Dependencies, dependencies, dependencies
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The document discusses OSGi dependency management and design patterns, highlighting key concepts such as components, services, and various types of dependencies. It introduces the Dependency Manager, a subproject of Apache Felix, nearing its 3.0 release, which offers improved functionality for managing service dependencies in OSGi applications. Lastly, the document emphasizes building higher-level abstractions and utilizing design patterns to simplify interactions with the OSGi framework.
Dependencies, dependencies, dependencies
- 1. Dependencies, dependencies, dependencies Marcel Offermans TECHNOLOGIES
- 2. Marcel Offermans • Fellow and Software Architect at Luminis Technologies marcel.offermans@luminis.nl ! •Member and Committer at Apache Software Foundation marrs@apache.org
- 3. Agenda • Introduction • Basic Concepts • Dependencies • Design Patterns • Custom Dependencies • Add-ons • Wrap-up Software distribution: copy both zip archives from the memory stick
- 4. Introduction
- 5. Framework Service Life$cycle Module Security OSGi$framework Bundle Bundle Bundle Java$Virtual$Machine
- 6. Service Dependencies • framework: ServiceListener – notification whenever there is a change in the service registry – but: only changes that occur while you are listening • utility: ServiceTracker – solves the listening issue – adds ability to customize services
- 7. Problem • using the framework supplied tooling, you are dealing with dependencies at a very low level • a lot of boiler plate code is needed to implement real world scenarios • there are very few design patterns that deal with composing an application based on services
- 8. Declaring your dependencies • Service Binder • Dependency Manager • Declarative Services • iPOJO • Blueprint • ...many more
- 9. Dependency Manager • Subproject of Apache Felix • Nearing a 3.0 release • Some interesting new features ...I’m not unbiased, being it’s author
- 10. Basic Concepts
- 11. Basic Concepts • Component: class that implements certain behaviour (a POJO) • Dependency: something our component needs or wants (ie. a service) • Service: OSGi service interface(s) under which our component is registered
- 12. Declarative API • Declarative ≠ XML • Using a Java API has many advantages: – less error prone because of syntax and compile time checking – refactoring support, completion – very readable through fluid API – everything in one place, no magic
- 13. On the memory stick: ! a) an Eclipse update site for BndTools b) a set of Eclipse projects Example code • Projects – Download: http://www.xs4all.nl/~mfo/projects.zip • Uses Eclipse + BndTools – Homepage: http://njbartlett.name/bndtools.html – Update Site: http://bndtools-updates.s3.amazonaws.com • Based on a snapshot release of the upcoming Dependency Manager 3.0 • During the presentation, we will switch between slides and Eclipse to show running examples
- 14. Using the Dependency Manager Import-Package = org.apache.felix.dm;version="[3.0,4)" public class Activator extends DependencyActivatorBase { public void init(BundleContext context, DependencyManager manager) throws Exception { manager.add(createComponent() .setImplementation(new HelloWorld()) ); } ! public void destroy(BundleContext context, DependencyManager manager) throws Exception { } }
- 15. Basic Use Cases • Declare a component HelloWorld helloWorld = new HelloWorld(); manager.add(createComponent() ! ! ! • Declare it lazily .setImplementation(helloWorld) ); manager.add(createComponent() .setImplementation(HelloWorld.class) );
- 16. Component Life Cycle •methods of the component public static class HelloWorldLifeCycle { ! ! ! • setCallbacks(“init”, “start”, ...) setCallbacks(inst, “init”, “start”, ...) private void init() { System.out.println("init"); } private void start() { System.out.println("start"); } private void stop() { System.out.println("stop"); } private void destroy() { System.out.println("destroy"); } } – to invoke the methods on ‘inst’ • ComponentStateListener – if you want to listen from the outside
- 17. Declaring as a service • setInterface(...) – allows you to declare multiple services – allows you to specify service properties manager.add(createComponent() .setInterface(LogService.class.getName(), new Properties() {{ put(Constants.SERVICE_RANKING, 20); }}) .setImplementation(MyLogService.class)
- 18. Declaring Dependencies • Adding a dependency manager.add(createComponent() ! ! ! • Injects dependency .setImplementation(HelloWorldLogger.class) .add(createServiceDependency() .setService(LogService.class) ) ); – uses null object pattern – injects other “OSGi” instances • setCallbacks(...) setCallbacks(inst, ...)
- 19. Dependencies
- 20. Dependencies • Different types: – Service Dependencies – Configuration Dependencies – Bundle Dependencies – Resource Dependencies
- 21. Configuration Dependencies • Based on Configuration Admin – designed for required dependencies – service.pid to identify the configuration – allows you to only accept *valid* configurations •update() throws ConfigurationException
- 22. Bundle Dependencies • Depend on a bundle: – in certain states – with certain manifest entries • Bundle instance can be injected
- 23. Resource Dependencies • Resources are modeled as URLs • Are provided by a repository – another bundle – an Eclipse workspace – some external source • Filter on host, port, protocol, path and URL
- 24. Design Patterns
- 25. Design Patterns •Moving up a level in abstraction • OSGi is too low level to expose to everybody, but it is a great platform to build on • Patterns provide a common language and describe solutions in context
- 26. Overview of Design Patterns •Whiteboard Pattern • Null Object Pattern • “Singleton” Service • Aspect Service • Adapter Service • Resource Adapter Service
- 27. Whiteboard Pattern “don’t call us... we’ll call you”
- 28. Null Object Pattern • An object that implements a certain interface, can be safely invoked and does nothing
- 29. “Singleton” Service • Publishes a component as a service • Ties its life cycle to that of its dependencies Document Repository get() getVersions() get(version) store(Document) Singleton Storage requires
- 30. Aspect Service •Works at the service level • “intercepts” certain services • can be chained based on rankings • completely dynamic Repository Cache get() getVersions() get(version) store(Document) Aspect Repository get() getVersions() get(version) store(Document) intercepts
- 31. Adapter Service •Works on existing services • Adapts them, publishes a different service for each existing one Repository Cache Manageable getCacheHits() setSize() setTTL() flush() Adapter Repository Cache get() getVersions() get(version) store(Document) adapts
- 32. Resource Adapter Service • Adapts resources (instead of services) • Allows you to expose the behavior of certain resources instead of their “inner guts” Audio Track play() pause() stop() Resource Adapter MP3 File adapts
- 34. Custom Dependencies • Dependency Manager is extensible with custom dependencies: – depend on time of day – depend on some custom instance / condition (start level, app state)
- 35. Add-ons
- 36. Add-ons • Shell (Felix, Equinox, Gogo) • Annotation based (Java 5 required) • Legacy support (2.x API adapter)
- 37. Wrap-up
- 38. Wrap-up • Points to take away: – do not expose every developer to the OSGi API – build higher level abstractions, use design patterns – consider the dependency manager, it is very flexible
- 39. More about OSGi • ApacheCon 2010 North America November 1-5, Atlanta – OSGi tutorial – full day OSGi track •Masterclass on OSGi October 12-15, Girona – Neil Bartlett and Peter Kriens