A seminar presented in Damascus University faculty of Information Technology about the OSGi technology in software engineering.

1. OSGi
SOFTWARE ENGINEERING SEMINAR

2. Overview
•What is OSGi?
•Architecture
•Module Layer
•Lifecycle Layer
•Service Layer
•OSGi Specifications
•OSGi Framework Implementation
•Projects using OSGi
•Pros and Cons
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3. What is OSGi?
•First appeared in 1999.
•OSGi is a group of standards for writing modular applications on top of the JVM.
•You develop your application as set of modules.
•These units are then deployed into the runtime.
•Your application, if written in a reasonable way, may be extendable by other modules.
•OSGi provides a service oriented development model allowing for SoA within a virtual machine.
•As a simple example – Eclipse is an OSGi based application.
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4. Architecture
•Bundles: Normal jar components with extra manifest headers.
•Services: The services layer connects bundles in a dynamic way.
•Services Registry: The application programming interface for service management.
•Life-Cycle: (install, start, stop, update, and uninstall) for bundles.
•Modules: The layer that defines encapsulation and declaration of dependencies.
•Security: The layer that handles the security aspects by limiting bundle functionality to pre-
defined capabilities.
•Execution Environment: Defines what methods and classes are available in a specific platform.
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5. Architecture
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6. Module Layer
•OSGi module definition is the bundle.
•OSGi’s core component is the Bundle.
•Bundles are structurally similar to a JAR with additional metadata.
•Modules promote code reuse.
•Modules promote high cohesion.
•Modules enforce logical boundaries for code location.
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7. Lifecycle Layer
•Provides the ability to dynamically install and manage bundles on the OSGi framework.
•Provides the interaction between bundles by allowing bundles to register with the framework
and gain access to the execution environment.
•Provides flexibility to architect a system with the knowledge that components can be updated,
removed or plugged-in without experiencing a system outage.
•OSGi containers implement the lifecycle specification.
•Containers provide command line and web based interfaces for administration of the container.
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8. Lifecycle Layer
•Installing Bundles:
•Bundle Installed.
•Bundle dependencies are resolved (remain in installed state
if missing dependencies)
•Bundle can be refreshed.
•Bundle uninstalled.
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9. Lifecycle Layer
•Starting Bundles:
•Once a bundle has resolved its
dependencies it is ready to start.
•A Bundle will be “activated” and
transition to an active state.
•An installed bundle cannot
become active if it depends on
another bundle that cannot resolve.
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10. Service Layer
•OSGi services enable a single-JVM service oriented architecture.
•OSGi services follow a publish, find and bind paradigm:
•Services are published to the service registry.
•Clients search the registry for needed/available services depending on the need.
• A service embodies a contract between a provider and a caller.
•Promotes loose coupling and code reuse.
•In OSGi a service can be an interface or a superclass, but the preferred methodology is to code
to an interface.
•Services provide a reliable way for bundles to communicate.
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11. Service Layer
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12. Service Layer
•Publishing/Registering Services:
• Services are described and published through interfaces.
• Services are registered by a bundle in order to make the API of the service available.
• Registration done through a simple method call.
•Finding and using services:
• OSGi provides the ServiceReferences interface that defines two methods:
• getRegisteredServices: Returns a list of services.
• getServicesInUse: Gets services being utilized by other bundles.
• OSGi provides the ServiceTracker class, a dynamic class that gets services based on events during
runtime.
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13. OSGi Specifications
•Core specification: Module Layer, Life Cycle Layer, Resource API, …
•Compendium Specification: HTTP Service, Log Service, User Admin, Wire Admin, …
•Enterprise Specification: Remote Services, JDBC
•Mobile Specification: Metatype Service, Foreign Applications Access
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14. OSGi Framework Implementations
•Apache Felix
•Concierge
•Eclipse Equinox
•Eclipse Gemini
•Hitachi Super J Engine
•Knopflerfish
•ProSyst
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15. Projects using OSGi
•Dotcms
•Eclipse
•Netbeans
•GlassFish
•Intellij
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16. Pros and Cons
•Reuse components to build and manage your highly complex systems.
•Make code easier to write, test and reuse.
•Detect bugs earlier.
•Deploy remotely.
•Detect and solve problems you might not be aware of right now.
•Developing with OSGi is a bit more complex.
•You need to think more about how you design your system.
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