OSGi has gained popularity over the last two years. The platform has some very interesting features like versioning, dynamic updates and it's service oriented nature. These characteristics however come with a price. A firm understanding of how and why OSGi works how it works, is a necessity if you plan on getting into OSGi. This talk will start with some basic principals on the java platform and will gradually move towards the OSGi infrastructure explaining the OSGi fundamentals. The following topics will be covered: * Classloading in OSGi * Lifecycle management of OSGi bundles * OSGi Service, the service registry and service composition models Afterwards, we will explain the generally accepted best practices and OSGi design patterns.

1. Introduction in OSGi
2009
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2. Speaker’s qualifications
 Lead architect & project manager @ Ebit
 10 years experience in JSE/JEE
development
 Steering member of the Devoxx conference
and BeJUG
 Talks at Bejug & SpringOne
 Prince2 and Scrum certified
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3. Imtech ICT Belgium
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4. 3 Companies – 1 Mindset
Developmen Data
Infrastruct t of Warehousi
ure Software ng
Solutions Solutions
and Business
Services Java Intelligenc
Open Source e
ICT Solutions and Services - Since 1996
Specific and in-depth knowledge – 78
employees
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5. Agenda
 Modularity in Java
 OSGi Basics
 OSGi Best Practices
 Conclusion
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6. Modularity in Java
 Modularity Matters
– Component reuse
– Consistency
– Efficiency
– Robustness
– Maintainability
– Scalability
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7. Modularity in Java
 Different approaches:
– OSGi
– HK2 (Apache Harmony)
– JSR 277/294
– JSR 232/291
– Spar
– Jigsaw project (only JDK)
– …
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8. Modularity in Java
 Common problems in Java
– JAR HELL
– No versioning
– The Classpath
– Class visibility
– Hot deployment
– Write once, run anywhere
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9. Agenda
 Modularity in Java
 OSGi Basics
 OSGi Best Practices
 Conclusion
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10. OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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11. What is OSGi
 OSGi is a service oriented platform
 On top of a unique classloading mechanism
 Enabling features like:
– Versioning
– Encapsulation
– Runtime Dynamics
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12. What is OSGi
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13. OSGi Basics
 What is OSGi
 Classloading in OSGi
Welcome to the World of
ClassNotFoundExceptions and
ClassDefNotFoundErrors
 The Bundle Lifecycle
 OSGi Services
 Demo
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14. Classloading in OSGi
 Classloading 1 O 1
– What is a class?
 Compile time
 Runtime
– A classloader defines an isolated class space.
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15. Classloading in OSGi
 How can a classloader share it’s class space
with other classloaders?
 Classloader hierarchy
• Linear hierarchy
• Tree hierarchy
 Classloader delegation model
• Parent first delegation model
• Child first delegation model
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16. Classloading in OSGi
 Default linear parent first delegation model
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17. Classloading in OSGi
 Linear parent first delegation model
 no versioning
 Breaking the linear structure to allow
versioning
public static void main(String[] args) throws Exception {
URL[] urls = ((URLClassLoader)TestApp.class.getClassLoader()).getURLs();
URLClassLoader urlCL = new URLClassLoader(urls, null);
Class personClass = urlCL.loadClass("be.ebit.bejug.model.Person");
Object personInstance =
personClass.newInstance();
Person p = (Person)
personInstance;
}
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18. Classloading in OSGi
 Tree parent delegation model:
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19. Classloading in OSGi
 Moving towards OSGi:
public static void main(String[] args) throws Exception {
//Path to model.jar version 1
URL[] urls = new URL[]{new URL(“<path>/modelv1.jar”)}
//Path to model.jar version 2
URL[] urls = new URL[]{new URL(“<path>/modelv2.jar”)}
URLClassLoader urlCL1 = new URLClassLoader(urls,null);
URLClassLoader urlCL2 = new URLClassLoader(urls,null);
Class personClassV1 =
urlCL1.loadClass("be.ebit.springone.model.Person");
Class personClassV2 =
urlCL2.loadClass("be.ebit.springone.model.Person");
}
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20. Classloading in OSGi
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21. Classloading in OSGi
 How does OSGi fit in?
– ModelV1.jar  Bundle A
– ModelV2.jar  Bundle B
 Each bundle with its own ClassLoader (and thread)
 The non-linear classloader structure allows
versioning within one JVM.
 Basic principle behind the versioning capabilities in
OSGi.
 Cfr. Servlet containers
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22. Classloading in OSGi
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23. Classloading in OSGi
 The parent first delegation model does NOT
allow communication between bundle
classloaders
– The FrontV1 bundle classloader cannot get a
Person class
 Solution?
– Extending the classloading mechanism
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24. Classloading in OSGi
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25. Classloading in OSGi
 Bundle classloaders can delegate
classloading to:
– The parent classloader
 parent first delegation
– Other bundle classloaders
 bundle delegation
 The problem is choice:
– Parent or bundle delegation?
– If bundle delegation: to which bundle classloader?
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26. Classloading in OSGi
 OSGi Delegation Rules:
– Parent Delegation Model is used if:
– Otherwise Bundle Delegation Model is used
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27. Classloading in OSGi
 The Bundle Delegation Model:
– To which bundle classloader to delegate?
– Metadata in the Bundle Manifest to the rescue:
 1 Import/export-packages
– what packages are needed by the bundle.
– what packages are provided by the bundle.
 2 Required bundles
 3 Bundle classpath
 4 Fragment bundles
 5 Dynamic imports
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28. Classloading in OSGi
 How to wire the classloaders?
 Classloader infrastructure
 General wiring resolution rules:
– Ensures that only one version of a class is visible
within a classloader:
 class space consistency
– Uses the highest version within the constraints
specified
 Version ranges
 Custom qualifiers
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29. Classloading in OSGi
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30. Classloading in OSGi
 Constraint solving: an example
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31. Classloading in OSGi
 For each imported package a wire exists to
the providing bundle classloader
 The wires are calculated at bundle resolution
 This wiring is NOT static:
– Bundles can come and go
– Dynamic updates
 This wiring process is repeated (partially)
with each refresh without a JVM restart.
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32. Classloading in OSGi
 Bundle Delegation
Model (Runtime)
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33. Classloading in OSGi
 The total picture:
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34. Classloading in OSGi
 Fine-tuning the choice of delegation model:
– Bootdelegation:
 Additional classes to be loaded by the parent classloader
 System property: org.osgi.framework.bootdelegation
 No versioning
– System packages:
 Exported by the system bundle (bundle 0)
 System property: org.osgi.framework.system.packages
 Versioning
 Takes part in the wiring process
 Last option is preferred
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35. Classloading in OSGi
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36. Classloading in OSGi
 Parent first delegation model:
– No versioning
– Static ‘wiring’
– See-all approach
 Bundle delegation model:
– Versioning
– By default, only the exports are visible
– Support for dynamic updates/dynamic wiring
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37. OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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38. The Bundle Lifecycle
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39. The Bundle Lifecycle
 The Bundle Activator
– Start() & Stop()
– Resource Mgnt, Thread Mgnt, Services Lifecycle Mgnt
 Events:
– Bundle events
– Framework events
 The Bundle Context
– Environment information
– Installing bundles
– Installing services
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40. The Bundle Lifecycle
 Bundle updates, uninstalls & refreshes
– Update: migrate from one version to another
 New exported packages are made available
 Old exported packages remain when used
– Uninstall
 New exported packages are made avialable
 Old exported packages remain when used
– RefreshPackages: The effective update & removal
 A package dependency graph is created
 The exporting bundle & all dependent bundles are stopped
and restarted.
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41. The Bundle Lifecycle
 Uninstall Model V1 Bundle
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42. The Bundle Lifecycle
 Model V1 Bundle is marked for deleting
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43. The Bundle Lifecycle
 Install & Start SecondFrontV1 Bundle
– (imports be.ebit.bejug.model)
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44. The Bundle Lifecycle
 RefreshPackages (or restart framework)
– Resolution (wiring) process is redone
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45. OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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46. OSGi Services
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47. OSGi Services
 Services
 Service interface
 Service implementation
 ServiceFactories
 ServiceRegistry
 Registration/Unregistration
 ServiceReference: search base
 Service Events/Listeners
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48. OSGi Services
 Service lookup is text-based
– Filter (interface + criteria)
 Services can come and go
 Stale references
 Services are fully loaded before registration
 Multi-threading
 OSGi development is NOT trivial!
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49. OSGi Services
 Extension Points:
– Not part of the specification
– Extension points and extensions
– One-to-many
– Lazy loading
– Less support for service dynamics (restart of
eclipse)
 Extension Points doesn’t solve all problems!
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50. OSGi Services
 Easing OSGi Component Development:
– Declarative Services
– Spring DM
– iPOJO
 Seperation of responsibilities:
– Implementation of the service
– Publication of the service
 Two component models in the specification:
– Declarative Services
– RFC 124 (Spring DM-based).
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51. OSGi Services
 Common characteristics:
– Declaration of services
– Creation of services externalized (outside the
bundle)
– Lazy loading
– Service composition
– Service dependencies & lifecycle management
– Dynamic dependencies
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52. OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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53. Demo
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54. Demo
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55. Agenda
 Modularity in Java
 OSGi Basics
 OSGi Best Practices
 Conclusion
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56. OSGi Patterns & Best Practices
 OSGi Patterns
– Extender Pattern
– Configuration Fragment Bundles
– Whiteboard Pattern
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57. Extender Pattern
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58. Configuration Fragment Bundles
 Examples:
– Log4J configuration,
– Custom configuration of the spring extender bundle
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59. Whiteboard Pattern
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60. OSGi Patterns & Best Practices
 Best Practices:
– Use imports instead of the parent classloader
(except java.*)
– Minimize dependencies
– Hide implementation details
– Start ordering dependencies
– Thread safety
– Framework Callbacks
– Classloader hierarchy dependencies
– OSGi Framework coupling
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61. Conclusion
 OSGi is a platform full of potential:
– Enhances encapsulation
– Versioning
– Runtime dynamics
– Service-oriented
 A different mindset is needed!
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62. Q&A
– http://belgium.osgiusers.org
– www.ouforum.be
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63. Links
 http://www.osgi.org
 http://www.springsource.org/osgi
 http://www.dynamicjava.org
 http://belgium.osgiusers.org
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