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<h1><a name="WhiteBoardPatternHandler-Thewhiteboardpatternhandler"></a>The white board pattern handler</h1>
<p>The objective of this handler is to simplify the development of
white-board architecture. This architecture-style is based is very
close to the extender architecture style but use services instead of
bundles.</p>
<div class="toc">
<ul>
<li><a href="#WhiteBoardPatternHandler-Thewhiteboardpattern">The whiteboard pattern</a></li>
<li><a href="#WhiteBoardPatternHandler-Usingthehandler">Using the handler</a></li>
<li><a href="#WhiteBoardPatternHandler-Configuration">Configuration</a></li>
<li><a href="#WhiteBoardPatternHandler-Configuringthehandlerwithannotations">Configuring the handler with annotations</a></li>
<li><a href="#WhiteBoardPatternHandler-Download">Download</a></li>
</ul></div>
<div class="panelMacro"><table class="infoMacro"><colgroup><col width="24"><col></colgroup><tbody><tr><td valign="top"><img src="white-board-pattern-handler_files/information.gif" alt="" border="0" align="absmiddle" height="16" width="16"></td><td><b>Change in the 1.2.0</b><br><p>The 1.2.0 uses the namespace <tt>org.apache.felix.ipojo.whiteboard</tt> instead of <tt>org.apache.felix.ipojo.white-board-pattern</tt></p></td></tr></tbody></table></div>
<h2><a name="WhiteBoardPatternHandler-Thewhiteboardpattern"></a>The whiteboard pattern</h2>
<p>A whiteboard is based on two different roles:</p>
<ul>
<li>A consumer looking to special services or a services published with a special mark</li>
<li>Looked services</li>
</ul>
<p>More information on this pattern is available in this <a href="http://www.osgi.org/wiki/uploads/Links/whiteboard.pdf" rel="nofollow">document</a><br>
Implementing a white board pattern could be complex as the extender
needs to track these services dynamically. Indeed looked services can
be highly dynamic; they can arrive, leave or be modified at runtime.<br>
Several services specified in the OSGi specification use white board pattern such as the Device Access Manager.</p>
<h2><a name="WhiteBoardPatternHandler-Usingthehandler"></a>Using the handler</h2>
<p>First of all, you need to configure the component type to use the handler such as:</p>
<div class="code">
<div class="dp-highlighter"><table class="dp-xml" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td class="line1"><span></span><span class="tag">&lt;</span><span>ipojo&nbsp;xmlns:</span><span class="attribute">wbp</span><span>=</span><span class="attribute-value">"org.apache.felix.ipojo.whiteboard"</span><span>&gt;&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="tag">&lt;</span><span>component&nbsp;&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="attribute">className</span><span>=</span><span class="attribute-value">"org.apache.felix.ipojo.test.FooWhiteBoardPattern"</span><span>&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="tag">&gt;</span><span>&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="tag">&lt;</span><span>wbp:wbp&nbsp;&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="attribute">filter</span><span>=</span><span class="attribute-value">"(my.property=1)"</span><span>&nbsp;&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="attribute">onArrival</span><span>=</span><span class="attribute-value">"onArrival"</span><span>&nbsp;&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="attribute">onDeparture</span><span>=</span><span class="attribute-value">"onDeparture"</span><span>&nbsp;&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="attribute">onModification</span><span>=</span><span class="attribute-value">"onModification"</span><span>&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="tag">/&gt;</span><span>&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="tag">&lt;</span><span>provides</span><span class="tag">/&gt;</span><span>&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="tag">&lt;/</span><span>component</span><span class="tag">&gt;</span><span>&nbsp;</span></td></tr></tbody></table></div><textarea style="display: none;" name="newcodemacro" class="xml:nocontrols:nogutter" rows="10" readonly="readonly">&lt;ipojo xmlns:wbp="org.apache.felix.ipojo.whiteboard"&gt;
&lt;component
className="org.apache.felix.ipojo.test.FooWhiteBoardPattern"
&gt;
&lt;wbp:wbp
filter="(my.property=1)"
onArrival="onArrival"
onDeparture="onDeparture"
onModification="onModification"
/&gt;
&lt;provides/&gt;
&lt;/component&gt;</textarea>
<script class="javascript">
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window.newcodemacro_initialised = true;
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</div>
<p>Notice that, this handler is an external handler. So, it uses the "org.apache.felix.ipojo.whiteboard" namespace.<br>
Once described, you can implement your component. The methods specified
methods will be called when a matching services arrives or leaves or
are modified. The modification callback is optional. A matching service
is detected by confronting the service reference against the specified
filter.<br>
The filter can target specific service interface (with the objectclass property) or property values.<br>
In the previous example, these methods could be: </p>
<div class="code">
<div class="dp-highlighter"><table class="dp-c" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td class="line1"><span></span><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;FooWhiteBoardPattern&nbsp;</span><span class="keyword">implements</span><span>&nbsp;Observable&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;List&nbsp;list&nbsp;=&nbsp;new&nbsp;ArrayList();&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;modifications&nbsp;=&nbsp;0;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">synchronized</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;onArrival(ServiceReference&nbsp;ref)&nbsp;{&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;list.add(ref);&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">synchronized</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;onDeparture(ServiceReference&nbsp;ref)&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;list.remove(ref);&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">synchronized</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;onModification(ServiceReference&nbsp;ref)&nbsp;{&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;modifications&nbsp;=&nbsp;modifications&nbsp;+&nbsp;1;&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">synchronized</span><span>&nbsp;Map&nbsp;getObservations()&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Map&nbsp;map&nbsp;=&nbsp;new&nbsp;HashMap();&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;map.put(<span class="string">"list"</span><span>,&nbsp;list);&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;map.put(<span class="string">"modifications"</span><span>,&nbsp;new&nbsp;Integer(modifications));&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">return</span><span>&nbsp;map;&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;</td></tr></tbody></table></div><textarea style="display: none;" name="newcodemacro" class="java:nocontrols:nogutter" rows="10" readonly="readonly">public class FooWhiteBoardPattern implements Observable {
List list = new ArrayList();
int modifications = 0;
public synchronized void onArrival(ServiceReference ref) {
list.add(ref);
}
public synchronized void onDeparture(ServiceReference ref) {
list.remove(ref);
}
public synchronized void onModification(ServiceReference ref) {
modifications = modifications + 1;
}
public synchronized Map getObservations() {
Map map = new HashMap();
map.put("list", list);
map.put("modifications", new Integer(modifications));
return map;
}</textarea>
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<p>All method received the arriving, leaving or modified service reference.</p>
<h2><a name="WhiteBoardPatternHandler-Configuration"></a>Configuration</h2>
<p>The handler has only three mandatory attributes:</p>
<ul>
<li>Filter: filter use to discover matching filter.</li>
<li>onArrival: declaring the method to invoke when a matching service arrives</li>
<li>onDeparture: declaring the method to invoke when a matching service leaves</li>
</ul>
<p>The onModification attribute is optional. This method is called when
an injected service reference is modified but stills valid against the
filter.</p>
<div class="panelMacro"><table class="noteMacro"><colgroup><col width="24"><col></colgroup><tbody><tr><td valign="top"><img src="white-board-pattern-handler_files/warning.gif" alt="" border="0" align="absmiddle" height="16" width="16"></td><td><b>Notifications</b><br><p>The
implementation will be notified of arrivals, modifications and
departures, despite the instance is invalid. Indeed, the implementation
must release all objects created from another bundle as soon it leaves.</p></td></tr></tbody></table></div>
<h2><a name="WhiteBoardPatternHandler-Configuringthehandlerwithannotations"></a>Configuring the handler with annotations</h2>
<p>It is possible to configure the handler with a simple annotation
available in the annotation pack ('annotation' project in the iPOJO
trunk). Here is an example of usage:</p>
<div class="code">
<div class="dp-highlighter"><table class="dp-c" border="0" cellpadding="0" cellspacing="0"><tbody><tr><td class="line1"><span></span><span class="keyword">import</span><span>&nbsp;org.apache.felix.ipojo.annotations.Component;&nbsp;</span></td></tr><tr><td class="line2"><span class="keyword">import</span><span>&nbsp;org.osgi.framework.ServiceReference;&nbsp;</span></td></tr><tr><td class="line1"><span class="preprocessor"></span></td></tr><tr><td class="line2"><span class="preprocessor">@Component</span><span>&nbsp;</span></td></tr><tr><td class="line1"><span class="preprocessor">@org.apache.felix.ipojo.whiteboard.Wbp(filter="(foo=true)",&nbsp;</span><span>&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;onArrival=<span class="string">"onArrival"</span><span>,&nbsp;&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;onDeparture=<span class="string">"onDeparture"</span><span>,&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;onModification=<span class="string">"onModification"</span><span>)&nbsp;</span></td></tr><tr><td class="line1"><span class="keyword">public</span><span>&nbsp;</span><span class="keyword">class</span><span>&nbsp;WhiteBoardExemple&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;onArrival(ServiceReference&nbsp;ref)&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="comment">//&nbsp;do&nbsp;something</span><span>&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;onDeparture(ServiceReference&nbsp;ref)&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="comment">//&nbsp;do&nbsp;something</span><span>&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;</td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;<span class="keyword">public</span><span>&nbsp;</span><span class="keyword">void</span><span>&nbsp;onModification(ServiceReference&nbsp;ref)&nbsp;{&nbsp;</span></td></tr><tr><td class="line2">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<span class="comment">//&nbsp;do&nbsp;something</span><span>&nbsp;</span></td></tr><tr><td class="line1">&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;</td></tr><tr><td class="line2">&nbsp;</td></tr><tr><td class="line1">}&nbsp;</td></tr></tbody></table></div><textarea style="display: none;" name="newcodemacro" class="java:nocontrols:nogutter" rows="10" readonly="readonly">import org.apache.felix.ipojo.annotations.Component;
import org.osgi.framework.ServiceReference;
@Component
@org.apache.felix.ipojo.whiteboard.Wbp(filter="(foo=true)",
onArrival="onArrival",
onDeparture="onDeparture",
onModification="onModification")
public class WhiteBoardExemple {
public void onArrival(ServiceReference ref) {
// do something
}
public void onDeparture(ServiceReference ref) {
// do something
}
public void onModification(ServiceReference ref) {
// do something
}
}</textarea>
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<p>The <tt>onModification</tt> attribute is optional.The <tt>filter</tt> attribute allows setting the service filter.</p>
<h2><a name="WhiteBoardPatternHandler-Download"></a>Download</h2>
<p>The handler is available on the <a href="http://felix.apache.org/site/download.html" title="Download">download</a> page.<br>
Sources are available on the Felix trunk at the following location: <a href="http://svn.apache.org/repos/asf/felix/trunk/ipojo/handler/whiteboard" rel="nofollow">http://svn.apache.org/repos/asf/felix/trunk/ipojo/handler/whiteboard</a>.
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