This document provides an overview of OSGi and modularity in Java. It discusses how OSGi addresses some of Java's limitations regarding modularity through the use of bundles and services. Key points include: Bundles define modules through a MANIFEST file and lifecycle methods; services allow for loose coupling between bundles; and Declarative Services simplify working with OSGi by declaring component configuration in an XML file rather than programmatically. The document also demonstrates OSGi concepts through code examples and encourages best practices for dynamism, versioning, and dependency management.

1. OSGi Training
Aruna Karunarathna
Senior Software Engineer

2. Modularity in Java

3. Why Modularity
o Ease of development (interactions are clearly defined)
o Pluggable modules
o Reusable modules

4. Is Java providing the modularity?
o Compile time vs Runtime difference
o Compiled and run under single/flat classpath
o Alternatives
o Write custom classloaders / non standard way to handle

5. Java Classloading...
http://www.techjava.de/topics/2008/01/java-class-loading/

6. Java Compile Time vs Runtime...
axiom-1.2.0.jar axis2-1.4.5.jar log4j.1.4.1..jar saxon.14.1..jar algpr.1.3.2..jar spring.0.2.1..jar
spring-core.0.34.1..jar abdera_1.0.0..jar ajaxtags_1.3.0.beta-rc7-.jar annogen_0.1.0..jar antlr_3.2.0..jar apache-zookeeper_3.3.4..jar

7. JAR (Java Archive)
o Development units in java
o Application contains several jar files
o At runtime there is no real meaning for a jar file
o All the developed Jar files treated as a single, ordered,
global list which is called the classpath.

8. JAR (Java Archive) cntd..
o Problems with JAR Files
- Multiple versions of JAR files cannot be loaded at same time
- A JAR cannot declare dependencies on other JARs.
- No mechanism for information hiding
- Jars cannot be treated as modules.
o Hence Java is suffering from modularity.. :(

9. Dynamism in Java
o Can a Jar file updated while the application is running?
o Can new functionality be added while the application is
running? (Update a class file with new version)
o The application (JVM), needs to be restarted.
o Java lacks Dynamism.

10. OSGi for the rescue?
o Widely accepted way (A specification) of developing modular
and dynamic systems using Java [1].
o OSGi Framework implementations (Open Source)
o Equinox
o Knopflerfish
o Apache
[1]. https://www.osgi.org/developer/specifications/

11. Bundles
o Bundles are the development unit of OSGi
o Typical OSGi application consists with multiple bundles
o Bundles can share packages and hide packages in a consistent
manner
Most Importantly
o What changes there in bundles, when comparing to JAR files?

12. MANIFEST.MF
o Use some standard MANIFEST Headers.
o Defines the bundle’s identity
o Specifies the packages on which
this bundle depends
o Specifies the packages on which this
bundle exports
o Declares additional human-readable
information
Manifest-Version: 1.0
Bnd-LastModified: 1405099135557
Build-Jdk: 1.7.0_55
Built-By: aruna
Bundle-Activator: org.sample.Activator
Bundle-ManifestVersion: 2
Bundle-Name: sample
Bundle-SymbolicName: sample
Bundle-Vendor: WSO2 Inc
Bundle-Version: 1.0.0
Created-By: Apache Maven Bundle Plugin
Export-Package:
org.sample;uses:="org.osgi.framework";
version="1.0.0"
Import-Package:
org.osgi.framework;version="[1.7,2)"
Tool: Bnd-2.1.0.20130426-122213

13. Bundles cntd..
o Standard Java - Place Jar files in classpath
o OSGi Framework - Install bundles to framework

14. Java class loading with OSGi...

15. Lifecycle State Diagram

16. DEMO
o OSGi Framework
o First Look at a Sample Bundle
o Try out the lifecycle events
of a bundle
http://www.slideshare.net/rossmason/realtime-apis

17. Bundle Activator Manifest Entry
o Bundle Activator Manifest Entry is the hook to the Bundle execution entry
point.
o Similar to the Main Method of a Jar file?..
o Header value specifies the name of a reachable class
o Either a class in an Imported Package
o A class in the Bundle class path
o This class implements the org.osgi.framework.BundleActivator interface, and
implements the following two methods.
o Can use these two methods to customize the behaviours when the bundle is
started and stopped.
public interface BundleActivator {
void start(org.osgi.framework.BundleContext bundleContext) throws java.lang.Exception;
void stop(org.osgi.framework.BundleContext bundleContext) throws java.lang.Exception;
}

18. Bundle Activator Manifest Entry cntd...
o When the bundle installed and started, OSGi Framework creates an instance of
Bundle Activator class and call the start() method.
o When the bundle stop is initiated, OSGi Framework call the stop() method.
o In practice when the stop is called, you should undo everything you did in
the start method.
o The activator instance on which start() is called is the same instance
on which stop() is called.
o The activator instance is discarded and isn’t used again, after stop()
is called.
o If the bundle is subsequently restarted after being stopped, a new
activator instance is created, and the start() and stop() methods are
invoked on it as appropriate.

19. Bundle Context
o Can be accessed via start() and stop() of Bundle Activator class
o Bundle context object is its role as the unique execution context of its
associated bundle
o Valid while the bundle is active. (The time range from the framework call
start() up until to the stop() is called )
o Gain access to two types of methods
o Related to deployment and lifecycle management.
o Related to bundle interaction via services.
o They should be treated as sensitive or private objects and not passed freely
among bundles

20. Listening for events
o OSGi framework supports two types of events: BundleEvents and FrameworkEvents
o The former event type reports changes in the lifecycle of bundles, whereas the
latter reports framework-related changes
o The BundleContext object has methods to register BundleListener and
FrameworkListener objects for receiving BundleEvent and FrameworkEvent
notifications, respectively
o How to implement BundleListener and FrameworkListener in your code?.
a. Create a new class which Implements BundleListener or FrameworkListener
b. Implement the bundleChanged(BundleEvent) or frameworkEvent(FrameworkEvent)
methods respectively.
c. Register by creating a new Object of the implemented class to the
BundleContext as a Listener using the bundleContext.addBundleListener() or
bundleContext.addFrameworkListener() respectively.
o No need to call the removeBundleListener() or removeFrameworkListener(), which will
automatically removed when the bundle is stopped.

21. Relationship between carbon & OSGi
Non OSGi Portion OSGi Portion
./carbon.sh
org.wso2.carbon.l
auncher-5.2.0.jar
Equinox + Some
Carbon Bundles
Bundles developed
by Component
Developers

22. What about Dynamism?
o After the Objects are instantiated, they will tightly coupled
with the modules.
o What if we want to change the module or part of the module,
dynamically at the runtime?

23. OSGi Services Layer
o Kind of in VM SOA model
o OSGi Registry
o OSGi Service is simply a Java Object
published to the service registry
o Services are published/looked up
Using an exported interface.
OSGi Service
Registry
Service Provider Service Consumer
Publish Find

24. Registering an OSGi Service
public class Activator implements BundleActivator {
ServiceRegistration serviceRegistration;
public void start(BundleContext bundleContext) throws Exception {
HelloWorld helloWorld = new HelloWorld();
serviceRegistration = bundleContext.registerService(Hello.class, helloWorld, null);
}
public void stop(BundleContext bundleContext) throws Exception {
serviceRegistration.unregister();
}
}

25. Using an OSGi Service
public void start(BundleContext bundleContext) throws Exception {
ServiceReference reference = bundleContext.getServiceReference(Hello.class.getName());
Hello helloService = null;
if (reference != null) {
helloService = (Hello) bundleContext.getService(reference);
} else {
System.out.println("Service reference is null...");
}
if (helloService != null) {
System.out.println(helloService.sayHello());
} else {
System.out.println("Error...");
}
}

26. DEMO
o OSGi Services
o Sample Hello Service
o diag command to diagnose
o p command to check exports and
imports
http://www.slideshare.net/rossmason/realtime-apis

27. Handle Dynamism?
o Services can register/unregister at anytime.
o Service may not be available when tries to use the service
because of the startup order of bundles
o Somehow the service was acquired, but service get unregistered
and now we are having a stale service reference?
o Developer has to handle such scenarios?

28. Slate References
o Stale references are a result of an object either,
o A bundle which the class loader has stopped
o A service object which the unregistered
o Can be result in significant increase of memory
o Developer should follow the best practices to keep away from
slate references,
HOW?
o Listen to the relevant events and act accordingly.

29. Monitoring OSGi Services
o ServiceListener
o Service Trackers
o Declarative Services

30. ServiceListener
o A listener interface that can be used to capture service
lifecycle events.
public interface ServiceListener{
public void serviceChanged(ServiceEvent event);
}
o When an osgi service events are fired; will get a synchronous
callback to the serviceChanged(ServiceEvent event)
method

31. ServiceListener cntd...
public class SampleServiceListener implements ServiceListener , BundleActivator{
public void start(BundleContext bc) throws Exception {
bc.addServiceListener(this);
}
public void stop(BundleContext bc) throws Exception {
bc.removeServiceListener(this);
}
public void serviceChanged(ServiceEvent event) {
switch(event.getType()){
case ServiceEvent.REGISTERED:
break;
case ServiceEvent.UNREGISTERING:
break;
case ServiceEvent.MODIFIED:
break;
}
}
}

32. DEMO
o OSGi Service Listener
http://www.slideshare.net/rossmason/realtime-apis

33. ServiceListener cntd...
o Any concerns/issues regarding
the Service Listener approach?...
https://www.pinterest.com/salenka/meme-faces/

34. ServiceListener cntd...
o What if the service has registered
Before the service listener?
o You probably missed the Service
Registration event
Now What?....
https://www.pinterest.com/zmeers/meme-faces/

35. Working with OSGi Services
o Some major issues with listeners and trackers
o Memory Leaks
o Race conditions
o Working with OSGI services API complex and error prone
o Bundle developers tend to make optimistic assumptions regarding
the availability of services in an attempt to simplify their
code.

36. Declarative Services
o Alternative approach for OSGi Services API’s
o The bundle declare the services; it registers and acquires in an
xml file
o Less effort for developers
o Developers can focus more on the business logic, rather worrying
about the OSGi concerns.

37. Declarative Services cntd...
o Additional resources required in a bundle
o OSGI-INF/example.xml
o Framework reads the configuration file and enables the dynamism
o A service component contains a description that is interpreted at
run time to create and dispose objects
o Maven SCR Plugin to generate the OSGI-INF/example.xml

38. Sample Declarative Service
public class SampleServiceComponent {
private Hello hello;
protected void activate(ComponentContext context) { }
protected void deactivate(ComponentContext context) { }
public void setHelloService(Hello hello) {
this.hello = hello;
}
public void unsetHelloService(Hello hello) {
this.hello = null;
}
}

39. Sample Declarative Service -scrdescriptor
<?xml version="1.0" encoding="UTF-8"?>
<components>
<scr:component enabled="true" immediate="true" name="org.sample.sc.SampleServiceComponent">
<implementation class="org.sample.sl.SampleServiceComponent"/>
<property name="service.pid" value="org.sample.sc.SampleServiceComponent"/>
<reference name="org.sample.api.Hello"
interface="org.sample.api.Hello"
cardinality="1..1"
policy="dynamic"
target="(name=impl2)"
bind="setHelloService"
unbind="unsetHelloService"/>
</scr:component>
</components>

40. SCR Component Lifecycle
o SCR Component become satisfied if the following conditions are
satisfied,
o The component should be enabled
o All the component’s references are satisfied.
A reference is satisfied when,
1. The reference specifies optional cardinality or
2. There is at least one target service for the reference.

41. References to Services
o Cardinality
o 0..1 –> optional and unary
o 1..1 – mandatory and unary (Default)
o 0..n – optional and multiple
o 1..n – mandatory and multiple.
o Policy
o Static
o Dynamic
o Target
o Filter out the necessary services specifying a service property

42. DEMO
o Sample OSGi Service Component
o SCR JavaDoc Tags
o Maven SCR Plugin
o Troubleshoot issues with,
o ls command
o comp command
o Target property?
http://www.slideshare.net/rossmason/realtime-apis

43. Jar to Bundle Conversion
o Used to convert a Jar to Bundle
o Export all packages in jar file
o Export version is set to 0.0.0
o Added the DynamicImport-Package header
o No control over
o This is just evil :(

44. Jar to Bundle Conversion cntd...
o Hence introduced the WSO2 Orbit Project
o Just a pom.xml file with
o Proper dependencies
o Proper Information Hiding
o Most importantly with proper versions.
o Use the Maven-Bundle-Plugin to create orbit bundles

45. Best Practices
o Dynamism comes with the cost, so handle it properly
o Proper imports and exports
o Proper usage of versioning
o Manage the dependencies
o Use services when and where it requires

46. DEMO
o Annotation based SCR
o Sample Code , POM Config
o Enabling OSGI Debug Logs
http://www.slideshare.net/rossmason/realtime-apis

47. References
OSGi Alliance Specifications http://www.osgi.org/Specifications/HomePage
OSGI in Action - CREATING MODULAR APPLICATIONS IN JAVA
Carbon Documentation - https://docs.wso2.com/display/Carbon500/WSO2+Carbon+Documentation