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<h1><a name="LifecycleCallbackHandler-Lifecyclecallbacks"></a>Life cycle callbacks</h1>
<p>It is often necessary to create a POJO object as soon the instance
becomes valid. Moreover, it is also often needed to be able to stop it
nicely. The lifecycle callback handler allows invoking methods
(callbacks) on the instance when instance's state changed. For example,
it allows invoking a <em>start</em> method when the instance becomes valid and a <em>stop</em> method when the instance becomes invalid. Moreover, this handler allows the creation of <em>immediate</em> component. This page presents how to use this handler.</p>
<h2><a name="LifecycleCallbackHandler-InstanceLifecycle"></a>Instance Lifecycle</h2>
<p>IPOJO instances have a very simple lifecycle. This lifecycle
contains two states: INVALID and VALID. Once an instance is created,
this instance can only be valid if all its plugged handlers are valid.
For example, an instance requiring a service (and so using the
dependency handler) cannot be valid if the required service is
unavailable. Indeed, the dependency handler will be invalid.</p>
<p>An instance starts and stops in the invalid state.</p>
<p><img src="lifecycle-callback-handler_files/lifecycle.png" align="absmiddle" border="0"></p>
<h1><a name="LifecycleCallbackHandler-Lifecyclecallback"></a>Lifecycle callback</h1>
<p>This handler supports two kinds of callback. The INVALID=&gt;VALID
callback are invoked when the instance becomes valid (at starting or
when an event allows the instance to become valid). The
VALID=&gt;INVALID callback are invoked when the instance becomes
invalid (at stopping or when an event invalids the instance).</p>
<p><img src="lifecycle-callback-handler_files/callback.png" align="absmiddle" border="0"></p>
<p><img src="lifecycle-callback-handler_files/callbackOnTransition.png" align="absmiddle" border="0"></p>
<h2><a name="LifecycleCallbackHandler-Anexample"></a>An example</h2>
<p>Let's take an example. The following class requires a FooService and has two lifecycle callbacks: start and stop.</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class Foo {
FooService fs;
<span class="code-keyword">private</span> void start() {
<span class="code-comment">// Starting method
</span> <span class="code-comment">//...
</span> fs.foo();
<span class="code-comment">//...
</span> }
<span class="code-keyword">protected</span> void stop() {
<span class="code-comment">// Stopping method
</span> <span class="code-keyword">if</span>(fs!=<span class="code-keyword">null</span>) { fs.foo(); }
}
}</pre>
</div></div>
<p>For this class, we define the following component type:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component className=<span class="code-quote">"...Foo"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"fs"</span>/&gt;</span>
<span class="code-tag">&lt;callback transition=<span class="code-quote">"validate"</span> method=<span class="code-quote">"start"</span>/&gt;</span>
<span class="code-tag">&lt;callback transition=<span class="code-quote">"invalidate"</span> method=<span class="code-quote">"stop"</span>/&gt;</span>
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>When an instance of this component type is created, the start method is called as soon as the <em>Foo</em> Service (service requirement) becomes available. If the <em>Foo</em> Service is no more available or when the instance is stopped, the stop method is called.</p>
<p>The invoked methods have no argument, but could be private,
protected or public. Public methods can be in parent classes too.
Moreover, the INVALID=&gt;VALID (validate) method can use service
dependencies (the instance becomes valid means that all required
services are available); however, in the stop method (invalidate) it is
possible that one of these dependency can be <em>null</em>. Indeed, the departure of a service can be the cause of the instance invalidation.</p>
<h2><a name="LifecycleCallbackHandler-Managingthreads"></a>Managing threads</h2>
<p>One usage of lifecycle callback is when the instance needs to create
threads. Indeed, the thread can be created in the validate callback,
and stopped in the invalidate method. The next class shows an example
of a class handling a thread by using lifecycle callbacks.</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloRequesterImpl <span class="code-keyword">implements</span> <span class="code-object">Runnable</span> {
<span class="code-keyword">final</span> <span class="code-keyword">static</span> <span class="code-object">int</span> DELAY=10000;
HelloService\[\] m_hello;&amp;nbsp; <span class="code-comment">// Service Dependency
</span> <span class="code-object">boolean</span> end;
<span class="code-keyword">public</span> void run() {
<span class="code-keyword">while</span> (\!end) {
<span class="code-keyword">try</span> {
<span class="code-keyword">synchronized</span> (<span class="code-keyword">this</span>) {
<span class="code-keyword">for</span>(<span class="code-object">int</span> i = 0; i &lt; m_hello.length; i++) {
<span class="code-object">System</span>.out.println(m_hello\[i\].sayHello(<span class="code-quote">"Clement"</span>));
}
}
<span class="code-object">Thread</span>.sleep(DELAY);
} <span class="code-keyword">catch</span> (InterruptedException ie) {
/* will recheck quit */
}
}
}
<span class="code-keyword">public</span> void starting() {
<span class="code-object">Thread</span> T = <span class="code-keyword">new</span> <span class="code-object">Thread</span>(<span class="code-keyword">this</span>);
end = <span class="code-keyword">false</span>;
T.start();
}
<span class="code-keyword">public</span> void stopping() { end = <span class="code-keyword">true</span>; }</pre>
</div></div>
<p>For this component type, the metadata are :</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component className=<span class="code-quote">"... HelloRequesterImpl"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"HelloService"</span>/&gt;</span>
<span class="code-tag">&lt;callback transition=<span class="code-quote">"validate"</span> method=<span class="code-quote">"starting"</span>/&gt;</span>
<span class="code-tag">&lt;callback transition=<span class="code-quote">"invalidate"</span> method=<span class="code-quote">"stopping"</span>/&gt;</span>
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<h2><a name="LifecycleCallbackHandler-Immediatecomponent"></a>Immediate component</h2>
<p>An instance of an immediate component type is instantiated as soon
it becomes valid. It means that, when the instance becomes valid, the
constructor of the implementation class is called. This can replace the
validate callback. However, it stills a difference between the
immediate and the validate callback. The constructor is call only once
time. The validate callback is re-called each time the instance becomes
valid.</p>
<p><img src="lifecycle-callback-handler_files/immediate.png" align="absmiddle" border="0"></p>
<p>However as there is no destructor in Java, the invalidate callback is necessary if some actions are needed when stopping.</p>
<h2><a name="LifecycleCallbackHandler-Callbackonseveralobjects"></a>Callback on several objects</h2>
<p>If you instance has created several objects (called the
implementation class constructor several times), the callback is called
on each object in the creation order.</p></td>
<td class="confluenceTd" valign="top" width="20%">
<h6><a name="LifecycleCallbackHandler-Overview"></a><b>Overview</b></h6>
<ul>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo.html" title="Apache Felix iPOJO">Home Page</a></li>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo-feature-overview.html" title="Apache Felix iPOJO Feature Overview">iPOJO Feature Overview</a></li>
<li><a href="http://felix.apache.org/site/download.html" title="Download">Download &amp; Install </a></li>
</ul>
<h6><a name="LifecycleCallbackHandler-GettingStarted"></a><b>Getting Started</b></h6>
<ul>
<li><a href="http://felix.apache.org/site/ipojo-in-10-minutes.html" title="iPOJO in 10 minutes">iPOJO in 10 minutes</a></li>
<li><a href="http://felix.apache.org/site/how-to-use-ipojo-annotations.html" title="How to use iPOJO Annotations">How to use iPOJO Annotations</a></li>
<li><a href="http://felix.apache.org/site/ipojo-hello-word-maven-based-tutorial.html" title="iPOJO Hello Word (Maven-Based) tutorial">iPOJO Hello Word (Maven-Based) tutorial</a></li>
<li><a href="http://felix.apache.org/site/ipojo-advanced-tutorial.html" title="iPOJO Advanced Tutorial">iPOJO Advanced Tutorial</a></li>
<li><a href="http://felix.apache.org/site/ipojo-composition-tutorial.html" title="iPOJO Composition Tutorial">iPOJO Composition Tutorial</a></li>
</ul>
<h6><a name="LifecycleCallbackHandler-UserGuide"></a><b>User Guide</b></h6>
<ul>
<li><a href="http://felix.apache.org/site/describing-components.html" title="Describing components">Describing components (handler list) </a></li>
<li><a href="http://felix.apache.org/site/using-xml-schemas.html" title="Using XML Schemas">Using XML Schemas</a></li>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo-testing-components.html" title="apache-felix-ipojo-testing-components">Testing components</a></li>
<li><a href="http://felix.apache.org/site/ipojo-advanced-topics.html" title="iPOJO Advanced Topics">Advanced Topics</a></li>
<li><a href="http://felix.apache.org/site/ipojo-faq.html" title="iPOJO FAQ">FAQ</a></li>
</ul>
<h6><a name="LifecycleCallbackHandler-Tools"></a><b>Tools</b></h6>
<ul>
<li><a href="http://felix.apache.org/site/ipojo-eclipse-plug-in.html" title="iPOJO Eclipse Plug-in">iPOJO Eclipse Plug-in</a></li>
<li><a href="http://felix.apache.org/site/ipojo-ant-task.html" title="iPOJO Ant Task">iPOJO Ant Task</a></li>
<li><a href="http://felix.apache.org/site/ipojo-maven-plug-in.html" title="iPOJO Maven Plug-in">iPOJO Maven Plug-in</a></li>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo-junit4osgi.html" title="apache-felix-ipojo-junit4osgi">Junit4OSGi</a></li>
<li><a href="http://felix.apache.org/site/ipojo-concepts-overview.html" title="iPOJO Concepts Overview">iPOJO concepts overview</a></li>
</ul>
<h6><a name="LifecycleCallbackHandler-DeveloperGuide"></a><b>Developer Guide</b></h6>
<ul>
<li>API: <span class="nobr"><a href="http://people.apache.org/%7Eclement/ipojo/api/1.0/" title="Visit page outside Confluence" rel="nofollow">1.0<sup><img class="rendericon" src="lifecycle-callback-handler_files/linkext7.gif" alt="" align="absmiddle" border="0" width="7" height="7"></sup></a></span></li>
<li><a href="http://felix.apache.org/site/how-to-write-your-own-handler.html" title="How to write your own handler">How to write your own handler</a></li>
<li><a href="http://felix.apache.org/site/how-to-use-ipojo-manipulation-metadata.html" title="How to use iPOJO Manipulation Metadata">How to use iPOJO Manipulation Metadata</a></li>
<li><a href="http://felix.apache.org/site/dive-into-the-ipojo-manipulation-depths.html" title="Dive into the iPOJO Manipulation depths">Dive into the iPOJO Manipulation depths</a></li>
</ul>
<h6><a name="LifecycleCallbackHandler-Misc&amp;Contact"></a><b>Misc &amp; Contact</b></h6>
<ul>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo-issuestracker.html" title="apache-felix-ipojo-issuestracker">Issues Tracker</a></li>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo-supportedvms.html" title="apache-felix-ipojo-supportedVMs">Supported JVMs</a></li>
<li><a href="http://felix.apache.org/site/apache-felix-ipojo-supportedosgi.html" title="apache-felix-ipojo-supportedOSGi">Supported OSGi Implementations</a></li>
<li><span class="nobr"><a href="http://ipojo-dark-side.blogspot.com/" title="Visit page outside Confluence" rel="nofollow">iPOJO's Dark Side Blog<sup><img class="rendericon" src="lifecycle-callback-handler_files/linkext7.gif" alt="" align="absmiddle" border="0" width="7" height="7"></sup></a></span></li>
<li><a href="http://felix.apache.org/site/future-ideas.html" title="Future Ideas">Future Ideas</a></li>
<li><a href="http://felix.apache.org/site/contact.html" title="Contact">Contact</a></li>
<li><a href="http://felix.apache.org/site/related-works.html" title="Related Works">Related Works</a></li>
<li><a href="http://felix.apache.org/site/article-presentations.html" title="Article &amp; Presentations">Article &amp; Presentations</a></li>
</ul>
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