Event Notifications in JCache

Event Notifications refer to a mechanism that allows listeners to be notified of cache events. These events represent multiple operations or updates that occur within the cache, enabling actions to be taken when such events happen. To get such event notifications, client applications must include the specific event handler methods to respond to the event(s) raised. Whenever these events are raised, the target event handler method executes the entire code written in the method body. By default, these notifications are disabled for all cache configurations (except for clearCache) and must be enabled for events to be published.

Prerequisites for Event Notifications in JCache

For using JCache with NCache Professional, replace the ncache-client with ncache-professional-client in your pom.xml.

<dependency>
    <groupId>com.alachisoft.ncache</groupId>
    <artifactId>ncache-client</artifactId>
    <version>x.x.x</version>
</dependency>

Import the following packages in your application:
- import javax.cache.CacheManager;
- import javax.cache.Caching;
- import javax.cache.spi.CachingProvider;
Make sure that the data is serialized or registered with NCache Serialization format.
Make sure that the cache is running and initialized.

Register Cache-Level Events

The JCache event model allows applications to receive and process events that represent changes to the entries in a cache. The event model uses standard Java events and event listener conventions. The following code snippet demonstrates how to register events in JCache. The code first gets the instance of JCache Manager, creates a listener config, registers a specific event through registerCacheEntryListener, and adds a value to get that event using the put method.

// Get instance of JCacheManager
CacheManager manager = Caching.getCachingProvider().getCacheManager();
// Get a cache from manager via its string name.
javax.cache.Cache jCache = manager.getCache("demoCache");

boolean isOldValueRequired = false;
boolean isSynchronous = false;

CacheEntryEventFilterImpl eventListener = new CacheEntryEventFilterImpl();

// Making listener config
MutableCacheEntryListenerConfiguration listenerConfig = new MutableCacheEntryListenerConfiguration(eventListener, new CacheEntryEventFilterImpl<String, Object>(), isOldValueRequired, isSynchronous);

// Registering events
jCache.registerCacheEntryListener(listenerConfig);

// Adding a value to get an event
jCache.put("key", "value");

Unregister Cache-Level Events

You can also unregister a previously registered Cache-level Event using the JCache deregisterCacheEnteryListener method with a registered cache entry listener instance to unregister.

// Get instance of JCacheManager
CacheManager manager = Caching.getCachingProvider().getCacheManager();
// Get a cache from manager via its string name.
javax.cache.Cache jCache = manager.getCache("demoCache");

boolean isOldValueRequired = false;
boolean isSynchronous = false;

// Making listener config
MutableCacheEntryListenerConfiguration listenerConfig = new MutableCacheEntryListenerConfiguration(eventListener, new CacheEntryEventFilterImpl<String, Object>(), isOldValueRequired, isSynchronous);

// Unregistering events
jCache.deregisterCacheEntryListener(listenerConfig);

Implement CacheEntryListenerFactory

You can implement the CacheEntryListenerFactory through the CacheEntryListenerFactoryImpl class that serves as a listener for cache entry events. The class reacts to certain changes in a cache, such as item creation, updation, or removal.

 public class CacheEntryListenerFactoryImpl<K, V> implements CacheEntryCreatedListener, CacheEntryUpdatedListener, CacheEntryRemovedListener, Factory<CacheEntryListener> {
    @Override
    public void onCreated(Iterable iterable) throws CacheEntryListenerException {
        // Business Logic
        System.out.println("Item Created");
    }

    @Override
    public void onRemoved(Iterable iterable) throws CacheEntryListenerException {
        // Business Logic
        System.out.println("Item Removed");
    }

    @Override
    public void onUpdated(Iterable iterable) throws CacheEntryListenerException {
        // Business Logic
        System.out.println("Item Updated");
    }

    @Override
    public CacheEntryListener create() {
        // Business Logic
        return new CacheEntryListenerFactoryImpl();
    }
}

Implement CacheEntryEventFilter

You can implement the CacheEntryEventFilter through the CacheEntryEventFilterImpl class, by implementing the CacheEntryEventFilter interface to determine whether a specific cache event should be considered or ignored by a cache entry listener. The class also implements the Factory<CacheEntryEventFilter> which creates objects of the CacheEntryEventFilter type.

public class CacheEntryEventFilterImpl<K, V> implements CacheEntryEventFilter, Factory<CacheEntryEventFilter> {

    @Override
    public boolean evaluate(CacheEntryEvent event) throws CacheEntryListenerException {
        // Business Logic
        return true;
    }

    @Override
    public CacheEntryEventFilter create() {
        // Business Logic
        return new CacheEntryEventFilterImpl();
    }
}

Register Item-Level Events

Along with JCache native events, NCache offers advanced Item-level events through which your client-applications can get notified about various operations taking place in the cache against specific key(s). Item-Level Events provide updates about specified keys, rather than bombarding the application with notifications for every key.

Item-level Events must be registered against some specific key(s) and these are the events published for these specific key(s) in the NCache API. To leverage NCache's Item-Level events, you can utilize the 'unwrap' method provided by NCache. This method allows you to access and effectively use the advanced events through your NCache cache instance. In the following example, the cache will generate a notification when a specific operation is performed on the key in a question.

// Get instance of JCacheManager
CacheManager manager = Caching.getCachingProvider().getCacheManager();
// Get a cache from manager via its string name.
javax.cache.Cache jCache = manager.getCache("demoCache");

// Unwrapping NCache's cache instance from JCache's cache instance
Cache ncache = (Cache) jCache.unwrap(Cache.class);

// Product to be registered for events
String key = "Product:1001";

CacheDataModificationListenerImpl eventListener = new CacheDataModificationListenerImpl();

// Registering events
ncache.getMessagingService().addCacheNotificationListener(key, eventListener, EnumSet.of(EventType.ItemUpdated, EventType.ItemRemoved), EventDataFilter.DataWithMetadata);

Unregister Item-level Events

You can also unregister previously registered Item-level Events using the removeCacheNotificationListener method.

// Get instance of JCacheManager
CacheManager manager = Caching.getCachingProvider().getCacheManager();
// Get a cache from manager via its string name.
javax.cache.Cache jCache = manager.getCache("demoCache");

// Unwrapping NCache's cache instance from JCache's cache instance
Cache ncache = (Cache) jCache.unwrap(Cache.class);

// Product to be unregistered for events
String key = "Product:1001";

// Unregistering events
ncache.getMessagingService().removeCacheNotificationListener(key, eventListener, EnumSet.of(EventType.ItemUpdated, EventType.ItemRemoved));

Implement CacheDataModificationListener

You can implement the CacheDataModificationListener through the CacheDataModificationListenerImpl class that implements a single method from the CacheDataModificationListener interface, called onCacheDataModified. The class monitors and reacts to different changes in a cache, such as updating or removing items.

public class CacheDataModificationListenerImpl implements CacheDataModificationListener {
    @Override
    public void onCacheDataModified(String key, CacheEventArg eventArgs) {
        switch (eventArgs.getEventType())
        {
            case ItemUpdated:
            // 'key' has been updated in the cache
            break;

            case ItemRemoved:
            // 'key' has been removed from the cache
            break;
        }
    }
}