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h1. Providing OSGi services

_This handler allows publishing and providing OSGi services. It manages:_
* _service publication_
* _service properties publication and management_
* _service object creation and creation strategies_
* _service un-registration_
* _configuration property propagation_

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h2. A simple example

The following code snippet shows a simple class implementing the FooService interface:
{code}
public class FooProviderType1 implements FooService {
            private String m_foo = "foo";

            public void foo() {
                        System.out.println("foo  " + m_foo);
            }

}
{code}
To provide a service, the implementation class *NEEDS* to implement the service interface. By the way, it guaranties that each methods of the service interface are implemented.

To provide the service, the component type needs to declare the providing:
{code:xml}
<component className="...FooProviderType1">
        <provides/>
</component>
{code}
!ps-foo.png!

The <provides/> element suffice to declare that each instance of this type will provide the FooService. Indeed, the provided specification can be discovered by analyzing the implementation class. By default, all implemented interface are published in the same service registration. iPOJO looks down the entire inheritance tree.

h2. Service Publication

The provided service handler manages the service publication and providing. For each declared {{provides}}, the handler register a service. The service is published as long as the instance is valid. If the instance becomes invalid, the service is removed from the service registry.

By default, it publishes all interfaces implemented by the implementation class of the component class. It collects all super-interfaces (interfaces implemented by implemented interfaces and by the super class). However it is possible to set exposed specifications with the {{specifications}} attribute to avoid to expose all collected interfaces.

{info:title=Change in the 1.2.0}
In the 1.0.0 version, the {{specifications}} attribute was named {{interface}}.
{info}

The following xml snippet is equivalent to the previous example:
{code:xml}
<component classname="...FooProviderType1">
            <provides specifications="...FooService "/>
</component>
{code}
If the implementation class implements several interfaces, all these interfaces are published by default in the same service publication. You can use the {{specifications}} attribute to set published service interfaces. If you want to publish several interfaces, you can use the following syntax:
{code:xml}
<component classname="...FooProviderType1">
            <provides specifications="{...FooService, ...BarService}"/>
</component>
{code}
!ps-foobar.png!

{info:title=Specification checking}
If you use the {{specifications}} attribute, the handler checks that all declared interfaces are really implemented by the implementation class. If an interface is not implemented, the handler log a warning.
{info}

{info:title=No service}
If the implementation class does not implement any interface, you cannot provide a service. In this case, the handler throws an error.
{info}

h2. Service Properties

The handler can manage service properties. Service properties are attached to published service and allow consumer filtering providers. A property can be attached to a field (contained in the component implementation class), and so by handle dynamically.

Let's take a new example very closed of the last one:
{code}
public class FooProviderType1 implements FooService {
	private String m_foo;
	public void foo() {
		System.out.println("foo  " + m_foo);
                m_foo = "bar";
	}
}
{code}
Remark that the {{m_foo}} field does not have any value. The following snippet shows a component publishing the {{FooService}} with two properties:
{code:xml}
<component classname="...FooProviderType1">
            <provides>
                        <property name="foo" field="m_foo" value="Foo"/>
                        <property name="intProps" type="int" value="5"/>
            </provides>
</component>
{code}
The first declared property will be attached to the {{m_foo}} field. This property is published with the name {{foo}}. This property has a default value "Foo". This value will be injected in the {{m_foo}} field, when this field asks for a value. A property with a field attribute does not need to declare a type (the type can be discovered by analyzing the implementation class).

The second property is published with the name {{intProps}}. This property is not attached to a field, so, we need to declare the property type. All primitive types or objects can be used has property type (for object, the qualified name of the class is used as java.lang.String).

!ps-foo2.png!

The implementation class set a new value to the {{m_foo}} field in the code. When this action occurs, the handler will modify the service publication to update the {{foo}} property published value. If a published property value becomes {{null}}, the property is unpublished since it has a new value.

!ps-foo3.png!

If property does not have default value, the instance configuration needs to set a value for each unvalued property. Moreover, the instance can override the property value. The following xml snippet shows the declaration of an instance overriding the property values:
{code:xml}
<instance component="...FooProviderType1" name="myFooServiceProvider">
            <property name="foo" value="baz"/>
            <property name="intProps" value="2"/>
</instance>
{code}
!ps-foo4.png!

h2. Advanced features

h3. Service Serving & Object Creation

When a consumer requires the published service, the handler sends an object (instance) of the implementation class. By default, it is always the same instance. If no instance already exists, an instance is created.

However, the handler supports the OSGi _Service Factory_. In this case, for each requester bundle, the handler sends a new object. To activate this policy, add the {{strategy}} attribute in the {{provides}} element:
{code:xml}
<provides strategy="SERVICE"/>
{code}

Other strategies are available:
 * {{strategy="instance"}} allows creating one service object per asking iPOJO instance (despite they are in the same bundle)
 * it is possible to create your own creation strategy by extending the {{org.apache.felix.ipojo.handlers.providedservice.CreationStrategy}} class and by indicating the qualified class name in the {{strategy}} attribute.

h3. Several Service Providing

You can declare several {{provides}} inside the same component. All this provided service will be manage by the same handler but separately. Several services will be published (with different service registrations). This case is useful when service properties are different for the different services.
{code:xml}
<component classname="...FooProviderType1">
                <provides specifications="...Foo"/>
                <provides specifications="...Bar">
                               <property name="foo" value="baz"/>
                </provides>
</component>
{code}
!ps-foobar2.png!

h3. Service Property Propagation

The configuration handler has the possibility to propagate received properties to service publication. So, when the propagation is activated, all properties received by the configuration handler will be propagated to all published service. If some properties are mapped on methods, these methods are invoked with the new value in argument.

!ps-propagation.png!

h3. Instance reconfiguration

The handler supports instance reconfiguration. When an instance is dynamically reconfigured, if the new configuration updates property values, these value are take into account (both for field, and service publication). If some of these properties have methods, these methods are invoked with the new value in argument.

h3. Publishing abstract and concrete class as services

It is also possible to expose concrete and abstract class as services. To to this, just specify the published class in the {{specification}} attribute:
{code:xml}
<component classname="...FooProviderType1">
                <provides specifications="...AbstractFoo"/>
</component>
<component classname="...FooBarProviderType1">
                <provides specifications="[...AbstractFoo, ...Bar]"/>
</component>
{code}
The component can also publish itself as a service. However, such practice are not recommended.
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