CacheManager Event Listeners

Introduction

CacheManager event listeners allow implementers to register callback methods that will be executed when a CacheManager event occurs. Cache listeners implement the CacheManagerEventListener interface. The events include:

  • adding a Cache

  • removing a Cache

Callbacks to these methods are synchronous and unsynchronized. It is the responsibility of the implementer to safely handle the potential performance and thread safety issues depending on what their listener is doing.

Configuration

One CacheManagerEventListenerFactory and hence one CacheManagerEventListener can be specified per CacheManager instance. The factory is configured as below:

         <cacheManagerEventListenerFactory class="" properties=""/>

The entry specifies a CacheManagerEventListenerFactory which will be used to create a CacheManagerEventListener, which is notified when Caches are added or removed from the CacheManager. The attributes of a CacheManagerEventListenerFactory are:

  • class - a fully qualified factory class name

  • properties - comma separated properties having meaning only to the factory.

Callbacks to listener methods are synchronous and unsynchronized. It is the responsibility of the implementer to safely handle the potential performance and thread safety issues depending on what their listener is doing. If no class is specified, or there is no cacheManagerEventListenerFactory element, no listener is created. There is no default.

Implementing a CacheManagerEventListenerFactory and CacheManagerEventListener

CacheManagerEventListenerFactory is an abstract factory for creating cache manager listeners. Implementers should provide their own concrete factory extending this abstract factory. It can then be configured in ehcache.xml. The factory class needs to be a concrete subclass of the abstract factory CacheManagerEventListenerFactory, which is reproduced below:

         /**
 * An abstract factory for creating {@link CacheManagerEventListener}s. Implementers should
 * provide their own concrete factory extending this factory. It can then be configured in
 * ehcache.xml
 *
 * @author Greg Luck
 * @version $Id: cachemanager_event_listeners.apt 4369 2011-07-15 19:59:14Z ilevy $
 * @see "http://ehcache.org/documentation/2.8/cachemanager_event_listeners.html"
 */
public abstract class CacheManagerEventListenerFactory {
    /**
     * Create a <code>CacheManagerEventListener</code>
     *
     * @param properties implementation specific properties. These are configured as comma
     *                   separated name value pairs in ehcache.xml. Properties may be null
     * @return a constructed CacheManagerEventListener
     */
    public abstract CacheManagerEventListener
           createCacheManagerEventListener(Properties properties);
}

The factory creates a concrete implementation of CacheManagerEventListener, which is reproduced below:

         /**
 * Allows implementers to register callback methods that will be executed when a
 * <code>CacheManager</code> event occurs.
 * The events include:
 * <ol>
 * <li>adding a <code>Cache</code>
 * <li>removing a <code>Cache</code>
 * </ol>
 * <p/>
 * Callbacks to these methods are synchronous and unsynchronized. It is the responsibility of
 * the implementer to safely handle the potential performance and thread safety issues
 * depending on what their listener is doing.
 * @author Greg Luck
 * @version $Id: cachemanager_event_listeners.apt 4369 2011-07-15 19:59:14Z ilevy $
 * @since 1.2
 * @see CacheEventListener
 */
public interface CacheManagerEventListener {
    /**
     * Called immediately after a cache has been added and activated.
     * <p/>
     * Note that the CacheManager calls this method from a synchronized method. Any attempt to
     * call a synchronized method on CacheManager from this method will cause a deadlock.
     * <p/>
     * Note that activation will also cause a CacheEventListener status change notification
     * from {@link net.sf.ehcache.Status#STATUS_UNINITIALISED} to
     * {@link net.sf.ehcache.Status#STATUS_ALIVE}. Care should be taken on processing that
     * notification because:
     * <ul>
     * <li>the cache will not yet be accessible from the CacheManager.
     * <li>the addCaches methods whih cause this notification are synchronized on the
     * CacheManager. An attempt to call {@link net.sf.ehcache.CacheManager#getCache(String)"/>
     * will cause a deadlock.
     * </ul>
     * The calling method will block until this method returns.
     * <p/>
     * @param cacheName the name of the <code>Cache</code> the operation relates to
     * @see CacheEventListener
     */
    void notifyCacheAdded(String cacheName);

    /**
     * Called immediately after a cache has been disposed and removed. The calling method will
     * block until this method returns.
     * <p/>
     * Note that the CacheManager calls this method from a synchronized method. Any attempt to
     * call a synchronized method on CacheManager from this method will cause a deadlock.
     * <p/>
     * Note that a {@link CacheEventListener} status changed will also be triggered. Any
     * attempt from that notification to access CacheManager will also result in a deadlock.
     * @param cacheName the name of the <code>Cache</code> the operation relates to
     */
    void notifyCacheRemoved(String cacheName);
}

The implementations need to be placed in the classpath accessible to ehcache. Ehcache uses the ClassLoader returned by Thread.currentThread().getContextClassLoader() to load classes.