OSGi
Michal Malohlava
Charles University in Prague
Faculty of Mathematics and Physics
Czech Republic

Introduction
OSGi overview CZJUG, Prague – May 28, 2008
• Distributed Systems Research Group
 http://dsrg.mff.cuni.cz/
 Formal methods sub-group
 Benchmarking gang
 Components team
• Components team
 SOFA2 component system
• http://sofa.ow2.org/
2

Outline
OSGi overview CZJUG, Prague – May 28, 2008
• OSGi platform
 motivation
• OSGi architecture overview
 Bundles
 Services
 Example
 Technical background
• OSGi related projects
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OSGi history
OSGi overview CZJUG, Prague – May 28, 2008
• OSGi alliance
 Open Services Gateway initiative
• Founded 1999
• OSGi specification release 1 – 2000
• OSGi specification target
 Define Java-based service platform, full dynamic
component model
 Why?
• JVM does not support natively dynamic module system -
starting, stopping, updating application at runtime
• JAR dependencies management missing
 No way how to restrict exported packages, enforce imports
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OSGi motivation
OSGi overview CZJUG, Prague – May 28, 2008
• OSGi – kind of Java Operating System
• Configuration of application
 “LEGO” principle :-)
• Reusability
• Scalability
• Portability
 Abstraction of underlying hardware
 Standardized execution environment
• Application isolation/security
 Well defined classloading
• Easy managing life-cycle of application
 Dynamic updates, modification of running modules
 Not-only for enterprise solutions
• Embedded devices
 Service oriented architecture 5

OSGi current state
OSGi overview CZJUG, Prague – May 28, 2008
• OSGi specification
 Release 4.1 – May 2007
• core + compendium (contains mobile spec. from R3)
• Implementation
 Open source
• Eclipse Equinox
 Many extensions of OSGi, e.g. bundles aspects
• Apache Felix
 Based on Oscar (implementation of OSGi R3)
• Knopflerfish
• Concierge
 Implementation of OSGi R3, optimized for embedded devices
 Commercial
• ProSyst, Knopflerfish Pro
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OSGi architecture
OSGi overview CZJUG, Prague – May 28, 2008
• OSGi framework
 Bundles (modules)
 Execution environment
 Application life cycle
 Services
• Service registry
 Security
• Application share
the same JVM
 Isolation/security
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Execution environment
OSGi overview CZJUG, Prague – May 28, 2008
• OSGi platform can run on different target devices
 Profiles in specification
• OSGi/Minimum-1.1, CDC-1.1/Foundation-1.1
 Other profiles
• J2SE-1.4, J2SE-1.5
 OSGi APIs only use a subset of J2SE and J2ME
• Matches most profiles
• Application can enforce the profile
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Bundle
OSGi overview CZJUG, Prague – May 28, 2008
• Bundle
 Basic deployment entity (~ application, library)
 Versioned (1.0.2.rc3 – major, minor, micro, qualifier)
 Declaratively specified dependencies
 Represented as JAR file of Manifest-Version: 1.0
• Code, Resources Bundle-Name: LogTargetBundle
Bundle-Activator: LogTargetActivator
• Extended manifest file Bundle-SymbolicName: LogTargetBundle
• Bundle fragments Bundle-Version: 1.0.0
Import-Package: org.osgi.framework;
 Similar to bundles version=”1.3.1”
• Allow extending an existing bundle
 represent e.g. native implementation, require a host bundle
• Bundle context
 Representation of bundle instance in the framework
 Installation of new bundle, bundle properties
 Access to persistent storage
 Services management, Listeners management 9

Bundle life-cycle
OSGi overview CZJUG, Prague – May 28, 2008
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Bundle Activator
OSGi overview CZJUG, Prague – May 28, 2008
• Class defined in MANIFEST
 Bundle-Activator header
public class SimpleLogTargetActivator {
• Can be an external class
@Override
• Handle bundle start/stop public void start(BundleContext context){
/* ... */
}
 Start @Override
• Register services public void stop(BundleContext context) {
/* ... */
• Create service trackers }
}
• Start threads
 Stop
• Release resources
 Unregister services
 Release services
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Bundle dependencies
OSGi overview CZJUG, Prague – May 28, 2008
Export-Package: cz.*;
• Expose packages exclude=”*Impl”
 List all of packages + versions + attributes
 Fine grained package filtering
• Import bundle Import-Package: cz.mff.*;
version=”[1.0,1.3.1)”
 Require specific version(s)
• e.g. [1.0, 2.0)
• Require bundle Require-Bundle: logger-api-bundle
 Not recommended because it restricts further changes
in API
• Bundle class path Bundle-Classpath: ., lib/bsh.jar
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Technical background of classloading
OSGi overview CZJUG, Prague – May 28, 2008
• Separated class loaders for each bundle
• Defined lookup order
 Parent (only for class from java.* package)
 Imported packages
 Required bundles
 Local class path
Image from the book OSGi in Practice, Neil Barlett
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Service
OSGi overview CZJUG, Prague – May 28, 2008
• Bundles – static entities
 Static dependencies through packages
 How to communicate between bundles?
• Services – dynamics in OSGi
 Can appear, disappear at runtime in according to a condition
 For one service name multiple providers can exist
 Bundles collaboration
• Well defined communication points
 Services permit bundles to detect environment and adapt their behavior
• Query language
• Service
 Object, registered by a bundle
• BundleContext.registerService(iface, implementation, properties)
• Service registry
• Framework automatically unregister all services of stopped bundle
 Service has properties (can be modified)
 Framework manages relation service <-> using bundle 14

Registering service (1)
OSGi overview CZJUG, Prague – May 28, 2008
• Programatically in BundleActivator
public void start(BundleContext context) {
SimpleLogTargetImpl logTargetImpl = new SimpleLogTargetImpl();
registration = context.registerService(
ILogTarget.class.getName(),
logTargetImpl,
null);
}
• Problems
 Code
• Semantics is not clear
 dependencies, properties, implementation v. provided interface
• Simplicity
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Registering service (2)
OSGi overview CZJUG, Prague – May 28, 2008
• Declaratively
 Automated service management by framework
• Declarative service
• Dependency injection of required services
 Services provided by components
• Component
 Entity providing/requiring exactly specified interfaces
 Optional, mandatory, collection interface
<component name=\"logger-component\">
<implementation class=\"cz.cuni...LoggerImpl\"/>
<service>
<provide interface=\"cz...ILogger\"/>
</service>
<reference name=\"TARGET\"
interface=\"cz...ILogTarget\"
bind=\"addLogTarget\"
unbind=\"removeLogTarget\"
cardinality=\"0..n\"
policy=\"dynamic\"/>
</component>
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Service consuming
OSGi overview CZJUG, Prague – May 28, 2008
• Bundle can search for service which implements
specific interface (defines semantics of the
service) Service reference ref =
context.getServiceRef(“cz.bar”);
• Several bad solution if (ref!=null) {
Bar bar = (Bar) context.getService(ref);
if (bar != null) {
 context.getService(...) ...
context.ungetService(ref)
• Nasty code }
}
 Listeners
• Just inform about changes
• Components <component name=”getServiceComp”>
<implementation class=”GetLoggerService”>
<reference name=”log”
 Declare getter component interface=”org.osgi...LogService”
bind=”setLog”
• Service tracker unbind=”unsetLog”
</component>
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Service tracker
OSGi overview CZJUG, Prague – May 28, 2008
• Tracking for service
 Filters (name, id, property, owning bundle, ...)
• LDAP syntax (e.g. (&(objectName=”foo”)(property1=”Xyz”)) )
 Adding service to registry, removing, update
//In Bundle Activator - start Name of service to track
tracker = new ServiceTracker(context,
ILogger.class.getName(),
null);
tracker.open(); ServiceTrackerCustomizer
- customize behavior of adding,
// get the service(s) removing, modifying the service
ILogger log = (ILogger)tracker.getService();
Ilogger log = (Ilogger)
tracker.waitForService(1000);
// stop tracking
tracker.close();
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Whiteboard pattern
OSGi overview CZJUG, Prague – May 28, 2008
• Services dependencies
 Content provider v. content consumers
• e.g. Register new service if and only if the specified service
appears
 “Don't look for content providers, let them to register as
services and track for the services”
 ServiceTracker capture service life-cycle
• via ServiceTrackerCustomizer
 Capture process of adding/removing/modifying service
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Example
OSGi overview CZJUG, Prague – May 28, 2008
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Services
OSGi overview CZJUG, Prague – May 28, 2008
• Http
 Exposing registered servlets
• Event
 Messaging Producer <-> Consumer
• Device manager
• Diagnostics/Monitoring
 JMX (in Equinox sandbox)
• Application manager
 Application package – set of resource (bundles, data,...)
• Can be deployed/install
• Location/measurement services
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Security layer
OSGi overview CZJUG, Prague – May 28, 2008
• Optional
• Based on Java 2 security architecture
• 4 roles
 Developer
• Adds local permissions to the bundle by signing
 Admin, Service, Bundle permissions
 Deployer
• Signs the bundle and deploys it
• Resulting bundle permissions = intersection of bundle and
target environment permissions
 Operator
• Full control all the time
 End user
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Issues of OSGi R4
OSGi overview CZJUG, Prague – May 28, 2008
• No repository defined
 No single trusted point for downloading bundles
• Each implementation has its own OBR (OSGi bundle
repo.)
 No automatic download of required bundles/packages
 Currently just RFC-112
• Searching by capabilities
 Planned in OSGi R5
• Bundle dependency resolver
 Resolving bundles can take long time
 Maximal sharing of packages
• Stale service problem
 Services can exchange objects connected to a bundle classloader.
What happens when this bundle is stopped? 23

Related projects
OSGi overview CZJUG, Prague – May 28, 2008
• rOSGi
 Access services in a remote OSGi
• Glassfish v3
 OSGi replace HK2 (module subsystem)
 Uses Apache Felix
• SpringDM (Spring-OSGi)
 Integration of OSGi inside Spring
• Spring application platform
 Similar concept to OSGi, cooperates with it, reuses bundles
 Defines bundles repository
• JSR 277
 Java Modules, proposed OSGi interoperability
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Q&A
OSGi overview CZJUG, Prague – May 28, 2008
Thank you!
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