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<document>
	<properties>
		<title>Element Event Handling</title>
		<author email="ASmuts@apache.org">Aaron Smuts</author>
	</properties>

	<body>
		<section name="Element Event Handling">
			<p>
				JCS allows you to attach event handlers to elements in
				the local memory cache.
			</p>
			<p>
				There are several events that you can listen for. All of
				the events are local memory related events. Element
				event handlers are not transmitted to other caches via
				lateral or remote auxiliaries, nor are they spooled to
				disk.
			</p>
			<p>
				You can register multiple handlers for a single item.
				Although the handlers are associated with particular
				items, you can also setup default handlers for any
				region. Each item put into the region, that will take
				the default element attributes, will be assigned the
				event default event handlers.
			</p>
			<p>
				The various events that you can handle have all been
				assigned integer codes. The codes are defined in the
				org.apache.commons.jcs.engine.control.event.behavior.IElementEventConstants
				interface. The events are named descriptively and
				include:
			</p>
			<table>
				<tr>
					<th>Name</th>
					<th>Description</th>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_EXCEEDED_MAXLIFE_BACKGROUND</td>
					<td>
						The element exceeded its max life. This was
						detected in a background cleanup.
					</td>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_EXCEEDED_MAXLIFE_ONREQUEST</td>
					<td>
						The element exceeded its max life. This was
						detected on request.
					</td>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_EXCEEDED_IDLETIME_BACKGROUND</td>
					<td>
						The element exceeded its max idle. This was
						detected in a background cleanup.
					</td>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_EXCEEDED_IDLETIME_ONREQUEST</td>
					<td>
						The element exceeded its max idle time. This was
						detected on request.
					</td>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_SPOOLED_DISK_AVAILABLE</td>
					<td>
						The element was pushed out of the memory store,
						there is a disk store available for the region,
						and the element is marked as spoolable.
					</td>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_SPOOLED_DISK_NOT_AVAILABLE</td>
					<td>
						The element was pushed out of the memory store,
						and there is not a disk store available for the
						region.
					</td>
				</tr>
				<tr>
					<td>ELEMENT_EVENT_SPOOLED_NOT_ALLOWED</td>
					<td>
						The element was pushed out of the memory store,
						there is a disk store available for the region,
						but the element is marked as not spoolable.
					</td>
				</tr>
			</table>
			<p>
				To create an event handler you must implement the
				org.apache.commons.jcs.engine.control.event.behavior.IElementEventHandler
				interface. This interface contains only one method:
			</p>
			<source>
				<![CDATA[
    public void handleElementEvent( IElementEvent event );
        		]]>
			</source>
			<p>
				The IElementEvent object contains both the event code
				and the source. The source is the element for which the
				event occurred. The code is the type of event. If you
				have an event handler registered, it will be called
				whenever any event occurs. It is up to the handler to
				decide what it would like to do for the particular
				event. Since there are not that many events, this does
				not create too much activity. Also, the event handling
				is done asynchronously. Events are added to an event
				queue and processed by background threads.
			</p>
            <p>
                Here is how to extract the event and source from the 
                IElementEvent:
            </p>
            <source>
                <![CDATA[
    public void handleElementEvent( IElementEvent event )
    {
        int eventType = event.getElementEvent();
        CacheElement element = (CacheElement)((EventObject)event).getSource();
        . . .
    }
                ]]>
            </source>
			<p>
				Once you have an IElementEventHandler implementation,
				you can attach it to an element via the Element
				Attributes. You can either add it to the element
				attributes when you put an item into the cache, add it
				to the attributes of an item that exist in the cache
				(which just results in a re-put), or add the event
				handler to the default element attributes for a region.
				If you add it to the default attributes, then all
				elements subsequently added to the region that do not
				define their own element attributes will be assigned the
				default event handlers.
			</p>
			<source>
				<![CDATA[
    CacheAccess<String, String> jcs = JCS.getInstance( "myregion" );

    . . .

    MyEventHandler meh = new MyEventHandler();

    // jcs.getDefaultElementAttributes returns a copy not a reference
    IElementAttributes attributes = jcs.getDefaultElementAttributes();
    attributes.addElementEventHandler( meh );
    jcs.put( "key", "data", attributes );
        		]]>
			</source>
			<p>
				Here is how to setup an event handler as a default
				setting for a region:
			</p>
			<source>
				<![CDATA[
    CacheAccess<String, String> jcs = JCS.getInstance( "myregion" );

    . . .

    MyEventHandler meh = new MyEventHandler();

    // this should add the event handler to all items as
    //they are created.
    // jcs.getDefaultElementAttributes returns a copy not a reference
    IElementAttributes attributes = jcs.getDefaultElementAttributes();
    attributes.addElementEventHandler( meh );
    jcs.setDefaultElementAttributes( attributes );
        		]]>
			</source>

		</section>
	</body>
</document>