Event Admin Handlers

The goal of the Event Admin Handlers is to allow event communications between iPOJO component instances. The implementation of these handlers relies on an event admin services. It enables the iPOJO component to listen to a list of topics and to receive all related event. It also allows components to send events in an easy way.

UPDATE : the 1.1.0-SNAPSHOT version has a new namespace : org.apache.felix.ipojo.handlers.event instead of org.apache.felix.ipojo.handlers.event.EventAdminHandler

Hereafter is presented a small example of the metadata.xml file :

<ipojo
    xmlns:ev="org.apache.felix.ipojo.handlers.event.EventAdminHandler">
	<component className="...MyComponent">
		<ev:subscriber
			name="mySubscriber"
			callback="receive"
			topics="foo"/>
		<ev:publisher
			name="myPublisher"
			field="m_publisher"
			topics="bar,nuts"/>
	</component>
	<instance component="...MyComponent"/>
</ipojo>

You need to specify the namespace of the Handler. You can find here one event subscriber (named mySubscriber) and one event publisher (named myPublisher). In these handler configurations, the name parameter is mandatory. The topics parameter is optional as it can be specified in the instance configuration. The callback parameter of the mySubscriber element is mandatory and indicates the method that handles received events. In this case, this method must have a single argument of type org.osgi.service.event.Event. The field parameter of the myPublisher element indicates the field (of type org.apache.felix.ipojo.handlers.event.publisher.Publisher) that is used by the POJO to send events on the specified topics. All type compliance will be checked by the handler at component instantiation time.

Here is an example of the component implementation, compatible with the given description :

import org.apache.felix.ipojo.handlers.event.publisher.Publisher;
import org.osgi.service.event.Event;

public class MyComponent ... {
    private Publisher m_publisher;
    public void receive(Event e) {
       // Event received
       // Do something with the event}

    public void doSomething() {
        Dictionary e = new Properties();
        //...
        // Fill out the event

        // Send event
        m_publisher.send(e);
    }
}

Download

The event admin handlers (to send and receive events) are available in the Felix trunk in the iPOJO project. See the Download page to download and compile these sources.

How does it work?

The handler will parse the description provided in the metadata, and register for you the EventHandler in the OSGi Registry. On one hand, your POJO will receive each event through the handler. With this handler you can specify different callback methods for different topics. On the other side, the handler instantiates and injects configured Publisher references in your POJO, so you can send events transparently through these publishers.

EventHandler Specification

Here you can find all configuration options of the EventAdmin handler. As seen before, the handler contains two components : the event subscriber and the event publisher. These components can be configured, using several attributes, as described below. Some of these attributes can be (re)defined in the instance configuration.

Handler namespace : org.apache.felix.ipojo.handlers.event.EventAdminHandler

Event subscriber attributes

Attribute name	Required	Description
name	YES	The name of the event subscriber, acting as a unique identifier.
callback	YES	The name of the POJO's method that will be called each time an event is received. This method takes only one parameter, of typeorg.osgi.service.event.Eventby default, but this type can be overridden by defining the data-key and/or the data-type attributes.
topics	YES*	The comma-separated-list of the topics that the handler will listen to. Each event sent on a topic present in this list will be sent to the specified callback method.
data-key	NO	The data key is used when you want to receive data events. This attribute's value is the key corresponding to the received data in the event's dictionary. If you use this attribute, the parameter passed to the callback method is the the value associated to this key, not the whole event. This attribute is generally used with the data-type attribute to specify the received object type. If an event is received and it does not contain such a key, it is ignored (with a warning message).
data-type	NO	This attribute is associated to the data-key attribute. It specifies the type of objects (java.lang.Objectby default) that the callback expects. It is used to determine the unique callback method (in case of multiple methods with the same name) and to check type compliance at event reception. Data events that are not corresponding to the specified type will be ignored (with a warning message).
filter	NO*	The event filter is used to filter incoming events before sending them to the callback. The syntax of this field is described in the OSGi EventAdmin Specification. If you don't specify a filter, all events sent on the listened topics will be considered.

* These attributes can be (re)defined in the instance configuration.

Event publisher attributes

Attribute name	Required	Description
name	YES	The name of the event publisher, acting as a unique identifier.
field	YES	The name of the POJO's field that will be used to send events. The field is initialized at component instantiation time. The type of the field must be : org.apache.felix.ipojo.handlers.event.publisher.Publisher. Despite it creates a dependency between the component code and the handler, this system allows hiding the whole complexity of event sending.
topics	YES*	The comma-separated-list of the topics on which events will be sent.
data-key	NO	The data key is used when you want to send data events. This attribute's value is the key, in the event's dictionary, in which sent data are stored. When you use the sendData method of the Publisher, the given object is placed in the event dictionary, associated with the specified data-key. The default value of this attribute is user.data.
synchronous	NO	Determines if event sending is synchronous or not. By default, events are sent asynchronously, but you can specify there the desired behaviour of the Publisher. The default value of this attribute is "false".

* These attributes can be (re)defined in the instance configuration.

Instance configuration

Some of the described attributes can be (re)defined in the instance configuration section of your metadata file. Its permits to configure event management instance by instance. The following properties are used by the handler :

event.topics : overrides topics attribute, available for both subscribers and publishers configuration
event.filter : overrides filter attribute, available for subscribers configuration only.

Publisher interface

The Publisher interface is the link between the component code and the handler. It permits to publish events on the topics specified in the component's description (or instance configuration). The implemented methods are :

public void send(Dictionary content);
This method is used to send a standard event, with the specified content. Some specific properties may be added in the content to satisfy EventAdmin specification. (e.g., event.topic).
public void sendData(Object o);
This method is the easier way to send data. The given object is placed in the event dictionary according to the data-key attribute (or its default value). Then, this dictionary is sent as a normal event.

Handler Architecture

Here is shown the global architecture of the EventHandler : the interactions between the user components (i.e., POJO), the handler and the OSGi runtime environment.

EventHandler Features

In this section, you will find some examples of the handler's features.

Instance customization

As described in the 'Instance configuration' section, you can (re)define some of the subscribers or publishers attributes. You can notice that required attributes that are not defined in the component description must be defined in the instance configuration section. Hereafter is an example of an instance configuration of this handler :

<ipojo
    xmlns:ev="org.apache.felix.ipojo.handlers.event.EventAdminHandler">
	<component className="...MyComponent">
		<ev:subscriber
			name="mySubscriber"
			callback="handleEvent"/>
		<ev:publisher
			name="myPublisher"
			field="m_publisher"/>
	</component>
	<instance component="...MyComponent">
		<property name="event.topics">
			<property name="mySubscriber" value="foo"/>
			<property name="myPublisher" value="bar,nuts"/>
		</property>
		<property name="event.filter">
			<property name="mySubscriber"
				    value="|((arg=Minibar)(arg=Coconuts))"/>
		</property>
	</instance>
</ipojo>

Data events

One of the most important features of the EventHandler is the capability of sending and receiving data events. You may know that the OSGi EventAdmin Service allows bundles to send custom objects in events, inserting them in the event's dictionary. The EventHandler hides the dictionary manipulation and allows iPOJO components to receive custom objects at any time.

First, you have define the data-key attribute in the publisher configuration. Sent objects will be contained in the event dictionary and are accessible with the "user.data" key.

<ipojo
    xmlns:ev="org.apache.felix.ipojo.handlers.event.EventAdminHandler">
    <component className="...DataPublisher">
		<ev:publisher
			name="myPublisher"
			field="m_publisher"
			topics="myTopic"
			data-key="my.data"/>
	</component>
	<instance component="...DataPublisher"/>
</ipojo>

Then you can use the sendData method of your configured publisher.

import org.apache.felix.ipojo.handlers.event.publisher.Publisher;
//...
public class DataPublisher ... {
    private Publisher m_publisher;

    public void doSomething() {
      // MyFavoriteType extends MyFavoriteInterface
      MyFavoriteType data = new MyFavoriteType(...);
      //...
      // Send a data event
      m_publisher.sendData(data);
    }
}

The second step is to configure an event subscriber to receive such events. The data-key attribute's value of the subscriber must be the same than the publisher's one. The data-typedescribe the type of received data events, and thus, must be compatible with the sent object's type (i.e., super-class or inherited interface). Then you can finally receive the sent object in the callback method. The parameter type of the callback must be the same than the data-type attribute value.

<ipojo
   xmlns:ev="org.apache.felix.ipojo.handlers.event.EventAdminHandler">
	<component className="...DataEventSubscriber">
		<ev:subscriber
			name="mySubscriber"
			callback="handleData"
			topics="myTopic"
			data-key="my.data"
			data-type="my.package.MyFavoriteInterface"/>
	</component>
	<instance component="...DataEventSubscriber"/>
</ipojo>

import my.package.MyFavoriteInterface;
//...
public class DataEventSubscriber ... {
  public void handleData(MyFavoriteInterface o) {
     // Object received
     //...
   }
}

Note on synchronous event sending

By default, events are sent using asynchronous sending (a.k.a.post in OSGi EventAdmin). You can use synchronous sending by defining the synchronous attribute of your publisher to true.

The behaviour of synchronous event sending is particular when you specify several topics in the publisher description. The event is synchronously sent to each topic, one by one. So when you return from this function, you can be sure that the event has been delivered to each topic.

Publisher instance information

All events sent by a publisher contains the name of the component instance that sent them. Its enables to filter received events depending the sender instance. The instance name is accessible in the event dictionary by the key publisher.instance.name. Despite it goes against MOM principles, this property is useful to trace events and especially event sources.

Configuring the handler with annotations [New in 1.1.0-SNAPSHOT]

It is possible to configure the handler with a simple annotations available in the annotation pack ('annotation' project in the iPOJO trunk). Here is an example of usage:

import org.apache.felix.ipojo.annotations.Component;
import org.apache.felix.ipojo.handlers.event.Subscriber;
import org.osgi.service.event.Event;


@Component
public class PubSub {
    @org.apache.felix.ipojo.handlers.event.Publisher(name="p1", synchronous=true)
    org.apache.felix.ipojo.handlers.event.publisher.Publisher publisher1;
    
    @org.apache.felix.ipojo.handlers.event.Publisher(name="p2", synchronous=false, topics="foo,bar", data_key="data")
    org.apache.felix.ipojo.handlers.event.publisher.Publisher publisher2;
    
    @org.apache.felix.ipojo.handlers.event.Publisher(name="p3", synchronous=true, topics="bar")
    org.apache.felix.ipojo.handlers.event.publisher.Publisher publisher3;
    
    @Subscriber(name="s1", data_key="data")
    public void receive1(Object foo) {
        // Process event
    }
    
    @Subscriber(name="s2", topics="foo,bar", filter="(foo=true)")
    public void receive2(Event foo) {
        // Process event
    }
    
    
    @Subscriber(name="s3", topics="foo", data_key="data", data_type="java.lang.String")
    public void receive3(String foo) {
        // Process event
    }
}