events:
    public class UserEvent { 
        /* Additional fields if required */ 
    }
    
Create subscriber:
    public class Listener {
        
        @Subscribe
        private void listen(UserEvent event) {
            /* Your event handling code goes here */
        }
    }
Register subscriber:
    
     JBus jbus = new JBus();
     jbus.register(new Listener());   
     // or to register with weak reference use jbus.registerWeak(new Listener());
Post event:
    jbus.post(new UserEvent());

It is that simple.

Installation

JBus is available from the Maven Central Repository using the following coordinates:

    <dependency>
        <groupId>org.dizitart</groupId>
        <artifactId>jbus</artifactId>
        <version>{latest.tag.release}</version>
    </dependency>

Details

To receive events from event bus, an object must:

  • register itself with the event bus via jbus.register(Object) or jbus.registerWeak(Object) method
  • have at least one method marked with @Subscribe annotation in its class hierarchy
  • subscribed method should accept only one parameter having the type of the event

Registration will scan the input object for any method which has been marked with @Subscribe annotation and it will keep track of all such methods found. If the subscribing method has more than one parameter, runtime will throw a JBusException during registration. A subscriber method can have any access modifier. Registration will scan through full class hierarchy of the input object including any super class and interfaces.

Upon successful registration, the runtime will keep track of all subscriber methods found along with a strong reference of the input object for future invocation. To store a weak reference of the input object instead of a strong one, use the jbus.registerWeak(Object) variant.

A developer must use jbus.deregister(Object) for the object to stop receiving events. The behavior of de-registration is not deterministic in the case of weak registration of the object. As the runtime automatically cleans up any invalid weak references and any subscriber methods associated with it as it goes, so a call to jbus.deregister(Object) might not do anything if the object has already been garbage collected and the event bus runtime has cleared up its records of subscriber methods already.

To post an event to the event bus, simply call jbus.post(Object) passing the event object. Event bus will automatically route the event depending on its type to a handler chain. Handler chain is a collection of registered subscribers of the event. By design, event bus does not support inheritance for the event object.

If an event implements HandlerChainAware interface then before each invocation, the runtime will check if an interruption has been signalled from the subscriber code via HandlerChain.interrupt() call. If interrupted, further invocation of the handler chain will be barred until the next jbus.post(Object) call for the event.

Subscriber execution mode can be either synchronous or asynchronous depending on the @Subscribe annotation declaration.

In case of any error from subscriber code during invocation, the runtime will first search for any ExceptionEvent handler registered into the system and dispatch the error along with relevant information in ExceptionContext to the handler if found. If no such error handler is found, runtime will just log the error and move on.

Contribute

Please feel free to contribute by creating a pull request. Please make sure your pull request is small and provided with test code.

License

This project is distributed under the terms of the Apache License, Version 2.0. See file "LICENSE" for further reference.