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Nhan AEM Notes

AEM provides two event systems for reacting to changes in the repository and the platform. Understanding

Event Listeners and Handlers

AEM provides two event systems for reacting to changes in the repository and the platform. Understanding when to use which is critical for building robust, performant integrations.

SystemInterfaceWhen to use
OSGi EventHandlerorg.osgi.service.event.EventHandlerLightweight, fire-and-forget notifications (replication events, resource changes, custom topics)
JCR ObservationManagerjavax.jcr.observation.EventListenerLow-level JCR node/property change events with fine-grained filtering
Sling Resource Change Listenerorg.apache.sling.api.resource.observation.ResourceChangeListenerModern, Sling-level resource change events (preferred over JCR observation)

Content Change

JCR Observation

Sling Resource Events

OSGi Event Admin

Your EventHandler / Listener

All three approaches ultimately receive their events from JCR observation, but the abstraction level and API differ. For most AEM projects, OSGi EventHandler and Sling ResourceChangeListener are the right choices.

OSGi EventHandler

The OSGi Event Admin service delivers events on topics (string identifiers). You register an EventHandler that listens to one or more topics, and the framework calls your handleEvent() method whenever an event matching those topics is published.

Event topics reference

Topic constantFires whenPackage
SlingConstants.TOPIC_RESOURCE_ADDEDA resource is createdorg.apache.sling.api
SlingConstants.TOPIC_RESOURCE_CHANGEDA resource is modifiedorg.apache.sling.api
SlingConstants.TOPIC_RESOURCE_REMOVEDA resource is deletedorg.apache.sling.api
ReplicationAction.EVENT_TOPICA replication (activation/deactivation) occurscom.day.cq.replication
PageEvent.EVENT_TOPICA page is created, modified, moved, or deletedcom.day.cq.wcm.api
NotificationConstants.TOPIC_JOB_FINISHEDA Sling Job finishesorg.apache.sling.event.jobs
NotificationConstants.TOPIC_JOB_FAILEDA Sling Job failsorg.apache.sling.event.jobs
DamEvent.EVENT_TOPICA DAM asset event occurscom.day.cq.dam.api

Minimal example: Resource change handler

package com.myproject.core.listeners;

import org.apache.sling.api.SlingConstants;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventConstants;
import org.osgi.service.event.EventHandler;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Component(
    service = EventHandler.class,
    immediate = true,
    property = {
        EventConstants.EVENT_TOPIC + "=" + SlingConstants.TOPIC_RESOURCE_CHANGED
    }
)
public class ResourceChangeHandler implements EventHandler {

    private static final Logger LOG = LoggerFactory.getLogger(ResourceChangeHandler.class);

    @Override
    public void handleEvent(Event event) {
        String path = (String) event.getProperty(SlingConstants.PROPERTY_PATH);
        LOG.debug("Resource changed: {}", path);
    }
}

Filtering events by path

Use the EventConstants.EVENT_FILTER property to apply an LDAP-style filter so your handler only fires for specific paths:

@Component(
    service = EventHandler.class,
    immediate = true,
    property = {
        EventConstants.EVENT_TOPIC + "=" + SlingConstants.TOPIC_RESOURCE_CHANGED,
        EventConstants.EVENT_FILTER + "=(path=/content/myproject/*)"
    }
)
public class FilteredChangeHandler implements EventHandler {

    @Override
    public void handleEvent(Event event) {
        String path = (String) event.getProperty(SlingConstants.PROPERTY_PATH);
        // Only fires for paths under /content/myproject/
    }
}

The filter syntax is LDAP, supporting & (AND), | (OR), and ! (NOT):

// Multiple conditions
EventConstants.EVENT_FILTER + "=(&(path=/content/myproject/*)(resourceType=myproject/*))"

// Exclude paths
EventConstants.EVENT_FILTER + "=(&(path=/content/*)(!(path=/content/dam/*)))"

Event properties

Each event carries properties that describe what happened. Access them via event.getProperty():

PropertyTypeDescription
SlingConstants.PROPERTY_PATHStringPath of the affected resource
SlingConstants.PROPERTY_RESOURCE_TYPEStringsling:resourceType of the resource
SlingConstants.PROPERTY_ADDED_ATTRIBUTESString[]Properties that were added
SlingConstants.PROPERTY_CHANGED_ATTRIBUTESString[]Properties that changed
SlingConstants.PROPERTY_REMOVED_ATTRIBUTESString[]Properties that were removed
SlingConstants.PROPERTY_USERIDStringUser who made the change
@Override
public void handleEvent(Event event) {
    String path = (String) event.getProperty(SlingConstants.PROPERTY_PATH);
    String userId = (String) event.getProperty(SlingConstants.PROPERTY_USERID);
    String[] changedProps = (String[]) event.getProperty(
        SlingConstants.PROPERTY_CHANGED_ATTRIBUTES);

    LOG.info("User {} modified {} (changed: {})",
        userId, path, Arrays.toString(changedProps));
}

Sling ResourceChangeListener

The ResourceChangeListener is the modern, recommended way to observe resource changes in Sling. It replaces the older OSGi SlingConstants.TOPIC_RESOURCE_* event topics and provides a cleaner API with explicit change types and path restrictions.

package com.myproject.core.listeners;

import org.apache.sling.api.resource.observation.ResourceChange;
import org.apache.sling.api.resource.observation.ResourceChangeListener;
import org.osgi.service.component.annotations.Component;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;

@Component(
    service = ResourceChangeListener.class,
    immediate = true,
    property = {
        // Only listen to paths under /content/myproject
        ResourceChangeListener.PATHS + "=/content/myproject",
        // Change types to listen for (ADDED, REMOVED, CHANGED, PROVIDER_ADDED, PROVIDER_REMOVED)
        ResourceChangeListener.CHANGES + "=ADDED",
        ResourceChangeListener.CHANGES + "=CHANGED",
        ResourceChangeListener.CHANGES + "=REMOVED"
    }
)
public class ContentChangeListener implements ResourceChangeListener {

    private static final Logger LOG = LoggerFactory.getLogger(ContentChangeListener.class);

    @Override
    public void onChange(List<ResourceChange> changes) {
        for (ResourceChange change : changes) {
            LOG.info("Resource {} was {} by {}",
                change.getPath(),
                change.getType(),
                change.getUserId());
        }
    }
}

ResourceChange properties

Each ResourceChange object provides:

MethodReturnsDescription
getPath()StringPath of the changed resource
getType()ChangeTypeADDED, REMOVED, CHANGED, PROVIDER_ADDED, PROVIDER_REMOVED
getUserId()StringUser who performed the change
isExternal()booleanWhether the change originated from a different cluster node
getAddedPropertyNames()Set<String>Properties that were added
getChangedPropertyNames()Set<String>Properties that changed
getRemovedPropertyNames()Set<String>Properties that were removed

[!-success] -success tip

Prefer ResourceChangeListener over the older SlingConstants.TOPIC_RESOURCE_* OSGi events for resource observation. It provides a richer API, better performance, and clearer semantics.

JCR ObservationManager

The lowest-level approach. Use this only when you need fine-grained control over JCR events that the Sling APIs don't expose (e.g., filtering by node type, or listening for property-level changes on specific node types).

package com.myproject.core.listeners;

import org.apache.sling.jcr.api.SlingRepository;
import org.osgi.service.component.annotations.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.jcr.*;
import javax.jcr.observation.*;

@Component(service = {}, immediate = true)
public class JcrNodeListener implements EventListener {

    private static final Logger LOG = LoggerFactory.getLogger(JcrNodeListener.class);

    @Reference
    private SlingRepository repository;

    private Session session;

    @Activate
    protected void activate() throws RepositoryException {
        session = repository.loginService("my-service-user", null);

        session.getWorkspace().getObservationManager().addEventListener(
            this,                              // listener
            Event.NODE_ADDED | Event.NODE_REMOVED | Event.PROPERTY_CHANGED, // event types
            "/content/myproject",              // absolute path
            true,                              // isDeep (include descendants)
            null,                              // uuid filter
            new String[]{"cq:PageContent"},    // node type filter
            false                              // noLocal
        );

        LOG.info("JCR observation listener registered");
    }

    @Deactivate
    protected void deactivate() throws RepositoryException {
        if (session != null) {
            session.getWorkspace().getObservationManager().removeEventListener(this);
            session.logout();
        }
    }

    @Override
    public void onEvent(EventIterator events) {
        while (events.hasNext()) {
            try {
                Event event = events.nextEvent();
                LOG.info("JCR event: type={}, path={}", event.getType(), event.getPath());
            } catch (RepositoryException e) {
                LOG.error("Error processing JCR event", e);
            }
        }
    }
}

JCR event types

ConstantValueDescription
Event.NODE_ADDED0x1A node was added
Event.NODE_REMOVED0x2A node was removed
Event.PROPERTY_ADDED0x4A property was added
Event.PROPERTY_REMOVED0x8A property was removed
Event.PROPERTY_CHANGED0x10A property value changed
Event.NODE_MOVED0x20A node was moved (ORDER_BEFORE)
Event.PERSIST0x40Changes were persisted (session.save())

Combine multiple types using bitwise OR: Event.NODE_ADDED | Event.PROPERTY_CHANGED

[!-warning] -warning warning

JCR observation requires an open JCR session for the lifetime of the listener. You must manage this session carefully - open it in @Activate and close it in @Deactivate. Use a service user for this session, never an admin session.

Practical Examples

Replication event handler (with OSGi config)

A production-quality example that triggers an external endpoint (e.g., a CI/CD pipeline) when content is replicated. Uses OSGi configuration for the endpoint URL and path filtering:

package com.myproject.core.listeners;

import com.day.cq.replication.ReplicationAction;
import com.day.cq.replication.ReplicationActionType;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;
import org.osgi.service.component.annotations.*;
import org.osgi.service.component.propertytypes.ServiceDescription;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventConstants;
import org.osgi.service.event.EventHandler;
import org.osgi.service.metatype.annotations.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.util.Arrays;

@Component(
    service = EventHandler.class,
    immediate = true,
    property = {
        EventConstants.EVENT_TOPIC + "=" + ReplicationAction.EVENT_TOPIC
    }
)
@ServiceDescription("Triggers a webhook when content is replicated to publish")
@Designate(ocd = ReplicationWebhookHandler.Config.class, factory = true)
public class ReplicationWebhookHandler implements EventHandler {

    private static final Logger LOG = LoggerFactory.getLogger(ReplicationWebhookHandler.class);

    @ObjectClassDefinition(
        name = "Replication Webhook Handler",
        description = "Triggers an external webhook on content replication events"
    )
    @interface Config {

        @AttributeDefinition(
            name = "Enabled",
            description = "Enable or disable this handler",
            type = AttributeType.BOOLEAN
        )
        boolean enabled() default false;

        @AttributeDefinition(
            name = "Webhook URL",
            description = "The external endpoint to call on replication"
        )
        String webhookUrl() default "";

        @AttributeDefinition(
            name = "Monitored paths",
            description = "Only trigger for replication events under these paths"
        )
        String[] monitoredPaths() default {"/content/myproject"};

        @AttributeDefinition(
            name = "Connection timeout (ms)",
            description = "HTTP connection timeout in milliseconds",
            type = AttributeType.INTEGER
        )
        int connectionTimeout() default 5000;

        @AttributeDefinition(
            name = "Socket timeout (ms)",
            description = "HTTP socket/read timeout in milliseconds",
            type = AttributeType.INTEGER
        )
        int socketTimeout() default 10000;
    }

    private boolean enabled;
    private String webhookUrl;
    private String[] monitoredPaths;
    private CloseableHttpClient httpClient;

    @Activate
    @Modified
    protected void activate(Config config) {
        enabled = config.enabled();
        webhookUrl = config.webhookUrl();
        monitoredPaths = config.monitoredPaths();

        RequestConfig requestConfig = RequestConfig.custom()
            .setConnectTimeout(config.connectionTimeout())
            .setSocketTimeout(config.socketTimeout())
            .build();

        httpClient = HttpClientBuilder.create()
            .setDefaultRequestConfig(requestConfig)
            .build();

        LOG.info("ReplicationWebhookHandler {} (url={}, paths={})",
            enabled ? "enabled" : "disabled", webhookUrl, Arrays.toString(monitoredPaths));
    }

    @Deactivate
    protected void deactivate() {
        if (httpClient != null) {
            try {
                httpClient.close();
            } catch (IOException e) {
                LOG.warn("Error closing HTTP client", e);
            }
        }
    }

    @Override
    public void handleEvent(Event event) {
        if (!enabled) {
            return;
        }

        ReplicationAction action = ReplicationAction.fromEvent(event);
        if (action == null) {
            return;
        }

        String path = action.getPath();
        ReplicationActionType type = action.getType();

        // Only trigger for Activate/Deactivate on monitored paths
        if ((type == ReplicationActionType.ACTIVATE || type == ReplicationActionType.DEACTIVATE)
                && isMonitoredPath(path)) {
            triggerWebhook(path, type.getName(), action.getUserId());
        }
    }

    private boolean isMonitoredPath(String path) {
        if (monitoredPaths == null || path == null) {
            return false;
        }
        return Arrays.stream(monitoredPaths).anyMatch(path::startsWith);
    }

    private void triggerWebhook(String path, String action, String userId) {
        String payload = String.format(
            "{\"path\":\"%s\",\"action\":\"%s\",\"user\":\"%s\"}",
            path, action, userId
        );

        HttpPost post = new HttpPost(webhookUrl);
        post.setHeader("Content-Type", "application/json");
        post.setEntity(new StringEntity(payload, StandardCharsets.UTF_8));

        try (CloseableHttpResponse response = httpClient.execute(post)) {
            int status = response.getStatusLine().getStatusCode();
            if (status >= 200 && status < 300) {
                LOG.info("Webhook triggered successfully for {} ({})", path, action);
            } else {
                LOG.warn("Webhook returned status {} for {} ({})", status, path, action);
            }
        } catch (IOException e) {
            LOG.error("Failed to trigger webhook for {} ({})", path, action, e);
        }
    }
}

Page event handler

Listen to page lifecycle events (create, modify, delete, move):

package com.myproject.core.listeners;

import com.day.cq.wcm.api.PageEvent;
import com.day.cq.wcm.api.PageModification;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventConstants;
import org.osgi.service.event.EventHandler;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Iterator;

@Component(
    service = EventHandler.class,
    immediate = true,
    property = {
        EventConstants.EVENT_TOPIC + "=" + PageEvent.EVENT_TOPIC
    }
)
public class PageEventHandler implements EventHandler {

    private static final Logger LOG = LoggerFactory.getLogger(PageEventHandler.class);

    @Override
    public void handleEvent(Event event) {
        PageEvent pageEvent = PageEvent.fromEvent(event);
        if (pageEvent == null) {
            return;
        }

        Iterator<PageModification> modifications = pageEvent.getModifications();
        while (modifications.hasNext()) {
            PageModification mod = modifications.next();
            String path = mod.getPath();
            PageModification.ModificationType type = mod.getType();

            switch (type) {
                case CREATED:
                    LOG.info("Page created: {}", path);
                    onPageCreated(path, mod.getUserId());
                    break;
                case MODIFIED:
                    LOG.debug("Page modified: {}", path);
                    break;
                case DELETED:
                    LOG.info("Page deleted: {}", path);
                    onPageDeleted(path, mod.getUserId());
                    break;
                case MOVED:
                    LOG.info("Page moved: {} -> {}", path, mod.getDestination());
                    break;
                case VERSION_CREATED:
                    LOG.debug("Page version created: {}", path);
                    break;
                default:
                    LOG.trace("Page event {}: {}", type, path);
            }
        }
    }

    private void onPageCreated(String path, String userId) {
        // Custom logic: set default properties, notify a team, etc.
    }

    private void onPageDeleted(String path, String userId) {
        // Custom logic: clean up references, update search index, etc.
    }
}

DAM asset event handler

React to asset upload, modification, and processing events:

package com.myproject.core.listeners;

import com.day.cq.dam.api.DamEvent;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventConstants;
import org.osgi.service.event.EventHandler;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Component(
    service = EventHandler.class,
    immediate = true,
    property = {
        EventConstants.EVENT_TOPIC + "=" + DamEvent.EVENT_TOPIC,
        // Filter to a specific DAM folder
        EventConstants.EVENT_FILTER + "=(path=/content/dam/myproject/*)"
    }
)
public class AssetEventHandler implements EventHandler {

    private static final Logger LOG = LoggerFactory.getLogger(AssetEventHandler.class);

    @Override
    public void handleEvent(Event event) {
        DamEvent damEvent = DamEvent.fromEvent(event);
        if (damEvent == null) {
            return;
        }

        String assetPath = damEvent.getAssetPath();
        DamEvent.Type type = damEvent.getType();

        switch (type) {
            case ACCEPTED:
                LOG.info("Asset accepted: {}", assetPath);
                break;
            case ASSET_CREATED:
                LOG.info("Asset created: {}", assetPath);
                break;
            case ASSET_MOVED:
                LOG.info("Asset moved: {}", assetPath);
                break;
            case METADATA_UPDATED:
                LOG.info("Asset metadata updated: {}", assetPath);
                break;
            case RENDITION_UPDATED:
                LOG.debug("Rendition updated: {}", assetPath);
                break;
            case RENDITION_REMOVED:
                LOG.debug("Rendition removed: {}", assetPath);
                break;
            case RESTORED:
                LOG.info("Asset restored from version: {}", assetPath);
                break;
            default:
                LOG.trace("DAM event {}: {}", type, assetPath);
        }
    }
}

Custom events: Publishing and consuming

You can publish your own events to decouple components:

Publishing a custom event:

package com.myproject.core.services.impl;

import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Reference;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventAdmin;

import java.util.HashMap;
import java.util.Map;

@Component(service = OrderService.class)
public class OrderServiceImpl implements OrderService {

    public static final String TOPIC_ORDER_PLACED = "com/myproject/order/PLACED";
    public static final String TOPIC_ORDER_SHIPPED = "com/myproject/order/SHIPPED";

    @Reference
    private EventAdmin eventAdmin;

    @Override
    public void placeOrder(String orderId, String userId) {
        // ... business logic ...

        // Publish event for other components to react to
        Map<String, Object> properties = new HashMap<>();
        properties.put("orderId", orderId);
        properties.put("userId", userId);
        properties.put("timestamp", System.currentTimeMillis());

        // sendEvent() is synchronous, postEvent() is asynchronous
        eventAdmin.postEvent(new Event(TOPIC_ORDER_PLACED, properties));
    }
}

Consuming the custom event:

@Component(
    service = EventHandler.class,
    immediate = true,
    property = {
        EventConstants.EVENT_TOPIC + "=com/myproject/order/PLACED",
        EventConstants.EVENT_TOPIC + "=com/myproject/order/SHIPPED"
    }
)
public class OrderNotificationHandler implements EventHandler {

    @Reference
    private EmailService emailService;

    @Override
    public void handleEvent(Event event) {
        String orderId = (String) event.getProperty("orderId");
        String userId = (String) event.getProperty("userId");
        String topic = event.getTopic();

        if (topic.endsWith("PLACED")) {
            emailService.sendOrderConfirmation(orderId, userId);
        } else if (topic.endsWith("SHIPPED")) {
            emailService.sendShipmentNotification(orderId, userId);
        }
    }
}

Sling ResourceChangeListener with external change detection

Detect changes that originate from other cluster nodes (e.g., replication from Author to Publish):

package com.myproject.core.listeners;

import org.apache.sling.api.resource.observation.ExternalResourceChangeListener;
import org.apache.sling.api.resource.observation.ResourceChange;
import org.apache.sling.api.resource.observation.ResourceChangeListener;
import org.osgi.service.component.annotations.Component;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;

@Component(
    service = ResourceChangeListener.class,
    immediate = true,
    property = {
        ResourceChangeListener.PATHS + "=/content/myproject",
        ResourceChangeListener.CHANGES + "=CHANGED"
    }
)
public class ExternalChangeListener
        implements ResourceChangeListener, ExternalResourceChangeListener {

    private static final Logger LOG = LoggerFactory.getLogger(ExternalChangeListener.class);

    @Override
    public void onChange(List<ResourceChange> changes) {
        for (ResourceChange change : changes) {
            if (change.isExternal()) {
                LOG.info("External change detected: {} (from another cluster node)",
                    change.getPath());
                // Invalidate local caches, update search index, etc.
            } else {
                LOG.debug("Local change: {}", change.getPath());
            }
        }
    }
}

[!-info] -info info

Implement ExternalResourceChangeListener in addition to ResourceChangeListener to also receive events originating from other cluster nodes. Without it, you only see local changes.

EventHandler vs Sling Jobs

OSGi events are fire-and-forget - if the instance crashes or the handler throws an exception, the event is lost. For operations that must complete (sending an email, updating an external system, writing to the JCR), use Sling Jobs instead.

FeatureOSGi EventHandlerSling Job
Delivery guaranteeNone - events can be lostAt least once - jobs are persisted and retried
Retry on failureNoYes (configurable retries, backoff)
Cluster awarenessFires on every nodeExecutes on one node only
PersistenceIn-memory onlyPersisted to JCR
Use caseLogging, cache invalidation, notificationsEmail sending, external API calls, JCR writes

Creating a Sling Job

Producer:

@Reference
private JobManager jobManager;

public void enqueueIndexUpdate(String path) {
    Map<String, Object> jobProps = new HashMap<>();
    jobProps.put("path", path);
    jobProps.put("timestamp", System.currentTimeMillis());

    jobManager.addJob("com/myproject/job/INDEX_UPDATE", jobProps);
}

Consumer:

package com.myproject.core.jobs;

import org.apache.sling.event.jobs.Job;
import org.apache.sling.event.jobs.consumer.JobConsumer;
import org.apache.sling.api.resource.ResourceResolver;
import org.apache.sling.api.resource.ResourceResolverFactory;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.Reference;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Map;

@Component(
    service = JobConsumer.class,
    property = {
        JobConsumer.PROPERTY_TOPICS + "=com/myproject/job/INDEX_UPDATE"
    }
)
public class IndexUpdateJobConsumer implements JobConsumer {

    private static final Logger LOG = LoggerFactory.getLogger(IndexUpdateJobConsumer.class);

    @Reference
    private ResourceResolverFactory resolverFactory;

    @Override
    public JobResult process(Job job) {
        String path = job.getProperty("path", String.class);
        LOG.info("Processing index update for: {}", path);

        try (ResourceResolver resolver = resolverFactory.getServiceResourceResolver(
                Map.of(ResourceResolverFactory.SUBSERVICE, "index-service"))) {

            // ... perform the index update ...

            return JobResult.OK;

        } catch (Exception e) {
            LOG.error("Index update failed for {}", path, e);
            // Return FAILED to let the queue retry (see queue.retries below), or
            // CANCEL for errors you know are not worth retrying (e.g. bad input).
            return JobResult.FAILED;
        }
    }
}

Job configuration

Retry behaviour is not configured on the consumer - it lives on the queue that processes the topic. Deploy a QueueConfiguration factory config that matches your topic:

{
    "queue.name": "Index Update Queue",
    "queue.topics": ["com/myproject/job/INDEX_UPDATE"],
    "queue.type": "UNORDERED",
    "queue.retries": 5,
    "queue.retrydelay": 30000
}

See Sling Jobs and Schedulers for the full list of queue properties.

EventHandler Lifecycle

Key points about the lifecycle:

  • @Activate is called when the component is registered
  • @Modified fires when OSGi configuration changes (no restart needed)
  • @Deactivate is called when the component is unregistered - clean up resources here
  • handleEvent() runs on the Event Admin thread pool - avoid blocking operations

Best Practices

Keep handlers fast

handleEvent() runs on the OSGi Event Admin thread pool. Blocking it delays all other event processing in the system:

// ❌ Bad - blocking HTTP call in the event handler thread
@Override
public void handleEvent(Event event) {
    httpClient.execute(new HttpGet(url)); // blocks the thread
}

// ✅ Good - delegate heavy work to a Sling Job
@Override
public void handleEvent(Event event) {
    Map<String, Object> props = new HashMap<>();
    props.put("path", event.getProperty(SlingConstants.PROPERTY_PATH));
    jobManager.addJob("com/myproject/job/HEAVY_WORK", props);
}

Never swallow exceptions

Always log errors. Silent catch blocks make debugging nearly impossible:

// ❌ Bad
try { ... } catch (IOException e) { }

// ✅ Good
try { ... } catch (IOException e) {
    LOG.error("Failed to process event for path {}", path, e);
}

Use postEvent() over sendEvent()

The EventAdmin service provides two methods:

MethodBehaviour
eventAdmin.sendEvent(event)Synchronous - blocks until all handlers finish
eventAdmin.postEvent(event)Asynchronous - returns immediately, handlers run in background

Prefer postEvent() unless you specifically need synchronous processing.

Guard against rapid-fire events

JCR changes can generate many events in quick succession (e.g., saving a dialog writes multiple properties). Debounce or batch your processing:

private final ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
private final Map<String, ScheduledFuture<?>> pendingTasks = new ConcurrentHashMap<>();

@Override
public void handleEvent(Event event) {
    String path = (String) event.getProperty(SlingConstants.PROPERTY_PATH);

    // Cancel any pending task for this path
    ScheduledFuture<?> existing = pendingTasks.get(path);
    if (existing != null) {
        existing.cancel(false);
    }

    // Schedule processing with a 2-second delay (debounce)
    ScheduledFuture<?> task = scheduler.schedule(
        () -> processChange(path),
        2, TimeUnit.SECONDS
    );
    pendingTasks.put(path, task);
}

Clean up resources in @Deactivate

If your handler opens HTTP clients, JCR sessions, thread pools, or other resources, close them:

@Deactivate
protected void deactivate() {
    if (httpClient != null) {
        try { httpClient.close(); } catch (IOException e) { LOG.warn("Error closing client", e); }
    }
    if (scheduler != null) {
        scheduler.shutdownNow();
    }
}

Use factory configurations for multi-tenant setups

Mark your handler with factory = true in @Designate to allow multiple instances with different configurations (e.g., different webhook URLs for different sites):

@Designate(ocd = MyHandler.Config.class, factory = true)
public class MyHandler implements EventHandler { ... }

This creates separate OSGi configuration entries in the Felix console, each spawning its own handler instance.

Common Pitfalls

PitfallSolution
Handler fires too often (event storm)Add path filtering, debounce, or move to Sling Jobs
Missing events after restartOSGi events are not persisted - use Sling Jobs for guaranteed delivery
Handler modifies content and triggers itselfCheck event.getProperty(PROPERTY_USERID) or use a flag path to prevent recursion
Slow handler blocks all event processingKeep handleEvent() fast; delegate heavy work to Sling Jobs
JCR session leak in observation listenerAlways close the session in @Deactivate
Handler works on Author but not PublishCheck that the OSGi config is deployed to the Publish run mode
Cluster: handler fires on every nodeUse Sling Jobs (cluster-aware) or check ResourceChange.isExternal()
AEMaaCS: Sling Jobs restrictedOn AEMaaCS, some job topics are reserved; test thoroughly in the cloud environment

Debugging Event Handlers

Felix Web Console

Navigate to http://localhost:4502/system/console/events to see:

  • All registered event topics and their handlers
  • Recent event history
  • Event delivery statistics

Logging

Enable debug logging for event processing:

{
    "org.apache.sling.commons.log.names": [
        "com.myproject.core.listeners",
        "org.apache.sling.event",
        "org.osgi.service.event"
    ],
    "org.apache.sling.commons.log.level": "DEBUG",
    "org.apache.sling.commons.log.file": "logs/events.log"
}

Groovy Console quick test

Fire a test event from the Groovy Console to verify your handler is working:

import org.osgi.service.event.Event
import org.osgi.service.event.EventAdmin

def eventAdmin = getService(EventAdmin.class)

def props = [
    "path": "/content/myproject/en/test-page",
    "resourceType": "myproject/components/page"
] as Map<String, Object>

eventAdmin.postEvent(new Event("org/apache/sling/api/resource/Resource/CHANGED", props))

println "Event posted"