Just a brief introduction I have given my team to get into the OSGi topic. Guess it is not the highest academic level, but should be enough to understand the fundamentals of OSGi.

1. OSGi
Create *real* modular
Java applications
- a brief introduction -
By Jeffrey Groneberg
@inkvine
jeffrey.groneberg@gmail.com

2. Table of Contents
• About me
• What is OSGi?
• Class Loader – Problems with “Twins”
• It’s all about bundles
• OSGi architecture and layers
• Services, Services, Services – Micro-SOA
• Building components with OSGi
– SCR
– SpringDM
• Tools
• How to learn?

3. About
me

4. About me
• Working at SAP AG as a developer for mobile healthcare solutions
(Android & iOS)
• MSc Computer Science
• Java enthusiast
• API creator ;)
• Reading and interested in everything related to technology
• Twitter: @Inkvine
• Mail: jeffrey.groneberg@gmail.com

5. What is
OSGi?

6. What is it!?
• Actually OSGi is not a framework
• Open Service Gateway initiative
• OSGi is a specification (http://www.osgi.org/Main/HomePage)
• Originally intended to be used within the Embedded-Device-Section
(Cars, Facility, Home-Environment)
• Extremely lose coupled and highly extendable (Communication via
services)
• Replace modules during runtime or extend (hard- & software)
• Extremely lightweight (limited resources on embedded devices)

7. Car example
Uses ABS-
Brakes
System
(Front)
(v1)
Fallback
Uses ABS-
Brakes
System
(BAck)
(v2)

9. Ever used ECLIPSE?
• Eclipse is completely build on OSGi
• Eclipse is running within an OSGi container
• Every time you install a plugin via “Update/Install”
an OSGi bundle is downloaded and installed
• OSGi is a specification therefore Eclipse uses a special implementation
• Container = Implementation running within the JVM
• OSGi = Java !
• Different implementations available (for different requirements)

10. Different Implementations
• A spec needs to be implemented. Therefore different implementations
are available on the market
Name Open Source Description
Equinox Yes Most spread OSGi container. Used in
Eclipse and so called “reference
implementation”
Felix Yes Apache implementation, formerly known
as “Oscar”
Knoplerfish Yes Originally used within embedded systems
now ported to “every day” usage.
mBedded Server No Commercial implementation with official
support and maintainance

11. Classloader
And Problems
with twins

12. The Java Classloader approach (1/2)
Bootstrap
classloader
Delegates to parent
Parent
Extension
classloader
Delegates to parent Parent
Application
Classloader

13. The Java Classloader approach (2/2)
• Bootstrap classloader:
all the java core libs that are located in $JAVA_HOME
• Extensions classloader:
lib/ext directory
• Application classloader:
all libs within the started application
• Every classloader asks his parent if he has already loaded the needed
class. If he receives a “no” he has to take care of it by himself.
• No twins possible!

14. JA A is stupid to identify twins
V
Load order
A D F
B C E
Jar 1 Jar 2 Jar 3
Classes:
First loaded – first seated!!
Merged
- Different shape means different class
- Same shape means similar class
(but different implementations)
A D
F
B

15. OSGi CLASSLOADER (1/2)
• “Non hierarchical” classloader
• Every component in OSGi is a bundle (JAR-file that contains
metadata)
• Every bundle has its own classloader (sandbox – no sight to outer
world)
• Classloader asks OSGi container for references
• If restrictions (given by the metadata) are fine the class will be
provided by another component and its classloader
• --> classloader chaining

16. OSGi CLASSLOADER (2/2)
Class not found
Component Component
classloader classloader
Classloader chaining
(if allowed)
Class loading Class resolution Class loading
OSGi component OSGI component
Classloader
isolation

17. It’S all
about bundles

18. AN osgi Bundle
• Physically it’s a JAR-File that contains:
Implementations & metadata
• Metadata (the MANIFEST.mf-File within the JAR) allows fine granularly
definitions of (just the most important definitions):
– What packages (classes) are needed that the bundle is able to run
– What packages (classes) are visible (and therefore exposed) to other
bundles within the OSGi container
– Which version of packages the bundle exposes
– Which version of packages the bundle needs to import
• Every bundle has to be installed within the OSGi container

19. EXAMPLE MANIFEST.MF
Bundle-Name: Toast Emergency
Bundle-SymbolicName:
org.equinoxosgi.toast.client.emergency
Bundle-Version: 1.0.0.qualifier
Bundle-RequiredExecutionEnvironment: J2SE-1.4
Bundle-Activator:
org.equinoxosgi.toast.client.emergency.Activator
Import-Package: org.equinoxosgi.toast.dev.airbag,
org.equinoxosgi.toast.dev.gps, org.osgi.framework;

20. Visibility
• Information Hiding
– Hide classes used internally and expose classes for reusability
– Encapsulate functionalities by interfaces, but hide implementations
• Best practice
– One bundle for the public API that exposes the package containing the
interfaces
– One bundle for the implementation
Export-Package: org.equinoxosgi.toast.dev.airbag

21. Dependencies
• Declare dependencies (What is needed for the bundle to be executed)
• Versions can be added easily (“I am running on legacy code”)
Import-Package: org.equinoxosgi.toast.dev.airbag,
org.equinoxosgi.toast.dev.gps, org.osgi.framework;

22. LIFE CYCLE AND STATES
• Every bundle has to be installed within the container
• It passes different states:
– Validating dependencies
– Exposing packages
– Checking versions
• States are:
– installed
– resolved
– Uninstalled
• A bundle has callback methods to react to the loss of dependencies
– Programmer has to take care of it

23. OSGi
Architecture
& Layers

24. Layers (1/2)
Services
Service
Registry
Security
Bundles Lifecycle
Modules
JVM
Operating System

25. Layers (2/2)
• Security
– Adopts the Java Security Standard (optional)
• JVM
– Different JREs can be supported therefore a kind of representation needs to be provided
within the OSGi container
• Modules
– All the bundles that are added to OSGi container
• Services Registry
– Functionalities can be provided within the OSGi container as services. The registry takes
care of registering and exposing those services and notifies bundles that consumes those
• Services
– OSGi provides (based on the implementation) services out of the box (HTTP e.g.).
• Life Cycle Management

26. Services,
services,
services
Micro SOA

27. What is a service?
“A service is a normal Java object that is registered under one or more
Java interfaces with the service registry. Bundles can register services,
search for them, or receive notifications when their registration state
changes.”

28. SOA

29. OSGi SOA
• Create a bundle with the interface (the public API) of your service
• Export the package containing the interface
• Create a bundle with the implementation of the interface (import the public API
package)
• Reference the OSGi Service Registry and deliver your implementation for the given
interface
• Reference the Service Registry and ask for an implementation for a given interface
• The implementation is always hidden to the consumer (lose coupling)

30. Two WAYS of service implementations
• Program the whole registration/referencing and state change listeners
by your own
– Think about it: Services can come and go or are never there. What shall your
bundle do if this happens?
– A lot of code to write
• Use Inversion of Code and Dependency Injection
– SpringDM or Declarative Services
(formerly known as SCR) (will be shown later)

31. Doing it the hard way
the service implementation

32. Doing it the hard way
The consumer

33. DOING it the hard way THE Service Tracker (1/2)

34. DOING it the hard way THE Service Tracker (2/2)

35. Downsides of the hard way
• Starting time is extended due to a lot of “management” code within the activator
– Think about a lot of bundles where each bundle runs a lot of initializing code
– The application takes a lot of time to start
• Allocations during runtime
– Trackers or other “flags” needs to be initialized to handle services even though the service
is never there
• Complexity
• As a programmer I do not want to take of all this stuff:
– Give me a service if it is there
– Take a service if I provide you one
– Declarations > Programming

36. Building
components
witH OSGI

37. CHOOSE WISELY
OSGi Alliance
with
SCR
“YOU SHALL NOT PASS! UAAH!!”
and btw: SpringSource
“One does not simple build an iOS framework” with
SpringDM

38. THE SCR
• By the OSGi Alliance
• A component consumes and/or provides services
• Using the Service Component Runtime
• SCR = Declarative Services
• SCR is a bundle that is installed within the OSGi container
– Extender Pattern
– Scans all bundles for metadata containing the component definition
(declaration)
– Registers provided services from bundles in the OSGi Service Registry
– Automatically binds services to consumers

39. What is an OSGI SERVICE component?
Bundle Bundle
Component Component
Component instance Component instance
Component declaration Component declaration
Provides Consumes
Services

40. SCR by example THE provider
• A component implementing a given API (interface) and registering
itself as service
Bundle structure Public API
component.xml

41. SCR BY Example THE CONSUMER
• A component consuming a service by a Public API (interface)
The consumer
component.xml

42. SPRING DM
• From SpringSource
• A lot more than just a component declaration
– SpringMVC
– Spring Security
– AOP
– Spring WS/RS
– Dependency Injection with Beans (most powerful feature)
• Not „component“, it is called „Bean“
• Using Extender Pattern
• Bundle within the OSGi container

43. Spring DM
Bundle with Spring config
Bean beans1.xml Searches for
beans2.xml
Bean uses
Bean Spring container
creates
Provides Consumes
Spring DM Extender
Configures and creates
Services

44. Spring DM Example The Provider
Bundle structure
Public API (interface)
Implementation
Beans declaration file
OSGi service declaration

45. SPRING DM EXAMPLE THE CONSUMER
Consumer
OSGi services declaration
Beans declaration

46. Tools

47. What do i need (not all)?
• Eclipse (all out of the box)
• SpringSource ToolSuite (my favorite Eclipse distribution)
• Maven (bundles with bundles need other bundles, that need bundles)
• SpringSource Enterprise Repository
• Virgo
– OSGi based application server
• Tomcat
– You need an OSGi Servlet Bridge to launch the OSGi Container within a web
application
• LeanDI? !

48. How to
learn OSGi?

49. I want to OSGI“
• Buy the book „OSGi & Equinox“
• Install SpringSource ToolSuite
• Install the book‘s plugin
• Work through the examples
• Buy „OSGi in Action“ as a great reference
• Buy „SpringDM in Action“
– Pure hardcore geek food
– If you understand everything you can call yourself „the shiat“

50. Great and what do I get from osgi
when programming non-java“?
• Think OSGi!
• Divide API and implementation
• Hide implementations behind registries and services
• Highly reusable components (technical)
• Seperate domain from the rest of the application
• A LOT of OSGi paradigms are adopted to our EMR Android app and will
„travel“ to iOS soon !

51. Thanks!
ANY QUESTIONS?

52. Disclaimer
• All photos/pictures within this slides are provided from flickr.com and
licenced under CC for commercial use.
• The code samples can be found in the books „OSGi & Equinox“ and
„SpringDM in Action“
• If you want to use these slides in any lecture do contact me before, pls.