Introduction in OSGi
2009
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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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Imtech ICT Belgium
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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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Agenda
 Modularity in Java
 OSGi Basics
 OSGi Best Practices
 Conclusion
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Modularity in Java
 Modularity Matters
– Component reuse
– Consistency
– Efficiency
– Robustness
– Maintainability
– Scalability
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Modularity in Java
 Different approaches:
– OSGi
– HK2 (Apache Harmony)
– JSR 277/294
– JSR 232/291
– Spar
– Jigsaw project (only JDK)
– …
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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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Agenda
 Modularity in Java
 OSGi Basics
 OSGi Best Practices
 Conclusion
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OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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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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What is OSGi
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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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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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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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Classloading in OSGi
 Default linear parent first delegation model
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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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Classloading in OSGi
 Tree parent delegation model:
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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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Classloading in OSGi
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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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Classloading in OSGi
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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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Classloading in OSGi
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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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Classloading in OSGi
 OSGi Delegation Rules:
– Parent Delegation Model is used if:
– Otherwise Bundle Delegation Model is used
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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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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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Classloading in OSGi
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Classloading in OSGi
 Constraint solving: an example
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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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Classloading in OSGi
 Bundle Delegation
Model (Runtime)
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Classloading in OSGi
 The total picture:
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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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Classloading in OSGi
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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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OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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The Bundle Lifecycle
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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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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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The Bundle Lifecycle
 Uninstall Model V1 Bundle
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The Bundle Lifecycle
 Model V1 Bundle is marked for deleting
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The Bundle Lifecycle
 Install & Start SecondFrontV1 Bundle
– (imports be.ebit.bejug.model)
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The Bundle Lifecycle
 RefreshPackages (or restart framework)
– Resolution (wiring) process is redone
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OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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OSGi Services
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OSGi Services
 Services
 Service interface
 Service implementation
 ServiceFactories
 ServiceRegistry
 Registration/Unregistration
 ServiceReference: search base
 Service Events/Listeners
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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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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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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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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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OSGi Basics
 What is OSGi
 Classloading in OSGi
 The Bundle Lifecycle
 OSGi Services
 Demo
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Demo
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Demo
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Agenda
 Modularity in Java
 OSGi Basics
 OSGi Best Practices
 Conclusion
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OSGi Patterns & Best Practices
 OSGi Patterns
– Extender Pattern
– Configuration Fragment Bundles
– Whiteboard Pattern
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Extender Pattern
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Configuration Fragment Bundles
 Examples:
– Log4J configuration,
– Custom configuration of the spring extender bundle
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Whiteboard Pattern
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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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Conclusion
 OSGi is a platform full of potential:
– Enhances encapsulation
– Versioning
– Runtime dynamics
– Service-oriented
 A different mindset is needed!
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Q&A
– http://belgium.osgiusers.org
– www.ouforum.be
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Links
 http://www.osgi.org
 http://www.springsource.org/osgi
 http://www.dynamicjava.org
 http://belgium.osgiusers.org
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