Introduction slides about Google's Guava Event Bus framework

1. Guava’s Event Bus
1

2. Traditional Events
Listener Listener
Activity
Service / Helper
Thread
Activity

3. Communication Issues?!
• Tight coupling of components
 Inflexible, changes are expensive
• Boiler plate code
– Define interfaces
– Callbacks for asynch. communication
– Listener management
– Propagation through all layers

4. EventBus Communication
Fragment Fragment
Activity
Service / Helper
Thread
Activity
Event
Bus

5. Guava’s EventBus
• A message dispatching system
– to allow publish-subscribe style of
communication between components
– No-nonsense
– Lightweight
– and very practical
• Everything happens within the run-time
boundaries of the same Java application.
https://code.google.com/p/guava-
libraries/wiki/EventBusExplained

6. Guava’s EventBus
• The Event Bus comes in two flavours:
– Synchronous (backed-up by the EventBus class), and
– Asynchronous (backed-up by the AsyncEventBus class which
extends EventBus)
• com.google.common.eventbus package
• Exposed API
– void register(Object) - registers and caches
subscribers for subsequent event handling
– void unregister(Object) - undoes the register action
– void post(Object) - posts an event (event) to all
registered subscribers

7. Building an Event Bus
• The Guava’s Event (Message) Bus itself
• The event (message), which can be any Java
object: a Date, a String, your POJO etc…
• The event subscriber (listener) – any
complexity Java class that must have a specially
annotated method for handling events (messages);

8. EventBus Configuration (Spring way)
• EventBus instances will create when
application is deploying
<bean id="eventBus" class="com.google.common.even
tbus.EventBus" />
<bean id="asyncEventBus"
class="com.google.common.eventbus.AsyncEventBus">
<constructor-arg name="executor" ref="executorService" />
</bean>

9. EventBus : Listener
• Define an event handler
@Component
Public class EventHandler
{
}
Event
Handler
E
v
e
n
t
B
u
s

10. EventBus: Listener Registration
• Register handler
@PostConstruct
public void registerHandler()
{
asyncEventBus.register(this);
}
Event
Handler
E
v
e
n
t
B
u
s
register

11. EventBus: Listener Subscription
• Subscribe for events a.k.a objects
@Subscribe
@AllowConcurrentEvents
public void handleEventMethod(ObjectA objA)
{
}
Event
Handler
E
v
e
n
t
B
u
s
subscribe
register

12. EventBus: Event Post
• Post an event
// Post information back to event bus
asyncEventBus.post(objA);
Event
Handler
E
v
e
n
t
B
u
s
subscribe
registerEvent
post

13. Guava Event Bus
E
v
e
n
t
B
u
s
Event
Event
Event
Event
Handler
Event
Handler
Event
Handler
post
post
post
subscribe
subscribe
subscribe
register
register
register

14. Event Handler Method
• EventBus scans subscribers
– During (first) registration of subscriber and
registers the event listener methods based on
the method’s parameter type
• Event handler methods
– public visibility
– No return value (void)
– Single parameter for the event to receive
14

15. Type-based Event Routing
• Event type: Java class of the event
• To receive an event, its type must match
• E.g.
post(new User());
handleUserXXX(User user) {…}
Post(new Address());
handleAddressXXX(Address address) {…}

16. Publish / Subscribe
Publisher
Event
Bus

17. Publish / Subscribe
Publisher
Event
Bus
Event
post(Object1)

18. Publish / Subscribe
Publisher
Event
Bus
Subscriber
Event
post(Object1)
Event
handleMethod1(Object1)
Subscriber
Event
handleMethod2(Object1)

19. Event Type is a Filter
Publisher
Event
Bus
Subscriber
Event
post
(user)
Event
handleUserXXX(User)
Subscriber
handleAddressXXX(Address)

20. It‘s time to see some
• CODE

21. EventBus Code: Sender (Spring)
@Autowired
private EventBus eventBus;
…
eventBus.post(user);
…
OR
@Autowired
private AsyncEventBus asyncEventBus;
…
asyncEventBus.post(user);
…

22. EventBus Code: Receiver (Spring)
@Component
public class UserListener
{
@Autowired
private AsyncEventBus asyncEventBus;
@PostConstruct
public void registerHandler()
{
asyncEventBus.register(this);
}
@PreDestroy
public void unRegisterHandler()
{
asyncEventBus.unregister(this);
}
@Subscribe
@AllowConcurrentEvents
public void handleUserXXX(User user)
{
// your logic
}
}