XWiki supports notifications (a.k.a Observation Events) and it's possible to do some action when a document is modified, when a document's objects are modified, when a wiki is created, etc. See the full documentation in the Observation reference documentation.

There are several ways to write an Event Listener to react to some events:

• In Java, as an XWiki Component
• In a wiki page, as a Wiki Component
• In a wiki page, using Groovy, by writing an XWiki Component and manually registering it against the Component Manager

This tutorial will demonstrate all techniques on several various examples.

Adding content to pages on save

The goal is to listen to DocumentCreatingEvent and DocumentUpdatingEvent events (note that these events are fired before the page is saved and thus our code doesn't need to perform the save itself since this is done automatically).

Let's implement this example using a Wiki Component.

Follow these steps:

• Create a page, for example EventListeners.DocumentSaveListener
• Add a XWiki.ComponentClass XObject in it
  • Component Role Type: org.xwiki.observation.EventListener
  • Component Role Hint: mytest (you can use whatever name you want, it's the technical id of your listener that you won't be using in this tutorial)
  • Component Scope: Current Wiki this means this listener will be active only in the current wiki). You'll need Admin rights for Current Wiki and you'd need Programming Rights for Global which would make the listener available in all the farm (i.e. all the wikis). For Current User you don't need any special permission but the listener will only be active for your user.
• Add a XWiki.ComponentMethodClass XObject for implementing the getEvents() method:
  • Method Name: getEvents
  • Method body code:
    {{groovy}}
    import org.xwiki.bridge.event.*
    
    xcontext.method.output.value = [new DocumentCreatingEvent(), new DocumentUpdatingEvent()]
    {{/groovy}}
• Add another XWiki.ComponentMethodClass XObject for implementing the getName() method:
  • Method Name: getName
  • Method body code:
    {{groovy}}
    xcontext.method.output.value = "mytest"
    {{/groovy}}

    Note that mytest should be the same technical id that you've used above.

• Add another XWiki.ComponentMethodClass XObject for implementing the onEvent() method:
  • Method Name: onEvent
  • Method body code:
    {{groovy}}
    def docSource = xcontext.method.input.get(1)
    if (docSource.space != "EventListeners") {
     docSource.setContent(docSource.content + "\n\nSome extra content...")
    }
    {{/groovy}}
• Save!

When you save the EventListeners.DocumentSaveListener page, the component you've defined (your Event Listener) is automatically registered and active.

You can verify it works by creating a new page or editing an existing page and you should see the text Some extra content... added at the bottom of your pages when you save them.

Log when a document is modified

In this example we want to log all document changes by adding a line in a page named Main.Logs.

We'll implement this use case using Groovy in a wiki page.

You can add other events to the getEvents returned list if you wish to listener to other events, for example:

And here's the full code with nice pretty-printing and a register button.

Send a mail whenever a comment is added

Let's implement this use case in Java this time!

Note: We're going to listen to CommentAddedEvent but in general if you wish to be notified when an XObject has been updated you should listen to XObjectUpdatedEvent (or XObjectPropertyUpdatedEvent when an XProperty has been updated). 

• Let's use the XWiki Maven Archetype for creating a skeleton project, by following this tutorial.
• Make sure you have the following 2 dependencies in your pom.xml file (note that I've used version 5.4.5 but you can use the version you wish):
  ...
     <dependency>
       <groupId>org.xwiki.platform</groupId>
       <artifactId>xwiki-platform-oldcore</artifactId>
       <version>5.4.5</version>
     </dependency>
     <dependency>
       <groupId>org.xwiki.platform</groupId>
       <artifactId>xwiki-platform-mailsender</artifactId>
       <version>5.4.5</version>
     </dependency>
  ...
• Create a CommentListener class in package org.xwiki.contrib.internal (pick the package name you wish!) with the following content:
  package org.xwiki.contrib.internal;
  
  import java.util.Arrays;
  import java.util.List;
  
  import javax.inject.Inject;
  import javax.inject.Named;
  import javax.inject.Singleton;
  
  import org.slf4j.Logger;
  import org.xwiki.component.annotation.Component;
  import org.xwiki.model.EntityType;
  import org.xwiki.model.reference.EntityReference;
  import org.xwiki.observation.EventListener;
  import org.xwiki.observation.event.Event;
  
  import com.xpn.xwiki.XWikiContext;
  import com.xpn.xwiki.doc.XWikiDocument;
  import com.xpn.xwiki.internal.event.XObjectAddedEvent;
  import com.xpn.xwiki.objects.BaseObject;
  import com.xpn.xwiki.plugin.mailsender.MailSenderPluginApi;
  
  @Component
  @Named("CommentEventListener")
  @Singleton
  public class CommentEventListener implements EventListener
  {
     @Inject
     private Logger logger;
  
     private EntityReference commentClassReference = new EntityReference("XWikiComments", EntityType.DOCUMENT,
         new EntityReference("XWiki", EntityType.SPACE));
  
     @Override public String getName()
     {
         return "CommentEventListener";
     }
  
     @Override public List<Event> getEvents()
     {
         return Arrays.<Event>asList(new XObjectAddedEvent());
     }
  
     @Override public void onEvent(Event event, Object source, Object data)
     {
          XWikiDocument document = (XWikiDocument) source;
          BaseObject commentObject = document.getXObject(this.commentClassReference);
         if (commentObject != null) {
             try {
                 // Get comment
                 String comment = commentObject.getStringValue("comment");
                 // Send email
                 XWikiContext xcontext = (XWikiContext) data;
                  MailSenderPluginApi mailSender = (MailSenderPluginApi) xcontext.getWiki().getPluginApi("mailsender", xcontext);
                  mailSender.sendTextMessage("XWiki <xwiki@no-reply>", "john@doe.com",
                     "[XWiki] Comment added to " + document.toString(), comment);
             } catch (Exception e) {
                 this.logger.error("Failure in comment listener", e);
             }
  
         }
     }
  }
• Don't forget to register your component in the META-INF/components.txt file in your project:
  org.xwiki.contrib.internal.CommentEventListener
• Build the project with maven: mvn clean install and copy the JAR generated in the target directory in your XWiki's WEB-INF/lib directory, and restart XWiki.

Before trying it, go to your wiki's administration page and make sure you've configured the Email properties. Try it by adding a new comment on a page. You should receive an email!