Apache Sling provides some mechanisms and support for managing events. This page describes ...

Sling makes a distinction between 2 types of events:

• standard events: no garantee of processing
• job events: garantee of processing. Someone has to do something with the event (do the job).

The event mechanism is leveraging the OSGi Event Admin Specification (OSGi Compendium 113).
The OSGi API is very simple and leightweight - sending an event is just generating the event object and calling the event admin. Receiving the event is implementing a single interface and declaring through properties which topics one is interested in. For more details please refer to the following javadocs:

• package org.osgi.service.event of the OSGI API.
• package org.apache.sling.event of the Sling API.

You can learn more in the Eventing, Jobs and Scheduling section (http://sling.apache.org/site/eventing-and-jobs.html).

To get started with the Sling eventing API, you will implement here a service that listens to files posted to /tmp/dropbox and moves them to the appropriate locations depending on the mime-type:

• images (.png) are moved to /dropbox/images/
• music (.mp3) are moved to /dropbox/music/
• movies (.avi) are moved to /dropbox/movies/
• otherwise the files are moved to /dropbox/other/

To do that, you will implement 2 services. The first one, called OsgiDropBoxService:

• listens to osgi event resource added
• sends a job event
  The second one, called DropBoxEventHandler:
• listens to job event file added to /tmp/dropbox
• moves the file according to its extension

Listening to OSGI Events

To listen to the specific osgi event resource added:

• The service needs to implement the org.osgi.service.event.EventHandler interface and its handleEvent(Event event) method.
• The property event.topics needs to be set to org.apache.sling.api.SlingConstants.TOPIC_RESOURCE_ADDED in the class annotations.

To get a list of possible events available in Sling (resource added, removed or changed), refer to the org.apache.sling.api.SlingConstants class in the Javadocs.

Sending a Job Event

To send a job event:

• The service needs to implement the org.apache.sling.event.JobProcessor interface and its process(org.osgi.service.event.Event job) method.

The first part of the code looks as follows:

/**
 * The <code>OsgiDropBoxService</code> is listening content added to /tmp/dropbox by using OSGI events
 * 
 * @scr.component  immediate="true"
 * @scr.service interface="org.osgi.service.event.EventHandler"
 * @scr.property name="event.topics" valueRef="org.apache.sling.api.SlingConstants.TOPIC_RESOURCE_ADDED"
 */
public class OsgiDropBoxService implements JobProcessor, EventHandler {

The Event Admin service is needed to send the job event:

	/** 
	 * The OSGI event admin used for sending events 
	 * @scr.reference
	 */
	private EventAdmin eventAdmin;

The job topic for dropbox job events needs to be defined:

    /** The job topic for dropbox job events. */
    public static final String JOB_TOPIC = "com/sling/eventing/dropbox/job";

The org.osgi.service.event.EventHandler#handleEvent(Event event) method needs to be implemented:

	public void handleEvent(Event event) {
	    if (EventUtil.isLocal(event)) {
	        EventUtil.processJob(event, this);
	    }
	}

The org.apache.sling.event.JobProcessor#process(Event event) method needs to be implemented.

The logic is as follows:

• the OSGI event is analyzed
• if the event is a file that has been added to "/tmp/dropbox":
  • an event is created with 2 properties:
    • one to flag the event as a job event
    • the file path
  • the job event is sent to all the listeners that subscribe to the topic of the event.

	public boolean process(Event event) {
		String propPath = (String) event.getProperty(SlingConstants.PROPERTY_PATH);
		String propResType = (String) event.getProperty(SlingConstants.PROPERTY_RESOURCE_TYPE);
		// an event is sent if a file is added to /tmp/dropbox
		if (propPath.startsWith("/tmp/dropbox") && propResType.equals("nt:file")) {
    		final Dictionary<String, Object> props = new Hashtable<String, Object>();
            props.put(EventUtil.PROPERTY_JOB_TOPIC, JOB_TOPIC);
    		props.put("resourcePath", propPath);
    		Event dropboxJobEvent = new Event(EventUtil.TOPIC_JOB, props);
    		eventAdmin.sendEvent(dropboxJobEvent);
    		log.info("the dropbox job has been sent: {}", propPath);
		}
		return true;
	}

The whole service looks now as follows:
...

Now that you have a service that sends job events whenever a file is uploaded to /tmp/dropbox, you will create the service DropBoxEventHandler that listens to those job events and moves the files to a location corresponding to their mime-types.

Listening to Job Events

To listen to the job events that have been defined before:

• The service also needs to implement the org.osgi.service.event.EventHandler interface and its handleEvent(Event event) method.
• The property event.topics needs to be set to com.jck.sling.eventing.osgi.OsgiDropBoxService.JOB_TOPIC in the class annotations.

To move the files:

• The service needs to implement the org.apache.sling.event.JobProcessor interface and its process(org.osgi.service.event.Event job) method.

The first part of the code looks as follows:

/**
 * @scr.component  immediate="true"
 * @scr.service interface="org.osgi.service.event.EventHandler"
 * @scr.property name="event.topics" valueRef="com.jck.sling.eventing.osgi.OsgiDropBoxService.JOB_TOPIC"
 */

public class DropBoxEventHandler implements JobProcessor, EventHandler {

Several class fields are defined:

• for logging
• to reference the SlingRepository and the JcrResourceResolverFactory services, which are used in the implementation
• to define the destination path of the files

   /** Default log. */
    protected final Logger log = LoggerFactory.getLogger(this.getClass());

    private Session adminSession;
    
    /** @scr.reference */
    private SlingRepository repository;
    
    /**
     * @scr.reference
     */
    private JcrResourceResolverFactory resolverFactory;
    
    private final static String IMAGES_PATH = "/dropbox/images/";
    private final static String MUSIC_PATH = "/dropbox/music/";
    private final static String MOVIES_PATH = "/dropbox/movies/";
    private final static String OTHER_PATH = "/dropbox/other/";

Handling Job Events

The org.osgi.service.event.EventHandler#handleEvent(Event event) method needs to be implemented:

	public void handleEvent(Event event) {
	    if (EventUtil.isLocal(event)) {
	        EventUtil.processJob(event, this);
	    }
	}

The org.apache.sling.event.JobProcessor#process(Event event) method needs to be implemented.
The logic is as follows:

• the resource path is extracted from the job event property
• the resource is obtained from the file path
• if the resource is a file, the destination path is defined based on the file mime-type
• the file is moved to the new location

	public boolean process(Event event) {
		try {
			String resourcePath = (String) event.getProperty("resourcePath");
			String resourceName = resourcePath.substring(resourcePath.lastIndexOf("/") + 1);
        	adminSession = repository.loginAdministrative(null);
	        ResourceResolver resourceResolver = resolverFactory.getResourceResolver(adminSession);
	        Resource res = resourceResolver.getResource(resourcePath);
	        if (ResourceUtil.isA(res, "nt:file")) {
	        	String mimeType = res.getResourceMetadata().getContentType();
	        	String destDir;
	        	if (mimeType.equals("image/png")) {
	        		destDir = IMAGES_PATH;
	        	}
	        	else if (mimeType.equals("audio/mpeg")) {
	        		destDir = MUSIC_PATH;
	        	}
	        	else if (mimeType.equals("video/x-msvideo")) {
	        		destDir = MOVIES_PATH;
	        	}
	        	else {
	        		destDir = OTHER_PATH;
	        	}
        		adminSession.move(resourcePath, destDir + resourceName);
	        	adminSession.save();
	        	log.info("The file {} has been moved to {}", resourceName, destDir);
	        }
	        return true;
		} catch (RepositoryException e) {
			log.error("RepositoryException: " + e);
			return false;
        } finally {
            if (adminSession != null && adminSession.isLive()) {
            	adminSession.logout();
            	adminSession = null;
            }
        }
	}

The complete code for the service looks as follows:
...