This document provides an overview of declarative services in OSGi, including: - Dependency injection using declarative services allows for loose coupling between components. - Implementations for OSGi include Declarative Services, iPOJO, Spring DM, and others. - Declarative Services uses XML component descriptors to define lifecycles, dependencies, and configurations. - Version 1.1 of Declarative Services introduced improvements like configurable lifecycle methods. - Issues with the XML format led to the creation of the Maven SCR plugin to generate descriptors from Java code.

1. Declarative Services
Dependency Injection OSGi Style
Felix Meschberger | Day Management AG

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About Felix Meschberger
• Senior Developer at Day Management AG
• fmeschbe@day.com
• http://blog.meschberger.ch
• OSGi Implementations @ Apache Felix
– Declarative Services
– Configuration Admin
– Metatype Service

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Contents
• Dependency Injection
• Implementations for OSGi
• Declarative Services
• Issues
• Declarative Services 1.1
• Maven SCR Plugin
• Apache Felix Extensions

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Dependency Injection
• Loose Coupling
– „Don't call use, we'll call you“
• Easier Testing
– Inject different implementations
• Popular in Spring Framework
• Traditionally Descriptor Based
• Current Trend: Java Annotations

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Implementations for OSGi
• Dependency Manager
• Declarative Services
• iPOJO (Evolution of Declarative Services)
• Spring DM
• Blueprint Service (Evolution of Spring DM)
• Peaberry (based on Google Guice)
• … possibly more …

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Declarative Services
• Version 1.0, Compendium R4
• Version 1.1, Comendium R4.2
• XML Descriptor Based
• Lifecycle Management
• Dependency Injection (Services)
• Configuration Support

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Component Descriptor
• XML
• Descriptors may be embedded
• Namespace for Component
– http://www.osgi.org/xmlns/scr/v1.0.0
– http://www.osgi.org/xmlns/scr/v1.1.0
• Descriptors listed in Bundle Manifest Header
– Service-Component
• Multiple Components per Document

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Component Descriptor
<scr:component
name=“sample.component“
xmlns:scr=“http://www.osgi.org/xmlns/scr/v1.1.0“>
<implementation
class=“org.sample.Component“ />
<reference
interface=“org.osgi.service.log.LogService“
bind=“bindLog unbind=“unbindLoad“ />
<property name=“p1“ value=“sample“ />
<property name=“p2“ type=“Integer“>
1
2
</property>
</component>

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Lifecycle Management
• Load Descriptors on Bundle Start
• Instantiation
• Configuration
• Activation
• Dependency Injection
• Deactivation
• Unload on Bundle Stop

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Component Descriptor
<scr:component
name=“sample.component“
xmlns:scr=“http://www.osgi.org/xmlns/scr/v1.1.0“>
<implementation
class=“org.sample.Component“ />
<reference
interface=“org.osgi.service.log.LogService“
bind=“bindLog unbind=“unbindLoad“ />
<property name=“p1“ value=“sample“ />
<property name=“p2“ type=“Integer“>
1
2
</property>
</component>

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Lifecycle: Activation
package org.sample;
public class Component {
protected void activate(
ComponentContext c) {
System.out.println(„Activating“);
}
protected void deactivate(
ComponentContext c) {
System.out.println(„Deactivating“);
}
}

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Lifecycle: Binding
package org.sample;
public class Component {
protected void bindLog(
LogService ls) {
this.logService = ls
}
protected void unbindLog(
LogService ls) {
this.logService = null;
}
}

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Lifecycle: Configuration
package org.sample;
public class Component {
protected void activate(
ComponentContext c) {
Dictionary props = c.getProperties();
String p1 = (String) props.get(„p1“);
int[] p2 = (int[]) props.get(„p2“);
}
}

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Component Types
• Regular Component (non-service)
• Service
• Service Factory
• Component Factory

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Dependency Injection
• Event-based using bind/unbind methods
• Lookup oriented using ComponentContext
• Optionality
• Multiplicity
• Binding Policy

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Configuration
• Configuration from Configuration Admin
• Properties from Descriptor
• Provided through
ComponentContext.getProperties()

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Instantiation
• If Enabled and Satisfied
• Single Instance
– No Configuration (unless required)
– Singleton Configuration (service.pid)
• Multiple Instances
– Factory Configuration (service.factoryPid)

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Component Factory
• ComponentFactory.newInstance()
• ComponentInstance.dispose()
• Controlled by Application only
• Global Singleton Configuration only

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Descriptor Unvealed
<scr:component
name=
enabled=
immediate=
factory=
configuration-policy=
activate=
deactivate=
modified=
>
… Component Description …
• </scr:component>

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Descriptor Unvealed
<implementation
class=
/>

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Descriptor Unvealed
<property
name=
value=
type=
/>
<property
name=
type=
>
… values …
</property>

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Descriptor Unvealed
<properties
entry=
/>

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Descriptor Unvealed
<service
servicefactory=
>
<provide
interface=
/>
… More Provide Elements …
</service>

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Descriptor Unvealed
<reference
name=
interface=
cardinality=
policy=
target=
bind=
unbind=
/>

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Issues
• Configuration Data Types
• Components not really POJO (DS 1.0)
• XML Descriptor

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Configuration Data
• Wrapper of primitive types
– Byte, Short, Integer, Long, etc.
• String
• Array or Vector
– Primitive types
– Wrappers of primitive types
– String
• Aligned with...
– Metatype Service Specification
– Configuration Admin Specification

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DS 1.0 not really POJO
• Requires OSGi API for full functionality
• Activate and Deactivate method names
fixed and public or protected
• Configuration through ComponentContext
• Reference Service Properties through
ServiceReference
• Fixed in Declarative Services 1.1

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DS 1.1
• Configurable names for (de)activator
methods
• More (de)activator method arguments
– ComponentContext
– BundleContext
– Map
– int/Integer (deactivator only)
– Any combination

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DS 1.1 (cont.)
• More (un)bind method arguments
– ServiceReference
– Service instance
– Service instance and Map
• Configuration Dependency
– Optional
– Ignore
– Require
• Support for private properties

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DS 1.1 (cont.)
• Activator and bind methods may be
– public (discouraged)
– protected
– private (if in the component class)
– default (if in the same package)
• New features require DS 1.1 Namespace
• Service-Component supports Wildcards
– E.g. OSGi-INF/ds*.xml

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XML Descriptor
• Good to re-use legacy POJO
• Problematic to keep in-sync with DS Classes
• YAXF – Yet another XML File

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Maven SCR Plugin
• Generates Descriptors from Java Source
– JavaDoc tags
• @scr.component, @scr.property, ...
– Java Annotations
• @Component, @Property, ...
– High Level Annotations
• @SlingServlet
• Alternative: Peter Kriens' BND library

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Component Descriptor
<scr:component
name=“sample.component“
xmlns:scr=“http://www.osgi.org/xmlns/scr/v1.1.0“>
<implementation
class=“org.sample.component“ />
<reference
interface=“org.osgi.service.log.LogService“
bind=“bindLog unbind=“unbindLoad“ />
<property name=“p1“ value=“sample“ />
<property name=“p2“ type=“Integer“>
1
2
</property>
</component>

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SCR Annotations
package org.sample;
@Component
public class Component {
@Reference
private LogService log;
@Property(value=“sample“)
private String p1;
@Property(value=“1,2“)
private int[] p2;
protected void activate(
ComponentContext c) {
System.out.println(„Activating“);
}
protected void deactivate(
ComponentContext c) {
System.out.println(„Deactivating“);
}
}

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SCR JavaDoc Tags
package org.sample;
/** @scr.component */
public class Component {
/** @scr.reference /
private LogService log;
/** @scr.property value=“sample“ */
private String p1;
/** @scr.property values.0=“1“ values.1=“2“ */
private int[] p2;
protected void activate(
ComponentContext c) {
System.out.println(„Activating“);
}
protected void deactivate(
ComponentContext c) {
System.out.println(„Deactivating“);
}
}

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Apache Felix Extensions
• Management API
– Since Apache Felix SCR 1.0
– Now also in Equinox DS 1.2.0v20100125
• Service Reference: updated
– Since Apache Felix SCR 1.4.0
– OSGi Bug #63

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Questions

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Thank You!