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<h1><a name="ServiceRequirementHandler-ServiceDependencyManagement"></a>Service Dependency Management</h1>
<p>The dependency handler manages <em>OSGi service</em> <em>dependencies/requirements</em>.
It allows a component to consume service without managing service
discovery, tracking and binding. The handler manages all this
interaction and injects required service in the component.</p>
<h2><a name="ServiceRequirementHandler-ServiceRequirement"></a>Service Requirement</h2>
<h3><a name="ServiceRequirementHandler-What'saservicerequirement?"></a>What's a service requirement?</h3>
<p>A requirement represents a required service. Therefore, it manages
the service lookup and the service binding. When an instance requires a
service, the handler injects directly a service object inside a field,
or invokes a method when a consistent service appears (or disappears).
Service requirements can be:</p>
<ul>
<li>Simple / Aggregate : the component can require one or several service providers</li>
<li>Mandatory / Optional : a component can declare an optional dependency</li>
<li>Filtered : a component can filter available providers</li>
<li>Dynamic / Static / Dynamic-Priority : the component can specify the binding policy</li>
<li>Specific : the dependency targets a specific service provider</li>
</ul>
<h3><a name="ServiceRequirementHandler-Dynamism&amp;InstanceLifecycle"></a>Dynamism &amp; Instance Lifecycle</h3>
<p>In OSGi™, services can appear and disappear dynamically. This
implies dependencies can target a provider which can appear or
disappear dynamically.&nbsp; So, dependencies need to manage this
dynamism by tracking every time available services. At any moment, a
dependency can be unresolved (i.e. no more provider can fulfill the
requirement).&nbsp; In the case of a mandatory requirement, the
instance becomes invalid (an invalid instance is no more accessible
externally, for example provided service are unpublished). If a
service, resolving the unfilled dependency appears, the instance
becomes valid. In consequence, dependencies affect directly the
instance state, and must manage correctly OSGi dynamism to allow a
complete unloading when a service goes away. As soon a mandatory
dependency cannot be fulfilled, the instance is invalidated.</p>
<p>By default, dependencies are managed dynamically (as previously
explained). However, iPOJO supports two other types of binding
policies:&nbsp;</p>
<ul>
<li>Static : if a bound service disappears, the instance is invalidated and cannot be revalidated (binding broken)</li>
<li>Dynamic-Priority: at each injection, the <em>best</em> provider is injected, or the providers array is sorted according to the OSGi Ranking policy.</li>
</ul>
<h2><a name="ServiceRequirementHandler-ServiceRequirementInjectionMechanisms"></a>Service Requirement Injection Mechanisms</h2>
<p>The handler manages two types of injections:</p>
<ul>
<li>Field injection: a field contains the service object. As soon
as the field is used, a consistent service object is injected. This
injection type fully hides the dynamism</li>
<li>Method invocation:
when a service appears, or disappears a method in the component is
invoked. For each dependency, bind and unbind methods are invoke to
notify the component of the event.</li>
</ul>
<p>Moreover, the two injections type can be merged. A component can declare a requirement containing both a field and 'binding'.</p>
<h3><a name="ServiceRequirementHandler-Fieldinjection"></a>Field injection</h3>
<p>Imagine a Hello service with one method 'getMessage' returning a
"Hello Message". The following component implementation can use this
service by attaching this service to a field and by using the field:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hello;
<span class="code-keyword">public</span> doSomething() {
<span class="code-object">System</span>.out.println(m_hello.getMesage());
}
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hello"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>The metadata contains a 'requires' element (representing the service
dependency). This element has a field attribute. This attribute is the
name of the field representing the service dependency in the
implementation class. The implementation uses the field as a normal
field without managing service interactions.</p>
<h3><a name="ServiceRequirementHandler-Methodinvocation"></a>Method invocation</h3>
<p>The second injection mechanism uses methods in the implementation
class. By this way, the dynamics can be managed directly by the
developer. Each dependency can declare two methods:</p>
<ul>
<li>A bind method called when a service appears</li>
<li>An unbind method called when a service disappears</li>
</ul>
<p>Moreover, callbacks can be in the component super class (in this
case methods must be public). These methods can have one of these four
signatures:</p>
<ul>
<li>Without any argument: the method is just a notification (method())</li>
<li>With the service object : the object is the implicated service object (method(Service svc))</li>
<li>With an OSGi service reference: the service reference appearing or disappearing (method(ServiceReference ref))</li>
<li>With the service object and the OSGi service reference (method(Service svc, ServiceReference ref))</li>
<li>With the service object and the service properties inside a Map (method(Service svc, Map properties)) <b>[New in the 1.1.0-SNAPSHOT version]</b></li>
<li>With the service object and the service properties inside a Dictionary (method(Service svc, Dictionary properties)) <b>[New in the 1.1.0-SNAPSHOT version]</b></li>
</ul>
<p>The following component implementation shows an example of implementation using this mechanism:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hello;
<span class="code-keyword">public</span> void bindHello(Hello h) { m_hello = h; }
<span class="code-keyword">public</span> void unbindHello() { m_hello = <span class="code-keyword">null</span>; }
<span class="code-keyword">public</span> doSomething() { <span class="code-object">System</span>.out.println(m_hello.getMesage()); }
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>Note, that the bind the unbind method can be have different
signatures. By using this mechanism, you need to be sure to manage the
dynamism correctly.<br>
(<a href="#ServiceRequirementHandler-discovery" title="discovery on Service Requirement Handler">See note on type discovery</a>)</p>
<h3><a name="ServiceRequirementHandler-FieldinjectionsandMethodinvocations"></a>Field injections and Method invocations</h3>
<p>The two mechanisms can be used together. In this case, the field
receives the value before the bind method invocation. So, if the field
is use in the method, the returned value will be up to date. The
following component implementation uses this mechanism:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hello; <span class="code-comment">// Injected Field
</span>
<span class="code-keyword">public</span> void bindHello() { <span class="code-object">System</span>.out.println(<span class="code-quote">"Hello appears"</span>); }
<span class="code-keyword">public</span> void unbindHello() { <span class="code-object">System</span>.out.println(<span class="code-quote">"Hello disapears"</span>); }
<span class="code-keyword">public</span> doSomething() { <span class="code-object">System</span>.out.println(m_hello.getMesage()); }
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<h3><a name="ServiceRequirementHandler-Injectionmechanisms&amp;lazyobjectcreation"></a>Injection mechanisms &amp; lazy object creation</h3>
<p>IPOJO creates objects only when required. When needed, iPOJO invokes
the constructor of the implementation class. The implementation class
can use field requirement because values are already injected. However,
method dependencies are called just after the constructor. If the
service already presents, the invocation of the methods are delayed
just after the constructor invocation.</p>
<h2><a name="ServiceRequirementHandler-SomeExamples"></a>Some Examples</h2>
<h3><a name="ServiceRequirementHandler-SimpleRequirement"></a>Simple Requirement</h3>
<p>By default, a requirement is mandatory, non-filtered and simple
(non-aggregate). The two previous examples illustrate this kind of
dependency. When services goes away and appears, the service
substitution is hidden. Fields attached to simple requirement point
always a consistent service object. For a simple dependency, the bind
method is called once time when the service appears or just after the
POJO constructor invocation is the service is available. When the
service disappears the unbind method is called. The bind method is
re-invoked as soon as another service provider is available. This
invocation occurs immediately if another service provider if available.
In this case, the instance is not invalidated.</p>
<h3><a name="ServiceRequirementHandler-AggregateRequirement"></a>Aggregate Requirement</h3>
<p>When a component requires several providers of the same service, it declares an aggregate dependency.</p>
<h4><a name="ServiceRequirementHandler-AggregateDependencywithfieldinjection"></a>Aggregate Dependency with field injection</h4>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hellos[];
<span class="code-keyword">public</span> doSomething() {
<span class="code-keyword">for</span>(<span class="code-object">int</span> I = 0; I &lt; m_hellos.length; i++) {
<span class="code-object">System</span>.out.println(m_hellos[i].getMessage());
}
}
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hellos"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>To declare an aggregate field for field requirement, you only need
to declare an array (instead of a scalar type). IPOJO will create and
inject the service object array. iPOJO discover that the dependency is
aggregate during the bytecode introspection.</p>
<p><em>Note:</em> The synchronization is managed by iPOJO. As soon as
you are 'touching' a dependency in a method, iPOJO ensure that you will
keep these objects until the end of the method. Nested methods will
share the same service object set.</p>
<h4><a name="ServiceRequirementHandler-AggregateDependencywithfieldinjection:list,vector,collectionandset"></a>Aggregate Dependency with field injection: list, vector, collection and set</h4>
<p>It is also possible to inject service objects inside fields of the type:</p>
<ul>
<li>list</li>
<li>vector</li>
<li>collection</li>
<li>set</li>
</ul>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> List m_hellos;
<span class="code-keyword">public</span> doSomething() {
<span class="code-keyword">for</span>(<span class="code-object">int</span> I = 0; I &lt; m_hellos.size(); i++) {
<span class="code-object">System</span>.out.println(((Hello) m_hellos.get(i)).
getMessage());
}
}
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hellos"</span> specification=<span class="code-quote">"o.a.f.i.Hello"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>In this case, just use the supported type that you want. iPOJO will
automatically understand that it is an aggregate dependency, and will
create the collection object containing service objects.<br>
<em>Note:</em> The service specification (i.e. interface) cannot be
discovered when using these types as the bytecode does not provide
enough information. So, you have to indicate the required service
interface (with the 'specification' attribute) in the requirement
description.<br>
<em>Note:</em> As in the previous case, the synchronization is managed
by iPOJO. As soon as you are 'touching' a dependency in a method, iPOJO
ensure that you will keep these objects until the end of the method.
Nested methods will share the same service object set.</p>
<h4><a name="ServiceRequirementHandler-AggregateDependencywithmethodinvocation"></a>Aggregate Dependency with method invocation</h4>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> List m_hellos= <span class="code-keyword">new</span> ArrayList();
<span class="code-keyword">private</span> void bindHello(Hello h) { m_hellos.add(h); }
<span class="code-keyword">private</span> void unbindHello(Hello h) { m_hellos.remove(h); }
<span class="code-keyword">public</span> <span class="code-keyword">synchronized</span> doSomething() {
<span class="code-keyword">for</span>(<span class="code-object">int</span> I = 0; I &lt; m_hellos.size(); i++) {
<span class="code-object">System</span>.out.println(m_hellos.get(i).
getMessage());
}
}
}
}</pre>
</div></div>
<p>This requirement is configured as following:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;requires aggregate=<span class="code-quote">"true"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span></pre>
</div></div>
<p>In this case, iPOJO cannot detect if the dependency is aggregate or not. So, you need to add the '<em>aggregate</em>' attribute. The bindHello and unbindHello will be called each time a Hello service appears or disappears.<br>
<em>Note:</em> To avoid the list modification during the loop, you need
synchronized the block. Indeed, as the field is not an iPOJO
requirement, iPOJO will not manage the synchronization.</p>
<h3><a name="ServiceRequirementHandler-OptionalRequirement(nonaggregate)"></a>Optional Requirement (non-aggregate)</h3>
<p>An optional requirement does not invalidate the instance despite no
providers are available. Moreover, it is possible to inject a default
service implementation when no <em>real</em> providers are available.</p>
<h4><a name="ServiceRequirementHandler-OptionalRequirementwithfieldinjection"></a>Optional Requirement with field injection</h4>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hello;
<span class="code-keyword">public</span> doSomething() {
<span class="code-object">System</span>.out.println(m_hello.getMesage());
}
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hello"</span> optional=<span class="code-quote">"true"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>To declare an optional requirement, you need to add the <em>'optional'</em> attribute. To avoid null pointer exception, iPOJO injects a <em>Nullable</em> object in the field when no service provider is available. The <em>nullable</em>
object implements the service interface, but does nothing. Moreover, it
is possible to set a default-implementation for the service. A
default-implementation is a class implementing the service but used
only when no others service providers are available. The
default-implementation object will be injected instead of the <em>Nullable</em> objet. For further information <a href="#ServiceRequirementHandler-nullable" title="nullable on Service Requirement Handler">refer to the note about nullable object</a>.</p>
<h4><a name="ServiceRequirementHandler-OptionalDependencywithmethodinvocation"></a>Optional Dependency with method invocation</h4>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hello;
<span class="code-keyword">public</span> void bindHello(Hello h) { m_hello = h; }
<span class="code-keyword">public</span> void unbindHello() { m_hello = <span class="code-keyword">null</span>; }
<span class="code-keyword">public</span> doSomething() {
<span class="code-keyword">if</span>(m_hello != <span class="code-keyword">null</span>) {
<span class="code-object">System</span>.out.println(m_hello.getMesage());
}
}
}</pre>
</div></div>
<p>For this component, metadata should be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires optional=<span class="code-quote">"true"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>As for field requirement, the dependency metadata needs to contain
the optional attribute. IPOJO invokes the method only when a 'real'
service is available, so you need to test if m_hello is null before to
use it.</p>
<h3><a name="ServiceRequirementHandler-Aggregate&amp;OptionalRequirement"></a>Aggregate &amp; Optional Requirement</h3>
<p>A dependency can be both aggregate and optional.</p>
<h4><a name="ServiceRequirementHandler-Aggregate&amp;OptionalDependencywithfieldinjection"></a>Aggregate &amp; Optional Dependency with field injection</h4>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> Hello m_hellos[];
<span class="code-keyword">public</span> doSomething() {
<span class="code-keyword">for</span>(<span class="code-object">int</span> I = 0; I &lt; m_hellos.length; i++) {
<span class="code-object">System</span>.out.println(m_hellos[i].getMessage());
}
}
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hellos"</span> optional=<span class="code-quote">"true"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>To declare an optional &amp; aggregate field requirement you need to
write the optional attribute in the dependency metadata and to point on
a field array. If no service available, iPOJO injects an empty array.</p>
<h4><a name="ServiceRequirementHandler-Aggregate&amp;OptionalRequirementwithmethodinvocation"></a>Aggregate &amp; Optional Requirement with method invocation</h4>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HelloConsumer {
<span class="code-keyword">private</span> List m_hellos&lt;Hello&gt; = <span class="code-keyword">new</span> ArrayList&lt;Hello&gt;();
<span class="code-keyword">private</span> void bindHello(Hello h) { m_hellos.add(h); }
<span class="code-keyword">private</span> void unbindHello(Hello h) { m_hellos.remove(h); }
<span class="code-keyword">public</span> <span class="code-keyword">synchronized</span> doSomething() {
<span class="code-keyword">for</span>(<span class="code-object">int</span> I = 0; I &lt; m_hellos.size(); i++) {
<span class="code-object">System</span>.out.println(m_hellos.get(i).getMessage());
}
}
}</pre>
</div></div>
<p>For this component, metadata could be:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;requires aggregate=<span class="code-quote">"true"</span> optional=<span class="code-quote">"true"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span></pre>
</div></div>
<p>In this case, you need to add the _'aggregate'_attribute and the
_'optional'_attribute. The bindHello and unbindHello will be called
each time a Hello service appears or disappears. These bind / unbind
methods are not called when binding / unbinding a Nullable object (when
both field and method are used).</p>
<h3><a name="ServiceRequirementHandler-FilteredRequirement"></a>Filtered Requirement</h3>
<p>A filtered dependency applies an LDAP filter on service provider.
IPOJO reuses OSGi LDAP filter ability. The following metadata
illustrates how to use filters:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires filter=<span class="code-quote">"(language=fr)"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>To add a filter, just add a 'filter' attribute in your dependency
containing the LDAP filter. iPOJO will select only provider matching
with this filter.</p>
<p>Moreover, filters can be customized instance by instance. It is
possible to specialize / change / add the filter of a component in the
instance description. It is useful when you want to create different
instances of the same component, with different filter. To do it, you
have to identify your dependency with an 'id' attribute. Then, you can
adapt the filter of the dependency in the instance description by using
the property "requires.filters". In this property you can specify each
dependency identified by its id and the new value of the filter.</p>
<div class="code"><div class="codeContent">
<pre class="code-xml">&lt;component
className=<span class="code-quote">"org.apache.felix.ipojo.example.FilteredDependency"</span>
name=<span class="code-quote">"FOO"</span>&gt;
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_foo"</span> fiter=<span class="code-quote">"(foo.property=FOO)"</span> id=<span class="code-quote">"id1"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bind"</span>/&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbind"</span>/&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
<span class="code-tag">&lt;/component&gt;</span>
<span class="code-tag">&lt;instance name=<span class="code-quote">"FOO1"</span> component=<span class="code-quote">"FOO"</span>/&gt;</span>
<span class="code-tag">&lt;instance name=<span class="code-quote">"FOO2"</span> component=<span class="code-quote">"FOO"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"requires.filters"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"id1"</span> value=<span class="code-quote">"(foo.property=BAR)"</span>/&gt;</span>
<span class="code-tag">&lt;/property&gt;</span>
<span class="code-tag">&lt;/instance&gt;</span>
<span class="code-tag">&lt;instance name=<span class="code-quote">"FOO3"</span> component=<span class="code-quote">"FOO"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"requires.filters"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"id1"</span> value=<span class="code-quote">"(foo.property=BAZ)"</span>/&gt;</span>
<span class="code-tag">&lt;/property&gt;</span>
<span class="code-tag">&lt;/instance&gt;</span></pre>
</div></div>
<p>The FOO component type declares a service dependency with the 'id1'
id. This dependency has no filter by default. The first instance is
just an instance of the FOO component type and does not modify the
dependency. The second one adds a filter to the declared dependency to
target providers with foo.property = BAR. The last one adds another
filter to the declared dependency. By using instance filter
customization, it is possible to create complex applications where you
avoid binding problems by filtering dependencies instance by instance.</p>
<h3><a name="ServiceRequirementHandler-Targetingaspecificprovider"></a>Targeting a specific provider</h3>
<p>A service dependency can choose a specific provider. To achieve
this, add a 'from' attribute in your requirement description such as in:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires from=<span class="code-quote">"MyHelloProvider"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bindHello"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbindHello"</span>&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>iPOJO maps the from attribute to a specific filter :
'|(instance.name=MyHelloProvider)(service.pid=MyHelloProvider)'. Then
the dependency can only be fulfilled by a service matching this filter.</p>
<p>Moreover, from attributes can be customized instance by instance. It
is possible to specialize / change / add a 'from' attribute of a
component in the instance configuration. It is useful when you want to
create different instances of the same component, with different 'from'
clauses. To do it, you have to identify your dependency with an 'id'
attribute. Then, you can adapt the 'from' of the dependency in the
instance configuration by using the property "requires.from". In this
property you can specify each dependency identified by its id and the
'from' value.</p>
<div class="code"><div class="codeContent">
<pre class="code-xml">&lt;component
className=<span class="code-quote">"org.apache.felix.ipojo.example.FilteredDependency"</span>
name=<span class="code-quote">"FOO"</span>&gt;
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_foo"</span> id=<span class="code-quote">"id1"</span>&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"bind"</span> method=<span class="code-quote">"bind"</span>/&gt;</span>
<span class="code-tag">&lt;callback type=<span class="code-quote">"unbind"</span> method=<span class="code-quote">"unbind"</span>/&gt;</span>
<span class="code-tag">&lt;/requires&gt;</span>
<span class="code-tag">&lt;/component&gt;</span>
<span class="code-tag">&lt;instance name=<span class="code-quote">"FOO1"</span> component=<span class="code-quote">"FOO"</span>/&gt;</span>
<span class="code-tag">&lt;instance name=<span class="code-quote">"FOO2"</span> component=<span class="code-quote">"FOO"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"requires.from"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"id1"</span> value=<span class="code-quote">"myprovider"</span>/&gt;</span>
<span class="code-tag">&lt;/property&gt;</span>
<span class="code-tag">&lt;/instance&gt;</span>
<span class="code-tag">&lt;instance name=<span class="code-quote">"FOO3"</span> component=<span class="code-quote">"FOO"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"requires.from"</span>&gt;</span>
<span class="code-tag">&lt;property name=<span class="code-quote">"id1"</span> value=<span class="code-quote">"myotherprovider"</span>/&gt;</span>
<span class="code-tag">&lt;/property&gt;</span>
<span class="code-tag">&lt;/instance&gt;</span></pre>
</div></div>
<p>The FOO component type declares a service dependency with the 'id1'
id. This dependency has no 'from' attribute by default. The first
instance is just an instance of the FOO component type and does not
modify the dependency. The second one adds a 'from' attribute to the
declared dependency to target the 'myprovider' provider. The last one
adds another 'from' clause to the declared dependency.</p>
<h2><a name="ServiceRequirementHandler-BindingPolicies"></a>Binding Policies</h2>
<p>Three binding policies are supported inside iPOJO.</p>
<ul>
<li>Dynamic policy (default): the binding are managed
dynamically. At each injection, the same provider is injected if the
provider is always available. Else a new one is chosen. For aggregate
dependency, the array order does not change; new providers are placed
at the end of the array.</li>
<li>Static policy: the binding is
static. So, once bound a provider cannot disappear. If it disappears,
the instance is invalidated and cannot be revalidated without stopping
and restarting the instance.</li>
<li>Dynamic-priority policy: the
binding is managed dynamically but the injected provider is selected by
using a ranking policy. Two injections can return two different
providers, is a new provider is 'better' than the previous one, despite
the first one is always available. For aggregate dependency, the array
is sorted.</li>
</ul>
<p>A static binding is declared as following:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hellos"</span> policy=<span class="code-quote">"static"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>A dynamic-priority binding is declared as following:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hellos"</span> policy=<span class="code-quote">"dynamic-priority"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>By default, the dynamic-priority policy uses the OSGi service
ranking policy. However, it is possible to customize the policy by
adding the '<em>comparator</em>' attribute. This attribute indicates
the class name of a class implementing the java.util.Comparator
interface. IPOJO will create an instance of your comparator and use it
to sort service references (so your customized comparator needs to be
able to sort OSGi Service Reference).</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...HelloConsumer"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_hellos"</span> policy=<span class="code-quote">"dynamic-priority"</span> comparator=<span class="code-quote">"my.great.Comparator"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p><a name="ServiceRequirementHandler-nullable"></a></p>
<h2><a name="ServiceRequirementHandler-Noteaboutnullableobject&amp;defaultimplementation"></a>Note about nullable object &amp; default-implementation</h2>
<p>The instance implementation can use an optional dependency without
any checking. Indeed, when an instance declares an optional dependency
using field injection, iPOJO create on the fly a Nullable class
implementing the service specification but doing nothing (mock object).
Therefore, iPOJO cannot return a service to the instance, for an
optional dependency, it returns a nullable object.</p>
<p>A nullable object returns:</p>
<ul>
<li>Null when the method returns an object</li>
<li>0 when the method returns an int, log, byte, short, char, float or a double</li>
<li>False when the method return a boolean</li>
</ul>
<p>You can check if the returned object is a nullable object with the test: <em>"myservice instanceof Nullable"</em>.</p>
<p>You can disable the Nullable pattern too (activated by default). In this case, iPOJO will inject <em>null</em> instead of a <em>Nullable</em> object. So, you can just test if your field is equals to <em>null</em>
to check if the service is available. To disable the Nullable pattern,
you need to add the 'nullable="false"' attribute in your service
dependency description as follows:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...LogExample"</span>&gt;</span>
<span class="code-tag">&lt;requires field=<span class="code-quote">"m_log"</span> optional=<span class="code-quote">"true"</span> nullable=<span class="code-quote">"false"</span>/&gt;</span>
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>However, you can also indicate a <em>default-implementation</em> for
your optional service. In this case, if no providers are found, iPOJO
creates an instance of the default-implementation and injects it. The
default-implementation attribute describes the class name of your
implementation. The given class <b>MUST</b> implement the required service interface.</p>
<p>For example, the following component uses a default implementation for a Log Service dependency:</p>
<div class="code"><div class="codeContent">
<pre class="code-xml"><span class="code-tag">&lt;component classname=<span class="code-quote">"...LogExample"</span>&gt;</span>
&lt;requires field=<span class="code-quote">"m_log"</span> optional=<span class="code-quote">"true"</span>
default-implementation=
<span class="code-quote">"org.apache.felix.ipojo.example.default.MyLogService"</span>/&gt;
...
<span class="code-tag">&lt;/component&gt;</span></pre>
</div></div>
<p>If the log service is not available, iPOJO creates an object of the
'org.apache.felix.ipojo.example.default.MyLogService' class. This
object is injected instead of a Nullable object. For instance, the
default implementation can print messages on the System.err stream. The
nullable object does no display anything.</p>
<p><a name="ServiceRequirementHandler-callbacks"></a></p>
<h2><a name="ServiceRequirementHandler-NoteaboutCallbacks"></a>Note about Callbacks</h2>
<p>Dependency manages two type of callback: bind and unbind. A callback
with a type "bind" is called each type that a service provider arrives
and the binding is necessary. According to the cardinality of the
dependency it means:</p>
<ul>
<li>Simple dependency : at the firs binding and at each rebinding to another service provider</li>
<li>Aggregate dependencies: each time that a service provider arrives</li>
</ul>
<p>An unbind callback is called each time that a <b>used</b> service
provider goes away. For a simple dependency this method is called each
time that the used service provider goes away. For a multiple
dependency this method is called each time that a service provider goes
away.</p>
<p>The method can receive in argument the service object or the service
reference (in order to obtain service properties). The bind methods are
delayed since a POJO object is created.</p>
<p><a name="ServiceRequirementHandler-discovery"></a></p>
<h2><a name="ServiceRequirementHandler-Noteonserviceinterfacediscovery"></a>Note on service interface discovery</h2>
<p>The <tt>specification</tt> attribute is generally optional except
when iPOJO cannot discover the type of the service. iPOJO cannot infer
the type when the dependency has no field and callbacks do not receive
the service object in parameter. In this case, you need to declare the
service interface.</p>
</td>
<td class="confluenceTd" valign="top" width="20%">
<h6><a name="ServiceRequirementHandler-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="ServiceRequirementHandler-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="ServiceRequirementHandler-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="ServiceRequirementHandler-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="ServiceRequirementHandler-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="service-requirement-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="ServiceRequirementHandler-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="service-requirement-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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