Many people are drawn into OSGi because it provides class loaders on steroids, however, one of the often overlooked feature of OSGi is the Service Registry and the (µ)service model. µServices provide a much more powerful alternative to the traditional Java factories and listeners in your code or xml, which are the usual culprits for class loading headaches. Alas, you need to migrate your application to OSGi first in order to take advantage of µServices which would make it so much easier to migrate your application to OSGi in the first place. The idea of PojoSR (a.k.a OSGi Lite) is to remedy this situation by providing OSGi without the module layer. PojoSR is based on Apache Felix and runs in any standard Java environment: from the class path, inside a WAR, wherever your current Java runs because it never touches a class loader. It allows any application built in Java to reap the benefits of service-based modularity without first having to rid existing code of any class loader tricks. This approach also allows, many existing bundles work out of the box - just by adding them to the class path (PojoSR will detect bundles on the class path and execute their activators). It works so well that OSGi is thinking of standardizing this approach. This talk will show you how you can take advantage of µServices using PojoSR and present some examples where µServices and PojoSR wore used to modularize and OSGi'fy an existing application.

1. µServices For the Rest of Us
Karl Pauls
Luminis
21.09.2011, 12:40-13:30
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2. Karl
• Member Apache Software Foundation
• PMC: Felix, Sling, Incubator
• PPMC: Ace, Clerezza, Celix
• Fellow at Luminis
• Project Owner PojoSR
• Co-Author of “OSGi in Action”
• karl.pauls@luminis.eu
• karlpauls@gmail.com
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3. Luminis
http://luminis.eu
http://luminis-technologies.com
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4. HISTORY
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5. Agenda
• OSGi bootcamp
• OSGi µServices
• µServices For the Rest of Us
• Use Cases
• Implementation / Demo
• Closing Remarks
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6. OSGi
BOOTCAMP
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7. OSGi bootcamp
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8. Define Modules ...
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9. ... Compose Modules ...
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10. ... with OSGi
• L3 – Provides publish/find/bind
service service model to decouple
bundles
• L2 – Manages the life cycle of
life cycle bundle in a bundle cache
without requiring restarts
• L1 – Creates the concept of
module bundles that use classes from
each other in a controlled way
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11. OSGi
µSERVICES
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18. WHITEBOARD
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19. EXTENDER
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20. SERVICE COMPONENTS
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21. OSGi
µSERVICES FOR THE REST OF US
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27. OR CAN YOU?
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28. PojoSR (a.k.a OSGi Lite a.k.a lite a.k.a ???)
• OSGi µServices For the Rest of Us
• OSGi without the module layer
• Based on Apache Felix
• Runs in standard Java environment
• Doesn‘t create class loaders
• Apache License 2
• Exposes Service Registry to normal Java
• No need for bundles
• But bundles are supported up to a point
• Missing module layer
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29. USE CASES
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30. Common discovery (SPI)
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32. Migration
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33. Migration with and without PojoSR
• Start using Service first!
• Split into bundles later and work on trunk!
• Without PojoSR
• Turn application into one big bundle
• Split into several bundles
• Fix problems
• Split even more bundles (goto split)
• Eventually, start using services
• Allows to remove ugly hacks and crazy meta-data needed
to fix problems
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34. Use OSGi where you can‘t
OSGi (lite) on Google App Engine using PojoSR
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35. USAGE
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37. As an OSGi „light“ framework
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38. Standalone
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39. IMPLEMENTATION
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40. • Classloader.getResources(META-
INF/MANIFEST.MF)
• Find bundle on the classpath
• Provide Bundle Revision based on jar:
and file: url schemas (extendable)
• Use and expose Apache Felix service
registry for the rest
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41. DEMO
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42. CLOSING REMARKS
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43. Benefits and Drawbacks
• PojoSR provides part of the power of OSGi
• In a non-intrusive way.
• Allows to increase modularity
• without first ridding an existing code base of class loader hacks
• Drawbacks are
• Does not enforce module boundaries
• Does not allow multiple versions of the same package
• and other module layer features (so some are supported)
• Not support the Bundle-Classpath
• But you can use µServices to get rid of class loading hacks over time,
after which it will be easier to move to OSGi and get side by side
versioning and real module boundaries.
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44. http://pojosr.googlecode.com
QUESTIONS
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